Prof. Dr. Rossitza Pentcheva

Computational Materials Physics
University of Duisburg-Essen

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  • Angular dependence of Hall effect and magnetoresistance in SrRuO3- SrIrO3 heterostructures
    Esser, S. and Wu, J. and Esser, S. and Gruhl, R. and Jesche, A. and Roddatis, V. and Moshnyaga, V. and Pentcheva, R. and Gegenwart, P.
    Physical Review B 103 (2021)
    The perovskite SrRuO3 is a prototypical itinerant ferromagnet which allows interface engineering of its electronic and magnetic properties. We report the synthesis and investigation of atomically flat artificial multilayers of SrRuO3 with the spin-orbit semimetal SrIrO3 in combination with band-structure calculations with a Hubbard U term and topological analysis. The latter reveal an electronic reconstruction and emergence of flat Ru-4dxz bands near the interface, ferromagnetic interlayer coupling, and a negative Berry-curvature contribution to the anomalous Hall effect. We analyze the Hall effect and magnetoresistance measurements as a function of the field angle from an out-of-plane towards an in-plane orientation (either parallel or perpendicular to the current direction) by a two-channel model. The magnetic easy direction is tilted by about 20∘ from the sample normal for low magnetic fields, rotating towards the out-of-plane direction by increasing fields. Fully strained epitaxial growth enables a strong anisotropy of magnetoresistance. An additional Hall effect contribution, not accounted for by the two-channel model, is compatible with stable skyrmions only up to a critical angle of roughly 45∘ from the sample normal. Within about 20∘ from the thin film plane an additional peaklike contribution to the Hall effect suggests the formation of a nontrivial spin structure. © 2021 American Physical Society.
    view abstract10.1103/PhysRevB.103.214430
  • Chern insulating phases and thermoelectric properties of EuO/MgO(001) superlattices
    Köksal, O. and Pentcheva, R.
    Physical Review B 103 (2021)
    The topological and thermoelectric properties of (EuO)n/(MgO)m(001) superlattices (SLs) are explored using density functional theory calculations including a Hubbard U term together with Boltzmann transport theory. In (EuO)1/(MgO)3(001) SL at the lattice constant of MgO a sizable band gap of 0.51 eV is opened by spin-orbit coupling (SOC) due to a band inversion between occupied localized Eu 4f and empty 5d conduction states. This inversion between bands of opposite parity is accompanied by a reorientation in the spin texture along the contour of band inversion surrounding the Γ point and leads to a Chern insulator with C = -1, also confirmed by the single edge state. Moreover, this Chern insulating phase shows promising thermoelectric properties, e.g., a Seebeck coefficient between 400 and 800μVK-1. A similar SOC-induced band inversion takes place also in the ferromagnetic semimetallic (EuO)2/(MgO)2(001) SL. Despite the vanishing band gap, it leads to a substantial anomalous Hall conductivity with values up to -1.04 e2/h and somewhat lower Seebeck coefficient. Both cases emphasize the relation between nontrivial topological bands and thermoelectricity also in systems with broken inversion symmetry. © 2021 American Physical Society.
    view abstract10.1103/PhysRevB.103.045135
  • Correlated interface electron gas in infinite-layer nickelate versus cuprate films on SrTiO3(001)
    Geisler, B. and Pentcheva, R.
    Physical Review Research 3 (2021)
    Based on first-principles calculations including a Coulomb repulsion term, we identify trends in the electronic reconstruction of ANiO2/SrTiO3(001) (A=Pr, La) and ACuO2/SrTiO3(001) (A=Ca, Sr). Common to all cases is the emergence of a quasi-two-dimensional electron gas (q2DEG) in SrTiO3(001), albeit the higher polarity mismatch at the interface of nickelates versus cuprates to the nonpolar SrTiO3(001) substrate (3+/0 versus 2+/0) results in an enhanced q2DEG carrier density. The simulations reveal a significant dependence of the interfacial Ti 3dxy band bending on the rare-earth-metal ion in the nickelate films, being 20-30% larger for PrNiO2 and NdNiO2 than for LaNiO2. Contrary to expectations from the formal polarity mismatch, the electrostatic doping in the films is twice as strong in cuprates as in nickelates. We demonstrate that the depletion of the self-doping rare-earth-metal 5d states enhances the similarity of nickelate and cuprate Fermi surfaces in film geometry, reflecting a single hole in the Ni and Cu 3dx2-y2 orbitals. Finally, we show that NdNiO2 films grown on a polar NdGaO3(001) substrate feature a simultaneous suppression of q2DEG formation as well as Nd 5d self-doping. © 2021 authors.
    view abstract10.1103/PhysRevResearch.3.013261
  • Effect of lattice excitations on transient near-edge x-ray absorption spectroscopy
    Rothenbach, N. and Gruner, M.E. and Ollefs, K. and Schmitz-Antoniak, C. and Salamon, S. and Zhou, P. and Li, R. and Mo, M. and Park, S. and Shen, X. and Weathersby, S. and Yang, J. and Wang, X.J. and Šipr, O. and Ebert, H. and Sokolowski-Tinten, K. and Pentcheva, R. and Bovensiepen, U. and Eschenlohr, A. and Wende, H.
    Physical Review B 104 (2021)
    Time-dependent and constituent-specific spectral changes in soft near-edge x-ray absorption spectroscopy (XAS) of an metal/insulator heterostructure after laser excitation are analyzed at the O K-edge with picosecond time resolution. The oxygen absorption edge of the insulator features a uniform intensity decrease of the fine structure at elevated phononic temperatures, which can be quantified by a simple simulation and fitting procedure presented here. Combining XAS with ultrafast electron diffraction measurements and ab initio calculations demonstrates that the transient intensity changes in XAS can be assigned to a transient lattice temperature. Thus, the sensitivity of transient near-edge XAS to phonons is demonstrated. © 2021 American Physical Society
    view abstract10.1103/PhysRevB.104.144302
  • Influence of fe and ni doping on the oer performance at the co3o4(001) surface: Insights from DFT+u calculations
    Peng, Y. and Hajiyani, H. and Pentcheva, R.
    ACS Catalysis 11 (2021)
    Using density functional theory calculations with an on-site Hubbard U term, we study the oxygen evolution reaction (OER) at the Co3O4(001) surface. The stability of different surface terminations as a function of oxygen partial pressure as well as pH and applied voltage is compiled in a Pourbaix diagram. The termination with octahedral Co and O ions (B-layer) is found to have the lowest overpotential of 0.46 V for an octahedral Co reaction site, associated with its p-type conducting character and the higher oxidation state of the active site (+4) during OER. Furthermore, we systematically investigated the effect of Fe and Ni doping on the overpotential. Our results indicate that Ni doping at an octahedral site in the surface layer reduces the overpotential from 0.46 to 0.34 V. Likewise, Fe doping at an octahedral site at the tetrahedral Co termination (A-layer) lowers η from 0.63 to 0.37 V with octahedral Co remaining in the active site. We note that the potential determining step changes from ∗OH (B-layer) to ∗OOH formation (A-layer). While implicit solvation increases the overpotential by 0.2 V (B-layer) and 0.4 V (A-layer), which is attributed to enhanced binding energies of the intermediates, the general trends with respect to doping remain unchanged. The scaling relationship of the binding energies of ∗OOH and ∗OH is overall satisfied, with the doped systems lying close to the top of the volcano plot of the overpotential versus (ΔG∗O b-ΔG∗OH b ). A further insight into the origin of this behavior is gained by analyzing the changes in oxidation states of surface ions and, in particular, the Co active site during OER. © 2021 American Chemical Society. All rights reserved.
    view abstract10.1021/acscatal.1c00214
  • Orbital engineering in YVO3-LaAl O3 superlattices
    Radhakrishnan, P. and Geisler, B. and Fürsich, K. and Putzky, D. and Wang, Y. and Ilse, S.E. and Christiani, G. and Logvenov, G. and Wochner, P. and Van Aken, P.A. and Goering, E. and Pentcheva, R. and Benckiser, E.
    Physical Review B 104 (2021)
    Oxide heterostructures provide unique opportunities to modify the properties of quantum materials through a targeted manipulation of spin, charge, and orbital states. Here, we use resonant x-ray reflectometry to probe the electronic structure of thin slabs of YVO3 embedded in a superlattice with LaAlO3. We extend the previously established methods of reflectometry analysis to a general form applicable to t2g electron systems and extract quantitative depth-dependent x-ray linear dichroism profiles. Our data reveal an artificial, layered orbital polarization, where the average occupation of xz and yz orbitals in the interface planes next to LaAlO3 is inverted compared to the central part of the YVO3 slab. This phase is stable down to 30 K and the bulklike orbital ordering transitions are absent. We identify the key mechanism for the electronic reconstruction to be a combination of epitaxial strain and spatial confinement by the LaAlO3 layers, in good agreement with predictions from ab initio theory. © 2021 authors. Published by the American Physical Society.
    view abstract10.1103/PhysRevB.104.L121102
  • Surface oxygen Vacancies on Reduced Co3O4(100): Superoxide Formation and Ultra-Low-Temperature CO Oxidation
    Liu, Y. and Peng, Y. and Naschitzki, M. and Gewinner, S. and Schöllkopf, W. and Kuhlenbeck, H. and Pentcheva, R. and Roldan Cuenya, B.
    Angewandte Chemie - International Edition 60 (2021)
    The activation of molecular oxygen is a fundamental step in almost all catalytic oxidation reactions. We have studied this topic and the role of surface vacancies for Co3O4(100) films with a synergistic combination of experimental and theoretical methods. We show that the as-prepared surface is B-layer terminated and that mild reduction produces oxygen single and double vacancies in this layer. Oxygen adsorption experiments clearly reveal different superoxide species below room temperature. The superoxide desorbs below ca. 120 K from a vacancy-free surface and is not active for CO oxidation while superoxide on a surface with oxygen vacancies is stable up to ca. 270 K and can oxidize CO already at the low temperature of 120 K. The vacancies are not refilled by oxygen from the superoxide, which makes them suitable for long-term operation. Our joint experimental/theoretical effort highlights the relevance of surface vacancies in catalytic oxidation reactions. © 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.
    view abstract10.1002/anie.202103359
  • Synergistic Effects of Co and Fe on the Oxygen Evolution Reaction Activity of LaCoxFe1−xO3
    Füngerlings, A. and Koul, A. and Dreyer, M. and Rabe, A. and Morales, D.M. and Schuhmann, W. and Behrens, M. and Pentcheva, R.
    Chemistry - A European Journal (2021)
    In a combined experimental and theoretical study we assess the role of Co incorporation on the OER activity of LaCoxFe1−xO3. Phase pure perovskites were synthesized up to (Formula presented.) in 0.025/0.050 steps. HAADF STEM and EDX analysis points towards FeO2-terminated (001)-facets in LaFeO3, in accordance with the stability diagram obtained from density functional theory calculations with a Hubbard U term (DFT+U). Linear sweep voltammetry conducted in a rotating disk electrode setup shows a reduction of the OER overpotential and a nonmonotonic trend with x, with double layer capacitance measurements indicating an intrinsic nature of activity. This is supported by DFT+U results that show reduced overpotentials for both Fe and Co reaction sites with the latter reaching values of 0.32–0.40 V, ∼0.3 V lower than for Fe. This correlates with a stronger reduction of the binding energy difference of the *O and *OH intermediates towards an optimum value of 1.6 eV for (Formula presented.), the OH deprotonation being the potential limiting step in most cases. Significant variations of the magnetic moments of both surface and subsurface Co and Fe during OER demonstrate that the beneficial effect is a result of a concerted action involving many surrounding ions, which extends the concept of the active site. © 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
    view abstract10.1002/chem.202102829
  • Theoretical description of optical and X-ray absorption spectra of MgO including many-body effects
    Begum, V. and Gruner, M.E. and Vorwerk, C. and Draxl, C. and Pentcheva, R.
    Physical Review B 103 (2021)
    Here we report the optical and X-ray absorption spectra of the wide-band-gap oxide MgO using density functional theory and many-body perturbation theory (MBPT). Our comprehensive study of the electronic structure shows that while the band gap is underestimated with the exchange-correlation functional PBEsol (4.58 eV) and the hybrid functional HSE06 (6.58 eV) compared to the experimental value (7.7 eV), it is significantly improved (7.52 eV) and even overcompensated (8.53 eV) when quasiparticle corrections are considered. Inclusion of excitonic effects by solving the Bethe-Salpeter equation (BSE) yields the optical spectrum in excellent agreement with experiment. Excellent agreement is observed also for the O and Mg K-edge absorption spectra, demonstrating the importance of the electron-hole interaction within MBPT. Projection of the electron-hole coupling coefficients from the BSE eigenvectors on the band structure allows us to determine the origin of prominent peaks and identify the orbital character of the relevant contributions. The real-space projection of the lowest energy exciton wave function of the optical spectrum indicates a Wannier-Mott type, whereas the first exciton in the O K edge is more localized. © 2021 American Physical Society.
    view abstract10.1103/PhysRevB.103.195128
  • Tuning the Thermoelectric Properties of Transition Metal Oxide Thin Films and Superlattices on the Quantum Scale
    Geisler, B. and Yordanov, P. and Gruner, M.E. and Keimer, B. and Pentcheva, R.
    Physica Status Solidi (B) Basic Research (2021)
    Combining advanced growth and characterization techniques with state-of-the-art first-principles simulations in the frameworks of density functional theory and Boltzmann transport theory, recent advances in the field of transition metal oxide films and superlattices (SLs) as thermoelectric materials are discussed, with particular focus on a selection of quantum-scale approaches to tune their thermoelectric performance. Specifically, (Formula presented.) films grown on regular and miscut substrates have enabled experimental confirmation of the large predicted out-of-plane Seebeck coefficient of this anisotropic material and also reveal the necessity of a Hubbard-U parameter on the Co (Formula presented.) states. Furthermore, oxygen diffusion and incorporation from the (Formula presented.) substrate lead to a significant enhancement of the high-temperature Seebeck coefficient in (Formula presented.) SLs. Next, it is shown how n- and p-type materials can be achieved either by exploiting interface polarity in a (Formula presented.) SL or using epitaxial strain to shift orbital-dependent transport resonances across the Fermi level in (Formula presented.) SLs. Moreover, confinement- and strain-induced metal-to-insulator transitions induce high Seebeck coefficients and power factors in short-period (Formula presented.) and (Formula presented.) SLs ((Formula presented.) V, Cr, Mn). Finally, a relation between the topologically nontrivial Chern insulating behavior and enhanced thermoelectric response in (Formula presented.) SLs is established. The article concludes with a discussion of challenges and future topics of research in oxide thermoelectrics. © 2021 The Authors. physica status solidi (b) basic solid state physics published by Wiley-VCH GmbH
    view abstract10.1002/pssb.202100270
  • Competition of defect ordering and site disproportionation in strained LaCoO3 on SrTiO3 (001)
    Geisler, B. and Pentcheva, R.
