Prof. Dr. Anna Böhmer

Experimental Physics
Ruhr-Universiät Bochum


  • Editorial: Nematicity in iron-based superconductors
    Wang, Q. and Fanfarillo, L. and Böhmer, A.E.
    Frontiers in Physics 10 (2022)
    view abstract10.3389/fphy.2022.1038127
  • Edwards-Anderson parameter and local Ising nematicity in FeSe revealed via NMR spectral broadening
    Wiecki, P. and Zhou, R. and Julien, M.-H. and Böhmer, A.E. and Schmalian, J.
    Physical Review B 104 (2021)
    The NMR spectrum of FeSe shows a dramatic broadening on cooling towards the bulk nematic phase at Ts=90 K, due to the formation of a quasistatic, short-range-ordered nematic domain structure. However, a quantitative understanding of the NMR broadening and its relationship to the nematic susceptibility is still lacking. Here, we show that the temperature and pressure dependence of the broadening is in quantitative agreement with the mean-field Edwards-Anderson parameter of an Ising-nematic model in the presence of random-field disorder introduced by nonmagnetic impurities. Furthermore, these results reconcile the interpretation of NMR and Raman spectroscopy data in FeSe under pressure. © 2021 American Physical Society.
    view abstract10.1103/PhysRevB.104.125134
  • Emerging symmetric strain response and weakening nematic fluctuations in strongly hole-doped iron-based superconductors
    Wiecki, P. and Frachet, M. and Haghighirad, A.-A. and Wolf, T. and Meingast, C. and Heid, R. and Böhmer, A.E.
    Nature Communications 12 (2021)
    Electronic nematicity is often found in unconventional superconductors, suggesting its relevance for electronic pairing. In the strongly hole-doped iron-based superconductors, the symmetry channel and strength of the nematic fluctuations, as well as the possible presence of long-range nematic order, remain controversial. Here, we address these questions using transport measurements under elastic strain. By decomposing the strain response into the appropriate symmetry channels, we demonstrate the emergence of a giant in-plane symmetric contribution, associated with the growth of both strong electronic correlations and the sensitivity of these correlations to strain. We find weakened remnants of the nematic fluctuations that are present at optimal doping, but no change in the symmetry channel of nematic fluctuations with hole doping. Furthermore, we find no indication of a nematic-ordered state in the AFe2As2 (A = K, Rb, Cs) superconductors. These results revise the current understanding of nematicity in hole-doped iron-based superconductors. © 2021, The Author(s).
    view abstract10.1038/s41467-021-25121-5
  • Lattice-shifted nematic quantum critical point in FeSe1−xSx
    Chibani, S. and Farina, D. and Massat, P. and Cazayous, M. and Sacuto, A. and Urata, T. and Tanabe, Y. and Tanigaki, K. and Böhmer, A.E. and Canfield, P.C. and Merz, M. and Karlsson, S. and Strobel, P. and Toulemonde, P. and Paul, I. and Gallais, Y.
    npj Quantum Materials 6 (2021)
    We report the evolution of nematic fluctuations in FeSe1−xSx single crystals as a function of Sulfur content x across the nematic quantum critical point (QCP) xc ~ 0.17 via Raman scattering. The Raman spectra in the B1g nematic channel consist of two components, but only the low energy one displays clear fingerprints of critical behavior and is attributed to itinerant carriers. Curie–Weiss analysis of the associated nematic susceptibility indicates a substantial effect of nemato-elastic coupling, which shifts the location of the nematic QCP. We argue that this lattice-induced shift likely explains the absence of any enhancement of the superconducting transition temperature at the QCP. The presence of two components in the nematic fluctuations spectrum is attributed to the dual aspect of electronic degrees of freedom in Hund’s metals, with both itinerant carriers and local moments contributing to the nematic susceptibility. © 2021, The Author(s).
    view abstract10.1038/s41535-021-00336-3
  • Multi-atom quasiparticle scattering interference for superconductor energy-gap symmetry determination
    Sharma, R. and Kreisel, A. and Sulangi, M.A. and Böker, J. and Kostin, A. and Allan, M.P. and Eisaki, H. and Böhmer, A.E. and Canfield, P.C. and Eremin, I. and Séamus Davis, J.C. and Hirschfeld, P.J. and Sprau, P.O.
    npj Quantum Materials 6 (2021)
    Complete theoretical understanding of the most complex superconductors requires a detailed knowledge of the symmetry of the superconducting energy-gap Δkα, for all momenta k on the Fermi surface of every band α. While there are a variety of techniques for determining ∣Δkα∣, no general method existed to measure the signed values of Δkα. Recently, however, a technique based on phase-resolved visualization of superconducting quasiparticle interference (QPI) patterns, centered on a single non-magnetic impurity atom, was introduced. In principle, energy-resolved and phase-resolved Fourier analysis of these images identifies wavevectors connecting all k-space regions where Δkα has the same or opposite sign. But use of a single isolated impurity atom, from whose precise location the spatial phase of the scattering interference pattern must be measured, is technically difficult. Here we introduce a generalization of this approach for use with multiple impurity atoms, and demonstrate its validity by comparing the Δkα it generates to the Δkα determined from single-atom scattering in FeSe where s± energy-gap symmetry is established. Finally, to exemplify utility, we use the multi-atom technique on LiFeAs and find scattering interference between the hole-like and electron-like pockets as predicted for Δkα of opposite sign. © 2021, Crown.
    view abstract10.1038/s41535-020-00303-4
  • Dominant In-Plane Symmetric Elastoresistance in CsFe2As2
    Wiecki, P. and Haghighirad, A.-A. and Weber, F. and Merz, M. and Heid, R. and Böhmer, A.E.
    Physical Review Letters 125 (2020)
    We study the elastoresistance of the highly correlated material CsFe2As2 in all symmetry channels. Neutralizing its thermal expansion by means of a piezoelectric-based strain cell is demonstrated to be essential. The elastoresistance response in the in-plane symmetric channel is found to be large, while the response in the symmetry-breaking channels is weaker and provides no evidence for a divergent nematic susceptibility. Rather, our results can be interpreted naturally within the framework of a coherence-incoherence crossover, where the low-temperature coherent state is sensitively tuned by the in-plane atomic distances. © 2020 American Physical Society.
    view abstract10.1103/PhysRevLett.125.187001
  • Impact of nematicity on the relationship between antiferromagnetic fluctuations and superconductivity in FeSe0.91 S0.09 under pressure
    Rana, K. and Xiang, L. and Wiecki, P. and Ribeiro, R.A. and Lesseux, G.G. and Böhmer, A.E. and Bud'ko, S.L. and Canfield, P.C. and Furukawa, Y.
    Physical Review B 101 (2020)
    The sulfur-substituted FeSe system, FeSe1-xSx, provides a versatile platform for studying the relationship among nematicity, antiferromagnetism, and superconductivity. Here, by nuclear magnetic resonance (NMR) and resistivity measurements up to 4.73 GPa on FeSe0.91S0.09, we established the pressure-(p-) temperature (T) phase diagram in which the nematic state is suppressed with pressure showing a nematic quantum phase transition (QPT) around p=0.5GPa, two superconductivity (SC) regions separated by the QPT appear, and antiferromagnetic (AFM) phase emerges above ∼3.3GPa. From the NMR results up to 2.1 GPa, AFM fluctuations are revealed to be characterized by the stripe-type wave vector which remains the same for the two SC regions. Furthermore, the electronic state is found to change in character from non-Fermi liquid to Fermi liquid around the nematic QPT and persists up to ∼2.1GPa. In addition, although the AFM fluctuations correlate with Tc in both SC states, demonstrating the importance of the AFM fluctuations for the appearance of SC in the system, we found that, when nematic order is absent, Tc is strongly correlated with the AFM fluctuations whereas Tc weakly depends on the AFM fluctuations when nematic order is present. Our findings on FeSe0.91S0.09 were shown to be applied to the whole FeSe1-xSx system and provide an insight into the relationship between AFM fluctuations and SC in Fe-based superconductors. © 2020 American Physical Society.
    view abstract10.1103/PhysRevB.101.180503
  • Bulk Superconductivity and Role of Fluctuations in the Iron-Based Superconductor FeSe at High Pressures
    Gati, E. and Böhmer, A.E. and Bud'ko, S.L. and Canfield, P.C.
    Physical Review Letters 123 (2019)
    The iron-based superconductor FeSe offers a unique possibility to study the interplay of superconductivity with purely nematic as well magnetic-nematic order by pressure (p) tuning. By measuring specific heat under p up to 2.36 GPa, we study the multiple phases in FeSe using a thermodynamic probe. We conclude that superconductivity is bulk across the entire p range and competes with magnetism. In addition, whenever magnetism is present, fluctuations exist over a wide temperature range above both the bulk superconducting and the magnetic transitions. Whereas the magnetic fluctuations are likely temporal, the superconducting fluctuations may be either temporal or spatial. These observations highlight similarities between FeSe and underdoped cuprate superconductors. © 2019 American Physical Society.
    view abstract10.1103/PhysRevLett.123.167002
  • Distinct pressure evolution of coupled nematic and magnetic orders in FeSe
    Böhmer, A.E. and Kothapalli, K. and Jayasekara, W.T. and Wilde, J.M. and Li, B. and Sapkota, A. and Ueland, B.G. and Das, P. and Xiao, Y. and Bi, W. and Zhao, J. and Alp, E.E. and Bud'Ko, S.L. and Canfield, P.C. and Goldman, A.I. and Kreyssig, A.
    Physical Review B 100 (2019)
    We present a microscopic study of nematicity and magnetism in FeSe over a wide temperature and pressure range using high-energy x-ray diffraction and time-domain Mössbauer spectroscopy. The low-temperature magnetic hyperfine field increases monotonically up to ∼6 GPa. The orthorhombic distortion initially decreases under increasing pressure but is stabilized at intermediate pressures by cooperative coupling to the pressure-induced magnetic order. Close to the reported maximum of the superconducting critical temperature at p=6.8GPa, the orthorhombic distortion suddenly disappears and a new tetragonal magnetic phase occurs. The pressure and temperature evolution of the structural and magnetic order parameters suggests that they have distinct origins. © 2019 American Physical Society.
    view abstract10.1103/PhysRevB.100.064515
  • Electron-phonon coupling and superconductivity-induced distortion of the phonon lineshape in V3Si
    Sauer, A. and Zocco, D.A. and Said, A.H. and Heid, R. and Böhmer, A. and Weber, F.
