The magnetic order for several compositions of CaK(Fe 1-x Mn x )4As4 has been studied by nuclear magnetic resonance (NMR), Mössbauer spectroscopy, and neutron diffraction. Our observations for the Mn-doped 1144 compound are consistent with the hedgehog spin vortex crystal (hSVC) order which has previously been found for Ni-doped CaKFe4As4 . The hSVC state is characterized by the stripe-type propagation vectors (π0) and (0π) just as in the doped 122 compounds. The hSVC state preserves tetragonal symmetry at the Fe site, and only this SVC motif with simple antiferromagnetic (AFM) stacking along c is consistent with all our observations using NMR Mössbauer spectroscopy, and neutron diffraction. We find that the hSVC state in the Mn-doped 1144 compound coexists with superconductivity, and by combining the neutron scattering and Mössbauer spectroscopy data we can infer a quantum phase transition, hidden under the superconducting dome, associated with the suppression of the AFM transition temperature (T N) to zero for x ≈ 0.01. In addition, unlike several 122 compounds and Ni-doped 1144, the ordered magnetic moment is not observed to decrease at temperatures below the superconducting transition temperature (T c). © 2023 The Author(s). Published by IOP Publishing Ltd.

The high-temperature crystal growth of intermetallics often asks for sealing of the materials in a protective atmosphere. Here, we report on the development of a convenient sealing method for alkali-containing melts, with high vapor pressure and reactivity. Our newly designed container made of high-temperature resistant steel can be sealed manually and reliably without any air exposure of the containing material. The closed container may be heated in air up to at least 1150 (Formula presented.) C. The containers were applied for the development and optimization of a high-temperature self-flux growth of KFe (Formula presented.) Ag (Formula presented.) Ch (Formula presented.) (Ch = Se, Te) single crystals. Their crystal structure and the low-temperature electrical resistance are presented. The successful growths of these air-sensitive materials out of a reactive self-flux confirm the reliability of the container. © 2023 by the authors.

EuMnSb2 is a candidate topological material which can be tuned towards a Weyl semimetal, but there are differing reports for its antiferromagnetic (AFM) phases. The coupling of bands dominated by pure Sb layers hosting topological fermions to Mn and Eu magnetic states provides a potential path to tune the topological properties. Here we present single-crystal neutron diffraction, magnetization, and heat-capacity data as well as polycrystalline Eu151 Mössbauer data which show that three AFM phases exist as a function of temperature, and we present a detailed analysis of the magnetic structure in each phase. The Mn magnetic sublattice orders into a C-type AFM structure below TNMn=323(1)K with the ordered Mn magnetic moment μMn lying perpendicular to the layers. AFM ordering of the Eu sublattice occurs below TNEu1=23(1)K with the ordered Eu magnetic moment μEu canted away from the layer normal and μMn retaining its higher temperature order. μEu is ferromagnetically aligned within each Eu layer but exhibits a complicated AFM layer stacking. Both of these higher-temperature phases are described by magnetic space group (MSG) Pn′m′a′ with the chemical and magnetic unit cells having the same dimensions. Cooling below TNEu2=9(1)K reveals a third AFM phase where μMn remains unchanged but μEu develops an additional substantial in-plane canting. This phase has MSG P1121a′. We also find some evidence of short-range magnetic correlations associated with the Eu between 12K T 30K. Using the determined magnetic structures, we postulate the signs of nearest-neighbor intralayer and interlayer exchange constants and the magnetic anisotropy within a general Heisenberg model. We then discuss implications of the various AFM states in EuMnSb2 and their potential for tuning topological properties. © 2022 American Physical Society. All rights reserved.

The inline equation for the Ginzburg number, Gi, in the right column of page 9 of the paper had a typo, displaying a wrong power of θ0. (Formula Presented). This typo did not affect the value of Gi for CaKFe 4 As 4 evaluated in the paper or any discussions and conclusions. © 2022 American Physical Society. All rights reserved.

La2Ni7 is an intermetallic compound that is thought to have itinerant magnetism with small moment (∼0.15μB/Ni) ordering below 65 K. A recent study of single crystal samples by Ribeiro et al. [Phys. Rev. B 105, 014412 (2022)2469-995010.1103/PhysRevB.105.014412] determined detailed anisotropic H-T phase diagrams and revealed three zero-field magnetic phase transitions at T1∼61.0 K, T2∼56.5 K, and T3∼42 K. In that study only the highest temperature phase is shown to have a clear ferromagnetic component. Here we present a single crystal neutron diffraction study determining the propagation vector and magnetic moment direction of the three magnetically ordered phases, two incommensurate and one commensurate, as a function of temperature. The higher temperature phases have similar, incommensurate propagation vectors, but with different ordered moment directions. At lower temperatures, the magnetic order becomes commensurate with magnetic moments along the c direction as part of a first-order magnetic phase transition. We find that the low-temperature commensurate magnetic order is consistent with a proposal from earlier DFT calculations. © 2022 American Physical Society.

Canfieldite, Ag8SnS6, is a semiconducting mineral notable for its high ionic conductivity, photosensitivity, and low thermal conductivity. We report the solution growth of large single crystals of Ag8SnS6 of mass up to 1 g from a ternary Ag–Sn–S melt. On cooling from high temperature, Ag8SnS6 undergoes a known cubic (F4̅3m) to orthorhombic (Pna21) phase transition at ≈460 K. By studying the magnetization and thermal expansion between 5–300 K, we discover a second structural transition at ≈120 K. Single crystal X-ray diffraction reveals the low-temperature phase adopts a different orthorhombic structure with space group Pmn21 (a = 7.662 9(5) Å, b = 7.539 6(5) Å, c = 10.630 0(5) Å, Z = 2 at 90 K) that is isostructural to the room-temperature forms of the related Se-based compounds Ag8SnSe6 and Ag8GeSe6. The 120 K transition is first-order and has a large thermal hysteresis. On the basis of the magnetization and thermal expansion data, the room-temperature polymorph can be kinetically arrested into a metastable state by rapidly cooling to temperatures below 40 K. We last compare the room- and low-temperature forms of Ag8SnS6 with its argyrodite analogues, Ag8TQ6 (T = Si, Ge, Sn; Q = S, Se), and identify a trend relating the preferred structures to the unit cell volume, suggesting smaller phase volume favors the Pna21 arrangement. We support this picture by showing that the transition to the Pmn21 phase is avoided in Ge alloyed Ag8Sn1–xGexS6 samples as well as in pure Ag8GeS6 © 2021 American Chemical Society

We studied the electronic structure of PtPb4 using laser angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations. This material is closely related to PtSn4, which exhibits exotic topological properties such as Dirac node arcs. The Fermi surface (FS) of PtPb4 consists of two electron pockets at the center of the Brillouin zone (BZ) and several hole pockets around the zone boundaries. Our ARPES data reveal significant Rashba splitting at the Γ point, in agreement with DFT calculations. The presence of Rashba splitting may render this material of potential interest for spintronic applications. © 2021 American Physical Society.

LaCrGe3 has attracted attention as a paradigm example of the avoidance of ferromagnetic (FM) quantum criticality in an itinerant magnet. Here, we combined thermodynamic (specific heat and thermal expansion), transport, x-ray, and neutron scattering as well as μSR measurements to obtain insights on the temperature-pressure phase diagram of LaCrGe3. Consistent with previous studies of the phase diagram by transport measurements, our thermodynamic data shows clearly that the FM transition at TFM changes its character from second order to first order when it is suppressed to low temperatures by pressure. In addition, previous studies demonstrated that for high pressures a new phase occurs below T2, which was proposed to be a long-wavelength antiferromagnetic state (AFMq). In this paper, we provide evidence from our thermodynamic data that this phase transition is preceded by yet another phase transition at T1>T2. Our μSR data indicate that full magnetic volume fraction is only established below T2, but that this magnetism is characterized by a short correlation length. Within the experimental resolution, our neutron-scattering data is not able to identify any magnetic Bragg peaks. Overall, the microscopic magnetic data is therefore consistent with the formation of FM clusters in the proximity of the avoided FM quantum critical point in LaCrGe3. This conclusion is at odds with the previous proposal of AFMq order and raises questions on the role of disorder in this stochiometric compound. © 2021 American Physical Society.

Knowledge of magnetic symmetry is vital for exploiting nontrivial surface states of magnetic topological materials. EuIn2As2 is an excellent example, as it is predicted to have collinear antiferromagnetic order where the magnetic moment direction determines either a topological-crystalline-insulator phase supporting axion electrodynamics or a higher-order-topological-insulator phase with chiral hinge states. Here, we use neutron diffraction, symmetry analysis, and density functional theory results to demonstrate that EuIn2As2 actually exhibits low-symmetry helical antiferromagnetic order which makes it a stoichiometric magnetic topological-crystalline axion insulator protected by the combination of a 180∘ rotation and time-reversal symmetries: C2× T= 2 ′. Surfaces protected by 2 ′ are expected to have an exotic gapless Dirac cone which is unpinned to specific crystal momenta. All other surfaces have gapped Dirac cones and exhibit half-integer quantum anomalous Hall conductivity. We predict that the direction of a modest applied magnetic field of μ0H ≈ 1 to 2 T can tune between gapless and gapped surface states. © 2021, This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply.

We report the magnetic ordering and structural distortion in PrFeAsO crystals, the basis compound for one of the oxypnictide superconductors, using high-resolution X-ray diffraction, neutron diffraction, and X-ray resonant magnetic scattering (XRMS). We find the structural tetragonal-to-orthorhombic phase transition at TS=147K, the AFM phase transition of the Fe moments at TFe=72K, and the Pr AFM phase transition at TPr=21K. Combined high-resolution neutron diffraction and XRMS show unambiguously that the Pr moments point parallel to the longer orthorhombic a axis and order antiferromagnetically along the a axis but ferromagnetically along the b and c directions in the stripelike AFM order. The temperature-dependent magnetic order parameter of the Pr moments shows no evidence for a reorientation of moments. © 2021 American Physical Society.

This erratum concerns the elastoresistivity coefficients reported in Fig. 14 of the paper. The absolute value of the coefficients had been taken without proper indication. This does not affect the conclusions of the paper, nor indeed any of the points of the discussion. In fact, 2m66 is positive, whereas m11 - m 12 was found to be negative. The oversight was revealed by comparison with newly published results in Ref. [1] with which our results are consistent. The revised Fig. 14 of the paper and its revised caption are given here. (Figure Presented). © 2020 American Physical Society. All rights reserved.