    Physical Review B 101 (2020)
    The origin of the 3×1 reconstruction observed in epitaxial LaCoO3 films on SrTiO3(001) is assessed by first-principles calculations including a Coulomb repulsion term. We compile a phase diagram as a function of the oxygen pressure, which shows that (3×1)-ordered oxygen vacancies (LaCoO2.67) are favored under commonly used growth conditions, while stoichiometric films emerge under oxygen-rich conditions. Growth of further reduced LaCoO2.5 brownmillerite films is impeded by phase separation. We report two competing ground-state candidates for stoichiometric films: a semimetallic phase with 3×1 low-spin/intermediate-spin/intermediate-spin (LS/IS/IS) magnetic order and a semiconducting phase with IS/IS/IS magnetic order. This demonstrates that tensile strain induces ferromagnetism even in the absence of oxygen vacancies. Both phases exhibit an intriguing (3×1)-reconstructed octahedral rotation pattern and accordingly modulated La-La distances. In particular, charge and bond disproportionation and concomitant orbital order of the t2g hole emerge at the Co sites that are also observed for unstrained bulk LaCoO3 in the IS state and explain structural data obtained by x-ray diffraction at elevated temperature. Site disproportionation drives a metal-to-semiconductor transition that reconciles the IS state with the experimentally observed low conductivity during spin-state crossover without the presence of Jahn-Teller distortions. © 2020 American Physical Society. ©2020 American Physical Society.
    view abstract10.1103/PhysRevB.101.165108
  • Extreme tensile strain states in La0.7Ca0.3MnO3 membranes
    Hong, S.S. and Gu, M. and Verma, M. and Harbola, V. and Wang, B.Y. and Lu, D. and Vailionis, A. and Hikita, Y. and Pentcheva, R. and Rondinelli, J.M. and Hwang, H.Y.
    Science 368 (2020)
    A defining feature of emergent phenomena in complex oxides is the competition and cooperation between ground states. In manganites, the balance between metallic and insulating phases can be tuned by the lattice; extending the range of lattice control would enhance the ability to access other phases. We stabilized uniform extreme tensile strain in nanoscale La0.7Ca0.3MnO3 membranes, exceeding 8% uniaxially and 5% biaxially. Uniaxial and biaxial strain suppresses the ferromagnetic metal at distinctly different strain values, inducing an insulator that can be extinguished by a magnetic field. Electronic structure calculations indicate that the insulator consists of charge-ordered Mn4+ and Mn3+ with staggered strain-enhanced Jahn-Teller distortions within the plane. This highly tunable strained membrane approach provides a broad opportunity to design and manipulate correlated electron states. © 2020 American Association for the Advancement of Science. All rights reserved.
    view abstract10.1126/science.aax9753
  • Fundamental difference in the electronic reconstruction of infinite-layer versus perovskite neodymium nickelate films on SrTiO3 (001)
    Geisler, B. and Pentcheva, R.
    Physical Review B 102 (2020)
    Motivated by recent reports of superconductivity in Sr-doped NdNiO2 films on SrTiO3(001) [Nature (London) 572, 624 (2019)NATUAS0028-083610.1038/s41586-019-1496-5], we explore the role of the polar interface on the structural and electronic properties of NdNiOn/SrTiO3(001) (n=2,3) by performing first-principles calculations including a Coulomb repulsion term. For infinite-layer nickelate films (n=2), electronic reconstruction drives the surprising emergence of a two-dimensional electron gas (2DEG) at the interface involving a strong occupation of the Ti 3d states. This effect is more pronounced than in LaAlO3/SrTiO3(001) and accompanied by a substantial reconstruction of the Fermi surface: a depletion of the self-doping Nd 5d states and an enhanced Ni eg orbital polarization reaching up to 35% at the surface, reflecting a single hole in the 3dx2-y2 states, i.e., cupratelike behavior. In contrast, no 2DEG forms for perovskite films (n=3) or if a single perovskite layer persists at the interface. We show that the topotactic reaction from the perovskite to the infinite-layer phase is confined to the nickelate film, whereas the SrTiO3 substrate remains intact. © 2020 American Physical Society.
    view abstract10.1103/PhysRevB.102.020502
  • How photocorrosion can trick you: A detailed study on low-bandgap Li doped CuO photocathodes for solar hydrogen production
    Kampmann, J. and Betzler, S. and Hajiyani, H. and Häringer, S. and Beetz, M. and Harzer, T. and Kraus, J. and Lotsch, B.V. and Scheu, C. and Pentcheva, R. and Fattakhova-Rohlfing, D. and Bein, T.
    Nanoscale 12 (2020)
    The efficiency of photoelectrochemical tandem cells is still limited by the availability of stable low band gap electrodes. In this work, we report a photocathode based on lithium doped copper(ii) oxide, a black p-type semiconductor. Density functional theory calculations with a Hubbard U term show that low concentrations of Li (Li0.03Cu0.97O) lead to an upward shift of the valence band maximum that crosses the Fermi level and results in a p-type semiconductor. Therefore, Li doping emerged as a suitable approach to manipulate the electronic structure of copper oxide based photocathodes. As this material class suffers from instability in water under operating conditions, the recorded photocurrents are repeatedly misinterpreted as hydrogen evolution evidence. We investigated the photocorrosion behavior of LixCu1-xO cathodes in detail and give the first mechanistic study of the fundamental physical process. The reduced copper oxide species were localized by electron energy loss spectroscopy mapping. Cu2O grows as distinct crystallites on the surface of LixCu1-xO instead of forming a dense layer. Additionally, there is no obvious Cu2O gradient inside the films, as Cu2O seems to form on all LixCu1-xO nanocrystals exposed to water. The application of a thin Ti0.8Nb0.2Ox coating by atomic layer deposition and the deposition of a platinum co-catalyst increased the stability of LixCu1-xO against decomposition. These devices showed a stable hydrogen evolution for 15 minutes. © The Royal Society of Chemistry .
    view abstract10.1039/c9nr10250g
  • Influence of 3 d, 4 d, and 5 d dopants on the oxygen evolution reaction at α-Fe2O3(0001) under dark and illumination conditions
    Hajiyani, H. and Pentcheva, R.
    Journal of Chemical Physics 152 (2020)
    Using density functional theory+U (DFT+U) calculations, we explore the effect of dopants on the performance of α-Fe2O3(0001) as an anode material for the oxygen evolution reaction (OER). Systematic screening of 3d, 4d, and 5d transition metal dopants indicates general trends with dopant band filling and allows us to identify the most efficient dopants with respect to the overpotential and relate those to the solution energy and electronic properties. Different conditions (electrochemical vs photoelectrochemical) are accounted for by considering hydroxylated, hydrated, and oxygenated terminations. Based on the DFT+U results, we identify Rh as the most promising dopant that can reduce the overpotential both under dark and illumination conditions: from 0.56 V to 0.48 V for the hydroxylated surface and quite substantially from 1.12 V to 0.31 V for the hydrated termination and from 0.81 V to 0.56 V for the oxygenated surface. The origin of this improvement is attributed to the modification of the binding energy of chemisorbed species to the Fe2O3(0001) surface. Investigation of the spin density of intermediate steps during the OER shows that surface iron ions adopt a wide range of oxidation states (+2, +3, and +4) in pure hematite, depending on the termination and chemisorbed species on the surface, but a Fe+3 state is stabilized predominantly upon doping. While Rh is in the +3 state in the bulk, it transforms to +4 at the surface and acquires a finite magnetic moment in several intermediate steps. © 2020 Author(s).
    view abstract10.1063/1.5143236
  • Interface-related magnetic and vibrational properties in Fe/MgO heterostructures from nuclear resonant spectroscopy and first-principles calculations
    Eggert, B. and Gruner, M.E. and Ollefs, K. and Schuster, E. and Rothenbach, N. and Hu, M.Y. and Zhao, J. and Toellner, T.S. and Sturhahn, W. and Pentcheva, R. and Cuenya, B.R. and Alp, E.E. and Wende, H. and Keune, W.
    Physical Review Materials 4 (2020)
    We combine Fe57 Mössbauer spectroscopy and Fe57 nuclear resonant inelastic x-ray scattering (NRIXS) on nanoscale polycrystalline [bcc-Fe57/MgO] multilayers with various Fe-layer thicknesses and layer-resolved density-functional-theory (DFT)-based first-principles calculations of a (001)-oriented [Fe(8 ML)/MgO(8 ML)](001) heterostructure (where ML denotes monolayer) to unravel the interface-related atomic vibrational properties of a multilayer system. Being consistent in theory and experiment, we observe enhanced hyperfine magnetic fields Bhf in the multilayers as compared to Bhf in bulk bcc Fe; this effect is associated with the Fe/MgO interface layers. NRIXS and DFT both reveal a strong reduction of the longitudinal acoustic phonon peak in combination with an enhancement of the low-energy vibrational density of states (VDOS) suggesting that the presence of interfaces and the associated increase in the layer-resolved magnetic moments results in drastic changes in the Fe-partial VDOS. From the experimental and calculated VDOS, vibrational thermodynamic properties have been determined as a function of Fe thickness and were found to be in excellent agreement. © 2020 American Physical Society.
    view abstract10.1103/PhysRevMaterials.4.044402
  • Single-layer Janus black arsenic-phosphorus (b-AsP): Optical dichroism, anisotropic vibrational, thermal, and elastic properties
    Li, L.L. and Bacaksiz, C. and Nakhaee, M. and Pentcheva, R. and Peeters, F.M. and Yagmurcukardes, M.
    Physical Review B 101 (2020)
    By using density functional theory (DFT) calculations, we predict a puckered, dynamically stable Janus single-layer black arsenic-phosphorus (b-AsP), which is composed of two different atomic sublayers, arsenic and phosphorus atoms. The calculated phonon spectrum reveals that Janus single-layer b-AsP is dynamically stable with either pure or coupled optical phonon branches arising from As and P atoms. The calculated Raman spectrum indicates that due to the relatively strong P-P bonds, As atoms have no contribution to the high-frequency optical vibrations. In addition, the orientation-dependent isovolume heat capacity reveals anisotropic contributions of LA and TA phonon branches to the low-temperature thermal properties. Unlike pristine single layers of b-As and b-P, Janus single-layer b-AsP exhibits additional out-of-plane asymmetry which leads to important consequences for its electronic, optical, and elastic properties. In contrast to single-layer b-As, Janus single-layer b-AsP is found to possess a direct band gap dominated by the P atoms. Moreover, real and imaginary parts of the dynamical dielectric function, including excitonic effects, reveal the highly anisotropic optical feature of the Janus single-layer. A tight-binding (TB) model is also presented for Janus single-layer b-AsP, and it is shown that, with up to seven nearest hoppings, the TB model reproduces well the DFT band structure in the low-energy region around the band gap. This TB model can be used in combination with the Green's function approach to study, e.g., quantum transport in finite systems based on Janus single-layer b-AsP. Furthermore, the linear-elastic properties of Janus single-layer b-AsP are investigated, and the orientation-dependent in-plane stiffness and Poisson ratio are calculated. It is found that the Janus single layer exhibits strong in-plane anisotropy in its Poisson ratio much larger than that of single-layer b-P. This Janus single layer is relevant for promising applications in optical dichroism and anisotropic nanoelasticity. © 2020 American Physical Society.
    view abstract10.1103/PhysRevB.101.134102
  • Chern and Z 2 topological insulating phases in perovskite-derived 4d and 5d oxide buckled honeycomb lattices
    Köksal, O. and Pentcheva, R.
    Scientific Reports 9 (2019)
    Based on density functional theory calculations including a Coulomb repulsion parameter U, we explore the topological properties of (LaXO3)2/(LaAlO3)4 (111) with X = 4d and 5d cations. The metastable ferromagnetic phases of LaTcO3 and LaPtO3 with preserved P321 symmetry emerge as Chern insulators (CI) with C = 2 and 1 and band gaps of 41 and 38 meV at the lateral lattice constant of LaAlO3, respectively. Berry curvatures, spin textures as well as edge states provide additional insight into the nature of the CI states. While for X = Tc the CI phase is further stabilized under tensile strain, for X = Pd and Pt a site disproportionation takes place when increasing the lateral lattice constant from aLAO to aLNO. The CI phase of X = Pt shows a strong dependence on the Hubbard U parameter with sign reversal for higher values associated with the change of band gap opening mechanism. Parallels to the previously studied (X2O3)1/(Al2O3)5 (0001) honeycomb corundum layers are discussed. Additionally, non-magnetic systems with X = Mo and W are identified as potential candidates for Z2 topological insulators at aLAO with band gaps of 26 and 60 meV, respectively. The computed edge states and Z2 invariants underpin the non-trivial topological properties. © 2019, The Author(s).
    view abstract10.1038/s41598-019-53125-1
  • Dynamics of optical excitations in a Fe/MgO(001) heterostructure from time-dependent density functional theory
    Gruner, M.E. and Pentcheva, R.
    Physical Review B 99 (2019)
    In the framework of real-time time-dependent density functional theory we unravel the layer-resolved dynamics of excited carriers in a (Fe)1/(MgO)3(001) multilayer after an optical excitation with a frequency below the band gap of bulk MgO. Substantial transient changes to the electronic structure, which persist after the duration of the pulse, are mainly observed for in-plane polarized electric fields, corresponding to a laser pulse arriving perpendicular to the interface. While the strongest charge redistribution takes place in the Fe layer, a time-dependent change in the occupation numbers is visible in all layers, mediated by the presence of interface states. The time evolution of the layer-resolved time-dependent occupation numbers indicates a strong orbital dependence with the depletion from in-plane orbitals (e.g., dx2-y2 of Fe) and accumulation in out-of-plane orbitals (d3z2-r2 of Fe and pz of apical oxygen). We also observe a small net charge transfer of less than one percent of an electron away from oxygen towards the Mg sites, even for MgO layers which are not directly in contact with the metallic Fe. © 2019 American Physical Society.
    view abstract10.1103/PhysRevB.99.195104
  • Effect of confinement and octahedral rotations on the electronic, magnetic, and thermoelectric properties of correlated SrX O3/SrTiO3(001) superlattices (X= V, Cr, or Mn)
    Verma, M. and Geisler, B. and Pentcheva, R.