    Physical Review B 99 (2019)
    Phonon measurements in the A15-type superconductors were complicated in the past because of the unavailability of large single crystals for inelastic neutron scattering, e.g., in the case of Nb3Sn, or unfavorable neutron scattering properties in the case of V3Si. Hence, only few studies of the lattice dynamical properties with momentum resolved methods were published, in particular below the superconducting transition temperature Tc. Here, we overcome these problems by employing inelastic x-ray scattering and report a combined experimental and theoretical investigation of lattice dynamics in V3Si with the focus on the temperature-dependent properties of low-energy acoustic phonon modes in several high-symmetry directions. We paid particular attention to the evolution of the soft phonon mode of the structural phase transition observed in our sample at Ts=18.9K, i.e., just above the measured superconducting phase transition at Tc=16.8K. Theoretically, we predict lattice dynamics including electron-phonon coupling based on density-functional-perturbation theory and discuss the relevance of the soft phonon mode with regard to the value of Tc. Furthermore, we explain superconductivity-induced anomalies in the lineshape of several acoustic phonon modes using a model proposed by Allen et al, [Phys. Rev. B 56, 5552 (1997)10.1103/PhysRevB.56.5552]. © 2019 American Physical Society.
    view abstract10.1103/PhysRevB.99.134511
  • Ferroelasticity, anelasticity and magnetoelastic relaxation in Co-doped iron pnictide: Ba(Fe0.957Co0.043)2As2
    Carpenter, M.A. and Evans, D.M. and Schiemer, J.A. and Wolf, T. and Adelmann, P. and Böhmer, A.E. and Meingast, C. and Dutton, S.E. and Mukherjee, P. and Howard, C.J.
    Journal of Physics Condensed Matter 31 (2019)
    The hypothesis that strain has a permeating influence on ferroelastic, magnetic and superconducting transitions in 122 iron pnictides has been tested by investigating variations of the elastic and anelastic properties of a single crystal of Ba(Fe0.957Co0.043)2As2 by resonant ultrasound spectroscopy as a function of temperature and externally applied magnetic field. Non-linear softening and stiffening of C 66 in the stability fields of both the tetragonal and orthorhombic structures has been found to conform quantitatively to the Landau expansion for a pseudoproper ferroelastic transition which is second order in character. The only exception is that the transition occurs at a temperature (T S ≈ 69 K) ∼10 K above the temperature at which C 66 would extrapolate to zero ( ≈ 59 K). An absence of anomalies associated with antiferromagnetic ordering below T N ≈ 60 K implies that coupling of the magnetic order parameter with shear strain is weak. It is concluded that linear-quadratic coupling between the structural/electronic and antiferromagnetic order parameters is suppressed due to the effects of local heterogeneous strain fields arising from the substitution of Fe by Co. An acoustic loss peak at ∼50-55 K is attributed to the influence of mobile ferroelastic twin walls that become pinned by a thermally activated process involving polaronic defects. Softening of C 66 by up to ∼6% below the normal - superconducting transition at T c ≈ 13 K demonstrates an effective coupling of the shear strain with the order parameter for the superconducting transition which arises indirectly as a consequence of unfavourable coupling of the superconducting order parameter with the ferroelastic order parameter. Ba(Fe0.957Co0.043)2As2 is representative of 122 pnictides as forming a class of multiferroic superconductors in which elastic strain relaxations underpin almost all aspects of coupling between the structural, magnetic and superconducting order parameters and of dynamic properties of the transformation microstructures they contain. © 2019 IOP Publishing Ltd.
    view abstract10.1088/1361-648X/aafe29
  • Strain relaxation behaviour of vortices in a multiferroic superconductor
    Evans, D.M. and Schiemer, J.A. and Wolf, T. and Adelmann, P. and Böhmer, A.E. and Meingast, C. and Dutton, S.E. and Mukherjee, P. and Hsu, Y.-T. and Carpenter, M.A.
    Journal of Physics Condensed Matter 31 (2019)
    The elastic and anelastic properties of a single crystal of Co-doped pnictide Ba(Fe 0.957 Co 0.043 ) 2 As 2 have been determined by resonant ultrasound spectroscopy in the frequency range 10-500 kHz, both as a function of temperature through the normal-superconducting transition (T c ≈ 12.5 K) and as a function of applied magnetic field up to 12.5 T. Correlation with thermal expansion, electrical resistivity, heat capacity, DC and AC magnetic data from crystals taken from the same synthetic batch has revealed the permeating influence of strain on coupling between order parameters for the ferroelastic (Q E ) and superconducting (Q SC ) transitions and on the freezing/relaxation behaviour of vortices. Elastic softening through T c in zero field can be understood in terms of classical coupling of the order parameter with the shear strain e 6 , λe 6 , which means that there must be a common strain mechanism for coupling of the form λ Q E . At fields of ∼5 T and above, this softening is masked by Debye-like stiffening and acoustic loss processes due to vortex freezing. The first loss peak may be associated with the establishment of superconductivity on ferroelastic twin walls ahead of the matrix and the second is due to the vortex liquid-vortex glass transition. Strain contrast between vortex cores and the superconducting matrix will contribute significantly to interactions of vortices both with each other and with the underlying crystal structure. These interactions imply that iron-pnictides represent a class of multiferroic superconductors in which strain-mediated coupling occurs between the multiferroic properties (ferroelasticity, antiferromagnetism) and superconductivity. © 2019 IOP Publishing Ltd.
    view abstract10.1088/1361-648X/aafbd7
  • Antiferromagnetic order in CaK(Fe1-xNix)4As4 and its interplay with superconductivity
    Kreyssig, A. and Wilde, J.M. and Böhmer, A.E. and Tian, W. and Meier, W.R. and Li, B. and Ueland, B.G. and Xu, M. and Bud'Ko, S.L. and Canfield, P.C. and McQueeney, R.J. and Goldman, A.I.
    Physical Review B 97 (2018)
    The magnetic order in CaK(Fe1-xNix)4As4 (1144) single crystals (x=0.051 and 0.033) has been studied by neutron diffraction. We observe magnetic Bragg peaks associated with the same propagation vectors as found for the collinear stripe antiferromagnetic (AFM) order in the related BaFe2As2 (122) compound. The AFM state in 1144 preserves tetragonal symmetry and only a commensurate, noncollinear structure with a hedgehog spin-vortex crystal (SVC) arrangement in the Fe plane and simple AFM stacking along the c direction is consistent with our observations. The SVC order is promoted by the reduced symmetry in the FeAs layer in the 1144 structure. The long-range SVC order coexists with superconductivity, however, similar to the doped 122 compounds, the ordered magnetic moment is gradually suppressed with the developing superconducting order parameter. This supports the notion that both collinear and noncollinear magnetism and superconductivity are competing for the same electrons coupled by Fermi surface nesting in iron arsenide superconductors. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.97.224521
  • Direct visualization of phase separation between superconducting and nematic domains in Co-doped CaFe2As2 close to a first-order phase transition
    Fente, A. and Correa-Orellana, A. and Böhmer, A.E. and Kreyssig, A. and Ran, S. and Bud'Ko, S.L. and Canfield, P.C. and Mompean, F.J. and García-Hernández, M. and Munuera, C. and Guillamón, I. and Suderow, H.
    Physical Review B 97 (2018)
    We show that biaxial strain induces alternating tetragonal superconducting and orthorhombic nematic domains in Co-substituted CaFe2As2. We use atomic force, magnetic force, and scanning tunneling microscopy to identify the domains and characterize their properties, finding in particular that tetragonal superconducting domains are very elongated, more than several tens of micrometers long and about 30 nm wide; have the same Tc as unstrained samples; and hold vortices in a magnetic field. Thus, biaxial strain produces a phase-separated state, where each phase is equivalent to what is found on either side of the first-order phase transition between antiferromagnetic orthorhombic and superconducting tetragonal phases found in unstrained samples when changing Co concentration. Having such alternating superconducting domains separated by normal conducting domains with sizes of the order of the coherence length opens opportunities to build Josephson junction networks or vortex pinning arrays and suggests that first-order quantum phase transitions lead to nanometric-size phase separation under the influence of strain. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.97.014505
  • Doping evolution of spin fluctuations and their peculiar suppression at low temperatures in Ca(Fe1-xCox)2As2
    Sapkota, A. and Das, P. and Böhmer, A.E. and Ueland, B.G. and Abernathy, D.L. and Bud'Ko, S.L. and Canfield, P.C. and Kreyssig, A. and Goldman, A.I. and McQueeney, R.J.
    Physical Review B 97 (2018)
    Results of inelastic neutron scattering measurements are reported for two annealed compositions of Ca(Fe1-xCox)2As2,x=0.026 and 0.030, which possess stripe-type antiferromagnetically ordered and superconducting ground states, respectively. In the AFM ground state, well-defined and gapped spin waves are observed for x=0.026, similar to the parent CaFe2As2 compound. We conclude that the well-defined spin waves are likely to be present for all x corresponding to the AFM state. This behavior is in contrast to the smooth evolution to overdamped spin dynamics observed in Ba(Fe1-xCox)2As2, wherein the crossover corresponds to microscopically coexisting AFM order and SC at low temperature. The smooth evolution is likely absent in Ca(Fe1-xCox)2As2 due to the mutual exclusion of AFM ordered and SC states. Overdamped spin dynamics characterize paramagnetism of the x=0.030 sample and high-temperature x=0.026 sample. A sizable loss of magnetic intensity is observed over a wide energy range upon cooling the x=0.030 sample, at temperatures just above and within the superconducting phase. This phenomenon is unique amongst the iron-based superconductors and is consistent with a temperature-dependent reduction in the fluctuating moment. One possible scenario ascribes this loss of moment to a sensitivity to the c-axis lattice parameter in proximity to the nonmagnetic collapsed tetragonal phase and another scenario ascribes the loss to a formation of a pseudogap. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.97.174519
  • Hedgehog Spin-Vortex Crystal Antiferromagnetic Quantum Criticality in CaK (Fe1-xNix)4As4 Revealed by NMR
    Ding, Q.-P. and Meier, W.R. and Cui, J. and Xu, M. and Böhmer, A.E. and Bud'Ko, S.L. and Canfield, P.C. and Furukawa, Y.