We have used neutron powder diffraction to demonstrate the existence of long-range antiferromagnetic order of Ising-like Dy moments in the DyCd6 quasicrystal approximant phase. This cubic compound undergoes a slight distortion to a monoclinic cell at low temperatures. The Néel temperature is 18.0(2) K and the magnetic order of the Dy sublattice may be described in the parent cubic Im3 structure using a combination of two propagation vectors, k1 = [0 0 0] and k2 = [12 0 12], yielding 'anti-I' order. Alternatively, when referred to the monoclinic C2/c cell, the magnetic structure may be described by a single propagation vector: k = [1 0 0]. © 2019 Author(s).

In search of a quantum phase transition between the two-dimensional (2D) ferromagnetism of CaCo2-yAs2 and stripe-type antiferromagnetism in SrCo2As2, we instead find evidence for 1D magnetic frustration between magnetic square Co layers. We present neutron-diffraction data for Ca1-xSrxCo2-yAs2 that reveal a sequence of x-dependent magnetic transitions which involve different stacking of 2D ferromagnetically aligned layers with different magnetic anisotropy. We explain the x-dependent changes to the magnetic order by utilizing classical analytical calculations of a 1D Heisenberg model where single-ion magnetic anisotropy and frustration of antiferromagnetic nearest- and next-nearest-layer exchange interactions are all composition dependent. © 2019 American Physical Society.

Whereas magnetic frustration is typically associated with local-moment magnets in special geometric arrangements, here we show that SrCo2As2 is a candidate for frustrated itinerant magnetism. Using inelastic neutron scattering (INS), we find that antiferromagnetic (AF) spin fluctuations develop in the square Co layers of SrCo2As2 below T≈100 K centered at the stripe-type AF propagation vector of (12,12), and that their development is concomitant with a suppression of the uniform magnetic susceptibility determined via magnetization measurements. We interpret this switch in spectral weight as signaling a temperature-induced crossover from an instability toward ferromagnetism ordering to an instability toward stripe-type AF ordering on cooling, and show results from Monte-Carlo simulations for a J1-J2 Heisenberg model that illustrates how the crossover develops as a function of the frustration ratio -J1/(2J2). By putting our INS data on an absolute scale, we quantitatively compare them and our magnetization data to exact-diagonalization calculations for the J1-J2 model [N. Shannon, Eur. Phys. J. B 38, 599 (2004).EPJBFY1434-602810.1140/epjb/e2004-00156-3], and show that the calculations predict a lower level of magnetic frustration than indicated by experiment. We trace this discrepancy to the large energy scale of the fluctuations (Javg 75 meV), which, in addition to the steep dispersion, is more characteristic of itinerant magnetism. © 2019 American Physical Society.

We report on the crystal and magnetic structures and magnetic and transport properties of SrMnSb2 single crystals grown by the self-flux method. Magnetic susceptibility measurements reveal an antiferromagnetic (AFM) transition at TN=295(3) K. Above TN, the susceptibility slightly increases and forms a broad peak at T∼420 K, which is a typical feature of two-dimensional magnetic systems. Neutron diffraction measurements on single crystals confirm the previously reported C-type AFM structure below TN. Both de Haas-van Alphen (dHvA) and Shubnikov-de Haas (SdH) effects are observed in SrMnSb2 single crystals. Analysis of the oscillatory component by a Fourier transform shows that the prominent frequencies obtained by the two different techniques are practically the same within error regardless of sample size or saturated magnetic moment. Transmission electron microscopy (TEM) reveals the existence of stacking faults in the crystals, which result from a horizontal shift of Sb atomic layers suggesting possible ordering of Sb vacancies in the crystals. Increase of temperature in susceptibility measurements leads to the formation of a strong peak at T∼570 K that upon cooling under magnetic field the susceptibility shows a ferromagnetic transition at TC∼580 K. Neutron powder diffraction on crushed single crystals does not support a ferromagnetic phase above TN. Furthermore, x-ray magnetic circular dichroism (XMCD) measurements of a single crystal at the L2,3 edge of Mn shows a signal due to induced canting of AFM moments by the applied magnetic field. All evidence strongly suggests that a chemical transformation at the surface of single crystals occurs above 500 K concurrently producing a minute amount of ferromagnetic impurity phase. © 2019 American Physical Society.

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.

SrCo2As2 is a peculiar itinerant magnetic system that does not order magnetically, but inelastic neutron-scattering experiments observe the same stripe-type antiferromagnetic (AF) fluctuations found in many of the Fe-based superconductors along with evidence of magnetic frustration. Here we present results from neutron diffraction measurements on single crystals of Sr(Co1-xNix)2As2 that show the development of long-range AF order with Ni doping. However, the AF order is not stripe type. Rather, the magnetic structure consists of ferromagnetically aligned (FM) layers (with moments laying in the layer) that are AF arranged along c with an incommensurate propagation vector of (0 0 τ), i.e., a helix. Using high-energy x-ray diffraction, we find no evidence for a temperature-induced structural phase transition that would indicate a collinear AF order. This finding supports a picture of competing FM and AF interactions within the square transition-metal layers due to flat-band magnetic instabilities. However, the composition dependence of the propagation vector suggests that far more subtle Fermi surface and orbital effects control the interlayer magnetic correlations. © 2019 American Physical Society.

The magnetic structure of Nd in NdFeAsO compound has been investigated by X-ray resonant magnetic scattering at the Nd L2 edge (E=6.725 keV) at 1.7≤T≤15 K. At T=1.7 K we find that the Nd moments are aligned along the crystallographic c direction with the (1, 0, 0) propagation vector, and are arranged antiferromagnetically along the a direction and ferromagnetically along the b and c directions. At 1.7<T≤15 K, we observe an unusual temperature dependence of the magnetic Bragg peaks that appear differently at Q=(h,0,l) and (0,k,l). From the changes of magnetic intensities and our representation analysis, we find that the Nd moments are reoriented toward the a direction with a gradual decrease of the moment component in the c direction with increasing temperature. We conclude that the reorientation of the Nd moment is due to a strong interplay with the Fe moment which is fully ordered in this temperature range. © 2019 American Physical Society.

We present neutron-diffraction data for the cubic-heavy-fermion YbBiPt that show broad magnetic diffraction peaks due to the fragile short-range antiferromagnetic (AFM) order persist under an applied magnetic-field H. Our results for H [1̄10] and a temperature of T=0.14(1)K show that the (12,12,32) magnetic diffraction peak can be described by the same two-peak line shape found for μ0H=0T below the Néel temperature of TN=0.4K. Both components of the peak exist for μ0H1.4T, which is well past the AFM phase boundary determined from our new resistivity data. Using neutron-diffraction data taken at T=0.13(2)K for H [001] or [110], we show that domains of short-range AFM order change size throughout the previously determined AFM and non-Fermi liquid regions of the phase diagram, and that the appearance of a magnetic diffraction peak at (12,12,12) at μ0H≈0.4T signals canting of the ordered magnetic moment away from [111]. The continued broadness of the magnetic diffraction peaks under a magnetic field and their persistence across the AFM phase boundary established by detailed transport and thermodynamic experiments present an interesting quandary concerning the nature of YbBiPt's electronic ground state. © 2019 American Physical Society. US.

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.

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.

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.

The irreversible transformation from an icosahedral quasicrystal (i-QC) CaAu4.39Al1.61 to its cubic 2/1 crystalline approximant (CA) Ca13Au56.31(3)Al21.69 (CaAu4.33(1)Al1.67, Pa3 (No. 205); Pearson symbol: cP728; a = 23.8934(4)), starting at ∼570 °C and complete by ∼650 °C, is discovered from in situ, high-energy, variable-temperature powder X-ray diffraction (PXRD), thereby providing direct experimental evidence for the relationship between QCs and their associated CAs. The new cubic phase crystallizes in a Tsai-type approximant structure under the broader classification of polar intermetallic compounds, in which atoms of different electronegativities, viz., electronegative Au + Al vs electropositive Ca, are arranged in concentric shells. From a structural chemical perspective, the outermost shell of this cubic approximant may be described as interpenetrating and edge-sharing icosahedra, a perspective that is obtained by splitting the traditional structural description of this shell as a 92-atom rhombic triacontahedron into an 80-vertex cage of primarily Au [Au59.86(2)Al17.14□3.00] and an icosahedral shell of only Al [Al10.5□1.5]. Following the proposal that the cubic 2/1 CA approximates the structure of the i-QC and on the basis of the observed transformation, an atomic site analysis of the 2/1 CA, which shows a preference to maximize the number of heteroatomic Au-Al nearest neighbor contacts over homoatomic Al-Al contacts, implies a similar outcome for the i-QC structure. Analysis of the most intense reflections in the diffraction pattern of the cubic 2/1 CA that changed during the phase transformation shows correlations with icosahedral symmetry, and the stability of this cubic phase is assessed using valence electron counts. According to electronic structure calculations, a cubic 1/1 CA, "Ca24Au88Al64" (CaAu3.67Al2.67) is proposed. © 2017 American Chemical Society.

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).

Low temperature powder inelastic neutron scattering measurements were performed on three different powder samples; parent BaMn2As2,12.5% K-doped Ba0.875K0.125Mn2As2 and 25% K-doped Ba(0.75)K0.25Mn2As2. The Heisenberg Model involving J1‐J2‐Jz coupling constants were compared to the data by a powder integration routine using Monte Carlo integration methods. The best magnetic parameters were selected using a chi-square test where model intensities were compared to the full (q,E) dependence of magnetic scattering. A key step to this analysis is the characterization of the background which is formed mostly by phonon scattering intensities along with other sources including the magnetic impurity scattering events. The calculated powder magnetic intensities added to the estimated background obtained from the non-magnetic high momentum transfer region. The agreement between the simulated and the raw data enabled us to perform quantitative analysis of the unreacted MnO impurities. Overall, this is another confirmation along with earlier studies using this technique, that magnetic exchange constants can be calculated within an acceptable range with a very quick inelastic neutron powder experiment without need for a single crystal sample. © 2017 Elsevier B.V.