    Physical Review B 100 (2019)
    By using density functional theory calculations with an on-site Coulomb repulsion term combined with Boltzmann transport theory, we explore the effect of t2g orbital occupation on the electronic, magnetic, and thermoelectric properties of (SrXO3)1/(SrTiO3)n(001) superlattices with n=1,3 and X=V, Cr, and Mn. In order to disentangle the effect of quantum confinement and octahedral rotations and to account for a wider temperature range, P4/mmm (untilted) and P21/c (tilted) phases are considered. We find that the ground-state superlattice geometries always display finite octahedral rotations, which drive an orbital reconstruction and a concomitant metal-to-insulator transition in confined SrVO3 and SrCrO3 single layers with ferro- A nd antiferromagnetic spin alignments, respectively. On the other hand, the confined SrMnO3 single layer exhibits electronic properties similar to bulk. We show that confinement enhances the thermoelectric properties, particularly for SrVO3 and SrCrO3 due to the emergent Mott phase. Large room-temperature Seebeck coefficients are obtained for the tilted superlattices, ranging from 500 to 600μV/K near the band edges. The estimated attainable power factors of 27.9(26.6)μWK-2cm-1 in plane for the (SrCrO3)1/(SrTiO3)1(001) superlattice with P4/mmm(P21/c) symmetry and 28.1μWK-2cm-1 cross plane for the (SrMnO3)1/(SrTiO3)1(001) superlattice with P21/c symmetry compare favorably with some of the best-performing oxide thermoelectrics. This demonstrates that the idea to use quantum confinement to enhance the thermoelectric response in correlated transition-metal oxide superlattices [Phys. Rev. Mater. 2, 055403 (2018)2475-995310.1103/PhysRevMaterials.2.055403] can be applied to a broader class of materials combinations. © 2019 American Physical Society.
    view abstract10.1103/PhysRevB.100.165126
  • Electronic Structure of a Graphene-like Artificial Crystal of NdNiO3
    Arab, A. and Liu, X. and Köksal, O. and Yang, W. and Chandrasena, R.U. and Middey, S. and Kareev, M. and Kumar, S. and Husanu, M.-A. and Yang, Z. and Gu, L. and Strocov, V.N. and Lee, T.-L. and Minár, J. and Pentcheva, R. and Chakhalian, J. and Gray, A.X.
    Nano Letters 19 (2019)
    Artificial complex-oxide heterostructures containing ultrathin buried layers grown along the pseudocubic [111] direction have been predicted to host a plethora of exotic quantum states arising from the graphene-like lattice geometry and the interplay between strong electronic correlations and band topology. To date, however, electronic-structural investigations of such atomic layers remain an immense challenge due to the shortcomings of conventional surface-sensitive probes with typical information depths of a few angstroms. Here, we use a combination of bulk-sensitive soft X-ray angle-resolved photoelectron spectroscopy (SX-ARPES), hard X-ray photoelectron spectroscopy (HAXPES), and state-of-the-art first-principles calculations to demonstrate a direct and robust method for extracting momentum-resolved and angle-integrated valence-band electronic structure of an ultrathin buckled graphene-like layer of NdNiO3 confined between two 4-unit cell-thick layers of insulating LaAlO3. The momentum-resolved dispersion of the buried Ni d states near the Fermi level obtained via SX-ARPES is in excellent agreement with the first-principles calculations and establishes the realization of an antiferro-orbital order in this artificial lattice. The HAXPES measurements reveal the presence of a valence-band bandgap of 265 meV. Our findings open a promising avenue for designing and investigating quantum states of matter with exotic order and topology in a few buried layers. Copyright © 2019 American Chemical Society.
    view abstract10.1021/acs.nanolett.9b03962
  • Inducing n - And p -Type Thermoelectricity in Oxide Superlattices by Strain Tuning of Orbital-Selective Transport Resonances
    Geisler, B. and Pentcheva, R.
    Physical Review Applied 11 (2019)
    By combining first-principles simulations including an on-site Coulomb-repulsion term and Boltzmann theory, we demonstrate how the interplay of quantum confinement and epitaxial strain allows one to selectively design n- and p-type thermoelectric response in (LaNiO3)3/(LaAlO3)1(001) superlattices. In particular, varying strain from -4.9% to 2.9% tunes the Ni orbital polarization at the interfaces from -6% to 3%. This is caused by an electron redistribution among Ni 3dx2-y2- and 3dz2-derived quantum-well states, which respond differently to strain. Owing to this charge transfer, the position of emerging cross-plane-transport resonances can be tuned relative to the Fermi energy. For moderate compressive strains of -1.5% and -2.8%, the cross-plane Seebeck coefficient reaches approximately -60 and 100μV/K at around room temperature, respectively. This provides a compelling mechanism to tailor thermoelectric materials. Finally, we demonstrate the robustness of the proposed concept with respect to oxygen-vacancy formation. © 2019 American Physical Society.
    view abstract10.1103/PhysRevApplied.11.044047
  • Interaction-driven spin-orbit effects and Chern insulating phases in corundum-based 4d and 5d oxide honeycomb lattices
    Köksal, O. and Pentcheva, R.
    Journal of Physics and Chemistry of Solids 128 (2019)
    Using density functional theory calculations with a Hubbard U, we explore topologically nontrivial phases in X2O3 honeycomb layers with X= 4d and 5d cation inserted in the band insulator α-Al2O3 along the [0001]-direction. Several promising candidates for quantum anomalous Hall insulators (QAHI)are identified. In particular, for X = Tc and Pt spin-orbit coupling (SOC)opens a gap of 54 and 59 meV, respectively, leading to Chern insulators (CI)with C = −2 and −1. The nature of different Chern numbers is related to the corresponding spin textures. The Chern insulating phase is sensitive to the Coulomb repulsion strength: X = Tc undergoes a transition from a CI to a trivial metallic state beyond a critical strength of Uc=2.5 eV. A comparison between the isoelectronic metastable FM phases of X = Pd and Pt emphasizes the intricate balance between electronic correlations and SOC: while the former is a trivial insulator, the latter is a Chern insulator. In addition, X = Os turns out to be a FM Mott insulator with an unpaired electron in the t2g manifold where SOC induces an unusually high orbital moment of 0.34 μB along the z-axis. Parallels to the 3d honeycomb corundum cases are discussed. © 2018 Elsevier Ltd
    view abstract10.1016/j.jpcs.2018.01.049
  • Large thermopower anisotropy in PdCo O2 thin films
    Yordanov, P. and Sigle, W. and Kaya, P. and Gruner, M.E. and Pentcheva, R. and Keimer, B. and Habermeier, H.-U.
    Physical Review Materials 3 (2019)
    Motivated by recent theoretical studies predicting a large thermopower anisotropy in the layered delafossite PdCoO2, we have used pulsed laser deposition to synthesize thin films on (0001)-oriented and offcut Al2O3 substrates. By combining transport measurements on films with different offcut angles, tensor rotation relations, and an iterative fit procedure for the transport parameters, we have determined the resistivity and the thermopower along the main crystallographic axes in the temperature range 300-815 K. The data reveal a small positive Seebeck coefficient along the delafossite planes and a large negative Seebeck coefficient perpendicular to the planes, in excellent agreement with density functional calculations in the presence of moderate Coulomb correlations. The methodology introduced here is generally applicable for measurements of the thermoelectric properties of materials with highly anisotropic electronic structures. © 2019 American Physical Society.
    view abstract10.1103/PhysRevMaterials.3.085403
  • Microscopic nonequilibrium energy transfer dynamics in a photoexcited metal/insulator heterostructure
    Rothenbach, N. and Gruner, M.E. and Ollefs, K. and Schmitz-Antoniak, C. and Salamon, S. and Zhou, P. and Li, R. and Mo, M. and Park, S. and Shen, X. and Weathersby, S. and Yang, J. and Wang, X.J. and Pentcheva, R. and Wende, H. and Bovensiepen, U. and Sokolowski-Tinten, K. and Eschenlohr, A.
    Physical Review B 100 (2019)
    The element specificity of soft X-ray spectroscopy makes it an ideal tool for analyzing the microscopic origin of ultrafast dynamics induced by localized optical excitation in metal-insulator heterostructures. Using [Fe/MgO]n as a model system, we perform ultraviolet pump/soft X-ray probe experiments, which are sensitive to all constituents of these heterostructures, to probe both electronic and lattice excitations. Complementary ultrafast electron diffraction experiments independently analyze the lattice dynamics of the Fe constituent, and together with ab initio calculations yield comprehensive insight into the microscopic processes leading to local relaxation within a single constituent or nonlocal relaxation between two constituents. Besides electronic excitations in Fe, which are monitored at the Fe L3 absorption edge and relax within 1 ps by electron-phonon coupling, soft X-ray analysis identifies a change at the oxygen K absorption edge of the MgO layers which occurs within 0.5 ps. This ultrafast energy transfer across the Fe-MgO interface is mediated by high-frequency, interface vibrational modes, which are excited by hot electrons in Fe and couple to vibrations in MgO in a mode-selective, nonthermal manner. A second, slower timescale is identified at the oxygen K pre-edge and the Fe L3 edge. The slower process represents energy transfer by acoustic phonons and contributes to thermalization of the entire heterostructure. We thus find that the interfacial energy transfer is associated with nonequilibrium behavior in the phonon system. Because our experiments lack signatures of charge transfer across the interface, we conclude that phonon-mediated processes dominate the competition of electronic and lattice excitations in these nonlocal, nonequilibrium dynamics. © 2019 American Physical Society.
    view abstract10.1103/PhysRevB.100.174301
  • Role of the exchange-correlation functional on the structural, electronic, and optical properties of cubic and tetragonal SrTiO3 including many-body effects
    Begum, V. and Gruner, M.E. and Pentcheva, R.
    Physical Review Materials 3 (2019)
    SrTiO3 is a model perovskite compound with unique properties and technological relevance. At 105 K it undergoes a transition from a cubic to a tetragonal phase with characteristic antiferrodistortive rotations of the TiO6 octahedra. Here we study systematically the effect of different exchange-correlation functionals on the structural, electronic, and optical properties of cubic and tetragonal STO by comparing the recently implemented strongly constrained and appropriately normed (SCAN) meta-GGA functional with the generalized gradient approximation (PBE96 and PBEsol) and the hybrid functional (HSE06). SCAN is found to significantly improve the description of the structural properties, in particular the rotational angle of the tetragonal phase, comparable to HSE06 at a computational cost similar to GGA. The addition of a Hubbard U term (SCAN+U, U=7.45 eV) allows us to achieve the experimental band gap of 3.25 eV with a moderate increase in the lattice constant, whereas within GGA+U the gap is underestimated even for high U values. The effect of the exchange-correlation functional on the optical properties is progressively reduced from 1.5 eV variance in the onset of the spectrum in the independent particle picture to 0.3 eV upon inclusion of many-body effects within the framework of the GW approximation (single-shot G0W0) and excitonic corrections by solving the Bethe-Salpeter equation (BSE). Moreover, a model BSE approach is shown to reproduce the main features of the optical spectrum at a lower cost compared to G0W0+BSE. Strong excitonic effects are found in agreement with previous results and their origin is analyzed based on the contributing interband transitions. Last but not least, the effect of the tetragonal distortion on the optical spectrum is discussed and compared to available experimental data. © 2019 American Physical Society.
    view abstract10.1103/PhysRevMaterials.3.065004
  • Selective 2-Propanol Oxidation over Unsupported Co3O4 Spinel Nanoparticles: Mechanistic Insights into Aerobic Oxidation of Alcohols
    Anke, S. and Bendt, G. and Sinev, I. and Hajiyani, H. and Antoni, H. and Zegkinoglou, I. and Jeon, H. and Pentcheva, R. and Roldan Cuenya, B. and Schulz, S. and Muhler, M.
    ACS Catalysis 9 (2019)
    Crystalline Co3O4 nanoparticles with a uniform size of 9 nm as shown by X-ray diffraction (XRD) and transmission electron microscopy (TEM) were synthesized by thermal decomposition of cobalt acetylacetonate in oleylamine and applied in the oxidation of 2-propanol after calcination. The catalytic properties were derived under continuous flow conditions as a function of temperature up to 573 K in a fixed-bed reactor at atmospheric pressure. Temperature-programmed oxidation, desorption (TPD), surface reaction (TPSR), and 2-propanol decomposition experiments were performed to study the interaction of 2-propanol and O2 with the exposed spinel surfaces. Co3O4 selectively catalyzes the oxidative dehydrogenation of 2-propanol, yielding acetone and H2O and only to a minor extent the total oxidation to CO2 and H2O at higher temperatures. The high catalytic activity of Co3O4 reaching nearly full conversion with 100% selectivity to acetone at 430 K is attributed to the high amount of active Co3+ species at the catalyst surface as well as surface-bound reactive oxygen species observed in the O2 TPD, 2-propanol TPD, TPSR, and 2-propanol decomposition experiments. Density functional theory calculations with a Hubbard U term support the identification of the 5-fold-coordinated octahedral surface Co5c3+ as the active site, and oxidative dehydrogenation involving adsorbed atomic oxygen was found to be the energetically most favored pathway. The consumption of surface oxygen and reduction of Co3+ to Co2+ during 2-propanol oxidation derived from X-ray absorption spectroscopy and X-ray photoelectron spectroscopy measurements before and after reaction and poisoning by strongly bound carbonaceous species result in the loss of the low-temperature activity, while the high-temperature reaction pathway remained unaffected. © 2019 American Chemical Society.
    view abstract10.1021/acscatal.9b01048
  • Sn-Doped Hematite for Photoelectrochemical Water Splitting: The Effect of Sn Concentration
    Zhang, S. and Hajiyani, H. and Hufnagel, A.G. and Kampmann, J. and Breitbach, B. and Bein, T. and Fattakhova-Rohlfing, D. and Pentcheva, R. and Scheu, C.
    Zeitschrift fur Physikalische Chemie (2019)
    Hematite-based photoanodes have been intensively studied for photoelectrochemical water oxidation. The n-type dopant Sn has been shown to benefit the activity of hematite anodes. We demonstrate in this study that Sn-doped hematite thin films grown by atomic layer deposition can achieve uniform doping across the film thickness up to at least 32 mol%, far exceeding the equilibrium solubility limit of less than 1 mol%. On the other hand, with the introduction of Sn doping, the hematite crystallite size decreases and many twin boundaries form in the film, which may contribute to the low photocurrent observed in these films. Density functional theory calculations with a Hubbard U term show that Sn doping has multiple effects on the hematite properties. With increasing Sn4+ content, the Fe2+ concentration increases, leading to a reduction of the band gap and finally to a metallic state. This goes hand in hand with an increase of the lattice constant. ©2019 Walter de Gruyter GmbH, Berlin/Boston 2019.
    view abstract10.1515/zpch-2019-1482
  • Three-dimensional character of the Fermi surface in ultrathin LaTiO3/SrTiO3 heterostructures
    Veit, M.J. and Chan, M.K. and Ramshaw, B.J. and Arras, R. and Pentcheva, R. and Suzuki, Y.