    Physical Review Letters 121 (2018)
    Two ordering states, antiferromagnetism and nematicity, have been observed in most iron-based superconductors (SCs). In contrast to those SCs, the newly discovered SC CaK(Fe1-xNix)4As4 exhibits an antiferromagnetic (AFM) state, called hedgehog spin-vortex crystal (SVC) structure, without nematic order, providing the opportunity for the investigation into the relationship between spin fluctuations and SC without any effects of nematic fluctuations. Our As75 nuclear magnetic resonance studies on CaK(Fe1-xNix)4As4 (0≤x≤0.049) revealed that CaKFe4As4 is located close to a hidden hedgehog SVC AFM quantum-critical point (QCP). The magnetic QCP without nematicity in CaK(Fe1-xNix)4As4 highlights the close connection of spin fluctuations and superconductivity in iron-based SCs. The advantage of stoichiometric composition also makes CaKFe4As4 an ideal platform for further detailed investigation of the relationship between magnetic QCP and superconductivity in iron-based SCs without disorder effects. © 2018 American Physical Society.
    view abstract10.1103/PhysRevLett.121.137204
  • Hedgehog spin-vortex crystal stabilized in a hole-doped iron-based superconductor
    Meier, W.R. and Ding, Q.-P. and Kreyssig, A. and Bud'Ko, S.L. and Sapkota, A. and Kothapalli, K. and Borisov, V. and Valentí, R. and Batista, C.D. and Orth, P.P. and Fernandes, R.M. and Goldman, A.I. and Furukawa, Y. and Böhmer, A.E. and Canfield, P.C.
    npj Quantum Materials 3 (2018)
    Magnetism is widely considered to be a key ingredient of unconventional superconductivity. In contrast to cuprate high-temperature superconductors, antiferromagnetism in most Fe-based superconductors (FeSCs) is characterized by a pair of magnetic propagation vectors, (π,0) and (0,π). Consequently, three different types of magnetic order are possible. Of these, only stripe-type spin-density wave (SSDW) and spin-charge-density wave (SCDW) orders have been observed. A realization of the proposed spin-vortex crystal (SVC) order is noticeably absent. We report a magnetic phase consistent with the hedgehog variation of SVC order in Ni-doped and Co-doped CaKFe4As4 based on thermodynamic, transport, structural and local magnetic probes combined with symmetry analysis. The exotic SVC phase is stabilized by the reduced symmetry of the CaKFe4As4 structure. Our results suggest that the possible magnetic ground states in FeSCs have very similar energies, providing an enlarged configuration space for magnetic fluctuations to promote high-temperature superconductivity. © 2018 The Author(s).
    view abstract10.1038/s41535-017-0076-x
  • High-field superconductivity on iron chalcogenide FeSe
    Shi, A. and Kitagawa, S. and Ishida, K. and Böhmer, A.E. and Meingast, C. and Wolf, T.
    Journal of the Physical Society of Japan 87 (2018)
    We have performed ac-susceptibility and77Se-NMR measurements on single-crystal FeSe in the field range from 12.5 to 14.75T below 1.6K in order to investigate the superconducting properties of the B phase. Our results show that although superconductivity persists beyond the A–B transition line (H+), the broadening of the 77Se-NMR linewidth arising from the superconducting diamagnetic effect decreases at around H+, suggesting that superconducting character is changed at H+. © 2018 The Physical Society of Japan.
    view abstract10.7566/JPSJ.87.065002
  • Imaging orbital-selective quasiparticles in the Hund’s metal state of FeSe
    Kostin, A. and Sprau, P.O. and Kreisel, A. and Chong, Y.X. and Böhmer, A.E. and Canfield, P.C. and Hirschfeld, P.J. and Andersen, B.M. and Davis, J.C.S.
    Nature Materials 17 (2018)
    Strong electronic correlations, emerging from the parent Mott insulator phase, are key to copper-based high-temperature superconductivity. By contrast, the parent phase of an iron-based high-temperature superconductor is never a correlated insulator. However, this distinction may be deceptive because Fe has five actived d orbitals while Cu has only one. In theory, such orbital multiplicity can generate a Hund’s metal state, in which alignment of the Fe spins suppresses inter-orbital fluctuations, producing orbitally selective strong correlations. The spectral weights Zm of quasiparticles associated with different Fe orbitals m should then be radically different. Here we use quasiparticle scattering interference resolved by orbital content to explore these predictions in FeSe. Signatures of strong, orbitally selective differences of quasiparticle Zm appear on all detectable bands over a wide energy range. Further, the quasiparticle interference amplitudes reveal that Zx y< Zx z≪ Zy z, consistent with earlier orbital-selective Cooper pairing studies. Thus, orbital-selective strong correlations dominate the parent state of iron-based high-temperature superconductivity in FeSe. © 2018, The Author(s).
    view abstract10.1038/s41563-018-0151-0
  • Indication of subdominant D-wave interaction in superconducting CaKFe4As4
    Jost, D. and Scholz, J.-R. and Zweck, U. and Meier, W.R. and Böhmer, A.E. and Canfield, P.C. and Lazarević, N. and Hackl, R.
    Physical Review B 98 (2018)
    We report inelastic light scattering results on the stoichiometric and fully ordered superconductor CaKFe4As4 as a function of temperature and light polarization. In the energy range between 10 and 315 cm-1 (1.24 and 39.1 meV) we observe the particle-hole continuum above and below the superconducting transition temperature Tc and seven of the eight Raman active phonons. The main focus is placed on the analysis of the electronic excitations. Below Tc all three symmetries projected with in-plane polarizations display a redistribution of spectral weight characteristic for superconductivity. The energies of the pair-breaking peaks in A1g and B2g symmetry are in approximate agreement with the results from photoemission studies. In B1g symmetry the difference between the normal and superconducting state is most pronounced, and the feature is shifted downwards with respect to those in A1g and B2g symmetry. The maximum peaking at 134 cm-1 (16.6 meV) has a substructure on the high-energy side. We interpret the peak at 134 cm-1 in terms of a collective Bardasis-Schrieffer (BS) mode and the substructure as a remainder of the pair-breaking feature on the electron bands. There is a very weak peak at 50 cm-1 (6.2 meV) which is tentatively assigned to another BS mode. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.98.020504
  • Ingredients for the electronic nematic phase in FeSe revealed by its anisotropic optical response
    Chinotti, M. and Pal, A. and Degiorgi, L. and Böhmer, A.E. and Canfield, P.C.
    Physical Review B 98 (2018)
    The origin of the anisotropy in physical quantities related to a symmetry-broken (nematic) electronic state is still very much debated in high-temperature superconductors. FeSe at ambient pressure undergoes a structural, tetragonal-to-orthorhombic phase transition at Ts≃90 K without any magnetic ordering on further cooling, which leads to an ideal electronic nematicity. Our unprecedented optical results provide evidence that the low-energy excitation spectrum in the nematic phase is shaped by an important interplay of the anisotropic Drude weight and scattering rate. In the zero-frequency limit though, the temperature dependence of the anisotropic scattering rate plays the dominant role and, combined with the nematic order parameter as evinced from the high energy optical response, accounts for the anisotropic dc resistivity. This favors the scattering by anisotropic spin fluctuations as the prominent candidate in governing the properties of the nematic phase. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.98.094506
  • Nematicity, magnetism and superconductivity in FeSe
    Böhmer, A.E. and Kreisel, A.
    Journal of Physics Condensed Matter 30 (2018)
    Iron-based superconductors are well known for their complex interplay between structure, magnetism and superconductivity. FeSe offers a particularly fascinating example. This material has been intensely discussed because of its extended nematic phase, whose relationship with magnetism is not obvious. Superconductivity in FeSe is highly tunable, with the superconducting transition temperature, T c, ranging from 8 K in bulk single crystals at ambient pressure to almost 40 K under pressure or in intercalated systems, and to even higher temperatures in thin films. In this topical review, we present an overview of nematicity, magnetism and superconductivity, and discuss the interplay of these phases in FeSe. We focus on bulk FeSe and the effects of physical pressure and chemical substitutions as tuning parameters. The experimental results are discussed in the context of the well-studied iron-pnictide superconductors and interpretations from theoretical approaches are presented. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-648X/aa9caa
  • Persistent correlation between superconductivity and antiferromagnetic fluctuations near a nematic quantum critical point in FeSe1-x Sx
    Wiecki, P. and Rana, K. and Böhmer, A.E. and Lee, Y. and Bud'Ko, S.L. and Canfield, P.C. and Furukawa, Y.
    Physical Review B 98 (2018)
    We present Se77-NMR measurements on FeSe1-xSx samples with sulfur content x=0%, 9%, 15%, and 29%. Twinned nematic domains are observed in the NMR spectrum for all samples except x=29%. The NMR spin-lattice relaxation rate shows that antiferromagnetic (AFM) fluctuations are initially enhanced between x=0% and x=9%, but are strongly suppressed for higher x values. The observed behavior of the AFM fluctuations parallels the superconducting transition temperature Tc in these materials, providing strong evidence for the primary importance of AFM fluctuations for superconductivity, despite the presence of nematic quantum criticality in the FeSe1-xSx system. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.98.020507
  • Pseudogap behavior of the nuclear spin-lattice relaxation rate in FeSe probed by 77Se-NMR
    Shi, A. and Arai, T. and Kitagawa, S. and Yamanaka, T. and Ishida, K. and Böhmer, A.E. and Meingast, C. and Wolf, T. and Hirata, M. and Sasaki, T.