The microscopic details of the suppression of antiferromagnetic order in the Kondo-lattice series Ce1-xLaxCu2Ge2 due to nonmagnetic dilution by La are revealed through neutron diffraction results for x=0.20, 0.40, 0.75, and 0.85. Magnetic Bragg peaks are found for 0.20≤x≤0.75, and both the Néel temperature TN and the ordered magnetic moment per Ce μ linearly decrease with increasing x. The reduction in μ points to strong hybridization of the increasingly diluted Ce 4f electrons, and we find a remarkable quadratic dependence of μ on the Kondo-coherence temperature. We discuss our results in terms of local-moment- versus itinerant-type magnetism and mean-field theory and show that Ce1-xLaxCu2Ge2 provides an exceptional opportunity to quantitatively study the multiple magnetic interactions in a Kondo lattice. © 2018 American Physical Society.

The interplay between superconductivity and charge-density wave (CDW) in 2H-NbSe2 is not fully understood despite decades of study. Artificially introduced disorder can tip the delicate balance between two competing long-range orders, and reveal the underlying interactions that give rise to them. Here we introduce disorder by electron irradiation and measure in-plane resistivity, Hall resistivity, X-ray scattering, and London penetration depth. With increasing disorder, the superconducting transition temperature, T c, varies non-monotonically, whereas the CDW transition temperature, T CDW, monotonically decreases and becomes unresolvable above a critical irradiation dose where T c drops sharply. Our results imply that the CDW order initially competes with superconductivity, but eventually assists it. We argue that at the transition where the long-range CDW order disappears, the cooperation with superconductivity is dramatically suppressed. X-ray scattering and Hall resistivity measurements reveal that the short-range CDW survives above the transition. Superconductivity persists to much higher dose levels, consistent with fully gapped superconductivity and moderate interband pairing. © 2018 The Author(s).

The structural properties of LaRu2P2 under external pressure have been studied up to 14 GPa, employing high-energy x-ray diffraction in a diamond-anvil pressure cell. At ambient conditions, LaRu2P2 (I4/mmm) has a tetragonal structure with a bulk modulus of B=105(2) GPa and exhibits superconductivity at Tc=4.1 K. With the application of pressure, LaRu2P2 undergoes a phase transition to a collapsed tetragonal (cT) state with a bulk modulus of B=175(5) GPa. At the transition, the c-lattice parameter exhibits a sharp decrease with a concurrent increase of the a-lattice parameter. The cT phase transition in LaRu2P2 is consistent with a second-order transition, and was found to be temperature dependent, increasing from P=3.9(3) GPa at 160 K to P=4.6(3) GPa at 300 K. In total, our data are consistent with the cT transition being near, but slightly above 2 GPa at 5 K where superconductivity is suppressed. Finally, we compare the effect of physical and chemical pressure in the RRu2P2 (R = Y, La-Er, Yb) isostructural series of compounds and find them to be analogous. © 2017 American Physical Society.

Iron pnictides and related materials have been a topic of intense research for understanding the complex interplay between magnetism and superconductivity. Here we report on the magnetic structure of SrMn2As2 that crystallizes in a trigonal structure (P3m1) and undergoes an antiferromagnetic (AFM) transition at TN=118(2) K. The magnetic susceptibility remains nearly constant at temperatures T≤TN with H∥c whereas it decreases significantly with H≈ab. This shows that the ordered Mn moments lie in the ab plane instead of aligning along the -axis as in tetragonal BaMn2As2. Single-crystal neutron diffraction measurements on SrMn2As2 demonstrate that the Mn moments are ordered in a collinear Néel AFM phase with 180° AFM alignment between a moment and all nearest neighbor moments in the basal plane and also perpendicular to it. Moreover, quasi-two-dimensional AFM order is manifested in SrMn2As2 as evident from the temperature dependence of the order parameter. © 2016 IOP Publishing Ltd.

We have performed inelastic neutron scattering measurements on powder samples of the quasicrystal approximant, TbCd6, grown using isotopically enriched Cd112. Both quasielastic scattering and distinct inelastic excitations were observed below 3 meV. The intensity of the quasielastic scattering measured in the paramagnetic phase diverges as TN∼22 K is approached from above. The inelastic excitations, and their evolution with temperature, are well characterized by the leading term, B20O20, of the crystal electric field (CEF) level scheme for local pentagonal symmetry for the rare-earth ions [S. Jazbec, Phys. Rev. B 93, 054208 (2016)2469-995010.1103/PhysRevB.93.054208] indicating that the Tb moment is directed primarily along the unique local pseudofivefold axis of the Tsai-type clusters. We also find good agreement between the inverse susceptibility determined from magnetization measurements using a magnetically diluted Tb0.05Y0.95Cd6 sample and that calculated using the CEF level scheme determined from the neutron measurements. © 2017 American Physical Society.

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.

Inelastic neutron scattering measurements on the itinerant antiferromagnet CaCo2-yAs2 at a temperature of 8 K reveal two orthogonal planes of scattering perpendicular to the Co square lattice in reciprocal space, demonstrating the presence of effective one-dimensional spin interactions. These results are shown to arise from near-perfect bond frustration within the J1-J2 Heisenberg model on a square lattice with ferromagnetic J1 and hence indicate that the extensive previous experimental and theoretical study of the J1-J2 Heisenberg model on local-moment square spin lattices should be expanded to include itinerant spin systems. © 2017 American Physical Society.

In situ measurements of structure, density, and magnetization on samples of Fe83B17 using an electrostatic levitation furnace allow us to identify and correlate the magnetic and structural transitions in this system during its complex solidification process. In particular, we identify magnetic ordering in the metastable Fe23B6/fcc Fe coherently grown structures and primitive tetragonal Fe3B metastable phase in addition to characterizing the equilibrium Fe2B phase. Our measurements demonstrate that the incorporation of a tunnel-diode oscillator circuit within an electrostatic levitation furnace enables investigations of the physical properties of high-temperature metastable structures. © 2017 American Physical Society.

Neutron-diffraction and magnetic susceptibility studies of polycrystalline SrCr2As2 reveal that this compound is an itinerant G-type antiferromagnet below the Néel temperature TN = 590(5) K with the Cr magnetic moments aligned along the tetragonal c axis. The system remains tetragonal to the lowest measured temperature (∼12 K). The lattice parameter ratio c/a and the magnetic moment saturate at about the same temperature below ∼200 K, indicating a possible magnetoelastic coupling. The ordered moment μ=1.9(1)μB/Cr, measured at T=12 K, is significantly reduced compared to its localized value (4μB/Cr) due to the itinerant character brought about by hybridization between the Cr 3d and As 4p orbitals. © 2017 American Physical Society.

We report the temperature-pressure phase diagram of CaKFe4As4 established using high-pressure electrical resistivity, magnetization, and high-energy x-ray diffraction measurements up to 6 GPa. With increasing pressure, both resistivity and magnetization data show that the bulk superconducting transition of CaKFe4As4 is suppressed and then disappears at p≳4 GPa. High-pressure x-ray data clearly indicate a phase transition to a collapsed tetragonal phase in CaKFe4As4 under pressure that coincides with the abrupt loss of bulk superconductivity near 4 GPa. The x-ray data, combined with resistivity data, indicate that the collapsed tetragonal transition line is essentially independent of pressure, occurring at 4.0(5) GPa for temperatures below 150 K. Density functional theory calculations also find a sudden transition to a collapsed tetragonal state near 4 GPa, as As-As bonding develops across the Ca layer. Bonding across the K layer only occurs for p≥12 GPa. These findings demonstrate a different type of collapsed tetragonal phase in CaKFe4As4 as compared to CaFe2As2: a half-collapsed tetragonal phase. © 2017 American Physical Society.

BaMn2As2 is an antiferromagnetic insulator where a metal-insulator transition occurs with hole doping via the substitution of Ba with K. The metal-insulator transition causes only a small suppression of the Néel temperature (TN) and the ordered moment, suggesting that doped holes interact weakly with the Mn spin system. Powder inelastic neutron scattering measurements were performed on three different samples of Ba1-xKxMn2As2 with x=0, 0.125, and 0.25 to study the effect of hole doping and metallization on the spin dynamics. We compare the neutron intensities to a linear spin-wave theory approximation to the J1-J2-Jc Heisenberg model. Hole doping is found to introduce only minor modifications to the exchange energies and spin gap. The changes observed in the exchange constants are consistent with the small drop of TN with doping. © 2017 American Physical Society.

Shape memory materials have the ability to recover their original shape after a significant amount of deformation when they are subjected to certain stimuli, for instance, heat or magnetic fields. However, their performance is often limited by the energetics and geometry of the martensitic-austenitic phase transformation. Here, we report a unique shape memory behavior in CaFe2As2, which exhibits superelasticity with over 13% recoverable strain, over 3 GPa yield strength, repeatable stress-strain response even at the micrometer scale, and cryogenic linear shape memory effects near 50 K. These properties are acheived through a reversible uni-axial phase transformation mechanism, the tetragonal/orthorhombic-to-collapsed-tetragonal phase transformation. Our results offer the possibility of developing cryogenic linear actuation technologies with a high precision and high actuation power per unit volume for deep space exploration, and more broadly, suggest a mechanistic path to a class of shape memory materials, ThCr2Si2-structured intermetallic compounds. © 2017 The Author(s).

Magnetization, neutron diffraction, and high-energy x-ray diffraction results for Sn-flux grown single-crystal samples of Ca(Co1-xFex)yAs2, 0≤x≤1, 1.86≤y≤2, are presented and reveal that A-type antiferromagnetic order, with ordered moments lying along the c axis, persists for x 0.12(1). The antiferromagnetic order is smoothly suppressed with increasing x, with both the ordered moment and Néel temperature linearly decreasing. Stripe-type antiferromagnetic order does not occur for x≤0.25, nor does ferromagnetic order for x up to at least x=0.104, and a smooth crossover from the collapsed-tetragonal (cT) phase of CaCo1.86As2 to the tetragonal (T) phase of CaFe2As2 occurs. These results suggest that hole doping CaCo1.86As2 has a less dramatic effect on the magnetism and structure than steric effects due to substituting Sr for Ca. © 2017 American Physical Society.

We report on the results of a high-energy x-ray diffraction study of Al-Pd-Mn to investigate the solidification products obtained during free-cooling using an electrostatic levitation furnace. The primary solidification product from the melt is i-Al-Pd-Mn which coexists with a significant remaining liquid component. As the sample cools further, we find that the solidification pathway is consistent with the liquidus projection and pseudo-binary cut through the ternary phase diagram reported previously. At ambient temperature we have identified the major phase to be the ξ′-phase orthorhombic approximant, along with minor phases identified as Al and, most likely, the R-phase orthorhombic approximant. We have also observed a distinct prepeak in the liquid at high temperature, signifying the presence of extended atomic order. Interestingly, this prepeak was not observed in previous neutron diffraction measurements on the Al-Pd-Mn system. No undercooling was observed preceding the solidification of the i-Al-Pd-Mn phase from the melt which may signal the close similarity of the short-range order in the solid and liquid. However, this can not be clearly determined because of the potential for heterogenous nucleation associated with the presence of an Al2O3 impurity at the surface of the sample. © 2017 Walter de Gruyter GmbH, Berlin/Boston.