    Physical Review B 99 (2019)
    LaTiO3 films on SrTiO3 single crystal substrates exhibit metallic behavior attributed to the LaTiO3 film, the interface as well as part of the SrTiO3. In the limit of ultrathin LaTiO3 films on SrTiO3, the contribution to the metallicity from strain-induced electronic structure modification of the LaTiO3 film is minimized so that the dominant contribution to metallicity is from the interface and part of the SrTiO3 due to charge transfer of 3d electrons from LaTiO3 to SrTiO3. In such a limit, we observe quantum oscillations whose angular dependence indicates a three-dimensional Fermi surface. Such angular dependence is observed in two sets of quantum oscillations - one low frequency and one high frequency - that we have attributed to an inner and outer Fermi surface associated with a Rashba-like spin split hybridized dxz+yz band. © 2019 American Physical Society.
    view abstract10.1103/PhysRevB.99.115126
  • Viewpoint: Atomic-Scale Design Protocols toward Energy, Electronic, Catalysis, and Sensing Applications
    Belviso, F. and Claerbout, V.E.P. and Comas-Vives, A. and Dalal, N.S. and Fan, F.-R. and Filippetti, A. and Fiorentini, V. and Foppa, L. and Franchini, C. and Geisler, B. and Ghiringhelli, L.M. and Groß, A. and Hu, S. and Íñiguez, J. and Kauwe, S.K. and Musfeldt, J.L. and Nicolini, P. and Pentcheva, R. and Polcar, T. and Ren, W. and Ricci, F. and Ricci, F. and Sen, H.S. and Skelton, J.M. and Sparks, T.D. and Stroppa, A. and Urru, A. and Vandichel, M. and Vavassori, P. and Wu, H. and Yang, K. and Zhao, H.J. and Puggioni, D. and Cortese, R. and Cammarata, A.
    Inorganic Chemistry 58 (2019)
    Nanostructured materials are essential building blocks for the fabrication of new devices for energy harvesting/storage, sensing, catalysis, magnetic, and optoelectronic applications. However, because of the increase of technological needs, it is essential to identify new functional materials and improve the properties of existing ones. The objective of this Viewpoint is to examine the state of the art of atomic-scale simulative and experimental protocols aimed to the design of novel functional nanostructured materials, and to present new perspectives in the relative fields. This is the result of the debates of Symposium I "Atomic-scale design protocols towards energy, electronic, catalysis, and sensing applications", which took place within the 2018 European Materials Research Society fall meeting. © 2019 American Chemical Society.
    view abstract10.1021/acs.inorgchem.9b01785
  • Confinement- and strain-induced enhancement of thermoelectric properties in LaNiO3/LaAlO3(001) superlattices
    Geisler, B. and Pentcheva, R.
    Physical Review Materials 2 (2018)
    By combining ab initio simulations including an on-site Coulomb repulsion term and Boltzmann theory, we explore the thermoelectric properties of (LaNiO3)n/(LaAlO3)n(001) superlattices (n=1,3) and identify a strong dependence on confinement, spacer thickness, and epitaxial strain. While the system with n=3 shows modest values of the Seebeck coefficient and power factor, the simultaneous reduction of the LaNiO3 region and the LaAlO3 spacer thickness to single layers results in a strong enhancement, in particular of the in-plane values. This effect can be further tuned by using epitaxial strain as a control parameter: Under tensile strain corresponding to the lateral lattice constant of SrTiO3 we predict in- and cross-plane Seebeck coefficients of ±600μV/K and an in-plane power factor of 11μW/K2cm for an estimated relaxation time of τ=4 fs around room temperature. These values are comparable to some of the best performing oxide systems such as La-doped SrTiO3 or layered cobaltates and are associated with the opening of a small gap (0.29 eV) induced by the concomitant effect of octahedral tilting and Ni-site disproportionation. This establishes oxide superlattices at the verge of a metal-to-insulator transition driven by confinement and strain as promising candidates for thermoelectric materials. © 2018 American Physical Society.
    view abstract10.1103/PhysRevMaterials.2.055403
  • Design of Chern insulating phases in honeycomb lattices
    Pickett, W.E. and Lee, K.-W. and Pentcheva, R.
    Physica C: Superconductivity and its Applications 549 (2018)
    The search for robust examples of the magnetic version of topological insulators, referred to as quantum anomalous Hall insulators or simply Chern insulators, so far lacks success. Our groups have explored two distinct possibilities based on multiorbital 3d oxide honeycomb lattices. Each has a Chern insulating phase near the ground state, but materials parameters were not appropriate to produce a viable Chern insulator. Further exploration of one of these classes, by substituting open shell 3d with 4d and 5d counterparts, has led to realistic prediction of Chern insulating ground states. Here we recount the design process, discussing the many energy scales that are active in participating (or resisting) the desired Chern insulator phase. © 2018 Elsevier B.V.
    view abstract10.1016/j.physc.2018.02.048
  • Digital modulation of the nickel valence state in a cuprate-nickelate heterostructure
    Wrobel, F. and Geisler, B. and Wang, Y. and Christiani, G. and Logvenov, G. and Bluschke, M. and Schierle, E. and Van Aken, P.A. and Keimer, B. and Pentcheva, R. and Benckiser, E.
    Physical Review Materials 2 (2018)
    Layer-by-layer oxide molecular-beam epitaxy has been used to synthesize cuprate-nickelate multilayer structures of composition (La2CuO4)m/LaO/(LaNiO3)n. In a combined experimental and theoretical study, we show that these structures allow a clean separation of dopant and doped layers. Specifically, the LaO layer separating cuprate and nickelate blocks provides an additional charge that, according to density-functional theory calculations, is predominantly accommodated in the interfacial nickelate layers. This is reflected in an elongation of bond distances and changes in valence state, as observed by scanning transmission electron microscopy and x-ray absorption spectroscopy. Moreover, the predicted charge disproportionation in the nickelate interface layers leads to a metal-to-insulator transition when the thickness is reduced to n=2, as observed in electrical transport measurements. The results exemplify the perspectives of charge transfer in metal-oxide multilayers to induce doping without introducing chemical and structural disorder. © 2018 American Physical Society.
    view abstract10.1103/PhysRevMaterials.2.035001
  • Modulations in martensitic Heusler alloys originate from nanotwin ordering
    Gruner, M.E. and Niemann, R. and Entel, P. and Pentcheva, R. and Rößler, U.K. and Nielsch, K. and Fähler, S.
    Scientific Reports 8 (2018)
    Heusler alloys exhibiting magnetic and martensitic transitions enable applications like magnetocaloric refrigeration and actuation based on the magnetic shape memory effect. Their outstanding functional properties depend on low hysteresis losses and low actuation fields. These are only achieved if the atomic positions deviate from a tetragonal lattice by periodic displacements. The origin of the so-called modulated structures is the subject of much controversy: They are either explained by phonon softening or adaptive nanotwinning. Here we used large-scale density functional theory calculations on the Ni2MnGa prototype system to demonstrate interaction energy between twin boundaries. Minimizing the interaction energy resulted in the experimentally observed ordered modulations at the atomic scale, it explained that a/b twin boundaries are stacking faults at the mesoscale, and contributed to the macroscopic hysteresis losses. Furthermore, we found that phonon softening paves the transformation path towards the nanotwinned martensite state. This unified both opposing concepts to explain modulated martensite. © 2018 The Author(s).
    view abstract10.1038/s41598-018-26652-6
  • Nonzero Berry phase in quantum oscillations from giant Rashba-type spin splitting in LaTiO3/SrTiO3 heterostructures
    Veit, M.J. and Arras, R. and Ramshaw, B.J. and Pentcheva, R. and Suzuki, Y.
    Nature Communications 9 (2018)
    The manipulation of the spin degrees of freedom in a solid has been of fundamental and technological interest recently for developing high-speed, low-power computational devices. There has been much work focused on developing highly spin-polarized materials and understanding their behavior when incorporated into so-called spintronic devices. These devices usually require spin splitting with magnetic fields. However, there is another promising strategy to achieve spin splitting using spatial symmetry breaking without the use of a magnetic field, known as Rashba-type splitting. Here we report evidence for a giant Rashba-type splitting at the interface of LaTiO3 and SrTiO3. Analysis of the magnetotransport reveals anisotropic magnetoresistance, weak anti-localization and quantum oscillation behavior consistent with a large Rashba-type splitting. It is surprising to find a large Rashba-type splitting in 3d transition metal oxide-based systems such as the LaTiO3/SrTiO3 interface, but it is promising for the development of a new kind of oxide-based spintronics. © 2018 The Author(s).
    view abstract10.1038/s41467-018-04014-0
  • Surface Termination and Composition Control of Activity of the CoxNi1- xFe2O4(001) Surface for Water Oxidation: Insights from DFT+ U Calculations
    Hajiyani, H. and Pentcheva, R.
    ACS Catalysis 8 (2018)
    Using density functional theory calculations with an on-site Hubbard term (DFT+U), we explore the effect of surface termination and cation substitution on the performance of the CoxNi1-xFe2O4(001) surface (x = 0.0, 0.5, 1.0) as an anode material in the oxygen evolution reaction (OER). Different reaction sites (Fe, Co, Ni, and an oxygen vacancy) were investigated at three terminations: the B-layer with octahedrally coordinated Co/Ni and with an additional half and full monolayer of Fe (0.5A and A-layer, respectively). Ni substitution with an equal concentration of Co and Ni (x = 0.5) reduces the overpotential over the end members for the majority of reaction sites. Surface Co cations are identified as the active sites and the ones at the A-layer termination for x = 0.5 exhibit one of the lowest theoretically reported overpotentials of 0.26 V. The effect of the additional iron layer on the active site modification is 2-fold: analysis of the electronic properties and spin densities indicates that the additional Fe layer stabilizes a bulk-like oxidation state of +2 for Co and Ni at the A-layer termination, whereas at the B-layer termination, they are oxidized to 3+. Moreover, the unusual relaxation pattern enables the formation of a hydrogen bond of the OOH intermediate to a neighboring surface oxygen that lowers the reaction free energy of this formerly rate-limiting step, leading to a deviation from the scaling relationship and almost equidistant reaction free-energy steps of intermediates. This renders an example of how a selective surface modification can result in a significant improvement of OER performance. © 2018 American Chemical Society.
    view abstract10.1021/acscatal.8b00574
  • The Role of Composition of Uniform and Highly Dispersed Cobalt Vanadium Iron Spinel Nanocrystals for Oxygen Electrocatalysis
    Chakrapani, K. and Bendt, G. and Hajiyani, H. and Lunkenbein, T. and Greiner, M.T. and Masliuk, L. and Salamon, S. and Landers, J. and Schlögl, R. and Wende, H. and Pentcheva, R. and Schulz, S. and Behrens, M.
    ACS Catalysis 8 (2018)
    Cation substitution in transition-metal oxides is an important approach to improve electrocatalysts by the optimization of their composition. Herein, we report on phase-pure spinel-type CoV2-xFexO4 nanoparticles with 0 ≤ x ≤ 2 as a new class of bifunctional catalysts for the oxygen evolution (OER) and oxygen reduction reactions (ORR). The mixed-metal oxide catalysts exhibit high catalytic activity for both OER and ORR that strongly depends on the V and Fe content. CoV2O4 is known to exhibit a high conductivity, while in CoFe2O4 the cobalt cation distribution is expected to change due to the inversion of the spinel structure. The optimized catalyst, CoV1.5Fe0.5O4, shows an overpotential for the OER of â300 mV for 10 mA cm-2 with a Tafel slope of 38 mV dec-1 in alkaline electrolyte. DFT+U+SOC calculations on cation ordering confirm the tendency toward the inverse spinel structure with increasing Fe concentration in CoV2-xFexO4 that starts to dominate already at low Fe contents. The theoretical results also show that the variations of oxidation states are related to the surface region, where the redox activity was found experimentally to be manifested in the transformation of V3+ ↠V2+. The high catalytic activity, facile synthesis, and low cost of the CoV2-xFexO4 nanoparticles render them very promising for application in bifunctional electrocatalysis. © 2017 American Chemical Society.
    view abstract10.1021/acscatal.7b03529
  • Unexpected termination switching and polarity compensation in LaAlO3/SrTiO3 heterostructures
    Singh-Bhalla, G. and Rossen, P.B. and Pálsson, G.K. and Mecklenburg, M. and Orvis, T. and Das, S. and Tang, Y.-L. and Suresha, J.S. and Yi, D. and Dasgupta, A. and Doenning, D. and Ruiz, V.G. and Yadav, A.K. and Trassin, M. and Heron, J.T. and Fadley, C.S. and Pentcheva, R. and Ravichandran, J. and Ramesh, R.
    Physical Review Materials 2 (2018)
    Polar crystals composed of charged ionic planes cannot exist in nature without acquiring surface changes to balance an ever-growing dipole. The necessary changes can manifest structurally or electronically as observed in semiconductors and ferroelectric materials through screening charges and/or domain wall formation. In the case of prototypical polar complex oxides such as the LaAlO3/SrTiO3 system the nature of screening charges for different interface terminations is not symmetric. Electron accumulation is observed near the LaAlO3/TiO2-SrTiO3 interface, while the LaAlO3/SrO-SrTiO3 stack is insulating. Here, we observe evidence for an asymmetry in the surface chemical termination for nominally stoichiometric LaAlO3 films in contact with the two different surface layers of SrTiO3 crystals, TiO2 and SrO. Using several element-specific probes, we find that the surface termination of LaAlO3 remains AlO2 irrespective of the starting termination of SrTiO3 substrate surface. We use a combination of cross-plane tunneling measurements and first-principles calculations to understand the effects of this unexpected termination on band alignments and polarity compensation of LaAlO3/SrTiO3 heterostructures. An asymmetry in LaAlO3 polarity compensation and resulting electronic properties will fundamentally limit atomic level control of oxide heterostructures. © 2018 American Physical Society.
    view abstract10.1103/PhysRevMaterials.2.112001
  • Why Tin-Doping Enhances the Efficiency of Hematite Photoanodes for Water Splitting—The Full Picture
    Hufnagel, A.G. and Hajiyani, H. and Zhang, S. and Li, T. and Kasian, O. and Gault, B. and Breitbach, B. and Bein, T. and Fattakhova-Rohlfing, D. and Scheu, C. and Pentcheva, R.