    Journal of the Physical Society of Japan 87 (2018)
    We conducted 77Se-nuclear magnetic resonance studies of the iron-based superconductor FeSe in magnetic fields of 0.6 to 19 T to investigate the superconducting and normal-state properties. The nuclear spin-lattice relaxation rate divided by the temperature (T1T)-1 increases below the structural transition temperature Ts but starts to be suppressed below T∗, well above the superconducting transition temperature Tc(H), resulting in a broad maximum of (T1T)-1 at Tp(H). This is similar to the pseudogap behavior in optimally doped cuprate superconductors. Because T∗ and Tp(H) decrease in the same manner as Tc(H) with increasing H, the pseudogap behavior in FeSe is ascribed to superconducting fluctuations, which presumably originate from the theoretically predicted preformed pair above Tc(H). © 2018 The Physical Society of Japan.
    view abstract10.7566/JPSJ.87.013704
  • Spectral Evidence for Emergent Order in Ba1-xNaxFe2As2
    Yi, M. and Frano, A. and Lu, D.H. and He, Y. and Wang, M. and Frandsen, B.A. and Kemper, A.F. and Yu, R. and Si, Q. and Wang, L. and He, M. and Hardy, F. and Schweiss, P. and Adelmann, P. and Wolf, T. and Hashimoto, M. and Mo, S.-K. and Hussain, Z. and Le Tacon, M. and Böhmer, A.E. and Lee, D.-H. and Shen, Z.-X. and Meingast, C. and Birgeneau, R.J.
    Physical Review Letters 121 (2018)
    We report an angle-resolved photoemission spectroscopy study of the iron-based superconductor family, Ba1-xNaxFe2As2. This system harbors the recently discovered double-Q magnetic order appearing in a reentrant C4 phase deep within the underdoped regime of the phase diagram that is otherwise dominated by the coupled nematic phase and collinear antiferromagnetic order. From a detailed temperature-dependence study, we identify the electronic response to the nematic phase in an orbital-dependent band shift that strictly follows the rotational symmetry of the lattice and disappears when the system restores C4 symmetry in the low temperature phase. In addition, we report the observation of a distinct electronic reconstruction that cannot be explained by the known electronic orders in the system. © 2018 American Physical Society.
    view abstract10.1103/PhysRevLett.121.127001
  • Discovery of orbital-selective Cooper pairing in FeSe
    Sprau, P.O. and Kostin, A. and Kreisel, A. and Böhmer, A.E. and Taufour, V. and Canfield, P.C. and Mukherjee, S. and Hirschfeld, P.J. and Andersen, B.M. and Davis, J.C.S.
    Science 357 (2017)
    The superconductor iron selenide (FeSe) is of intense interest owing to its unusual nonmagnetic nematic state and potential for high-temperature superconductivity. But its Cooper pairing mechanism has not been determined. We used Bogoliubov quasiparticle interference imaging to determine the Fermi surface geometry of the electronic bands surrounding the Γ = (0, 0) and X = (π/aFe, 0) points of FeSe and to measure the corresponding superconducting energy gaps. We show that both gaps are extremely anisotropic but nodeless and that they exhibit gap maxima oriented orthogonally in momentum space. Moreover, by implementing a novel technique, we demonstrate that these gaps have opposite sign with respect to each other. This complex gap configuration reveals the existence of orbital-selective Cooper pairing that, in FeSe, is based preferentially on electrons from the dyz orbitals of the iron atoms. © 2017, American Association for the Advancement of Science. All rights reserved.
    view abstract10.1126/science.aal1575
  • Dome of magnetic order inside the nematic phase of sulfur-substituted FeSe under pressure
    Xiang, L. and Kaluarachchi, U.S. and Böhmer, A.E. and Taufour, V. and Tanatar, M.A. and Prozorov, R. and Bud'Ko, S.L. and Canfield, P.C.
    Physical Review B 96 (2017)
    The pressure dependence of the structural, magnetic, and superconducting transitions and of the superconducting upper critical field were studied in sulfur-substituted Fe(Se1-xSx). Resistance measurements were performed on single crystals with three substitution levels (x=0.043, 0.096, 0.12) under hydrostatic pressures up to 1.8 GPa and in magnetic fields up to 9 T and were compared to data on pure FeSe. Our results illustrate the effects of chemical and physical pressure on Fe(Se1-xSx). On increasing sulfur content, magnetic order in the low-pressure range is strongly suppressed to a small domelike region in the phase diagrams. However, Ts is much less suppressed by sulfur substitution, and Tc of Fe(Se1-xSx) exhibits similar nonmonotonic pressure dependence with a local maximum and a local minimum present in the low-pressure range for all x. The local maximum in Tc coincides with the emergence of the magnetic order above Tc. At this pressure the slope of the upper critical field decreases abruptly, which may indicate a Fermi-surface reconstruction. The minimum of Tc correlates with a broad maximum of the upper critical field slope normalized by Tc. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.96.024511
  • Effect of Biaxial Strain on the Phase Transitions of Ca (Fe1-xCox)2As2
    Böhmer, A.E. and Sapkota, A. and Kreyssig, A. and Bud'Ko, S.L. and Drachuck, G. and Saunders, S.M. and Goldman, A.I. and Canfield, P.C.
    Physical Review Letters 118 (2017)
    We study the effect of applied strain as a physical control parameter for the phase transitions of Ca(Fe1-xCox)2As2 using resistivity, magnetization, x-ray diffraction, and Fe57 Mössbauer spectroscopy. Biaxial strain, namely, compression of the basal plane of the tetragonal unit cell, is created through firm bonding of samples to a rigid substrate via differential thermal expansion. This strain is shown to induce a magnetostructural phase transition in originally paramagnetic samples, and superconductivity in previously nonsuperconducting ones. The magnetostructural transition is gradual as a consequence of using strain instead of pressure or stress as a tuning parameter. © 2017 American Physical Society.
    view abstract10.1103/PhysRevLett.118.107002
  • Growth and characterization of BaZnGa
    Jo, N.H. and Lin, Q. and Nguyen, M.C. and Kaluarachchi, U.S. and Meier, W.R. and Manni, S. and Downing, S.S. and Böhmer, A.E. and Kong, T. and Sun, Y. and Taufour, V. and Wang, C.-Z. and Ho, K.-M. and Bud’ko, S.L. and Canfield, P.C.
    Philosophical Magazine 97 (2017)
    We report the growth, structure and characterization of BaZnGa, identifying it as the sole known ternary compound in the Ba–Zn–Ga system. Single crystals of BaZnGa can be grown out of excess Ba–Zn and adopt a tI36 structure type. There are three unique Ba sites and three M = Zn/Ga sites. Using DFT calculations we can argue that whereas one of these three M sites is probably solely occupied by Ga, the other two M sites, most likely, have mixed Zn/Ga occupancy. Temperature-dependent resistivity and magnetization measurements suggest that BaZnGa is a poor metal with no electronic or magnetic phase transitions between 1.8 and 300 K. © 2017 Informa UK Limited, trading as Taylor & Francis Group.
    view abstract10.1080/14786435.2017.1380861
  • Magnetic fluctuations and superconducting properties of CaKFe4As4 studied by As 75 NMR
    Cui, J. and Ding, Q.-P. and Meier, W.R. and Böhmer, A.E. and Kong, T. and Borisov, V. and Lee, Y. and Bud'Ko, S.L. and Valentí, R. and Canfield, P.C. and Furukawa, Y.
    Physical Review B 96 (2017)
    We report As75 nuclear magnetic resonance (NMR) studies on a new iron-based superconductor, CaKFe4As4, with Tc=35 K. As75 NMR spectra show two distinct lines corresponding to the As(1) and As(2) sites close to the K and Ca layers, respectively, revealing that K and Ca layers are well ordered without site inversions. We found that nuclear quadrupole frequencies νQ of the As(1) and As(2) sites show an opposite temperature T dependence. Nearly T independent behavior of the Knight shifts K is observed in the normal state, and a sudden decrease in K in the superconducting (SC) state suggests spin-singlet Cooper pairs. As75 spin-lattice relaxation rates 1/T1 show a power-law T dependence with different exponents for the two As sites. The isotropic antiferromagnetic spin fluctuations characterized by the wave vector q=(π,0) or (0,π) in the single-iron Brillouin zone notation are revealed by 1/T1T and K measurements. Such magnetic fluctuations are necessary to explain the observed temperature dependence of the As75 quadrupole frequencies, as evidenced by our first-principles calculations. In the SC state, 1/T1 shows a rapid decrease below Tc without a Hebel-Slichter peak and decreases exponentially at low T, consistent with an s± nodeless two-gap superconductor. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.96.104512
  • NMR evidence for static local nematicity and its cooperative interplay with low-energy magnetic fluctuations in FeSe under pressure
    Wiecki, P. and Nandi, M. and Böhmer, A.E. and Bud'Ko, S.L. and Canfield, P.C. and Furukawa, Y.
    Physical Review B 96 (2017)
    We present Se77-NMR measurements on single-crystalline FeSe under pressures up to 2 GPa. Based on the observation of the splitting and broadening of the NMR spectrum due to structural twin domains, we discovered that static, local nematic ordering exists well above the bulk nematic ordering temperature, Ts. The static, local nematic order and the low-energy stripe-type antiferromagnetic spin fluctuations, as revealed by NMR spin-lattice relaxation rate measurements, are both insensitive to pressure application. These NMR results provide clear evidence for the microscopic cooperation between magnetism and local nematicity in FeSe. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.96.180502
  • NMR study of the new magnetic superconductor CaK(Fe0.951Ni0.049)4As4: Microscopic coexistence of the hedgehog spin-vortex crystal and superconductivity
    Ding, Q.-P. and Meier, W.R. and Böhmer, A.E. and Bud'Ko, S.L. and Canfield, P.C. and Furukawa, Y.