A new icosahedral quasicrystalline phase, CaAu4.5-xAl1.5+x [0.11 ≤ x ≤ 0.40(6); CaAu4.4Al1.6, aQC = 5.383(4) Å, and Pm3 5], and its lowest-order 1/0 cubic crystalline approximant phase, CaAu3+xAl1-x [0 ≤ x ≤ 0.31(1); a = 9.0766(5)-9.1261(8) Å, Pa3 (No. 205), and Pearson symbol cP40], have been discovered in the Ca-poor region of the Ca-Au-Al system. In the crystalline approximant, eight [Au3-xAl1+x] tetrahedra fill the unit cell, and each tetrahedron is surrounded by four Ca atoms, thus forming a three-dimensional network of {Ca4/4[Au3-xAl1+x]} tetrahedral stars. A computational study of Au and Al site preferences concurs with the experimental results, which indicate a preference for near-neighbor Au-Al interactions over Au-Au and Al-Al interactions. Analysis of the electronic density of states and the associated crystal orbital Hamilton population curves was used to rationalize the descriptions of CaAu4.5-xAl1.5+x [0.11 ≤ x ≤ 0.46(6)] and CaAu3+xAl1-x [0 ≤ x ≤ 0.31(1)] as polar intermetallic species, whereby Ca atoms engage in polar covalent bonding with the electronegative, electron-deficient [Au3-xAl1+x] tetrahedral clusters and the observed phase width of the crystalline approximant. © 2016 American Chemical Society.

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.

We report on the six-dimensional (6D) structural refinement of three members of the i-R-Cd quasicrystals (R = Gd, Dy, Tm) via synchrotron x-ray diffraction from single-grain samples, and show that this series is isostructural to the i-YbCd5.7 quasicrystal. However, our refinements suggest that the R occupancy on the Yb icosahedron sites within the Tsai-type atomic cluster is approximately 80%, with the balance taken up by Cd. Similarities between the i-R-Cd series and i-ScZn7.33, and their differences with i-YbCd5.7 and i-Ca15Cd85, indicate that there are at least two subclasses of Tsai-type icosahedral quasicrystals. We further show from x-ray resonant magnetic scattering (XRMS) measurements on a set of closely related Tb1-xYxCd6 1/1 approximants that the dilution of the magnetic R ions on the icosahedron within the Tsai-type cluster by nonmagnetic Y disrupts the commensurate magnetic ordering in the approximant phase. © 2016 American Physical Society.

We use angle resolved photoemission spectroscopy, Raman spectroscopy, low energy electron diffraction, and x-ray scattering to reveal an unusual electronically mediated charge density wave (CDW) in K0.9Mo6O17. Not only does K0.9Mo6O17 lack signatures of electron-phonon coupling, but it also hosts an extraordinary surface CDW, with TS-CDW=220 K nearly twice that of the bulk CDW, TB-CDW=115 K. While the bulk CDW has a BCS-like gap of 12 meV, the surface gap is 10 times larger and well in the strong coupling regime. Strong coupling behavior combined with the absence of signatures of strong electron-phonon coupling indicates that the CDW is likely mediated by electronic interactions enhanced by low dimensionality. © 2016 American Physical Society.

Neutron diffraction studies of metallic liquids provide valuable information about inherent topological and chemical ordering on multiple length scales as well as insight into dynamical processes at the level of a few atoms. However, there exist very few facilities in the world that allow such studies to be made of reactive metallic liquids in a containerless environment, and these are designed for use at reactor-based neutron sources. We present an electrostatic levitation facility, NESL (for Neutron ElectroStatic Levitator), which takes advantage of the enhanced capabilities and increased neutron flux available at spallation neutron sources (SNSs). NESL enables high quality elastic and inelastic neutron scattering experiments to be made of reactive metallic and other liquids in the equilibrium and supercooled temperature regime. The apparatus is comprised of a high vacuum chamber, external and internal neutron collimation optics, and a sample exchange mechanism that allows up to 30 samples to be processed between chamber openings. Two heating lasers allow excellent sample temperature homogeneity, even for samples approaching 500 mg, and an automated temperature control system allows isothermal measurements to be conducted for times approaching 2 h in the liquid state, with variations in the average sample temperature of less than 0.5%. To demonstrate the capabilities of the facility for elastic scattering studies of liquids, a high quality total structure factor for Zr64Ni36 measured slightly above the liquidus temperature is presented from experiments conducted on the nanoscale-ordered materials diffractometer (NOMAD) beam line at the SNS after only 30 min of acquisition time for a small sample (∼100 mg). © 2016 AIP Publishing LLC.

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.

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.

Detailed high-energy x-ray diffraction studies were performed to gain insight into the evolution of phase formation in undercooled Fe83B17 and the mechanism for the stabilization of face-centered cubic (fcc) Fe in the presence of Fe23B6. Fe83B17 solidifies directly into either the equilibrium Fe2B + Fe phases or the metastable Fe23B6 + Fe phases. When formed, the metastable Fe23B6 phase either rapidly transforms into the equilibrium Fe2B phase within the solidification plateau or can persist down to ambient temperature. Here, we detail these different solidification behaviors in a set of thermal cycles taken from one sample and demonstrate the absence of a direct correlation with cooling rate and thermal history. We show that the coherent growth of Fe23B6 and fcc Fe suppresses the allotropic transition from fcc Fe to bcc Fe. The temperature evolution of the phase fractions and lattice parameters is also presented. © 2016 Author(s).

Information on the lattice parameter of single crystals with known crystallographic structure allows for estimations of sample quality and composition. In many cases, it is sufficient to determine one lattice parameter or the lattice spacing along a certain, high-symmetry direction, e.g. in order to determine the composition in a substitution series by taking advantage of Vegard’s rule. Here we present a guide to accurate measurements of single crystals with dimensions ranging from 200 μm up to several millimetres using a standard powder diffractometer in Bragg–Brentano geometry. The correction of the error introduced by the sample height and the optimisation of the alignment are discussed in detail. In particular for single crystals with a plate-like habit, the described procedure allows for measurement of the lattice spacings normal to the plates with high accuracy on a timescale of minutes. © 2016 Informa UK Limited, trading as Taylor & Francis Group.

YbBiPt is a heavy-fermion compound possessing significant short-range antiferromagnetic correlations below a temperature of T∗=0.7K, fragile antiferromagnetic order below TN=0.4K, a Kondo temperature of TK≈1K, and crystalline-electric-field splitting on the order of E/kB=1-10K. Whereas the compound has a face-centered-cubic lattice at ambient temperature, certain experimental data, particularly those from studies aimed at determining its crystalline-electric-field scheme, suggest that the lattice distorts at lower temperature. Here, we present results from high-resolution, high-energy x-ray diffraction experiments which show that, within our experimental resolution of ≈6-10×10-5Å, no structural phase transition occurs between T=1.5 and 50K. In combination with results from dilatometry measurements, we further show that the compound's thermal expansion has a minimum at ≈18K and a region of negative thermal expansion for Ta18K. Despite diffraction patterns taken at 1.6K which indicate that the lattice is face-centered cubic and that the Yb resides on a crystallographic site with cubic point symmetry, we demonstrate that the linear thermal expansion may be modeled using crystalline-electric-field level schemes appropriate for Yb3+ residing on a site with either cubic or less than cubic point symmetry. © 2015 American Physical Society.

X-ray magnetic circular dichroism (XMCD) measurements on single-crystal and powder samples of Ba0.6K0.4Mn2As2 show that the ferromagnetism below TC≈100K arises in the As 4p conduction band. No XMCD signal is observed at the Mn x-ray absorption edges. Below TC, however, a clear XMCD signal is found at the As K edge which increases with decreasing temperature. The XMCD signal is absent in data taken with the beam directed parallel to the crystallographic c axis indicating that the orbital magnetic moment lies in the basal plane of the tetragonal lattice. These results show that the previously reported itinerant ferromagnetism is associated with the As 4p conduction band and that distinct local-moment antiferromagnetism and itinerant ferromagnetism with perpendicular easy axes coexist in this compound at low temperature. © 2015 American Physical Society.

We report on combined neutron and resonant x-ray scattering results, identifying the nature of the spin-orbital ground state and magnetic excitations in LuVO3 as driven by the orbital parameter. In particular, we distinguish between models based on orbital-Peierls dimerization, taken as a signature of quantum effects in orbitals, and Jahn-Teller distortions, in favor of the latter. In order to solve this long-standing puzzle, polarized neutron beams were employed as a prerequisite in order to solve details of the magnetic structure, which allowed quantitative intensity analysis of extended magnetic-excitation data sets. The results of this detailed study enabled us to draw definite conclusions about the classical versus quantum behavior of orbitals in this system and to discard the previous claims about quantum effects dominating the orbital physics of LuVO3 and similar systems. © 2015 American Physical Society.

We present high-energy x-ray diffraction data under applied pressures up to p=29GPa, neutron diffraction measurements up to p=1.1GPa, and electrical resistance measurements up to p=5.9GPa, on SrCo2As2. Our x-ray diffraction data demonstrate that there is a first-order transition between the tetragonal (T) and collapsed-tetragonal (cT) phases, with an onset above approximately 6 GPa at T=7K. The pressure for the onset of the cT phase and the range of coexistence between the T and cT phases appears to be nearly temperature independent. The compressibility along the a axis is the same for the T and cT phases, whereas, along the c axis, the cT phase is significantly stiffer, which may be due to the formation of an As-As bond in the cT phase. Our resistivity measurements found no evidence of superconductivity in SrCo2As2 for p≤5.9 GPa and T≥1.8 K. The resistivity data also show signatures consistent with a pressure-induced phase transition for p 5.5 GPa. Single-crystal neutron diffraction measurements performed up to 1.1 GPa in the T phase found no evidence of stripe-type or A-type antiferromagnetic ordering down to 10 K. Spin-polarized total-energy calculations demonstrate that the cT phase is the stable phase at high pressure with a ca ratio of 2.54. Furthermore, these calculations indicate that the cT phase of SrCo2As2 should manifest either A-type antiferromagnetic or ferromagnetic order. © 2015 American Physical Society.