    Advanced Functional Materials 28 (2018)
    The beneficial effects of Sn(IV) as a dopant in ultrathin hematite (α-Fe 2 O 3 ) photoanodes for water oxidation are examined. Different Sn concentration profiles are prepared by alternating atomic layer deposition of Fe 2 O 3 and SnO x . Combined data from spectrophotometry and intensity-modulated photocurrent spectroscopy yields the individual process efficiencies for light harvesting, charge separation, and charge transfer. The best performing photoanodes are Sn-doped both on the surface and in the subsurface region and benefit from enhanced charge separation and transfer. Sn-doping throughout the bulk of the hematite photoanode causes segregation at the grain boundaries and hence a lower overall efficiency. Fe 2 O 3 (0001) and terminations, shown to be dominant by microstructural analysis, are investigated by density functional theory (DFT) calculations. The energetics of surface intermediates during the oxygen evolution reaction (OER) reveal that while Sn-doping decreases the overpotential on the (0001) surface, the Fe 2 O 3 orientation shows one of the lowest overpotentials reported for hematite so far. Electronic structure calculations demonstrate that Sn-doping on the surface also enhances the charge transfer efficiency by elimination of surface hole trap states (passivation) and that subsurface Sn-doping introduces a gradient of the band edges that reinforces the band bending at the semiconductor/electrolyte interface and thus boosts charge separation. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adfm.201804472
  • Design of n - And p -type oxide thermoelectrics in LaNiO3/SrTiO3(001) superlattices exploiting interface polarity
    Geisler, B. and Blanca-Romero, A. and Pentcheva, R.
    Physical Review B - Condensed Matter and Materials Physics 95 (2017)
    We investigate the structural, electronic, transport, and thermoelectric properties of LaNiO3/SrTiO3(001) superlattices containing either exclusively n- or p-type interfaces or coupled interfaces of opposite polarity by using density functional theory calculations with an on-site Coulomb repulsion term. The results show that significant octahedral tilts are induced in the SrTiO3 part of the superlattice. Moreover, the La-Sr distances and Ni-O out-of-plane bond lengths at the interfaces exhibit a distinct variation by about 7% with the sign of the electrostatic doping. In contrast to the much studied LaAlO3/SrTiO3 system, the charge mismatch at the interfaces is exclusively accommodated within the LaNiO3 layers, whereas the interface polarity leads to a band offset and to the formation of an electric field within the coupled superlattice. Features of the electronic structure indicate an orbital-selective quantization of quantum well states. The potential- and confinement-induced multiband splitting results in complex cylindrical Fermi surfaces with a tendency towards nesting that depends on the interface polarity. The analysis of the thermoelectric response reveals a particularly large positive Seebeck coefficient (135μV/K) and a high figure of merit (0.35) for room-temperature cross-plane transport in the p-type superlattice that is attributed to the participation of the SrTiO3 valence band. Superlattices with either n- or p-type interfaces show cross-plane Seebeck coefficients of opposite sign and thus emerge as a platform to construct an oxide-based thermoelectric generator with structurally and electronically compatible n- and p-type oxide thermoelectrics. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.95.125301
  • Ordering tendencies and electronic properties in quaternary Heusler derivatives
    Neibecker, P. and Gruner, M.E. and Xu, X. and Kainuma, R. and Petry, W. and Pentcheva, R. and Leitner, M.
    Physical Review B 96 (2017)
    The phase stabilities and ordering tendencies in the quaternary full-Heusler alloys NiCoMnAl and NiCoMnGa have been investigated by in situ neutron diffraction, calorimetry, and magnetization measurements. NiCoMnGa was found to adopt the L21 structure, with distinct Mn and Ga sublattices but a common Ni-Co sublattice. A second-order phase transition to the B2 phase with disorder also between Mn and Ga was observed at 1160K. In contrast, in NiCoMnAl slow cooling or low-temperature annealing treatments are required to induce incipient L21 ordering, otherwise the system displays only B2 order. Linked to L21 ordering, a drastic increase in the magnetic transition temperature was observed in NiCoMnAl, while annealing affected the magnetic behavior of NiCoMnGa only weakly due to the low degree of quenched-in disorder. First principles calculations were employed to study the thermodynamics as well as order-dependent electronic properties of both compounds. It was found that a near half-metallic pseudogap emerges in the minority spin channel only for the completely ordered Y structure. However, this structure is energetically unstable compared to a tetragonal structure with alternating layers of Ni and Co, which is predicted to be the low-temperature ground state. The experimental inaccessibility of the totally ordered structures is explained by kinetic limitations due to the low ordering energies. © 2017 authors. Published by the American Physical Society.
    view abstract10.1103/PhysRevB.96.165131
  • Rock Salt Ni/Co Oxides with Unusual Nanoscale-Stabilized Composition as Water Splitting Electrocatalysts
    Fominykh, K. and Tok, G.C. and Zeller, P. and Hajiyani, H. and Miller, T. and Döblinger, M. and Pentcheva, R. and Bein, T. and Fattakhova-Rohlfing, D.
    Advanced Functional Materials 27 (2017)
    The influence of nanoscale on the formation of metastable phases is an important aspect of nanostructuring that can lead to the discovery of unusual material compositions. Here, the synthesis, structural characterization, and electrochemical performance of Ni/Co mixed oxide nanocrystals in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is reported and the influence of nanoscaling on their composition and solubility range is investigated. Using a solvothermal synthesis in tert-butanol ultrasmall crystalline and highly dispersible Ni x Co1− x O nanoparticles with rock salt type structure are obtained. The mixed oxides feature non-equilibrium phases with unusual miscibility in the whole composition range, which is attributed to a stabilizing effect of the nanoscale combined with kinetic control of particle formation. Substitutional incorporation of Co and Ni atoms into the rock salt lattice has a remarkable effect on the formal potentials of NiO oxidation that shift continuously to lower values with increasing Co content. This can be related to a monotonic reduction of the work function of (001) and (111)-oriented surfaces with an increase in Co content, as obtained from density functional theory (DFT+U) calculations. Furthermore, the electrocatalytic performance of the Ni x Co1− x O nanoparticles in water splitting changes significantly. OER activity continuously increases with increasing Ni contents, while HER activity shows an opposite trend, increasing for higher Co contents. The high electrocatalytic activity and tunable performance of the nonequilibrium Ni x Co1− x O nanoparticles in HER and OER demonstrate great potential in the design of electrocatalysts for overall water splitting. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adfm.201605121
  • Role of Composition and Size of Cobalt Ferrite Nanocrystals in the Oxygen Evolution Reaction
    Chakrapani, K. and Bendt, G. and Hajiyani, H. and Schwarzrock, I. and Lunkenbein, T. and Salamon, S. and Landers, J. and Wende, H. and Schlögl, R. and Pentcheva, R. and Behrens, M. and Schulz, S.
    ChemCatChem 9 (2017)
    Sub-10 nm CoFe2O4 nanoparticles with different sizes and various compositions obtained by (partial) substitution of Co with Ni cations have been synthesized by using a one-pot method from organic solutions by the decomposition of metal acetylacetonates in the presence of oleylamine. The electrocatalytic activity of CoFe2O4 towards the oxygen evolution reaction (OER) is clearly enhanced with a smaller size (3.1 nm) of the CoFe2O4 nanoparticles (compared with 4.5 and 5.9 nm). In addition, the catalytic activity is improved by partial substitution of Co with Ni, which also leads to a higher degree of inversion of the spinel structure. Theoretical calculations attribute the positive catalytic effect of Ni owing to the lower binding energy differences between adsorbed O and OH compared with pure cobalt or nickel ferrites, resulting in higher OER activity. Co0.5Ni0.5Fe2O4 exhibited a low overpotential of approximately 340 mV at 10 mA cm−2, a smaller Tafel slope of 51 mV dec−1, and stability over 30 h. The unique tunability of these CoFe2O4 nanocrystals provides great potential for their application as an efficient and competitive anode material in the field of electrochemical water splitting as well as for systematic fundamental studies aiming at understanding the correlation of composition and structure with performance in electrocatalysis. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cctc.201700376
  • Depth-Resolved Composition and Electronic Structure of Buried Layers and Interfaces in a LaNiO3/SrTiO3 Superlattice from Soft- and Hard- X-ray Standing-Wave Angle-Resolved Photoemission
    Eiteneer, D. and Pálsson, G.K. and Nemšák, S. and Gray, A.X. and Kaiser, A.M. and Son, J. and LeBeau, J. and Conti, G. and Greer, A.A. and Keqi, A. and Rattanachata, A. and Saw, A.Y. and Bostwick, A. and Rotenberg, E. and Gullikson, E.M. and Ueda, S. and Kobayashi, K. and Janotti, A. and Van de Walle, C.G. and Blanca-Romero, A. and Pentcheva, R. and Schneider, C.M. and Stemmer, S. and Fadley, C.S.
    Journal of Electron Spectroscopy and Related Phenomena 211 (2016)
    LaNiO3 (LNO) is an intriguing member of the rare-earth nickelates in exhibiting a metal-insulator transition for a critical film thickness of about 4 unit cells [Son et al., Appl. Phys. Lett. 96, 062114 (2010)]; however, such thin films also show a transition to a metallic state in superlattices with SrTiO3 (STO) [Son et al., Appl. Phys. Lett. 97, 202109 (2010)]. In order to better understand this transition, we have studied a strained LNO/STO superlattice with 10 repeats of [4 unit-cell LNO/3 unit-cell STO] grown on an (LaAlO3)0.3(Sr2AlTaO6)0.7 substrate using soft x-ray standing-wave-excited angle-resolved photoemission (SWARPES), together with soft- and hard- x-ray photoemission measurements of core levels and densities-of-states valence spectra. The experimental results are compared with state-of-the-art density functional theory (DFT) calculations of band structures and densities of states. Using core-level rocking curves and x-ray optical modeling to assess the position of the standing wave, SWARPES measurements are carried out for various incidence angles and used to determine interface-specific changes in momentum-resolved electronic structure. We further show that the momentum-resolved behavior of the Ni 3d eg and t2g states near the Fermi level, as well as those at the bottom of the valence bands, is very similar to recently published SWARPES results for a related La0.7Sr0.3MnO3/SrTiO3 superlattice that was studied using the same technique (Gray et al., Europhysics Letters 104, 17004 (2013)), which further validates this experimental approach and our conclusions. Our conclusions are also supported in several ways by comparison to DFT calculations for the parent materials and the superlattice, including layer-resolved density-of-states results. © 2016 Elsevier B.V.
    view abstract10.1016/j.elspec.2016.04.008
  • Design of Chern and Mott insulators in buckled 3d oxide honeycomb lattices
    Doennig, D. and Baidya, S. and Pickett, W.E. and Pentcheva, R.
    Physical Review B - Condensed Matter and Materials Physics 93 (2016)
    Perovskite (LaXO3)2/(LaAlO3)4(111) superlattices with X spanning the entire 3d transition-metal series combine the strongly correlated, multiorbital nature of electrons in transition-metal oxides with a honeycomb lattice as a key feature. Based on density functional theory calculations including strong interaction effects, we establish trends in the evolution of electronic states as a function of several control parameters: band filling, interaction strength, spin-orbit coupling (SOC), and lattice instabilities. Competition between local pseudocubic and global trigonal symmetry as well as the additional flexibility provided by the magnetic and spin degrees of freedom of 3d ions lead to a broad array of distinctive broken-symmetry ground states not accessible for the (001)-growth direction, offering a platform to design two-dimensional electronic functionalities. Constraining the symmetry between the two triangular sublattices causes X=Mn, Co, and Ti to emerge as Chern insulators driven by SOC. For X=Mn we illustrate how interaction strength and lattice distortions can tune these systems between a Dirac semimetal, a Chern and a trivial Mott insulator. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.93.165145
  • High Thermopower with Metallic Conductivity in p-Type Li-Substituted PbPdO2
    Lamontagne, L.K. and Laurita, G. and Gaultois, M.W. and Knight, M. and Ghadbeigi, L. and Sparks, T.D. and Gruner, M.E. and Pentcheva, R. and Brown, C.M. and Seshadri, R.
    Chemistry of Materials 28 (2016)
    PbPdO2 is a band semiconductor with a band gap arising from the filled d8 nature of square-planar Pd2+. We establish that hole doping through Li substitution for Pd in PbPdO2 results in a p-type metallic oxide with a positive temperature coefficient of resistance for substitution amounts as small as 2 mol % Li for Pd. Furthermore, PbPd1-xLixO2 demonstrates a high Seebeck coefficient and is therefore an oxide thermoelectric material with high thermopower despite the metallic conductivity. Up to 4 mol % Li is found to substitute for Pd as verified by Rietveld refinement of neutron diffraction data. At this maximal Li substitution, the resistivity is driven below the Mott metallic maximum to 3.5 × 10-3 ω cm with a Seebeck coefficient of 115 μV/K at room temperature, which increases to 175 μV/K at 600 K. These electrical properties are almost identical to those of the well-known p-type oxide thermoelectric NaxCoO2. Nonmagnetic Li-substituted PbPdO2 does not possess a correlated, magnetic state with high-spin degeneracy as found in some complex cobalt oxides. This suggests that there are other avenues to achieving high Seebeck coefficients with metallic conductivities in oxide thermoelectrics. The electrical properties coupled with the moderately low lattice thermal conductivities allow for a zT of 0.12 at 600 K, the maximal temperature measured here. The trend suggests yet higher values at elevated temperatures. First-principles calculations of the electronic structure and electrical transport provide insight into the observed properties. © 2016 American Chemical Society.
    view abstract10.1021/acs.chemmater.6b00447
  • Impact of lattice dynamics on the phase stability of metamagnetic FeRh: Bulk and thin films
    Wolloch, M. and Gruner, M.E. and Keune, W. and Mohn, P. and Redinger, J. and Hofer, F. and Suess, D. and Podloucky, R. and Landers, J. and Salamon, S. and Scheibel, F. and Spoddig, D. and Witte, R. and Roldan Cuenya, B. and Gutfleisch, O. and Hu, M.Y. and Zhao, J. and Toellner, T. and Alp, E.E. and Siewert, M. and Entel, P. and Pentcheva, R. and Wende, H.
    Physical Review B - Condensed Matter and Materials Physics 94 (2016)
    We present phonon dispersions, element-resolved vibrational density of states (VDOS) and corresponding thermodynamic properties obtained by a combination of density functional theory (DFT) and nuclear resonant inelastic x-ray scattering (NRIXS) across the metamagnetic transition of B2 FeRh in the bulk material and thin epitaxial films. We see distinct differences in the VDOS of the antiferromagnetic (AF) and ferromagnetic (FM) phases, which provide a microscopic proof of strong spin-phonon coupling in FeRh. The FM VDOS exhibits a particular sensitivity to the slight tetragonal distortions present in epitaxial films, which is not encountered in the AF phase. This results in a notable change in lattice entropy, which is important for the comparison between thin film and bulk results. Our calculations confirm the recently reported lattice instability in the AF phase. The imaginary frequencies at the X point depend critically on the Fe magnetic moment and atomic volume. Analyzing these nonvibrational modes leads to the discovery of a stable monoclinic ground-state structure, which is robustly predicted from DFT but not verified in our thin film experiments. Specific heat, entropy, and free energy calculated within the quasiharmonic approximation suggest that the new phase is possibly suppressed because of its relatively smaller lattice entropy. In the bulk phase, lattice vibrations contribute with the same sign and in similar magnitude to the isostructural AF-FM phase transition as excitations of the electronic and magnetic subsystems demonstrating that lattice degrees of freedom need to be included in thermodynamic modeling. © 2016 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License.
    view abstract10.1103/PhysRevB.94.174435
  • Mott Electrons in an Artificial Graphenelike Crystal of Rare-Earth Nickelate
    Middey, S. and Meyers, D. and Doennig, D. and Kareev, M. and Liu, X. and Cao, Y. and Yang, Z. and Shi, J. and Gu, L. and Ryan, P.J. and Pentcheva, R. and Freeland, J.W. and Chakhalian, J.