    Physical Review B 96 (2017)
    The coexistence of a new-type antiferromagnetic (AFM) state, the so-called hedgehog spin-vortex crystal (SVC), and superconductivity (SC) is evidenced by an As75 nuclear magnetic resonance study on single-crystalline CaK(Fe0.951Ni0.049)4As4. The hedgehog SVC order is clearly demonstrated by the direct observation of internal magnetic induction along the c axis at the As1 site (close to K) and a zero net internal magnetic induction at the As2 site (close to Ca) below an AFM ordering temperature TN∼52 K. The nuclear spin-lattice relaxation rate 1/T1 shows a distinct decrease below Tc∼10 K, providing also unambiguous evidence for the microscopic coexistence. Furthermore, based on the analysis of the 1/T1 data, the hedgehog SVC-type spin correlations are found to be enhanced below T∼150 K in the paramagnetic state. These results indicate the hedgehog SVC-type spin correlations play an important role for the appearance of SC in the new magnetic superconductor. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.96.220510
  • Optical anisotropy in the electronic nematic phase of FeSe
    Chinotti, M. and Pal, A. and Degiorgi, L. and Böhmer, A.E. and Canfield, P.C.
    Physical Review B 96 (2017)
    At ambient pressure, FeSe undergoes a structural, tetragonal-to-orthorhombic, phase transition at Ts≃90 K without any magnetic ordering on further cooling. FeSe thus provides an arena for examining the nematic phase without the complications following the reconstruction of the Fermi surface due to the antiferromagnetic order within the orthorhombic state. We perform an optical-reflectivity investigation across the structural transition, as a function of uniaxial stress in order to detwin the specimen. These measurements reveal a hysteretic behavior of the anisotropic optical response to uniaxial stress for T≤Ts, which extends to energy scales of about 0.5 eV. The sign changes of the optical anisotropy between distinct energy intervals suggest a complex evolution of the polarized electronic structure in the nematic phase. The temperature dependence of the optical anisotropy for the fully detwinned specimen is furthermore acting as a proxy for the order parameter of nematicity. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.96.121112
  • Absence of nematic order in the pressure-induced intermediate phase of the iron-based superconductor B a0.85 K0.15 F e2 A s2
    Zheng, Y. and Tam, P.M. and Hou, J. and Böhmer, A.E. and Wolf, T. and Meingast, C. and Lortz, R.
    Physical Review B 93 (2016)
    The hole doped Fe-based superconductors Ba1-xAxFe2As2 (where A=Na or K) show a particularly rich phase diagram. It was observed that an intermediate reentrant tetragonal phase, in which the C4 fourfold rotational symmetry is restored, forms within the orthorhombic antiferromagnetically ordered stripe-type spin density wave state above the superconducting transition [S. Avci, Nat. Commun. 5, 3845 (2014);10.1038/ncomms4845 A. E. Böhmer et al., Nat. Commun. 6, 7911 (2015)10.1038/ncomms8911]. A similar intermediate phase was reported to appear if pressure is applied to underdoped Ba1-xKxFe2As2 [E. Hassinger, Phys. Rev. B 86, 140502(R) (2012)10.1103/PhysRevB.86.140502]. Here we report data of the electric resistivity, Hall effect, specific heat, and the thermoelectric Nernst and Seebeck coefficients measured on a Ba0.85K0.15Fe2As2 single crystal under pressure up to 5.5 GPa. The data reveal a coexistence of the intermediate phase with filamentary superconductivity. The Nernst coefficient shows a large signature of nematic order that coincides with the stripe-type spin density wave state up to optimal pressure. In the pressure-induced intermediate phase the nematic order is removed, thus confirming that its nature is a reentrant tetragonal phase. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.93.104516
  • Anisotropic thermodynamic and transport properties of single-crystalline CaKFe4As4
    Meier, W.R. and Kong, T. and Kaluarachchi, U.S. and Taufour, V. and Jo, N.H. and Drachuck, G. and Böhmer, A.E. and Saunders, S.M. and Sapkota, A. and Kreyssig, A. and Tanatar, M.A. and Prozorov, R. and Goldman, A.I. and Balakirev, F.F. and Gurevich, A. and Bud'Ko, S.L. and Canfield, P.C.
    Physical Review B 94 (2016)
    Single-crystalline, single-phase CaKFe4As4 has been grown out of a high-temperature, quaternary melt. Temperature-dependent measurements of x-ray diffraction, anisotropic electrical resistivity, elastoresistivity, thermoelectric power, Hall effect, magnetization, and specific heat, combined with field-dependent measurements of electrical resistivity and field and pressure-dependent measurements of magnetization indicate that CaKFe4As4 is an ordered, stoichiometric, Fe-based superconductor with a superconducting critical temperature, Tc=35.0±0.2 K. Other than superconductivity, there is no indication of any other phase transition for 1.8K≤T≤300 K. All of these thermodynamic and transport data reveal striking similarities to those found for optimally or slightly overdoped (Ba1-xKx)Fe2As2, suggesting that stoichiometric CaKFe4As4 is intrinsically close to what is referred to as "optimal-doped" on a generalized, Fe-based superconductor, phase diagram. The anisotropic superconducting upper critical field, Hc2(T), of CaKFe4As4 was determined up to 630 kOe. The anisotropy parameter γ(T)=Hc2/Hc2 , for H applied perpendicular and parallel to the c axis, decreases from ≃2.5 at Tc to ≃1.5 at 25 K, which can be explained by interplay of paramagnetic pair breaking and orbital effects. The slopes of dHc2 /dT≃-44 kOe/K and dHc2/dT≃-109 kOe/K at Tc yield an electron mass anisotropy of m/m ≃1/6 and short Ginzburg-Landau coherence lengths ξ (0)≃5.8Å and ξ(0)≃14.3Å. The value of Hc2(0) can be extrapolated to ≃920 kOe, well above the BCS paramagnetic limit. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.94.064501
  • Complex phase diagram of Ba1-xNaxFe2As2: A multitude of phases striving for the electronic entropy
    Wang, L. and Hardy, F. and Böhmer, A.E. and Wolf, T. and Schweiss, P. and Meingast, C.
    Physical Review B 93 (2016)
    The low-temperature electronic phase diagram of Ba1-xNaxFe2As2, obtained using high-resolution thermal-expansion and specific-heat measurements, is shown to be considerably more complex than previously reported, containing nine different phases. Besides the magnetic C2 and reentrant C4 phases, we find evidence for an additional, presumably magnetic, phase below the usual spin-density-wave transition, as well as a possible incommensurate magnetic phase. All these phases coexist and compete with superconductivity, which is particularly strongly suppressed by the C4-magnetic phase due to a strong reduction of the electronic entropy available for pairing in this phase. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.93.014514
  • Electronic nematic susceptibility of iron-based superconductors
    Böhmer, A.E. and Meingast, C.
    Comptes Rendus Physique 17 (2016)
    We review our recent experimental results on the electronic nematic phase in electron- and hole-doped BaFe2As2 and FeSe. The nematic susceptibility is extracted from shear-modulus data (obtained using a three-point-bending method in a capacitance dilatometer) using Landau theory and is compared to the nematic susceptibility obtained from elastoresistivity and Raman data. FeSe is particularly interesting in this context, because of a large nematic, i.e., a structurally distorted but paramagnetic, region in its phase diagram. Scaling of the nematic susceptibility with the spin lattice relaxation rate from NMR, as predicted by the spin-nematic theory, is found in both electron- and hole-doped BaFe2As2, but not in FeSe. The intricate relationship of the nematic susceptibility to spin and orbital degrees of freedom is discussed. © 2015 Académie des sciences.
    view abstract10.1016/j.crhy.2015.07.001
  • Enhancement of superconducting transition temperature by pointlike disorder and anisotropic energy gap in FeSe single crystals
    Teknowijoyo, S. and Cho, K. and Tanatar, M.A. and Gonzales, J. and Böhmer, A.E. and Cavani, O. and Mishra, V. and Hirschfeld, P.J. and Bud'Ko, S.L. and Canfield, P.C. and Prozorov, R.
    Physical Review B 94 (2016)
    A highly anisotropic superconducting gap is found in single crystals of FeSe by studying the London penetration depth Δλ measured down to 50 mK in samples before and after 2.5 MeV electron irradiation. The gap minimum increases with introduced pointlike disorder, indicating the absence of symmetry-imposed nodes. Surprisingly, the superconducting transition temperature Tc increases by 0.4 K from Tc0≈8.8 K while the structural transition temperature Ts decreases by 0.9 K from Ts0≈91.2 K after electron irradiation. We discuss several explanations for the Tc enhancement and propose that local strengthening of the pair interaction by irradiation-induced Frenkel defects most likely explains the phenomenon. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.94.064521
  • Fermi surface reconstruction in FeSe under high pressure
    Terashima, T. and Kikugawa, N. and Kiswandhi, A. and Graf, D. and Choi, E.-S. and Brooks, J.S. and Kasahara, S. and Watashige, T. and Matsuda, Y. and Shibauchi, T. and Wolf, T. and Böhmer, A.E. and Hardy, F. and Meingast, C. and Löhneysen, H.V. and Uji, S.
    Physical Review B 93 (2016)
    We report Shubnikov-de Haas (SdH) oscillation measurements on FeSe under high pressure up to P=16.1 kbar. We find a sudden change in SdH oscillations at the onset of the pressure-induced antiferromagnetism at P∼8 kbar. We argue that this change can be attributed to a reconstruction of the Fermi surface by the antiferromagnetic order. The negative dTc/dP observed in a range between P∼8 and 12 kbar may be explained by the reduction in the density of states due to the reconstruction. The ratio of the transition temperature to the effective Fermi energy remains high under high pressure: kBTc/EF∼0.1 even at P=16.1 kbar. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.93.094505
  • Magnetization and transport properties of single crystalline RPd2P2 (R=Y, La–Nd, Sm–Ho, Yb)
    Drachuck, G. and Böhmer, A.E. and Bud'ko, S.L. and Canfield, P.C.