We present the results of elastic and inelastic neutron scattering measurements on nonsuperconducting Ba(Fe0.957Cu0.043)2As2, a composition close to a quantum critical point between antiferromagnetic (AFM) ordered and paramagnetic phases. By comparing these results with the spin fluctuations in the low-Cu composition as well as the parent compound BaFe2As2 and superconducting Ba(Fe1-xNix)2As2 compounds, we demonstrate that paramagnon-like spin fluctuations are evident in the antiferromagnetically ordered state of Ba(Fe0.957Cu0.043)2As2, which is distinct from the AFM-like spin fluctuations in the superconducting compounds. Our observations suggest that Cu substitution decouples the interaction between quasiparticles and the spin fluctuations. We also show that the spin-spin correlation length ξ(T) increases rapidly as the temperature is lowered and find ω/T scaling behavior, the hallmark of quantum criticality, at an antiferromagnetic quantum critical point. © 2015 American Physical Society.

We use nuclear magnetic resonance (NMR), high-resolution x-ray, and neutron scattering studies to study structural and magnetic phase transitions in phosphorus-doped BaFe2(As1-xPx)2. Previous transport, NMR, specific heat, and magnetic penetration depth measurements have provided compelling evidence for the presence of a quantum critical point (QCP) near optimal superconductivity at x=0.3. However, we show that the tetragonal-to-orthorhombic structural (Ts) and paramagnetic to antiferromagnetic (AF, TN) transitions in BaFe2(As1-xPx)2 are always coupled and approach TN≈Ts≥Tc (≈29K) for x=0.29 before vanishing abruptly for x≥0.3. These results suggest that AF order in BaFe2(As1-xPx)2 disappears in a weakly first-order fashion near optimal superconductivity, much like the electron-doped iron pnictides with an avoided QCP. © 2015 American Physical Society.

Previous investigations of undercooled liquid Fe83B17 near the eutectic composition have found that metastable crystalline phases, such as Fe23B6, can be formed and persist down to ambient temperature even for rather modest cooling rates. Using time-resolved high-energy x-ray diffraction on electrostatically levitated samples of Fe83B17, we demonstrate that the Fe23B6 metastable phase and fcc γ-Fe grow coherently from the undercooled Fe83B17 liquid and effectively suppress the formation of the equilibrium Fe2B-+-bcc α-Fe phases. The stabilization of γ-Fe offers another opportunity for experimental investigations of magnetism in metastable fcc iron. © 2015 AIP Publishing LLC.

Electronic inhomogeneity appears to be an inherent characteristic of the enigmatic cuprate superconductors. Here we report the observation of charge–density–wave correlations in the model cuprate superconductor HgBa2CuO4 + d (Tc = 72 K) via bulk Cu L3-edge-resonant X-ray scattering. At the measured hole-doping level, both the short-range charge modulations and Fermi-liquid transport appear below the same temperature of about 200 K. Our result points to a unifying picture in which these two phenomena are preceded at the higher pseudogap temperature by q = 0 magnetic order and the build-up of significant dynamic antiferromagnetic correlations. The magnitude of the charge modulation wave vector is consistent with the size of the electron pocket implied by quantum oscillation and Hall effect measurements for HgBa2CuO4 + d and with corresponding results for YBa2Cu3O6 + d, which indicates that charge–density–wave correlations are universally responsible for the low-temperature quantum oscillation phenomenon. © 2014 Macmillan Publishers Limited. All rights reserved.

Neutron diffraction measurements on a single crystal of CeGe1.76 reveal a complex series of magnetic transitions at low temperature. At TN≈7 K, there is a transition from a paramagnetic state at higher temperature to an incommensurate magnetic structure characterized by a magnetic propagation vector (0 0 τ) with τ≈14 and the magnetic moment along the a axis of the orthorhombic unit cell. Below TLI≈5 K, the magnetic structure locks in to a commensurate structure with τ=14 and the magnetic moment remains along the a axis. Below T4 K, we find additional half-integer and integer indexed magnetic Bragg peaks consistent with a second commensurately ordered antiferromagnetic state. © 2014 American Physical Society.

Using inelastic neutron scattering, we show that the onset of superconductivity in underdoped Ba(Fe1-xCox)2As2 coincides with a crossover from well-defined spin waves to overdamped and diffusive spin excitations. This crossover occurs despite the presence of long-range stripe antiferromagnetic order for samples in a compositional range from x=0.04 to 0.055, and is a consequence of the shrinking spin-density wave gap and a corresponding increase in the particle-hole (Landau) damping. The latter effect is captured by a simple itinerant model relating Co doping to changes in the hot spots of the Fermi surface. We argue that the overdamped spin fluctuations provide a pairing mechanism for superconductivity in these materials. © 2014 American Physical Society.

The crystallographic and physical properties of polycrystalline and single-crystal samples of BaCo2As2 and K-doped Ba1-xKxCo2As2 (x=0.06,0.22) are investigated by x-ray and neutron powder diffraction, magnetic susceptibility χ, magnetization, heat capacity Cp,As75 nuclear magnetic resonance (NMR), and electrical resistivity ρ measurements versus temperature T. The crystals were grown using both Sn flux and CoAs self-flux, where the Sn-grown crystals contain 1.6-2.0 mol% Sn. All samples crystallize in the tetragonal ThCr2Si2-type structure (space group I4/mmm). For BaCo2As2, powder neutron diffraction data show that the c-axis lattice parameter exhibits anomalous negative thermal expansion from 10 K to 300 K, whereas the a axis lattice parameter and the unit cell volume show normal positive thermal expansion over this T range. No transitions in BaCo2As2 were found in this T range from any of the measurements. Below 40-50 K, we find T2, indicating a Fermi liquid ground state. A large density of states at the Fermi energy D(EF)≈18 states/(eV f.u.) for both spin directions is found from low-T Cp(T) measurements, whereas the band-structure calculations give D(EF)=8.23 states/(eV f.u.). The enhancement of the former value above the latter is inferred to arise from electron-electron correlations and the electron-phonon interaction. The derived intrinsic χ(T) monotonically increases with decreasing T, with anisotropy χab>χc. The 75As-NMR shift data versus T have the same T dependence as the derived χ(T) data, demonstrating that the derived χ(T) data are intrinsic. The observed 75As nuclear spin dynamics rule out the presence of Néel-type antiferromagnetic electronic spin fluctuations, but are consistent with the presence of ferromagnetic and/or stripe-type antiferromagnetic spin fluctuations. The crystals of Ba0.78K0.22Co2As2 were grown in Sn flux and show properties very similar to those of undoped BaCo2As2. On the other hand, the crystals from two batches of Ba0.94K0.06Co2As2 grown in CoAs self-flux show evidence of weak ferromagnetism at 10 K with small ordered moments at 1.8 K of ≈0.007 and 0.03μB per formula unit, respectively. © 2014 American Physical Society.

We report results from neutron scattering experiments on single crystals of YbBiPt that demonstrate antiferromagnetic order characterized by a propagation vector, Ï.,AFM = (121212), and ordered moments that align along the [1 1 1] direction of the cubic unit cell. We describe the scattering in terms of a two-Gaussian peak fit, which consists of a narrower component that appears below TN≈0.4 K and corresponds to a magnetic correlation length of ξn≈ 80 Å, and a broad component that persists up to T*≈ 0.7 K and corresponds to antiferromagnetic correlations extending over ξb≈ 20 Å. Our results illustrate the fragile magnetic order present in YbBiPt and provide a path forward for microscopic investigations of the ground states and fluctuations associated with the purported quantum critical point in this heavy-fermion compound. © 2014 American Physical Society.

Large magnetic anisotropy and coercivity are key properties of functional magnetic materials and are generally associated with rare earth elements. Here we show an extreme, uniaxial magnetic anisotropy and the emergence of magnetic hysteresis in Li2(Li1-xFex)N. An extrapolated, magnetic anisotropy field of 220 T and a coercivity field of over 11 T at 2 K outperform all known hard ferromagnets and single-molecular magnets. Steps in the hysteresis loops and relaxation phenomena in striking similarity to single-molecular magnets are particularly pronounced for x 1 and indicate the presence of nanoscale magnetic centres. Quantum tunnelling, in the form of temperature-independent relaxation and coercivity, deviation from Arrhenius behaviour and blocking of the relaxation, dominates the magnetic properties up to 10 K. The simple crystal structure, the availability of large single crystals and the ability to vary the Fe concentration make Li2(Li 1-xFex)N an ideal model system to study macroscopic quantum effects at elevated temperatures and also a basis for novel functional magnetic materials. © 2014 Macmillan Publishers Limited. All rights reserved.

Ca10(Pt3As8)(Fe2As2)5 is the parent compound for a class of Fe-based high-temperature superconductors where superconductivity with transition temperatures up to 30 K can be introduced by partial element substitution. We present a combined high-resolution high-energy x-ray diffraction and elastic neutron scattering study on a Ca10(Pt3As8)(Fe2As2)5 single crystal. This study reveals the microscopic nature of two distinct and continuous phase transitions to be very similar to other Fe-based high-temperature superconductors: an orthorhombic distortion of the high-temperature tetragonal Fe-As lattice below TS=110(2) K followed by stripelike antiferromagnetic ordering of the Fe moments below TN=96(2) K. These findings demonstrate that major features of the Fe-based high-temperature superconductors are very robust against variations in chemical constitution as well as structural imperfection of the layers separating the Fe-As layers from each other and confirms that the Fe-As layers primarily determine the physics in this class of material. © 2014 American Physical Society.

We present a detailed characterization of the recently discovered i-R-Cd (R=Y,Gd-Tm) binary quasicrystals by means of x-ray diffraction, temperature-dependent dc and ac magnetization, temperature-dependent resistance, and temperature-dependent specific heat measurements. Structurally, the broadening of x-ray diffraction peaks found for i-R-Cd is dominated by frozen-in phason strain, which is essentially independent of R. i-Y-Cd is weakly diamagnetic and manifests a temperature-independent susceptibility. i-Gd-Cd can be characterized as a spin glass below 4.6 K via dc magnetization cusp, a third order nonlinear magnetic susceptibility peak, a frequency-dependent freezing temperature, and a broad maximum in the specific heat. i-R-Cd (R=Ho-Tm) is similar to i-Gd-Cd in terms of features observed in thermodynamic measurements. i-Tb-Cd and i-Dy-Cd do not show a clear cusp in their zero-field-cooled dc magnetization data, but instead show a more rounded, broad local maximum. The resistivity for i-R-Cd is of order 300μΩ cm and weakly temperature dependent. The characteristic freezing temperatures for i-R-Cd (R=Gd-Tm) deviate from the de Gennes scaling, in a manner consistent with crystal electric field splitting induced local moment anisotropy. © 2014 American Physical Society.