    Physical Review Letters 116 (2016)
    Deterministic control over the periodic geometrical arrangement of the constituent atoms is the backbone of the material properties, which, along with the interactions, define the electronic and magnetic ground state. Following this notion, a bilayer of a prototypical rare-earth nickelate, NdNiO3, combined with a dielectric spacer, LaAlO3, has been layered along the pseudocubic [111] direction. The resulting artificial graphenelike Mott crystal with magnetic 3d electrons has antiferromagnetic correlations. In addition, a combination of resonant X-ray linear dichroism measurements and ab initio calculations reveal the presence of an ordered orbital pattern, which is unattainable in either bulk nickelates or nickelate based heterostructures grown along the [001] direction. These findings highlight another promising venue towards designing new quantum many-body states by virtue of geometrical engineering. © 2016 American Physical Society.
    view abstract10.1103/PhysRevLett.116.056801
  • Tailoring magnetic frustration in strained epitaxial FeRh films
    Witte, R. and Kruk, R. and Gruner, M.E. and Brand, R.A. and Wang, D. and Schlabach, S. and Beck, A. and Provenzano, V. and Pentcheva, R. and Wende, H. and Hahn, H.
    Physical Review B - Condensed Matter and Materials Physics 93 (2016)
    We report on a strain-induced martensitic transformation, accompanied by a suppression of magnetic order in epitaxial films of chemically disordered FeRh. X-ray diffraction, transmission electron microscopy, and electronic structure calculations reveal that the lowering of symmetry (from cubic to tetragonal) imposed by the epitaxial relation leads to a further, unexpected, tetragonal-to-orthorhombic transition, triggered by a band-Jahn-Teller-type lattice instability. The collapse of magnetic order is a direct consequence of this structural change, which upsets the subtle balance between ferromagnetic nearest-neighbor interactions arising from Fe-Rh hybridization and frustrated antiferromagnetic coupling among localized Fe moments at larger distances. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.93.104416
  • The 2016 oxide electronic materials and oxide interfaces roadmap
    Lorenz, M. and Ramachandra Rao, M.S. and Venkatesan, T. and Fortunato, E. and Barquinha, P. and Branquinho, R. and Salgueiro, D. and Martins, R. and Carlos, E. and Liu, A. and Shan, F.K. and Grundmann, M. and Boschker, H. and Mukherjee, J. and Priyadarshini, M. and Dasgupta, N. and Rogers, D.J. and Teherani, F.H. and Sandana, E.V. and Bove, P. and Rietwyk, K. and Zaban, A. and Veziridis, A. and Weidenkaff, A. and Muralidhar, M. and Murakami, M. and Abel, S. and Fompeyrine, J. and Zuniga-Perez, J. and Ramesh, R. and Spaldin, N.A. and Ostanin, S. and Borisov, V. and Mertig, I. and Lazenka, V. and Srinivasan, G. and Prellier, W. and Uchida, M. and Kawasaki, M. and Pentcheva, R. and Gegenwart, P. and Miletto Granozio, F. and Fontcuberta, J. and Pryds, N.
    Journal of Physics D: Applied Physics 49 (2016)
    Oxide electronic materials provide a plethora of possible applications and offer ample opportunity for scientists to probe into some of the exciting and intriguing phenomena exhibited by oxide systems and oxide interfaces. In addition to the already diverse spectrum of properties, the nanoscale form of oxides provides a new dimension of hitherto unknown phenomena due to the increased surface-to-volume ratio. Oxide electronic materials are becoming increasingly important in a wide range of applications including transparent electronics, optoelectronics, magnetoelectronics, photonics, spintronics, thermoelectrics, piezoelectrics, power harvesting, hydrogen storage and environmental waste management. Synthesis and fabrication of these materials, as well as processing into particular device structures to suit a specific application is still a challenge. Further, characterization of these materials to understand the tunability of their properties and the novel properties that evolve due to their nanostructured nature is another facet of the challenge. The research related to the oxide electronic field is at an impressionable stage, and this has motivated us to contribute with a roadmap on 'oxide electronic materials and oxide interfaces'. This roadmap envisages the potential applications of oxide materials in cutting edge technologies and focuses on the necessary advances required to implement these materials, including both conventional and novel techniques for the synthesis, characterization, processing and fabrication of nanostructured oxides and oxide-based devices. The contents of this roadmap will highlight the functional and correlated properties of oxides in bulk, nano, thin film, multilayer and heterostructure forms, as well as the theoretical considerations behind both present and future applications in many technologically important areas as pointed out by Venkatesan. The contributions in this roadmap span several thematic groups which are represented by the following authors: novel field effect transistors and bipolar devices by Fortunato, Grundmann, Boschker, Rao, and Rogers; energy conversion and saving by Zaban, Weidenkaff, and Murakami; new opportunities of photonics by Fompeyrine, and Zuniga-Perez; multiferroic materials including novel phenomena by Ramesh, Spaldin, Mertig, Lorenz, Srinivasan, and Prellier; and concepts for topological oxide electronics by Kawasaki, Pentcheva, and Gegenwart. Finally, Miletto Granozio presents the European action 'towards oxide-based electronics' which develops an oxide electronics roadmap with emphasis on future nonvolatile memories and the required technologies. In summary, we do hope that this oxide roadmap appears as an interesting up-to-date snapshot on one of the most exciting and active areas of solid state physics, materials science, and chemistry, which even after many years of very successful development shows in short intervals novel insights and achievements. Guest editors: M S Ramachandra Rao and Michael Lorenz. © 2016 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/49/43/433001
  • Control of orbital reconstruction in (LaAlO3)M/(SrTiO3) N (001) quantum wells by strain and confinement
    Doennig, D. and Pentcheva, R.
    Scientific Reports 5 (2015)
    The diverse functionality emerging at oxide interfaces calls for a fundamental understanding of the mechanisms and control parameters of electronic reconstructions. Here, we explore the evolution of electronic phases in (LaAlO3) M/(SrTiO3) N (001) superlattices as a function of strain and confinement of the SrTiO 3 quantum well. Density functional theory calculations including a Hubbard U term reveal a charge ordered Ti3+ and Ti4+ state for N = 2 with an unanticipated orbital reconstruction, displaying alternating d xz and d yz character at the Ti3+ sites, unlike the previously reported d xy state, obtained only for reduced c-parameter at a STO. At a LAO c-compression leads to a Dimer-Mott insulator with alternating d xz, d yz sites and an almost zero band gap. Beyond a critical thickness of N = 3 (a STO) and N = 4 (a LAO) an insulator-to-metal transition takes place, where the extra e/2 electron at the interface is redistributed throughout the STO slab with a d xy interface orbital occupation and a mixed d xz + d yz occupation in the inner layers. Chemical variation of the SrTiO3 counterpart (LaAlO3 vs. NdGaO3) proves that the significant octahedral tilts and distortions in the SrTiO3 quantum well are induced primarily by the electrostatic doping at the polar interface and not by variation of the SrTiO3 counterpart.
    view abstract10.1038/srep07909
  • Electrostatic doping as a source for robust ferromagnetism at the interface between antiferromagnetic cobalt oxides
    Li, Z.-A. and Fontaíña-Troitiño, N. and Kovács, A. and Liébana-Viñas, S. and Spasova, M. and Dunin-Borkowski, R.E. and Müller, M. and Doennig, D. and Pentcheva, R. and Farle, M. and Salgueiriño, V.
    Scientific Reports 5 (2015)
    Polar oxide interfaces are an important focus of research due to their novel functionality which is not available in the bulk constituents. So far, research has focused mainly on heterointerfaces derived from the perovskite structure. It is important to extend our understanding of electronic reconstruction phenomena to a broader class of materials and structure types. Here we report from high-resolution transmission electron microscopy and quantitative magnetometry a robust - above room temperature (Curie temperature TC 蠑 300 K) - environmentally stable- ferromagnetically coupled interface layer between the antiferromagnetic rocksalt CoO core and a 2-4 nm thick antiferromagnetic spinel Co3O4 surface layer in octahedron-shaped nanocrystals. Density functional theory calculations with an on-site Coulomb repulsion parameter identify the origin of the experimentally observed ferromagnetic phase as a charge transfer process (partial reduction) of Co3+ to Co2+ at the CoO/Co3O4 interface, with Co2+ being in the low spin state, unlike the high spin state of its counterpart in CoO. This finding may serve as a guideline for designing new functional nanomagnets based on oxidation resistant antiferromagnetic transition metal oxides.
    view abstract10.1038/srep07997
  • Impact of strain-induced electronic topological transition on the thermoelectric properties of PtCoO2 and PdCoO2
    Gruner, M.E. and Eckern, U. and Pentcheva, R.
    Physical Review B - Condensed Matter and Materials Physics 92 (2015)
    By a combination of first-principles calculations and semiclassical Boltzmann transport theory, we investigate the effect of epitaxial strain on the electronic structure and transport properties of PtCoO2 and PdCoO2. In contrast to the rather uniform elastic response of both systems, we predict for PtCoO2 a high sensitivity of the out-of-plane transport properties to strain, which is not present in PdCoO2. At ambient temperature we identify a considerable absolute change in the thermopower from -107μV/K at -5% compressive strain to -303μV/K at +5% tensile strain. This remarkable response is related to distinct changes of the Fermi surface, which involve the crossing of two additional bands at a moderate compressive in-plane strain. Combining our transport results with available experimental data on electrical and lattice thermal conductivity we predict a thermoelectric figure of merit of up to ZT=0.25 at T=600 K for strained PtCoO2. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.92.235140
  • Insights into the structural, electronic, and magnetic properties of Fe2−xTixO3/Fe2O3 thin films with x = 0.44 grown on Al2O3 (0001)
    Dennenwaldt, T. and Lübbe, M. and Winklhofer, M. and Müller, A. and Döblinger, M. and Nabi, H.S. and Gandman, M. and Cohen-Hyams, T. and Kaplan, W.D. and Moritz, W. and Pentcheva, R. and Scheu, C.
    Journal of Materials Science 50 (2015)
    The interface between hematite (α-Fe2 IIIO3) and ilmenite (FeIITiO3), a weak ferrimagnet and an antiferromagnet, respectively, has been suggested to be strongly ferrimagnetic due to the formation of a mixed valence layer of Fe2+/Fe3+ (1:1 ratio) caused by compensation of charge mismatch at the chemically abrupt boundary. Here, we report for the first time direct experimental evidence for a chemically distinct layer emerging at heterointerfaces in the hematite—Ti-doped-hematite system. Using molecular beam epitaxy, we have grown thin films (~25 nm thickness) of α-Fe2O3 on α-Al2O3 (0001) substrates, which were capped with a ~25 nm thick Fe2−xTixO3 layer (x = 0.44). An additional 3 nm cap of α-Fe2O3 was deposited on top. The films were structurally characterized in situ with surface X-ray diffraction, which showed a partial low index orientation relationship between film and substrate in terms of the [0001] axis and revealed two predominant domains with (Formula presented.) one with (Formula presented.) and a twin domain with (Formula presented.). Electron energy loss spectroscopy profiles across the Fe2−xTixO3/Fe2O3 interface show that Fe2+/Fe3+ ratios peak right at the interface. This strongly suggests the formation of a chemically distinct interface layer, which might also be magnetically distinct as indicated by the observed magnetic enhancement in the Fe2−xTixO3/α-Fe2O3/Al2O3 system compared to the pure α-Fe2O3/Al2O3 system. © 2014, Springer Science+Business Media New York.
    view abstract10.1007/s10853-014-8572-x
  • Confinement-driven transitions between topological and Mott phases in (La Ni O 3) N / (La Al O 3) M (111) superlattices
    Doennig, D. and Pickett, W.E. and Pentcheva, R.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    A set of broken-symmetry two-dimensional ground states is predicted in (111)-oriented (LaNiO3)N/(LaAlO3)M (N/M) superlattices, based on density functional theory calculations including a Hubbard U term. An unanticipated Jahn-Teller distortion with dz2 orbital polarization and a ferromagnetic Mott-insulating (and multiferroic) phase emerges in the double perovskite (1/1), that shows strong susceptibility to strain-controlled orbital engineering. The LaNiO3 bilayer with graphene topology has a switchable multiferroic (ferromagnetic and ferroelectric) insulating ground state with inequivalent Ni sites. Beyond N=3 the confined LaNiO3 slab undergoes a metal-to-insulator transition through a half-semimetallic phase with conduction originating from the interfaces. Antiferromagnetic arrangements allow combining motifs of the bilayer and single trigonal layer band structures in designed artificial mixed phases. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.121110
  • Suppression of the critical thickness threshold for conductivity at the LaAlO3 /SrTiO3 interface
    Lesne, E. and Reyren, N. and Doennig, D. and Mattana, R. and Jaffrès, H. and Cros, V. and Petroff, F. and Choueikani, F. and Ohresser, P. and Pentcheva, R. and Barthélémy, A. and Bibes, M.
    Nature Communications 5 (2014)
    Perovskite materials engineered in epitaxial heterostructures have been intensely investigated during the last decade. The interface formed by an LaAlO3 thin film grown on top of a TiO2-terminated SrTiO3 substrate hosts a two-dimensional electronic system and has become the prototypical example of this field. Although controversy exists regarding some of its physical properties and their precise origin, it is universally found that conductivity only appears beyond an LaAlO 3thickness threshold of four unit cells. Here, we experimentally demonstrate that this critical thickness can be reduced to just one unit cell when a metallic film of cobalt is deposited on top of LaAlO3. First-principles calculations indicate that Co modifies the electrostatic boundary conditions and induces a charge transfer towards the Ti 3d bands, supporting the electrostatic origin of the electronic system at the LaAlO 3 /SrTiO3 interface. Our results expand the interest of this low-dimensional oxide system from in-plane to perpendicular transport and to the exploration of elastic and inelastic tunnel-type transport of (spin-polarized) carriers. © 2014 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/ncomms5291
  • DFT+ U study of arsenate adsorption on FeOOH surfaces: Evidence for competing binding mechanisms
    Otte, K. and Schmahl, W.W. and Pentcheva, R.