    Journal of Magnetism and Magnetic Materials 417 (2016)
    Single crystals of RPd2P2 (R=Y, La–Nd, Sm–Ho, Yb) were grown out of a high temperature solution rich in Pd and P and characterized by room-temperature powder X-ray diffraction, anisotropic temperature- and field-dependent magnetization and temperature-dependent in-plane resistivity measurements. In this series, YPd2P2 and LaPd2P2 YbPd2P2 (with Yb2+) are non-local-moment bearing. Furthermore, YPd2P2 and LaPd2P2 are found to be superconducting with Tc≃0.75 and 0.96 K respectively. CePd2P2 and PrPd2P2 magnetically order at low temperature with a ferromagnetic component along the crystallographic c-axis. The rest of the series manifest low temperature antiferromagnetic ordering. EuPd2P2 has Eu2+ ions and both EuPd2P2 and GdPd2P2 have isotropic paramagnetic susceptibilities consistent with L=0 and [formula presented] and exhibit multiple magnetic transitions. For R=Eu–Dy, there are multiple, T>1.8K transitions in zero applied magnetic field and for R=Nd, Eu, Gd, Tb, and Dy there are clear metamagnetic transitions at T=2.0 K for H<55kOe. Strong anisotropies arising mostly from crystal electric field (CEF) effects were observed for most magnetic rare earths with L≠0. The experimentally estimated CEF parameters B20 were calculated from the anisotropic paramagnetic θab and θc values and compared to theoretical trends across the rare earth series. The ordering temperatures as well as the polycrystalline averaged paramagnetic Curie–Weiss temperature, θave, were extracted from magnetization and resistivity measurements, and compared to the de-Gennes factor. © 2016 Elsevier B.V.
    view abstract10.1016/j.jmmm.2016.05.089
  • Nonmonotonic pressure evolution of the upper critical field in superconducting FeSe
    Kaluarachchi, U.S. and Taufour, V. and Böhmer, A.E. and Tanatar, M.A. and Bud'Ko, S.L. and Kogan, V.G. and Prozorov, R. and Canfield, P.C.
    Physical Review B 93 (2016)
    The pressure dependence of the upper critical field, Hc2,c, of single crystalline FeSe was studied using measurements of the interplane resistivity, ρc, in magnetic fields parallel to tetragonal c axis. Hc2,c(T) curves obtained under hydrostatic pressures up to 1.56 GPa, the range over which the superconducting transition temperature, Tc, of FeSe exhibits a nonmonotonic dependence with local maximum at p1≈0.8 GPa and local minimum at p2≈1.2 GPa. The slope of the upper critical field at Tc,dHc2,c/dTTc, also exhibits a nonmonotonic pressure dependence with distinct changes at p1 and p2. For p<p1 the slope can be described within a multiband orbital model. For both p1<p<p2 and p>p2 the slope is in good semiquantitative agreement with a single band, orbital Helfand-Werthamer theory with Fermi velocities determined from Shubnikov-de Haas measurements. This finding indicates that Fermi surface changes are responsible for the local minimum of Tc(p) at p2≈1.2 GPa. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.93.064503
  • Origin of the Resistivity Anisotropy in the Nematic Phase of FeSe
    Tanatar, M.A. and Böhmer, A.E. and Timmons, E.I. and Schütt, M. and Drachuck, G. and Taufour, V. and Kothapalli, K. and Kreyssig, A. and Bud'Ko, S.L. and Canfield, P.C. and Fernandes, R.M. and Prozorov, R.
    Physical Review Letters 117 (2016)
    The in-plane resistivity anisotropy is studied in strain-detwinned single crystals of FeSe. In contrast to other iron-based superconductors, FeSe does not develop long-range magnetic order below the tetragonal-to-orthorhombic transition at Ts≈90 K. This allows for the disentanglement of the contributions to the resistivity anisotropy due to nematic and magnetic orders. Comparing direct transport and elastoresistivity measurements, we extract the intrinsic resistivity anisotropy of strain-free samples. The anisotropy peaks slightly below Ts and decreases to nearly zero on cooling down to the superconducting transition. This behavior is consistent with a scenario in which the in-plane resistivity anisotropy is dominated by inelastic scattering by anisotropic spin fluctuations. © 2016 American Physical Society.
    view abstract10.1103/PhysRevLett.117.127001
  • Strong cooperative coupling of pressure-induced magnetic order and nematicity in FeSe
    Kothapalli, K. and Böhmer, A.E. and Jayasekara, W.T. and Ueland, B.G. and Das, P. and Sapkota, A. and Taufour, V. and Xiao, Y. and Alp, E. and Bud'ko, S.L. and Canfield, P.C. and Kreyssig, A. and Goldman, A.I.
    Nature Communications 7 (2016)
    A hallmark of the iron-based superconductors is the strong coupling between magnetic, structural and electronic degrees of freedom. However, a universal picture of the normal state properties of these compounds has been confounded by recent investigations of FeSe where the nematic (structural) and magnetic transitions appear to be decoupled. Here, using synchrotron-based high-energy x-ray diffraction and time-domain Mössbauer spectroscopy, we show that nematicity and magnetism in FeSe under applied pressure are indeed strongly coupled. Distinct structural and magnetic transitions are observed for pressures between 1.0 and 1.7 GPa and merge into a single first-order transition for pressures ≥31.7 GPa, reminiscent of what has been found for the evolution of these transitions in the prototypical system Ba(Fe1-xCox)2As2. Our results are consistent with a spin-driven mechanism for nematic order in FeSe and provide an important step towards a universal description of the normal state properties of the iron-based superconductors.
    view abstract10.1038/ncomms12728
  • Strong correlations, strong coupling, and s -wave superconductivity in hole-doped BaFe2As2 single crystals
    Hardy, F. and Böhmer, A.E. and De'Medici, L. and Capone, M. and Giovannetti, G. and Eder, R. and Wang, L. and He, M. and Wolf, T. and Schweiss, P. and Heid, R. and Herbig, A. and Adelmann, P. and Fisher, R.A. and Meingast, C.
    Physical Review B 94 (2016)
    We present a comprehensive study of the low-temperature heat capacity and thermal expansion of single crystals of the hole-doped Ba1-xKxFe2As2 series (0<x<1) and the end-members RbFe2As2 and CsFe2As2. A large increase of the Sommerfeld coefficient γn is observed with both decreasing band filling and isovalent substitution (K, Rb, and Cs) revealing a strong enhancement of electron correlations and the possible proximity of these materials to a Mott insulator. This trend is well reproduced theoretically by our density functional theory + slave-spin (DFT+SS) calculations, confirming that 122-iron pnictides are effectively Hund metals, in which sizable Hund's coupling and orbital selectivity are the key ingredients for tuning correlations. We also find direct evidence for the existence of a coherence-incoherence crossover between a low-temperature heavy Fermi liquid and a highly incoherent high-temperature regime similar to heavy fermion systems. In the superconducting state, clear signatures of multiband superconductivity are observed with no evidence for nodes in the energy gaps, ruling out the existence of a doping-induced change of symmetry (from s to d wave). We argue that the disappearance of the electron band in the range 0.4<x<1.0 is accompanied by a strong-to-weak coupling crossover and that this shallow band remains involved in the superconducting pairing, although its contribution to the normal state fades away. Differences between hole- and electron-doped BaFe2As2 series are emphasized and discussed in terms of strong pair breaking by potential scatterers beyond the Born limit. ©2016 American Physical Society.
    view abstract10.1103/PhysRevB.94.205113
  • Variation of transition temperatures and residual resistivity ratio in vapor-grown FeSe
    Böhmer, A.E. and Taufour, V. and Straszheim, W.E. and Wolf, T. and Canfield, P.C.
    Physical Review B 94 (2016)
    The study of the iron-based superconductor FeSe has blossomed with the availability of high-quality single crystals, obtained through flux/vapor-transport growth techniques below the structural transformation temperature of its tetragonal phase, T≈450°C. Here, we report on the variation of sample morphology and properties due to small modifications in the growth conditions. A considerable variation of the superconducting transition temperature Tc, from 8.8 K to 3 K, which cannot be correlated with the sample composition, is observed. Instead, we point out a clear correlation between Tc and disorder, as measured by the residual resistivity ratio. Notably, the tetragonal-to-orthorhombic structural transition is also found to be quite strongly disorder dependent (Ts≈72-90K) and linearly correlated with Tc. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.94.024526
  • Infrared Study of the Spin Reorientation Transition and Its Reversal in the Superconducting State in Underdoped Ba1-x KxFe2As2
    Mallett, B.P.P. and Marsik, P. and Yazdi-Rizi, M. and Wolf, Th. and Böhmer, A.E. and Hardy, F. and Meingast, C. and Munzar, D. and Bernhard, C.
    Physical Review Letters 115 (2015)
    With infrared spectroscopy we investigated the spin-reorientation transition from an orthorhombic antiferromagnetic (o-AF) to a tetragonal AF (t-AF) phase and the reentrance of the o-AF phase in the superconducting state of underdoped Ba1-xKxFe2As2. In agreement with the predicted transition from a single-Q to a double-Q AF structure, we found that a distinct spin density wave develops in the t-AF phase. The pair breaking peak of this spin density wave acquires much more low-energy spectral weight than the one in the o-AF state which indicates that it competes more strongly with superconductivity. We also observed additional phonon modes in the t-AF phase which likely arise from a Brillouin-zone folding that is induced by the double-Q magnetic structure with two Fe sublattices exhibiting different magnitudes of the magnetic moment. © 2015 American Physical Society. © 2015 American Physical Society.
    view abstract10.1103/PhysRevLett.115.027003
  • Origin of the tetragonal-to-orthorhombic phase transition in FeSe: A combined thermodynamic and NMR study of nematicity
    Böhmer, A.E. and Arai, T. and Hardy, F. and Hattori, T. and Iye, T. and Wolf, T. and Löhneysen, H.V. and Ishida, K. and Meingast, C.