We present a combined density functional perturbation theory and inelastic neutron scattering study of the lattice dynamical properties of YNi2B2C. In general, very good agreement was found between theory and experiment for both phonon energies and line widths. Our analysis reveals that the strong coupling of certain low energy modes is linked to the presence of large displacements of the light atoms, i.e., B and C, which is unusual in view of the rather low phonon energies. Specific modes exhibiting a strong coupling to the electronic quasiparticles were investigated as a function of temperature. Their energies and line widths showed marked changes on cooling from room temperature to just above the superconducting transition at Tc = 15.2 K. Calculations simulating the effects of temperature allow us to model the observed temperature dependence qualitatively. © 2014 American Physical Society.

We report an unusual coexistence of ferromagnetism and ferrimagnetism, and metamagnetism in Pr0.6Er0.4Al2. In addition, this compound retains a clear Griffiths phase behavior even at 1 kOe magnetic field and shows a large exchange bias after field cooling from the paramagnetic state. The crystal-field excitations and opposite exchange interactions between nearest-neighbor and next-nearest-neighbor rare earth sites explain these behaviors. © 2014 American Physical Society.

Examples of stable binary icosahedral quasicrystals are relatively rare, and at present there are no known examples featuring localized magnetic moments. These would represent an ideal model system for attaining a deeper understanding of the nature of magnetic interactions in aperiodic lattices. Here we report the discovery of a family of at least seven rare earth icosahedral binary quasicrystals, i-R-Cd (R=Gd to Tm, Y), six of which bear localized magnetic moments. Our work highlights the importance of carefully motivated searches through phase space and supports the proposal that, like icosahedral Sc 12 Zn 88 (ref.), binary quasicrystalline phases may well exist nearby known crystalline approximants, perhaps as peritectically forming compounds with very limited liquidus surfaces, offering very limited ranges of composition/ temperature for primary solidification. © 2013 Macmillan Publishers Limited. All rights reserved.

We present detailed temperature- and field-dependent data obtained from magnetization, resistivity, heat capacity, Hall resistivity and thermoelectric power measurements performed on single crystals of CeZn11. The compound orders antiferromagnetically at ∼2 K. The zero-field resistivity and thermoelectric power data show features characteristic of a Ce-based intermetallic with crystal-electric-field splitting and possible Kondo lattice effects. We constructed the T-H phase diagram for the magnetic field applied along the easy [110] direction, which shows that the magnetic field required to suppress TN below 0.4 K is in the range of 45-47.5 kOe. A linear behavior of the ρ(T) data, Hâ̂¥ [110], was observed only for H=45 kOe for 0.46 K ≤T≤ 1.96 K followed by the Landau-Fermi-liquid regime for a limited range of fields 47.5 kOe ≤H≤ 60 kOe. From the analysis of our data, it appears that CeZn11 is a local moment compound with little or no electronic correlations arising from the Ce 4f shell. The thermoelectric and transport properties of CeZn11 are mostly governed by the crystal-electric-field effects. Given the very high quality of our single crystals, quantum oscillations are found for both CeZn11 and its nonmagnetic analog LaZn11. © 2013 American Physical Society.

It has generally been accepted that the orientational ordering of the Cd4 tetrahedron within the Cd6 R quasicrystal approximants is kinetically inhibited for R = Ho, Er, Tm and Lu by steric constraints. Our high-resolution X-ray scattering measurements of the Cd6Ho quasicrystal approximant, however, reveal an abrupt (first-order) transition to a monoclinic structure below T S = 178 K for samples that have aged at room temperature for approximately one year, reopening this question. Using X-ray resonant magnetic scattering at the Ho L 3-edge we have elucidated the nature of the antiferromagnetic ordering below T N = 8.5 K in Cd6Ho. The magnetic Bragg peaks are found at the charge forbidden H + K + L = 2n + 1 positions, referenced to the high-temperature body-centred cubic structure. In general terms, this corresponds to antiferromagnetic arrangements of the Ho moments on adjacent clusters in the unit cell as previously found for Cd6Tb. © 2013 Copyright The work as part of Andreas Kreyssig, Guillaume Beutier, Takanobu Hiroto, Min Gyu Kim, Gregory S. Tucker, Marc de Boissieu, Ryuji Tamura and Alan I. Goldmans official duties as Federal Government Contractors is published by permission of the Ames Laboratory, U. S. DOE and Department of Physics and Astronomy under Contract Number DE-AC02-07CH11358. The US Government retains for itself, and others acting on its behalf, a paid-up, non-exclusive, and irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

A combined neutron and x-ray diffraction study of TbBaFe2O 5 reveals a rare checkerboard to charge ordering transition. TbBaFe2O5 is a mixed valent compound where Fe2 +/Fe3+ ions are known to arrange into a stripe charge ordered state below TV=291 K that consists of alternating Fe2 +/Fe3+ stripes in the basal plane running along the b direction. Our measurements reveal that the stripe charge ordering is preceded by a checkerboard charge ordered phase between TV&lt;T&lt;T*= 308 K. The checkerboard ordering is stabilized by intersite Coulomb interactions which give way to a stripe state stabilized by orbital ordering. © 2013 American Physical Society.

Magnetization, nuclear magnetic resonance, high-resolution x-ray diffraction, and magnetic field-dependent neutron diffraction measurements reveal a novel magnetic ground state of Ba0.60K0.40Mn 2As2 in which itinerant ferromagnetism (FM) below a Curie temperature TC≈100 K arising from the doped conduction holes coexists with collinear antiferromagnetism (AFM) of the Mn local moments that order below a Néel temperature TN=480 K. The FM ordered moments are aligned in the tetragonal ab plane and are orthogonal to the AFM ordered Mn moments that are aligned along the c axis. The magnitude and nature of the low-T FM ordered moment correspond to complete polarization of the doped-hole spins (half-metallic itinerant FM) as deduced from magnetization and ab-plane electrical resistivity measurements. © 2013 American Physical Society.

Neutron and x-ray diffraction measurements are presented for powders and single crystals of CaCo2As2. The crystal structure is a collapsed-tetragonal ThCr2Si2-type structure as previously reported, but with 7(1)% vacancies on the Co sites corresponding to the composition CaCo1.86(2)As2. The thermal expansion coefficients for both the a and c axes are positive from 10 to 300 K. Neutron diffraction measurements on single crystals demonstrate the onset of A-type collinear antiferromagnetic order below the Néel temperature T N=52(1) K with the ordered moments directed along the tetragonal c axis, aligned ferromagnetically in the ab plane and antiferromagnetically stacked along the c axis. © 2013 American Physical Society.

In tetragonal SrCo2As2 single crystals, inelastic neutron scattering measurements demonstrated that strong stripe-type antiferromagnetic (AFM) correlations occur at a temperature T=5 K that are the same as in the isostructural AFe2As2 (A = Ca, Sr, Ba) parent compounds of high-Tc superconductors. This surprising discovery suggests that SrCo2As2 may also be a good parent compound for high-Tc superconductivity. Here structural and thermal expansion, electrical resistivity ρ, angle-resolved photoemission spectroscopy (ARPES), heat capacity Cp, magnetic susceptibility χ, 75As NMR, and neutron diffraction measurements of SrCo2As 2 crystals are reported together with LDA band structure calculations that shed further light on this fascinating material. The c-axis thermal expansion coefficient αc is negative from 7 to 300 K, whereas αa (the a-axis thermal expansion coefficient) is positive over this T range. The ρ(T) shows metallic character. The ARPES measurements and band theory confirm the metallic character and in addition show the presence of a flat band near the Fermi energy EF. The band calculations exhibit an extremely sharp peak in the density of states D(E≈EF) arising from a flat dx2-y2 band, where the x and y axes are along the a and b axes of the Co square lattice, respectively. A comparison of the Sommerfeld coefficient of the electronic specific heat with χ(T→0) suggests the presence of strong ferromagnetic itinerant spin correlations, which on the basis of the Stoner criterion predicts that SrCo2As2 should be an itinerant ferromagnet, in conflict with the magnetization data. The χ(T) does have a large magnitude, but also exhibits a broad maximum at ≈115 K suggestive of dynamic short-range AFM spin correlations, in agreement with the neutron scattering data. The measurements show no evidence for any type of phase transition between 1.3 and 300 K and we suggest that metallic SrCo 2As2 has a gapless quantum spin-liquid ground state. © 2013 American Physical Society.

The relationship between antiferromagnetic spin fluctuations and superconductivity has become a central topic of research in studies of superconductivity in the iron pnictides. We present unambiguous evidence of the absence of magnetic fluctuations in the nonsuperconducting collapsed tetragonal phase of CaFe2As2 via inelastic neutron scattering time-of-flight data, which is consistent with the view that spin fluctuations are a necessary ingredient for unconventional superconductivity in the iron pnictides. We demonstrate that the collapsed tetragonal phase of CaFe 2As2 is nonmagnetic, and discuss this result in light of recent reports of high-temperature superconductivity in the collapsed tetragonal phase of closely related compounds. © 2013 American Physical Society.

Inelastic neutron scattering measurements on Ba(Fe0.963Ni 0.037)2As2 manifest a neutron spin resonance in the superconducting state with anisotropic dispersion within the Fe layer. Whereas the resonance is sharply peaked at the antiferromagnetic (AFM) wave vector QAFM along the orthorhombic a axis, the resonance disperses upwards away from QAFM along the b axis. In contrast to the downward dispersing resonance and hourglass shape of the spin excitations in superconducting cuprates, the resonance in electron-doped BaFe 2As2 compounds possesses a magnonlike upwards dispersion. © 2013 American Physical Society.

BaMn2As2 is a local-moment antiferromagnetic insulator with a Néel temperature TN of 625 K and a large ordered moment of 3.9 μB/Mn. Remarkably, this compound can be driven metallic by the substitution of as little as 1.6% K for Ba while retaining essentially the same ordered magnetic moment and Néel temperature, as previously reported. Here, using both powder and single crystal neutron diffraction we show that the local moment antiferromagnetic order in Ba1-xKxMn2As2 remains robust up to x=0.4. The ordered moment is nearly independent of x for 0 ≤x≤ 0.4 and TN decreases to 480 K at x=0.4. © 2013 American Physical Society.