    Journal of Physical Chemistry C 117 (2013)
    On the basis of periodic density functional theory (DFT) calculations including an on-site Coulomb repulsion term U, we study the adsorption mechanism of arsenate on the goethite (101), akaganeite (100), and lepidocrocite (010) surfaces. Mono- and bidentate binding configurations of arsenate complexes are considered at two distinct iron surface sites - directly at 5-fold coordinated Fe1 and/or 4-fold coordinated Fe2 as well as involving ligand exchange. The results obtained within ab initio thermodynamics shed light on the ongoing controversy on the arsenate adsorption configuration, and we identify monodentate adsorbed arsenate complexes as stable configurations at ambient conditions with a strong preference for protonated arsenate complexes: a monodentate mononuclear complex at Fe1 (dFe1-As = 3.45 Å) at goethite (101) and a monodentate binuclear complex at Fe2 (dFe2-As = 3.29 Å) at akaganeite (100). Repulsive interactions between the complexes limit the loading capacity and promote configurations with maximized distances between the adsorbates. With decreasing oxygen pressures, a mixed adsorption of bidentate binuclear complexes at Fe1 (dFe1-As = 3.26-3.34 Å) and monodentate binuclear arsenate at Fe2 (dFe2-As = 3.31-3.50 Å) and, finally, rows of protonated bidentate complexes at Fe1 with d Fe1-As = 3.55-3.59 Å are favored at α-FeOOH(101) and β-FeOOH(100). At lepidocrocite (010) with only Fe2 sites exposed, the surface phase diagram is dominated by alternating protonated monodentate binuclear complexes (dFe2-As = 3.38 Å) and hydroxyl groups. At low oxygen pressures, alternating rows of protonated bidentate mononuclear complexes (dFe2-As = 3.10 Å) and water are present. Hydrogen bond formation to surface hydroxyl groups and water plays a crucial role in the stabilization of these adsorbate configurations and together with steric effects of the oxygen lone pairs leads to tilting of the arsenate complex that significantly reduces the Fe-As distance. Our results show that the Fe-As bond length is mainly determined by the protonation state, arsenate coverage, steric effects, and hydrogen bonding to surface functional groups and to a lesser extent by the adsorption mode. This demonstrates that the Fe-As distance cannot be used as a unique criterion to discriminate between adsorption modes. © 2013 American Chemical Society.
    view abstract10.1021/jp400649m
  • Erratum: Probing the surface phase diagram of Fe3O 4(001) towards the Fe-rich limit: Evidence for progressive reduction of the surface (Physical Review B - Condensed Matter and Materials Physics (2013) 87 (195410))
    Novotny, Z. and Mulakaluri, N. and Edes, Z. and Schmid, M. and Pentcheva, R. and Diebold, U. and Parkinson, G.S.
    Physical Review B - Condensed Matter and Materials Physics 88 (2013)
    view abstract10.1103/PhysRevB.88.039902
  • Massive symmetry breaking in LaAlO3/SrTiO3(111) quantum wells: A three-orbital strongly correlated generalization of graphene
    Doennig, D. and Pickett, W.E. and Pentcheva, R.
    Physical Review Letters 111 (2013)
    Density functional theory calculations with an on-site Coulomb repulsion term reveal competing ground states in (111)-oriented (LaAlO3) M/(SrTiO3)N superlattices with n-type interfaces, ranging from spin, orbitally polarized (with selective eg′, a1g, or dxy occupation), Dirac point Fermi surface, to charge-ordered flat band phases. These phases are steered by the interplay of (i) Hubbard U, (ii) SrTiO3 quantum well thickness, and (iii) crystal field splitting tied to in-plane strain. In the honeycomb lattice bilayer N=2 under tensile strain, inversion symmetry breaking drives the system from a ferromagnetic Dirac point (massless Weyl semimetal) to a charge-ordered multiferroic (ferromagnetic and ferroelectric) flat band massive (insulating) phase. With increasing SrTiO3 quantum well thickness an insulator-to-metal transition occurs. © 2013 American Physical Society.
    view abstract10.1103/PhysRevLett.111.126804
  • Momentum-resolved electronic structure at a buried interface from soft X-ray standing-wave angle-resolved photoemission
    Gray, A.X. and Minár, J. and Plucinski, L. and Huijben, M. and Bostwick, A. and Rotenberg, E. and Yang, S.-H. and Braun, J. and Winkelmann, A. and Conti, G. and Eiteneer, D. and Rattanachata, A. and Greer, A.A. and Ciston, J. and Ophus, C. and Rijnders, G. and Blank, D.H.A. and Doennig, D. and Pentcheva, R. and Kortright, J.B. and Schneider, C. M. and Ebert, H. and Fadley, C.S.
    EPL 104 (2013)
    Angle-resolved photoemission spectroscopy (ARPES) is a powerful technique for the study of electronic structure, but it lacks a direct ability to study buried interfaces between two materials. We address this limitation by combining ARPES with soft X-ray standing-wave (SW) excitation (SWARPES), in which the SW profile is scanned through the depth of the sample. We have studied the buried interface in a prototypical magnetic tunnel junction La0.7Sr 0.3MnO3/SrTiO3. Depth-and momentum-resolved maps of Mn 3d eg and t2g states from the central, bulk-like and interface-like regions of La0.7Sr0.3MnO 3 exhibit distinctly different behavior consistent with a change in the Mn bonding at the interface. We compare the experimental results to state-of-the-art density-functional and one-step photoemission theory, with encouraging agreement that suggests wide future applications of this technique. © Copyright EPLA, 2013.
    view abstract10.1209/0295-5075/104/17004
  • Probing the surface phase diagram of Fe3O4(001) towards the Fe-rich limit: Evidence for progressive reduction of the surface
    Novotny, Z. and Mulakaluri, N. and Edes, Z. and Schmid, M. and Pentcheva, R. and Diebold, U. and Parkinson, G.S.
    Physical Review B - Condensed Matter and Materials Physics 87 (2013)
    Reduced terminations of the Fe3O4(001) surface were studied using scanning tunneling microscopy, x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). Fe atoms, deposited onto the thermodynamically stable, distorted B-layer termination at room temperature (RT), occupy one of two available tetrahedrally coordinated sites per (√2×√2)R45 unit cell. Further RT deposition results in Fe clusters. With mild annealing, a second Fe adatom per unit cell is accommodated, though not in the second tetrahedral site. Rather both Fe atoms reside in octahedral coordinated sites, leading to a "Fe-dimer" termination. At four additional Fe atoms per unit cell, all surface octahedral sites are occupied, resulting in a FeO(001)-like phase. The observed configurations are consistent with the calculated surface phase diagram. Both XPS and DFT+U results indicate a progressive reduction of surface iron from Fe3+ to Fe2+ upon Fe deposition. The antiferromagnetic FeO layer on top of ferromagnetic Fe3O4(001) suggests possible exchange bias in this system. Published by the American Physical Society under the terms of the http://creativecommons.org/licenses/by/3.0/ Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
    view abstract10.1103/PhysRevB.87.195410
  • Symmetrization driven spin transition in ε-FeOOH at high pressure
    Gleason, A.E. and Quiroga, C.E. and Suzuki, A. and Pentcheva, R. and Mao, W.L.
    Earth and Planetary Science Letters 379 (2013)
    Structural and electronic spin transitions in high-pressure ε-FeOOH are studied using a combination of high pressure X-ray emission spectroscopy (XES), X-ray diffraction (XRD) and density functional theory (DFT) calculations. Using XES, a high- to low-spin transition in trivalent iron is found in ε-FeOOH on compression between 40 and 60 GPa. This is accompanied by a sudden discontinuity in unit cell volume at 53( ± 2) GPa, obtained from XRD data collected over the same compression range. These results are consistent with DFT calculations using an on-site Coulomb repulsion term (GGA+U), which predict a spin transition in ε-FeOOH at 64.8 GPa. A second order phase transition from P21nm to Pnnm is predicted from DFT at ~43 GPa and evidenced in the XRD data from the anisotropic stiffening of the lattice parameters around the spin transition. In addition, the DFT results give evidence that the spin collapse is assisted by symmetrization of hydrogen bonds during the transition from P21nm to Pnnm. As the presence of hydrogen, even in small quantities, can affect phase relations, melting temperature, rheology, and other key properties of the Earth's mantle, our study unveils a connection between water (hydroxyl) content and the spin-transition pressure of Fe3+ in the Earth's mantle. © 2013 Elsevier B.V.
    view abstract10.1016/j.epsl.2013.08.012
  • X-ray photoemission and density functional theory study of the interaction of water vapor with the Fe3O4(001) surface at near-ambient conditions
    Kendelewicz, T. and Kaya, S. and Newberg, J.T. and Bluhm, H. and Mulakaluri, N. and Moritz, W. and Scheffler, M. and Nilsson, A. and Pentcheva, R. and Brown Jr., G.E.
    Journal of Physical Chemistry C 117 (2013)
    The interaction of water with the Fe3O4(001) surface was investigated in a combined ambient pressure X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) study. The uptake of molecular water and hydroxyl species on the (001) surface of a natural magnetite sample at near-ambient conditions was quantified using O 1s spectra taken in the p(H 2O) range from 10-9 to 2 Torr. At low p(H2O) (≤10-4-10-5 Torr) and room temperature, we found that water does not adsorb dissociatively on the surface, except on defect sites. In contrast, progressive dissociation into surface hydroxyl species was observed between 10-4 and 10-2 Torr p(H2O). The onset of hydroxylation coincides with the increasing presence of molecular water species on the surface, which demonstrates the key role played by cooperative interactions between adsorbed water molecules, leading to dissociation and surface hydroxylation. The measured O 1s chemical shifts of hydroxyl and molecular water species from both isotherm and isobar data are on average ∼1.2 eV and ∼3.3 eV, respectively, relative to lattice oxygen. The chemical shift of the hydroxyl species on magnetite(001) agrees with previously reported values for hydroxyl species on iron oxyhydroxides such as goethite (α-FeOOH). DFT calculations including an on-site Coulomb repulsion parameter (generalized gradient approximation (GGA) + U approach) predict O 1s surface core-level shifts (SCLS) at the clean (21/2×2 1/2)R45 reconstructed Fe3O4(001) surface of up to ∼-1 eV depending on the specific bonding configuration of the surface O atoms. Hydroxyl groups formed by the dissociation of isolated water molecules at O vacancies have an SCLS value of ∼1.2 eV. With increasing coverage there is a transition toward partial dissociation on the (001) surface. The calculated SCLS for hydroxyl and adsorbed water are 1.2-1.9 and 2.6-3.0 eV, respectively, and compare very well with our experimental results. Final-state effects obtained within the Slater-Janak approach thus have the dominant contribution. In addition, the modest reduction of the work function (∼0.5 eV) predicted by DFT calculations for the mixed adsorption of dissociated and intact water molecules agrees well with work function changes measured experimentally. Finally, the similarity between isotherm and isobar data and the DFT calculations for the C-free Fe3O4(001) surface indicate that surface hydroxylation is indeed substrate induced and not catalyzed by the presence of adventitious carbonaceous species. Both theory and experiment show the importance of cooperative effects between adjacent water molecules in the dissociation reaction. © 2013 American Chemical Society.
    view abstract10.1021/jp3078024
  • Density functional theory study of water adsorption on FeOOH surfaces
    Otte, K. and Schmahl, W.W. and Pentcheva, R.
    Surface Science 606 (2012)
    Using density functional theory (DFT) calculations with an on-site Coulomb repulsion term, we study the composition, stability, and electronic properties of the most common FeOOH surfaces goethite(101), akaganeite(100), and lepidocrocite(010), and their interaction with water. Despite the differences in surface structure, the trends in surface stability of these FeOOH polymorphs exhibit remarkable similarities. We find that the reactivity and the binding configuration of adsorbates depend strongly on the coordination of surface iron: at the fourfold coordinated Fe2 site water is chemisorbed, whereas at the fivefold coordinated Fe1 water is only loosely bound with hydrogen pointing towards the surface. Our results show that the oxidation state of surface iron can be controlled by the surface termination where ferryl (Fe 4 +) species emerge for oxygen terminated surfaces and ferrous iron (Fe 2 +) at iron and water terminations leading to a reduced band gap. In contrast, the fully hydroxylated surfaces, identified as stable surface configurations at standard conditions from the surface phase diagram, show electronic properties and band gaps closest to bulk FeOOH with ferric surface iron (Fe 3 +). Only in the case of goethite(101), a termination with mixed surface hydroxyl and aquo groups is stabilized. © 2012 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.susc.2012.07.009
  • Hydrogen adsorption and site-selective reduction of the Fe 3O 4(001) surface: Insights from first principles
    Mulakaluri, N. and Pentcheva, R.
    Journal of Physical Chemistry C 116 (2012)
    Density functional theory calculations including an on-site Hubbard term are used to explore hydrogen adsorption on the surface of Fe 3O 4(001). The adsorption energy exhibits a minimum for two hydrogen atoms per (√2 × √2)R45° surface unit cell and gets less favorable with increasing hydrogen coverage due to OH-OH repulsion. Terminations with two and four hydrogen atoms per surface unit cell are stable for moderate to high partial pressures of O and H. The strong tilt of the OH bond parallel to the surface facilitates hydrogen bonding to neighboring oxygen and hopping of the protons between surface oxygen sites. Furthermore, the formation of surface OH groups leads to a monotonic reduction of work function with increasing H coverage. The analysis of the electronic properties reveals selective switching of neighboring surface and subsurface Fe from Fe 3+ to Fe 2+ upon hydrogen adsorption. This provides a promising way to tune the catalytic activity of the Fe 3O 4(001) surface. © 2012 American Chemical Society.
    view abstract10.1021/jp302259d
  • Termination control of electronic phases in oxide thin films and interfaces: LaAlO 3/SrTiO 3(001)
    Pentcheva, R. and Arras, R. and Otte, K. and Ruiz, V.G. and Pickett, W.E.
    Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 370 (2012)
    A wealth of intriguing properties emerge in the seemingly simple system composed of the band insulators LaAlO 3 and SrTiO 3 such as a two-dimensional electron gas, superconductivity and magnetism. In this paper, we review the current insight obtained from first principles calculations on the mechanisms governing the behaviour of thin LaAlO 3 films on SrTiO 3(001). In particular, we explore the strong dependence of the electronic properties on the surface and interface termination, the finite film thickness, lattice polarization and defects. A further aspect that is addressed is how the electronic behaviour and functionality can be tuned by an SrTiO 3 capping layer, adsorbates and metallic contacts. Lastly, we discuss recent reports on the coexistence of magnetism and superconductivity in this system for what they might imply about the electronic structure of this system. © 2012 The Royal Society.
    view abstract10.1098/rsta.2012.0202
  • Tuning the two-dimensional electron gas at the LaAlO 3/SrTiO 3(001) interface by metallic contacts
    Arras, R. and Ruiz, V.G. and Pickett, W.E. and Pentcheva, R.