    Physical Review Letters 114 (2015)
    The nature of the tetragonal-to-orthorhombic structural transition at Ts≈90K in single crystalline FeSe is studied using shear-modulus, heat-capacity, magnetization, and nuclear magnetic resonance measurements. The transition is shown to be accompanied by a large shear-modulus softening, which is practically identical to that of underdoped Ba(Fe,Co)2As2, suggesting a very similar strength of the electron-lattice coupling. On the other hand, a spin-fluctuation contribution to the spin-lattice relaxation rate is only observed below Ts. This indicates that the structural, or "nematic," phase transition in FeSe is not driven by magnetic fluctuations. © 2015 American Physical Society.
    view abstract10.1103/PhysRevLett.114.027001
  • Pressure-induced antiferromagnetic transition and phase diagram in FeSe
    Terashima, T. and Kikugawa, N. and Kasahara, S. and Watashige, T. and Shibauchi, T. and Matsuda, Y. and Wolf, T. and Böhmer, A.E. and Hardy, F. and Meingast, C. and Löhneysen, H.V. and Uji, S.
    Journal of the Physical Society of Japan 84 (2015)
    We report measurements of resistance and ac magnetic susceptibility on FeSe single crystals under high pressure up to 27.2 kbar. The structural phase transition is quickly suppressed with pressure, and the associated anomaly is not seen above ∼18 kbar. The superconducting transition temperature evolves nonmonotonically with pressure, showing a minimum at ∼12 kbar. We find another anomaly at 21.2 K at 11.6 kbar. This anomaly most likely corresponds to the antiferromagnetic phase transition found in μSR measurements [M. Bendele et al., Phys. Rev. Lett. 104, 087003 (2010)]. The antiferromagnetic and superconducting transition temperatures both increase with pressure up to ∼25 kbar and then level off. The width of the superconducting transition anomalously broadens in the pressure range where the antiferromagnetism coexists. ©2015 The Physical Society of Japan.
    view abstract10.7566/JPSJ.84.063701
  • Resistivity and magnetoresistance of FeSe single crystals under helium-gas pressure
    Knöner, S. and Zielke, D. and Köhler, S. and Wolf, B. and Wolf, Th. and Wang, L. and Böhmer, A. and Meingast, C. and Lang, M.
    Physical Review B - Condensed Matter and Materials Physics 91 (2015)
    We present temperature-dependent in-plane resistivity measurements on FeSe single crystals under He-gas pressure up to 800 MPa and magnetic fields B≤10 T. A sharp phase transition anomaly is revealed at the tetragonal-to-orthorhombic transition at Ts slightly below 90 K. Ts becomes reduced with increasing pressure in a linear fashion at a rate dTs/dP≃-31 K/GPa. This is accompanied by a P-linear increase of the superconducting transition temperature at Tc∼8.6 K with dTc/dP≃+5.8 K/GPa. Pressure studies of the normal-state resistivity highlight two distinctly different regimes: for T>Ts, i.e., in the tetragonal phase, the in-plane resistivity changes strongly with pressure. This contrasts with the state deep in the orthorhombic phase at TTs, preceding the superconducting transition. Here, a T-linear resistivity is observed the slope of which does not change with pressure, pointing against a spin-fluctuation origin of this term. Resistivity studies in varying magnetic fields both at ambient and finite pressure reveal clear changes of the magnetoresistance, ΔρB2, upon cooling through Ts. Our data are consistent with a reconstruction of the Fermi surface accompanying the structural transition. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.91.174510
  • Superconductivity-induced re-entrance of the orthorhombic distortion in Ba1-xKxFe2As2
    Böhmer, A.E. and Hardy, F. and Wang, L. and Wolf, T. and Schweiss, P. and Meingast, C.
    Nature Communications 6 (2015)
    Detailed knowledge of the phase diagram and the nature of the competing magnetic and superconducting phases is imperative for a deeper understanding of the physics of iron-based superconductivity. Magnetism in the iron-based superconductors is usually a stripe-type spin-density-wave, which breaks the tetragonal symmetry of the lattice, and is known to compete strongly with superconductivity. Recently, it was found that in some systems an additional spin-density-wave transition occurs, which restores this tetragonal symmetry, however, its interaction with superconductivity remains unclear. Here, using thermodynamic measurements on Ba1-xKxFe2As2 single crystals, we show that the spin-density-wave phase of tetragonal symmetry competes much stronger with superconductivity than the stripe-type spin-density-wave phase, which results in a novel re-entrance of the latter at or slightly below the superconducting transition. © 2015 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/ncomms8911
  • Anomalous Fermi surface in FeSe seen by Shubnikov-de Haas oscillation measurements
    Terashima, T. and Kikugawa, N. and Kiswandhi, A. and Choi, E.-S. and Brooks, J.S. and Kasahara, S. and Watashige, T. and Ikeda, H. and Shibauchi, T. and Matsuda, Y. and Wolf, T. and Böhmer, A.E. and Hardy, F. and Meingast, C. and Löhneysen, H.V. and Suzuki, M.-T. and Arita, R. and Uji, S.
    Physical Review B - Condensed Matter and Materials Physics 90 (2014)
    We have observed Shubnikov-de Haas oscillations in FeSe. The Fermi surface deviates significantly from predictions of band-structure calculations and most likely consists of one electron and one hole thin cylinder. The carrier density is in the order of 0.01 carriers/Fe, an order of magnitude smaller than predicted. Effective Fermi energies as small as 3.6 meV are estimated. These findings call for elaborate theoretical investigations incorporating both electronic correlations and orbital ordering. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.90.144517
  • High-pressure evolution of the specific heat of a strongly underdoped Ba(Fe 0.963 Co 0.037)As 2 iron-based superconductor
    Zheng, Y. and Wang, Y. and Hardy, F. and Böhmer, A.E. and Wolf, T. and Meingast, C. and Lortz, R.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    We report on specific-heat experiments under the influence of high pressure on a strongly underdoped Co-substituted BaFe2As2 single crystal. This allows us to study the phase diagram of this iron pnictide superconductor with a bulk thermodynamic method and pressure as a clean control parameter. The data show large specific-heat anomalies at the superconducting transition temperature, which proves the bulk nature of pressure-induced superconductivity. The transitions in specific heat are sharper than those in resistivity, which demonstrates the necessity of employing bulk thermodynamic methods to explore the exact phase diagram of pressure-induced Fe-based superconductors. The Tc at optimal pressure and the superconducting condensation energy are found to be larger than those in optimally Co-doped samples at ambient pressure, which we attribute to a weak pair-breaking effect of the Co ions. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.054514
  • Lifting of xz / yz orbital degeneracy at the structural transition in detwinned FeSe
    Shimojima, T. and Suzuki, Y. and Sonobe, T. and Nakamura, A. and Sakano, M. and Omachi, J. and Yoshioka, K. and Kuwata-Gonokami, M. and Ono, K. and Kumigashira, H. and Böhmer, A.E. and Hardy, F. and Wolf, T. and Meingast, C. and Löhneysen, H.V. and Ikeda, H. and Ishizaka, K.
    Physical Review B - Condensed Matter and Materials Physics 90 (2014)
    We study superconducting FeSe (Tc=9K) exhibiting the tetragonal-orthorhombic structural transition (Ts∼90K) without any antiferromagnetic ordering, by utilizing angle-resolved photoemission spectroscopy. In the detwinned orthorhombic state, the energy position of the dyz orbital band at the Brillouin zone corner is 50meV higher than that of dxz, indicating the orbital order similar to the NaFeAs and BaFe2As2 families. Evidence of orbital order also appears in the hole bands at the Brillouin zone center. Precisely measured temperature dependence using strain-free samples shows that the onset of the orbital ordering (To) occurs very close to Ts, thus suggesting that the electronic nematicity above Ts is considerably weaker in FeSe compared to BaFe2As2 family. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.90.121111
  • Multiband superconductivity in KFe2As2: Evidence for one isotropic and several lilliputian energy gaps
    Hardy, F. and Ederl, R. and Jackson, M. and Aoki, D. and Paulsen, C. and Wolf, T. and Burger, P. and Böhmer, A. and Schweiss, P. and Adelmann, P. and Fisher, R.A. and Meingast, C.
    Journal of the Physical Society of Japan 83 (2014)
    We report a detailed low-temperature thermodynamic investigation (heat capacity and magnetization) of the superconducting state of KFe 2As2 for H||c axis. Our measurements reveal that the properties of KFe2As2 are dominated by a relatively large nodeless energy gap (δ0 = 1.9 κBTc) which excludes dx2-y2 symmetry. We prove the existence of several additional extremely small gaps (δ0 &lt; 1.0 k BTc) that have a profound impact on the low-temperature and lowfield behavior, similar to MgB2, CeCoIn5, and PrOs4Sb12. The zero-field heat capacity is analyzed in a realistic selfconsistent 4-band BCS model which reproduces only qualitatively the recent laser ARPES results of Okazaki et al. [Science 337, 1314 (2012)]. Our results show that extremely low-temperature measurements, i.e., T &lt; 0.1 K, are required in order to resolve the question of the existence of line nodes in this compound. © 2014 The Physical Society of Japan.
    view abstract10.7566/JPSJ.83.014711
  • Nematic susceptibility of hole-doped and electron-doped Ba F e 2 As 2 iron-based superconductors from shear modulus measurements
    Böhmer, A.E. and Burger, P. and Hardy, F. and Wolf, T. and Schweiss, P. and Fromknecht, R. and Reinecker, M. and Schranz, W. and Meingast, C.