We have employed the x-ray resonant magnetic scattering (XRMS) technique at the Ru L2 edge of the Ba(Fe1-xRux) 2As2 (x=0.205) superconductor. We show that pronounced resonance enhancements at the Ru L2 edge are observed at the wave vector which is consistent with the antiferromagnetic propagation vector of the Fe in the undoped BaFe2As2. We also demonstrate that the XRMS signals at the Ru L2 edge follow the magnetic ordering of the Fe with a long correlation length, ξab&gt;2850±400 Å. Our experimental observation shows that the Ru is spin polarized in Ba(Fe 1-xRux)2As2 compounds. © 2013 American Physical Society.

Inelastic neutron scattering measurements of paramagnetic SrCo 2As2 at T=5 K reveal antiferromagnetic (AFM) spin fluctuations that are peaked at a wave vector of QAFM=(1/2,1/2,1) and possess a large energy scale. These stripe spin fluctuations are similar to those found in AFe2As2 compounds, where spin-density wave AFM is driven by Fermi surface nesting between electron and hole pockets separated by QAFM. SrCo2As2 has a more complex Fermi surface and band-structure calculations indicate a potential instability toward either a ferromagnetic or stripe AFM ground state. The results suggest that stripe AFM magnetism is a general feature of both iron and cobalt-based arsenides and the search for spin fluctuation-induced unconventional superconductivity should be expanded to include cobalt-based compounds. © 2013 American Physical Society.

Inelastic neutron scattering measurements demonstrate that the magnetic interactions in antiferromagnetic LaFeAsO are two dimensional. Spin-wave velocities within the Fe layer and the magnitude of the spin gap are similar to the AFe2As2 based materials. However, the ratio of interlayer and intralayer exchange is found to be less than ∼10-4 in LaFeAsO, very similar to the cuprates, and ∼100 times smaller than that found in AFe2As2 compounds. The results suggest that the effective dimensionality of the magnetic system is highly variable in the parent compounds of the iron arsenides and weak three-dimensional interactions may limit the maximum attainable superconducting Tc. © 2013 American Physical Society.

Golden opportunity: Na13Au12Ga15 is the first Na-containing, thermodynamically stable quasicrystal and was discovered during systematic exploration of the polar intermetallic Na-Au-Ga system. Its electron-to-atom ratio, 1.75, is extremely low for Bergman-type icosahedral phases, but it is the substantial Au content that allows Hume-Rothery stabilization and promotes novel Na-Au polar-covalent interactions, which stabilize the icosahedral phase. Copyright © 2012 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.

We have used x-ray resonant magnetic scattering at the Tb L 2 edge to elucidate the nature of magnetic ordering in Cd 6Tb, a 1/1 approximant closely related to the Cd-Mg-R (R = rare earth) icosahedral alloys. Below T N≈24 K, the ordered moments associated with the icosahedral clusters at the corners and body center of the pseudocubic unit cell are antiferromagnetically correlated, and long-range magnetic order is realized. © 2012 American Physical Society.

The compound BaMn 2As 2 with the tetragonal ThCr 2Si 2 structure is a local-moment antiferromagnetic insulator with a Néel temperature T N=625K and a large ordered moment μ=3.9μ B/Mn. We demonstrate that this compound can be driven metallic by partial substitution of Ba by K while retaining the same crystal and antiferromagnetic structures together with nearly the same high T N and large μ. Ba 1-xK xMn 2As 2 is thus the first metallic ThCr 2Si 2-type MAs-based system containing local 3d transition metal M magnetic moments, with consequences for the ongoing debate about the local-moment versus itinerant pictures of the FeAs-based superconductors and parent compounds. The Ba 1-xK xMn 2As 2 class of compounds also forms a bridge between the layered iron pnictides and cuprates and may be useful to test theories of high T c superconductivity. © 2012 American Physical Society.

Inelastic neutron scattering measurements on Ba(Fe 0.925Mn 0.075)2As 2 manifest spin fluctuations at two different wave vectors, Q stripe=(12,12,1) and Q Néel=(1,0,1), corresponding to the expected stripe spin-density wave order and checkerboard antiferromagnetic order in the tetragonal I4/mmm cell, respectively. Below T N=80K, long-range stripe magnetic ordering occurs and sharp spin wave excitations appear at Q stripe while broad and diffusive spin fluctuations remain at Q Néel at all temperatures. Low concentrations of Mn dopants nucleate local moment spin fluctuations at Q Néel that compete with itinerant spin fluctuations at Q stripe and may disrupt the development of superconductivity. © 2012 American Physical Society.

We have grown single-crystal samples of Co substituted CaFe 2As 2 using an FeAs flux and systematically studied the effects of annealing/quenching temperature on the physical properties of these samples. Whereas the as-grown samples (quenched from 960C) all enter the collapsed tetragonal phase upon cooling, annealing/quenching temperatures between 350 and 800C can be used to tune the system to low-temperature antiferromagnetic/orthorhomic or superconducting states as well. The progression of the transition temperature versus annealing/quenching temperature (T-T anneal) phase diagrams with increasing Co concentration shows that, by substituting Co, the antiferromagnetic/orthorhombic and the collapsed tetragonal phase lines are separated and bulk superconductivity is revealed. We established a 3D phase diagram with Co concentration and annealing/quenching temperature as two independent control parameters. At ambient pressure, for modest x and T anneal values, the Ca(Fe 1-xCo x) 2As 2 system offers ready access to the salient low-temperature states associated with Fe-based superconductors: antiferromagnetic/orthorhombic, superconducting, and nonmagnetic/collapsed tetragonal. © 2012 American Physical Society.

We report the growth of single crystals of SrCu2As2, SrCu2Sb2, SrCu2(As0.84Sb 0.16)2, and BaCu2Sb2 using the self-flux technique and their structural, magnetic, thermal, and transport properties that were investigated by powder x-ray diffraction (XRD), magnetic susceptibility χ, specific heat Cp, and electrical resistivity ρ measurements versus temperature T from 1.8 to 350 K. Rietveld refinements of XRD patterns for crushed crystals confirm that SrCu2As2 crystallizes in the ThCr2Si2-type body-centered tetragonal structure (space group I4/mmm) and SrCu2Sb2 crystallizes in the CaBe2Ge2-type primitive tetragonal structure (space group P4/nmm). However, as reported previously, BaCu 2Sb2 is found to have a large unit cell consisting of three blocks. Here a ThCr2Si2-type block is sandwiched between two CaBe2Ge2-type blocks along the c axis with an overall symmetry of I4/mmm, as reported, but likely with a monoclinic distortion. The χ data of all these compounds are diamagnetic and reveal nearly T-independent anisotropic behavior. The χ of SrCu2As 2 is found to be larger in the ab plane than along the c axis, as also previously reported for pure and doped BaFe2As2, whereas the χ values of SrCu2Sb2 and BaCu 2Sb2 are larger along the c axis. This difference in anisotropy appears to arise from the differences between the crystal structures. The finite values of the Sommerfeld linear specific heat coefficients γ and the T dependences of ρ reveal metallic character of all four compounds. The electronic and magnetic properties indicate that these compounds are sp metals with Cu in the nonmagnetic 3d10 electronic configuration corresponding to the oxidation state Cu+1, as previously predicted theoretically for SrCu2As2 by Singh. We present a brief review of theoretical and experimental work on the doping character of transition metals for Fe in BaFe2As2. The As-As covalent interlayer bond distances in the collapsed-tetragonal (Ca,Sr,Ba)Cu2As2 compounds are much shorter than the nonbonding As-As distances in BaFe 2As2. Thus, the electronic character of the Cu and the strength of the As-As interlayer bonding are both expected to drastically change between weakly Cu-substituted BaFe2As2 and pure BaCu 2As2, perhaps via a first-order lattice instability such as a miscibility gap in the Ba(Fe1-xCux) 2As2 system. © 2012 American Physical Society.

The effect of uniaxial tensile stress and the resultant strain on the structural/magnetic transition in the parent compound of the iron arsenide superconductor BaFe 2As 2 is characterized by temperature-dependent electrical resistivity, x-ray diffraction, and quantitative polarized light imaging. We show that strain induces a measurable uniaxial structural distortion above the first-order magnetic transition and significantly smears the structural transition. This response is different from that found in another parent compound, SrFe 2As 2, where the coupled structural and magnetic transitions are strongly first order. This difference in the structural responses explains the in-plane resistivity anisotropy above the transition in BaFe 2As 2. This conclusion is supported by the Ginzburg-Landau-type phenomenological model for the effect of the uniaxial strain on the resistivity anisotropy. © 2012 American Physical Society.

The spin fluctuation spectra from nonsuperconducting Cu-substituted, and superconducting Co-substituted, BaFe 2As 2 are compared quantitatively by inelastic neutron scattering measurements and are found to be indistinguishable. Whereas diffraction studies show the appearance of incommensurate spin-density wave order in Co and Ni substituted samples, the magnetic phase diagram for Cu substitution does not display incommensurate order, demonstrating that simple electron counting based on rigid-band concepts is invalid. These results, supported by theoretical calculations, suggest that substitutional impurity effects in the Fe plane play a significant role in controlling magnetism and the appearance of superconductivity, with Cu distinguished by enhanced impurity scattering and split-band behavior. © 2012 American Physical Society.

We report an inelastic neutron scattering investigation of phonons with energies up to 159meV in the conventional superconductor YNi 2B 2C. Using the sweep mode, a newly developed time-of-flight technique involving the continuous rotation of a single crystal specimen, allowed us to measure a four-dimensional volume in (Q, E) space and, thus, determine the dispersion surface and linewidths of the A 1g (102meV) and A u (159meV) type phonon modes over the whole Brillouin zone. Despite of having linewidths of Γ=10meV, A 1g modes do not strongly contribute to the total electron-phonon coupling constant λ. However, experimental linewidths show a remarkable agreement with abinitio calculations over the complete phonon energy range, demonstrating the accuracy of such calculations in a rare comparison to a comprehensive experimental data set. © 2012 American Physical Society.

We present an X-ray resonant magnetic scattering study at the Fe-K absorption edge of the BaFe2As2 compound. The energy spectrum of the resonant scattering, together with our calculation using the full-potential linear-augmented plane wave method with a local density functional suggests that the observed resonant scattering arises from electric dipole (E1) transitions. We discuss the role of Fe K-edge X-ray resonant magnetic scattering in understanding the relationship between the structure and the antiferromagnetic transition in the doped Ba(Fe1-xCox)2As2 superconductors. © 2012 EDP Sciences, Springer-Verlag.