    Physical Review B - Condensed Matter and Materials Physics 85 (2012)
    Density functional theory (DFT) calculations reveal that adding a metallic overlayer on LaAlO 3/SrTiO 3(001) alters significantly the electric field within the polar LaAlO 3 film. For Al or Ti metal contacts the electric field is eliminated, leading to a suppression of the thickness-dependent insulator-to-metal transition observed in uncovered films. Independent of the LaAlO 3 thickness, both the surface and the interface are metallic, with an enhanced carrier density at the interface relative to LaAlO 3/SrTiO 3(001) after the metallization transition. Monolayer thick contacts of Ti develop a finite magnetic moment and for a thin SrTiO 3 substrate induce a spin-polarized two-dimensional electron gas at the n-type interface, due to confinement effects in the SrTiO 3 slab. For transition (Fe, Co, Pt) and noble metal contacts (Cu, Ag, Au) a finite and even enhanced (Au) internal electric field develops within LaAlO 3. Results for a representative series of metallic overlayers on LaAlO 3/SrTiO 3(001) (Na, Al; Ti, Fe, Co, Pt; Cu, Ag, Au) reveal broad variation of band alignment, size of Schottky barrier, carrier concentration and lattice polarization at the LaAlO 3/ SrTiO 3(001) interface. The identified relationship to the size of work function of the metal on LaAlO 3 provides guidelines on how the carrier density at the LaAlO 3/SrTiO 3 interface can be controlled by the choice of the metal contact. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.85.125404
  • Confinement-induced metal-to-insulator transition in strained LaNiO 3/LaAlO3 superlattices
    Blanca-Romero, A. and Pentcheva, R.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    Using density functional theory calculations including a Hubbard U term we explore the effect of strain and confinement on the electronic ground state of superlattices containing the band insulator LaAlO3 and the correlated metal LaNiO3. Besides a suppression of holes at the apical oxygen, a central feature is the asymmetric response to strain in single unit cell superlattices: For tensile strain a band gap opens due to charge disproportionation at the Ni sites with two distinct magnetic moments of 1.45μB and 0.71μB. Under compressive stain, charge disproportionation is nearly quenched and the band gap collapses due to overlap of d3z2-r2 bands through a semimetallic state. This asymmetry in the electronic behavior is associated with the difference in octahedral distortions and rotations under tensile and compressive strain. The ligand hole density and the metallic state are quickly restored with increasing thickness of the (LaAlO3)n/(LaNiO3)n superlattice from n=1 to n=3. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.195450
  • Energetic stability and magnetic coupling in (Cr1-xFe x)2O3: Evidence for a ferrimagnetic ilmenite-type superlattice from first principles
    Sadat Nabi, H. and Pentcheva, R.
    Physical Review B - Condensed Matter and Materials Physics 83 (2011)
    Based on density functional theory (DFT) calculations with a Hubbard U-term, we explore the possibility to design an artificial ferrimagnet FeCrO3 of ilmenite type out of the two antiferromagnets α-Fe2O3 and α-Cr2O3. By varying the concentration of Fe in α-Cr2O3, we provide a phase diagram of the relative stability of different chemical and magnetic arrangements with respect to the end members. At 50% Fe-doped α-Cr2O3, the ilmenite-like structure with alternating Fe and Cr layers and antiparallel magnetic moments competes energetically with a phase-separated structure containing a mixed Fe-Cr interface layer. The magnetic interaction parameters between Fe(3d5) and Cr(3d3) ions in the digital ferrimagnetic heterostructure, extracted by mapping the DFT total energies to a Heisenberg Hamiltonian, indicate a hematite-like magnetic order with parallel intralayer and antiparallel interlayer alignment. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.83.214424
  • Erratum: Confinement-induced metal-to-insulator transition in strained LaNiO 3/LaAlO 3 superlattices (Physical Review B - Condensed Matter and Materials Physics (2011) 84:23 (239902))
    Blanca-Romero, A. and Pentcheva, R.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    view abstract10.1103/PhysRevB.84.239902
  • Orbital control in strained ultra-thin LaNiO3/LaAlO3 superlattices
    Freeland, J.W. and Liu, J. and Kareev, M. and Gray, B. and Kim, J.W. and Ryan, P. and Pentcheva, R. and Chakhalian, J.
    EPL 96 (2011)
    In pursuit of rational control of orbital polarization, we present a combined experimental and theoretical study of single-unit-cell superlattices of the correlated metal LaNiO3 and the band insulator LaAlO 3. Polarized X-ray absorption spectra show a distinct asymmetry in the orbital response under strain. A splitting of orbital energies consistent with octahedral distortions is found for the case of compressive strain. In sharp contrast, for tensile strain, no splitting is found although a strong orbital polarization is present. Density functional theory calculations including a Hubbard U-term reveal that this asymmetry is a result of the interplay of strain and confinement that induces octahedral rotations and distortions and altered covalency in the bonding across the interfacial Ni-O-Al apical oxygen, leading to a charge disproportionation at the Ni sites for tensile strain. © Europhysics Letters Association.
    view abstract10.1209/0295-5075/96/57004
  • Resistive memory switching in layered oxides: A nB nO 3n+2 perovskite derivatives and Bi 2Sr 2CaCu 2O 8+δ high-T c superconductor
    Koval, Y. and Chowdhury, F. and Jin, X. and Simsek, Y. and Lichtenberg, F. and Pentcheva, R. and Müller, P.
    Physica Status Solidi (A) Applications and Materials Science 208 (2011)
    Resistive memory switching was investigated in titanates and niobates of the type A nB nO 3n+2 and in the high-T c superconductor Bi 2Sr 2CaCu 2O 8+δ. We studied the switching by current injection perpendicular to the layers. Both dc and pulsed measurements were performed. Out-of-plane transport properties were investigated by measurements of the resistance and current-voltage characteristics (IVs) vs. temperature for different resistive states. The critical temperature of superconducting transition and the critical current of intrinsic Josephson junctions were also analyzed for different resistive states in Bi 2Sr 2CaCu 2O 8+δ. The resistive memory switching was explained in terms of doping of the conducting layers, which is induced by trapped charges in the insulating layers. The charged insulating layers act as a floating gate and reduce or increase the carrier concentration in the conducting layers, respectively. We found that all studied materials demonstrate a different type of non-persistent resistive switching at low temperatures. This type of switching shows up in a specific form of current-voltage characteristics with a pronounced back-bending often called s-shaped IV. Both types of resistive switching with and without memory effect were analyzed in terms of electron overheating. We examine the role of hot electrons and discuss additional factors, which might lead to persistent resistive states. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201026757
  • Ab initio electronic structures of rhombohedral and cubic HgXO3 (X = Ti, Pb)
    Nabi, H. S. and Pentcheva, R. and Ranjan, R.
    Journal of Physics-condensed Matter 22 (2010)
    First-principles calculations were performed for orthorhombic HgO, rhombohedral and cubic phases of HgTiO3 (HTO) and HgPbO3 (HPO). The calculations show that in the rhombohedral phase HTO is a direct gap insulator with a gap of similar to 1.6 eV. The rhombohedral phase of HPO, on the other hand, shows a weak metallic character. The results provide an explanation for the electrical properties of these compounds. The cubic phases of HTO and HPO are invariably metallic in nature, thereby suggesting that for HTO the rhombohedral-cubic transition must also be accompanied by a change in the electrical state. Examination of the electronic density of states of these systems revealed no significant on-site mixing of Hg 5d and Hg 6s states in any of these materials.
    view abstract10.1088/0953-8984/22/4/045504
  • Coverage-dependent adsorption mode of water on Fe3O 4(001): Insights from first principles calculations
    Mulakaluri, N. and Pentcheva, R. and Scheffler, M.
    Journal of Physical Chemistry C 114 (2010)
    Using density functional theory calculations together with an on-site Coulomb repulsion term (GGA+U), we investigate the adsorption of water on Fe3O4(001). Starting from a single water molecule per (√2 × √2)R45° unit cell, we vary the concentration and configuration of water and hydroxyl groups. Isolated water molecules on the clean surface tend to dissociate heterolytically with an OH group adsorbed on top of an octahedral iron and a proton donated to a surface oxygen. Furthermore, oxygen defects are found to promote strongly water dissociation. The released protons bind to distant surface oxygen to minimize the repulsive interaction between the surface OH groups. At higher coverages, the interplay between adsorbate-adsorbate and adsorbate-substrate interactions and the formation of hydrogen bonds between the surface species result in a crossover to a mixed adsorption mode where every second molecule is dissociated. The energetic trends are related to the underlying electronic mechanisms. © 2010 American Chemical Society.
    view abstract10.1021/jp100344n
  • Electronic phenomena at complex oxide interfaces: insights from first principles
    Pentcheva, R. and Pickett, A. E.
    Journal of Physics-condensed Matter 22 (2010)
    Oxide interfaces have attracted considerable attention in recent years due to the emerging novel behavior which does not exist in the corresponding bulk parent compounds. This opens possibilities for future applications in oxide-based electronics and spintronics devices. Among the different materials combinations, heterostructures containing the two simple band insulators LaAlO(3) and SrTiO(3) have advanced to a model system exhibiting unanticipated properties ranging from conductivity, to magnetism, even to superconductivity. Electronic structure calculations have contributed significantly towards understanding these phenomena and we review here the progress achieved in the past few years, also showing some future directions and perspectives. A central issue in understanding the novel behavior in these oxide heterostructures is to discover the way (or ways) that these heterostructures deal with the polar discontinuity at the interface. Despite analogies to polar semiconductor interfaces, transition metal oxides offer much richer possibilities to compensate the valence mismatch, including, for example, an electronic reconstruction. Moreover, electronic correlations can lead to additional complex behavior like charge disproportionation and order, magnetism and orbital order. We discuss in some detail the role of finite size effects in ultrathin polar films on a nonpolar substrate leading to another intriguing feature-the thickness-dependent insulator-to-metal transition in thin LaAlO(3) films on a SrTiO(3)(001) substrate, driven by the impending polar catastrophe. The strong and uniform lattice polarization that emerges as a response to the potential build-up enables the system to remain insulating up to a few layers. However, beyond a critical thickness there is a crossover from an ionic relaxation to an electronic reconstruction. At this point two bands of electron and hole character, separated both in real and in reciprocal space, have been shifted sufficiently by the internal field in LaAlO3 to impose the closing of the bandgap. We discuss briefly further parameters that allow one to manipulate this behavior, e. g. via vacancies, adsorbates or an oxide capping layer.
    view abstract10.1088/0953-8984/22/4/043001
  • Magnetic coupling parameters at an oxide-oxide interface from first principles: Fe2O3-FeTiO3
    Nabi, H.S. and Harrison, R.J. and Pentcheva, R.
    Physical Review B - Condensed Matter and Materials Physics 81 (2010)
    Charge mismatch at the interface between canted antiferromagnetic hematite (α -Fe2O3) and antiferromagnetic ilmenite (FeTiO3) is accommodated by the formation of mixed Fe2+ and Fe3+ contact layers, leading to uncompensated magnetic moments in the system. To derive the magnetic exchange interaction parameters of the end members and interface, we map total-energy differences of collinear spin arrangements obtained from density-functional theory calculations to a Heisenberg Hamiltonian using the least-squares method. Parameters for the end members, hematite (Jm3+,3+) and ilmenite (Jm 2+,2+) are in good agreement with the values obtained from inelastic neutron-scattering data. The magnetic interaction parameters between Fe2+ and Fe3+ (Jm2+,3+) in the contact layer show a strong antiferromagnetic coupling to the adjacent hematite layers and thus explain the ferrimagnetism in the system. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.81.214432
  • Origin of Interface Magnetism in Fe(2)O(3)/FeTiO(3) Heterostructures
    Nabi, H. S. and Pentcheva, R.
    High Performance Computing in Science and Engineering, Garching/munich 2009: Transactions of the Fourth Joint Hlrb and Konwihr Review and Results Workshop (2010)
    Nanoscale exsolutions of the canted antiferromagnet hematite (alpha-Fe(2)O(3)) and the room temperature paramagnet ilmenite (FeTiO(3)) show a surprisingly stable room temperature remanent magnetization, making the material interesting for spintronics applications. To understand the nature of this phenomenon at the atomic scale, density functional theory calculations with an on-site Coulomb repulsion parameter were performed on Fe(2-x)Ti(x)O(3), varying the concentration, distribution and charge state of cations. We find that the polar discontinuity at the interface is accommodated by the formation of a mixed Fe(2+)/Fe(3+) layer. The uncompensated interface moments give rise to ferrimagnetism in this system. We also explore the effect of strain, showing that it can be used to tune the electronic properties (e.g. band gap, position of impurity levels). Furthermore, we find that epitaxial growth on an Al(2)O(3)(0001) substrate is energetically unfavorable compared to substrates with a larger lateral lattice parameter, providing thereby a guideline for an optimal choice of the substrate in growth experiments.
    view abstract10.1007/978-3-642-13872-0_47
  • Parallel electron-hole bilayer conductivity from electronic interface reconstruction
    Pentcheva, R. and Huijben, M. and Otte, K. and Pickett, W.E. and Kleibeuker, J.E. and Huijben, J. and Boschker, H. and Kockmann, D. and Siemons, W. and Koster, G. and Zandvliet, H.J.W. and Rijnders, G. and Blank, D.H.A. and Hilgenkamp, H. and Brinkman, A.
    Physical Review Letters 104 (2010)
    The perovskite SrTiO3-LaAlO3 structure has advanced to a model system to investigate the rich electronic phenomena arising at polar oxide interfaces. Using first principles calculations and transport measurements we demonstrate that an additional SrTiO3 capping layer prevents atomic reconstruction at the LaAlO3 surface and triggers the electronic reconstruction at a significantly lower LaAlO3 film thickness than for the uncapped systems. Combined theoretical and experimental evidence (from magnetotransport and ultraviolet photoelectron spectroscopy) suggests two spatially separated sheets with electron and hole carriers, that are as close as 1 nm. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevLett.104.166804
  • Semiconductor-half metal transition at the Fe3O4(001 ) surface upon hydrogen adsorption
    Parkinson, G.S. and Mulakaluri, N. and Losovyj, Y. and Jacobson, P. and Pentcheva, R. and Diebold, U.
    Physical Review B - Condensed Matter and Materials Physics 82 (2010)
    The adsorption of H on the magnetite (001) surface was studied with photoemission spectroscopies, scanning tunneling microscopy, and density-functional theory. At saturation coverage the insulating (√2×√2) R45° reconstruction is lifted and the surface undergoes a semiconductor-half metal transition. This transition involves subtle changes in the local geometric structure linked to an enrichment of Fe2 + cations at the surface. The ability to manipulate the electronic properties by surface engineering has important implications for magnetite-based spintronic devices. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.82.125413
  • electrocatalysis

  • electronics

  • first-principles calculation

  • modelling and simulation

  • nanoparticles

  • perovskite

  • spintronics

  • topological nontrivial phases

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