    Physical Review Letters 112 (2014)
    The nematic susceptibility, χφ, of hole-doped Ba1-xKxFe2As2 and electron-doped Ba(Fe1-xCox)2As2 iron-based superconductors is obtained from measurements of the elastic shear modulus using a three-point bending setup in a capacitance dilatometer. Nematic fluctuations, although weakened by doping, extend over the whole superconducting dome in both systems, suggesting their close tie to superconductivity. Evidence for quantum critical behavior of χφ is, surprisingly, only found for Ba(Fe1-xCox)2As2 and not for Ba1-xKxFe2As2 - the system with the higher maximal Tc value. © 2014 American Physical Society.
    view abstract10.1103/PhysRevLett.112.047001
  • Unusual band renormalization in the simplest iron-based superconductor FeSe1-x
    Maletz, J. and Zabolotnyy, V.B. and Evtushinsky, D.V. and Thirupathaiah, S. and Wolter, A.U.B. and Harnagea, L. and Yaresko, A.N. and Vasiliev, A.N. and Chareev, D.A. and Böhmer, A.E. and Hardy, F. and Wolf, T. and Meingast, C. and Rienks, E.D.L. and Büchner, B. and Borisenko, S.V.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    The electronic structure of the iron chalcogenide superconductor FeSe1-x was investigated by high-resolution angle-resolved photoemission spectroscopy (ARPES). The results were compared to DFT calculations showing some significant differences between the experimental electronic structure of FeSe1-x, DFT calculations, and existing data on FeSexTe1-x. The bands undergo a pronounced orbital-dependent renormalization, different from what was observed for FeSexTe1-x and any other pnictides. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.220506
  • Evidence of strong correlations and coherence-incoherence crossover in the iron pnictide superconductor KFe2As2
    Hardy, F. and Böhmer, A.E. and Aoki, D. and Burger, P. and Wolf, T. and Schweiss, P. and Heid, R. and Adelmann, P. and Yao, Y.X. and Kotliar, G. and Schmalian, J. and Meingast, C.
    Physical Review Letters 111 (2013)
    Using resistivity, heat-capacity, thermal-expansion, and susceptibility measurements we study the normal-state behavior of KFe2As 2. Both the Sommerfeld coefficient (γ≈103 mJ mol -1 K-2) and the Pauli susceptibility (χ≈4× 10-4) are strongly enhanced, which confirm the existence of heavy quasiparticles inferred from previous de Haas-van Alphen and angle-resolved photoemission spectroscopy experiments. We discuss this large enhancement using a Gutzwiller slave-boson mean-field calculation, which shows the proximity of KFe2As2 to an orbital-selective Mott transition. The temperature dependence of the magnetic susceptibility and the thermal expansion provide strong experimental evidence for the existence of a coherence- incoherence crossover, similar to what is found in heavy fermion and ruthenate compounds, due to Hund's coupling between orbitals. © 2013 American Physical Society.
    view abstract10.1103/PhysRevLett.111.027002
  • Lack of coupling between superconductivity and orthorhombic distortion in stoichiometric single-crystalline FeSe
    Böhmer, A.E. and Hardy, F. and Eilers, F. and Ernst, D. and Adelmann, P. and Schweiss, P. and Wolf, T. and Meingast, C.
    Physical Review B - Condensed Matter and Materials Physics 87 (2013)
    The coupling between superconductivity and orthorhombic distortion is studied in vapor-grown FeSe single crystals using high-resolution thermal-expansion measurements. In contrast to the Ba122-based (Ba122) superconductors, we find that superconductivity does not reduce the orthorhombicity below Tc. Instead we find that superconductivity couples strongly to the in-plane area, which explains the large hydrostatic pressure effects. We discuss our results in light of the spin-nematic scenario and argue that FeSe has many features that are quite different from typical Fe-based superconductors. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.87.180505
  • Scaling between magnetic and lattice fluctuations in iron pnictide superconductors
    Fernandes, R.M. and Böhmer, A.E. and Meingast, C. and Schmalian, J.
    Physical Review Letters 111 (2013)
    The phase diagram of the iron arsenides is dominated by a magnetic and a structural phase transition, which need to be suppressed in order for superconductivity to appear. The proximity between the two transition temperature lines indicates correlation between these two phases, whose nature remains unsettled. Here, we find a scaling relation between nuclear magnetic resonance and shear modulus data in the tetragonal phase of electron-doped Ba(Fe1-xCox)2As2 compounds. Because the former probes the strength of magnetic fluctuations while the latter is sensitive to orthorhombic fluctuations, our results provide strong evidence for a magnetically driven structural transition. © 2013 American Physical Society.
    view abstract10.1103/PhysRevLett.111.137001
  • Strong Pauli-limiting behavior of Hc2 and uniaxial pressure dependencies in KFe2As2
    Burger, P. and Hardy, F. and Aoki, D. and Böhmer, A.E. and Eder, R. and Heid, R. and Wolf, T. and Schweiss, P. and Fromknecht, R. and Jackson, M.J. and Paulsen, C. and Meingast, C.
    Physical Review B - Condensed Matter and Materials Physics 88 (2013)
    KFe2As2 single crystals are studied using specific-heat, high-resolution thermal-expansion, magnetization, and magnetostriction measurements. The magnetization and magnetostriction data provide clear evidence for strong Pauli-limiting effects of the upper critical field for magnetic fields parallel to the FeAs planes, suggesting that KFe 2As2 may be a good candidate to search for the Fulde-Ferrell-Larkin-Ovchinnikov state. Using standard thermodynamic relations, the uniaxial pressure derivatives of the critical temperature Tc, the normal-state Sommerfeld coefficient γn, the normal-state susceptibility χ, and the thermodynamic critical field Hc are calculated from our data. We find that the close relationship between doping and pressure as found in other Fe-based systems does not hold for KFe 2As2. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.88.014517
  • High-resolution thermal expansion of isovalently substituted BaFe 2(As1-xPx)2
    Böhmer, A.E. and Burger, P. and Hardy, F. and Wolf, T. and Schweiss, P. and Fromknecht, R. and Von Löhneysen, H. and Meingast, C. and Kasahara, S. and Terashima, T. and Shibauchi, T. and Matsuda, Y.
    Journal of Physics: Conference Series 391 (2012)
    We have investigated the isovalently substituted system BaFe2(As1-xPx)2 by high-resolution thermal expansion using a home-built capacitive dilatometer. Accurate measurements succeeded despite the very small size of the available single crystals (∼ 500 × 500 × 100μm3). Information on the uniaxial pressure derivatives of the transition temperatures is obtained using thermodynamic relations. In-plane and out-of-plane pressure derivatives have opposite sign, which demonstrates the sensitivity of the compound to uniaxial pressure. The structural and the superconducting transition always respond oppositely to uniaxial pressure, which signals their coupling and competition. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1742-6596/391/1/012122
  • Superconducting and martensitic transitions of V3Si and Nb 3Sn under high pressure
    Tanaka, S. and Handoko and Miyake, A. and Kagayama, T. and Shimizu, K. and Böhmer, Anna.E. and Burger, P. and Hardy, F. and Meingast, C. and Tsutsumi, H. and Onuki, Y.
    Journal of the Physical Society of Japan 81 (2012)
    Electrical resistivity and specific heat measurements have been performed under high pressure to reveal the interplay between the martensitic transition and the superconductivity of V3Si and Nb3Sn. For V 3Si, superconducting transition temperature Tc increases with increasing pressure up to 4 GPa, where the martensitic transition temperature TM merges with Tc. Tc is insensitive to the pressure between 4 GPa and 9 GPa, and is suppressed on further compression accompanied with a decrease of the electrical resisitivity at Tc and an increase of the superconducting transition width. The similar tendency is also observed in Nb3Sn. The martensitic transition seems to play an important role in superconductivity. © 2012 The Physical Society of Japan.
    view abstract10.1143/JPSJS.81SB.SB026
  • Thermal expansion and grüneisen parameters of Ba(Fe 1-xCo x) 2As 2: A thermodynamic quest for quantum criticality
    Meingast, C. and Hardy, F. and Heid, R. and Adelmann, P. and Böhmer, A. and Burger, P. and Ernst, D. and Fromknecht, R. and Schweiss, P. and Wolf, T.
    Physical Review Letters 108 (2012)
    Thermal expansion data are used to study the uniaxial pressure dependence of the electronic-magnetic entropy of Ba(Fe 1-xCo x) 2As 2. Uniaxial pressure is found to be proportional to doping and, thus, also an appropriate tuning parameter in this system. Many of the features predicted to occur for a pressure-tuned quantum critical system, in which superconductivity is an emergent phase hiding the critical point, are observed. The electronic-magnetic Grüneisen parameters associated with the spin-density wave and superconducting transitions further demonstrate an intimate connection between both ordering phenomena. © 2012 American Physical Society.
    view abstract10.1103/PhysRevLett.108.177004
  • Thermodynamic phase diagram, phase competition, and uniaxial pressure effects in BaFe 2(As 1-xP x) 2 studied by thermal expansion
    Böhmer, A.E. and Burger, P. and Hardy, F. and Wolf, T. and Schweiss, P. and Fromknecht, R. and V. Löhneysen, H. and Meingast, C. and Mak, H.K. and Lortz, R. and Kasahara, S. and Terashima, T. and Shibauchi, T. and Matsuda, Y.
    Physical Review B - Condensed Matter and Materials Physics 86 (2012)
    High-resolution thermal-expansion and specific-heat data of isovalently substituted single-crystalline BaFe 2(As 1-xP x) 2 (0≤x≤0.33, x=1) are presented. We show that crystals can be detwinned in situ in the capacitance dilatometer, allowing a study of all three independent crystallographic directions. From the thermal-expansion data, we determine the phase diagram via a thermodynamic probe, study the coupling of the spin-density wave (SDW) and superconducting order parameters, and determine various pressure dependencies of the normal and superconducting states. Our results show that in the underdoped region, superconductivity and SDW order coexist and compete with each other. The resulting phase diagram, however, exhibits a smaller coexistence region of SDW and superconductivity with a steeper rise of T c on the underdoped side than in, e.g., Ba(Fe 1-xCo x) 2As 2. On the overdoped side, where there is no sign of SDW order, the lattice parameters respond to superconductivity in much the same way as to the SDW on the underdoped side, which demonstrates the intimate connection between both kinds of order. Using thermodynamic relations, the uniaxial pressure derivatives of the superconducting critical temperature and the electronic Sommerfeld coefficient are determined from our thermal-expansion data together with the specific-heat data. We find that uniaxial pressure is proportional to P substitution and that the electronic density of states has a maximum at optimal doping. Overall, the coupling of the SDW and superconducting order to the lattice parameters of BaFe 2(As 1-xP x) 2 is found to be qualitatively very similar to that of the well-studied, supposedly electron-doped Ba(Fe 1-xCo x) 2As 2 system. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.86.094521
  • correlated phases

  • magnetism

  • metallic compounds

  • quantum materials

  • superconductors

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