We investigate the magnetic polarization of the Ir 5d dopant states in the pnictide superconductor Ba(Fe 1-xIr x) 2As 2 with x=0.027(2) using Ir L 3 edge x-ray resonant magnetic scattering (XRMS). Despite the fact that doping partially suppresses the antiferromagnetic transition, we find that magnetic order survives around the Ir dopant sites. The Ir states are magnetically polarized with commensurate stripe-like antiferromagnetic order and long correlations lengths, ξ mag&gt;2800 and &gt;850, in the ab plane and along the c axis, respectively, driven by their interaction with the Fe spins. This Ir magnetic order persists up to the Néel transition of the majority Fe spins at T N=74(2) K. At 5 K we find that magnetic order coexists microscopically with superconductivity in Ba(Fe 1-xIr x) 2As 2. The energy dependence of the XRMS through the Ir L 3 edge shows a non-Lorentzian line shape, which we explain in terms of interference between Ir resonant scattering and Fe nonresonant magnetic scattering. © 2012 American Physical Society.

Single crystals of CaNi 2 and CaNi 3 are successfully grown out of excess Ca. Both compounds manifest a metallic ground state with enhanced, temperature-dependent magnetic susceptibility. The relatively high Stoner factors of Z=0.79 and 0.87 found for CaNi 2 and CaNi 3, respectively, reveal their close vicinity to ferromagnetic instabilities. The pronounced field dependence of the magnetic susceptibility of CaNi 3 at low temperatures (T&lt;25 K) suggests strong ferromagnetic fluctuations. A corresponding contribution to the specific heat with a temperature dependence of T3lnT is also observed. © 2012 American Physical Society.

We describe the development of a new method for measuring the electrical resistivity and magnetic susceptibility of high temperature liquids and solids. The technique combines a tunnel diode oscillator with an electrostatic levitation furnace to perform noncontact measurements on spherical samples 2-3 mm in diameter. The tank circuit of the oscillator is inductively coupled to the sample, and measurements of the oscillator frequency as a function of sample temperature can be translated into changes in the samples electrical resistivity and magnetic susceptibility. Particular emphasis is given on the need to improve the positional stability of the levitated samples, as well as the need to stabilize the temperature of the measurement coil. To demonstrate the validity of the technique, measurements have been performed on solid spheres of pure zirconium and low-carbon steel. In the case of zirconium, while absolute values of the resistivity were not determined, the temperature dependence of the resistivity was measured over the range of 640-1770 K and found to be in good agreement with literature data. In the case of low-carbon steel, the ferromagnetic-paramagnetic transition was clearly observable and, when combined with thermal data, appears to occur simultaneously with the solid-solid structural transition. © 2012 American Institute of Physics.

We present a combined high-resolution x-ray diffraction and x-ray resonant magnetic scattering study of as-grown BaFe2As2. The structural and magnetic transitions must be described as a two-step process. At TS = 134.5 K we observe the onset of a second-order structural transition from the high-temperature paramagnetic tetragonal structure to a paramagnetic orthorhombic phase, followed by a discontinuous step in the structural order parameter that is coincident with a first-order antiferromagnetic (AFM) transition at TN = 133.75 K. These data, together with detailed high-resolution x-ray studies of the structural transition in lightly doped Ba(Fe1-xCox) 2As2 and Ba(Fe1-xRhx) 2As2 compounds, show that the structural and AFM transitions do, in fact, occur at slightly different temperatures in the parent BaFe2As2 compound, and evolve toward split second-order transitions as the doping concentration is increased. We estimate the composition for the tricritical point for Co doping and employ a mean-field approach to show that our measurements can be explained by the inclusion of an anharmonic term in the elastic free energy and magnetoelastic coupling in the form of an emergent Ising-nematic degree of freedom. © 2011 American Physical society.

Although quasicrystals form in a wide variety of ternary and quaternary metallic alloys, examples of stable binary icosahedral quasicrystals are quite rare. Indeed, it has been a decade since the discovery of icosahedral phases in Yb-Cd and Ca-Cd. We have discovered millimeter-sized facetted grains of i- Sc12Zn88 with icosahedral (pentagonal dodecahedral and rhombic triacontahedral) morphologies in solution-grown samples. Structural characterization of the bulk icosahedral phase was accomplished through single-grain high-energy X-ray diffraction. For both growth morphologies, all diffraction peaks could be indexed by a primitive (P-type) icosahedral phase. The two types of morphology do, however, present interesting differences in their respective degrees of quasicrystalline order. © 2011 Taylor &amp; Francis.

Intrinsic, in-plane anisotropy of electrical resistivity was studied on mechanically detwinned single crystals of SrFe2As2 above and below the temperature of the coupled structural/magnetic transition, T TO. Resistivity is smaller for electrical current flow along the orthorhombic ao direction (direction of antiferromagnetically alternating magnetic moments) and is larger for transport along the b o direction (direction of ferromagnetic chains), which is similar to CaFe2As2 and BaFe2As2 compounds. A strongly first-order structural transition in SrFe2As2 was confirmed by high-energy x-ray measurements, with the transition temperature and character unaffected by moderate strain. For small strain levels, which are just sufficient to detwin the sample, we find a negligible effect on the resistivity above TTO. With the increase of strain, the resistivity anisotropy starts to develop above TTO, clearly showing the relation of anisotropy to an anomalously strong response to strain. Our study suggests that electronic nematicity cannot be observed in the FeAs-based compounds in which the structural transition is strongly first order. © 2011 American Physical society.

Neutron diffraction studies of Ba(Fe1-xCox) 2As2 reveal that commensurate antiferromagnetic order gives way to incommensurate magnetic order for Co compositions between 0.056&lt;x&lt;0.06. The incommensurability has the form of a small transverse splitting (0, ±, 0) from the commensurate antiferromagnetic propagation vector QAFM=(1,0,1) (in orthorhombic notation) where 0.02-0.03 and is composition dependent. The results are consistent with the formation of a spin-density wave driven by Fermi surface nesting of electron and hole pockets and confirm the itinerant nature of magnetism in the iron arsenide superconductors. © 2011 American Physical Society.

Rare-earth (R) half-Heusler compounds RBiPt exhibit a wide spectrum of interesting ground states. We have employed x-ray resonant magnetic scattering to elucidate the microscopic details of the magnetic structure in GdBiPt below T N=8.5 K. Experiments at the Gd L 2 absorption edge show that the Gd moments order in an antiferromagnetic stacking along the cubic diagonal [111] direction, satisfying one of the requirements for an antiferromagnetic topological insulator as proposed previously, where both time-reversal symmetry and lattice translational symmetry are broken, but their product is conserved. © 2011 American Physical Society.

We present a systematic investigation of the antiferromagnetic ordering and structural distortion for the series of Ba(Fe1-xRux) 2As2 compounds (0≤x≤0.246). Neutron and x-ray diffraction measurements demonstrate that, unlike for the electron-doped compounds, the structural and magnetic transitions remain coincident in temperature. Both the magnetic and structural transitions are gradually suppressed with increased Ru concentration and coexist with superconductivity. For samples that are superconducting, we find strong competition between superconductivity, the antiferromagnetic ordering, and the structural distortion. © 2011 American Physical Society.

Polarized neutron diffraction has been performed on a tetragonal Ba(Fe 1-xCox)2As2, x=0.066 single crystal under an applied magnetic field of 6 T directed along the [1̄10] direction to determine the magnetic structure factors of various Bragg reflections. The maximum entropy reconstruction based on bulk magnetization measurements and polarized neutron diffraction data reveal a small induced magnetic moment residing on the 4d Wyckoff site that is occupied by Fe/Co atoms. No significant magnetization density has been found on the Ba and As atomic positions. The small polarizability of Fe/Co sites leads to flipping ratios very close to 1.00. Our data suggest a non-zero orbital contribution to the Fe/Co magnetic form factor in good agreement with recent theoretical and experimental studies. Copyright © EPLA, 2011.

We report on neutron diffraction experiments on high quality CaFe2As2 single crystals performed under uniaxial pressure and combined with in-situ electrical resistance measurements. We conclude that the superconductivity is associated with the high-temperature tetragonal phase that is stabilized down to the lowest temperatures by a small uniaxial pressure of at least 0.075 GPa applied along the c-axis. © Published under licence by IOP Publishing Ltd.

The atomic structures in equilibrium and supercooled liquids of Zr 80Pt20 were determined as a function of temperature by in situ high-energy synchrotron diffraction studies of the levitated liquids (containerless processing) using the beamline electrostatic levitation (BESL) technique. The presence of a pronounced pre-peak at q ∼ 1.7 -1 in the static structure factor indicates medium-range order (MRO) in the liquid. The position and intensity of the pre-peak remain constant with cooling, indicating that the MRO is already present in the liquid above its melting temperature. An analysis of the liquid atomic structures obtained using the Reverse Monte Carlo method utilizing both the structure factor S(q) from x-ray diffraction experiments and the partial pair-correlation functions from ab initio molecular dynamics simulations show that the pre-peak arises from a Pt-Pt correlation that can be identified with icosahedral short-range order around the Pt atoms. The local atomic ordering is dominated by icosahedral-like structures, raising the nucleation barrier between the liquid and these phases, thus assisting glass formation. © 2011 American Physical Society.

We have found a remarkably large response of the transition temperature of CaFe2As2 single crystals grown from excess FeAs to annealing and quenching temperature. Whereas crystals that are annealed at 400̂C exhibit a first-order phase transition from a high-temperature tetragonal to a low-temperature orthorhombic and antiferromagnetic state near 170 K, crystals that have been quenched from 960̂C exhibit a transition from a high-temperature tetragonal phase to a low-temperature, nonmagnetic, collapsed tetragonal phase below 100 K. By use of temperature-dependent electrical resistivity, magnetic susceptibility, x-ray diffraction, Mössbauer spectroscopy, and nuclear magnetic resonance measurements we have been able to demonstrate that the transition temperature can be reduced in a monotonic fashion by varying the annealing or quenching temperature from 400̂ to 850 ̂C with the low-temperature state remaining antiferromagnetic for transition temperatures larger than 100 K and becoming collapsed tetragonal, nonmagnetic for transition temperatures below 90 K. This suppression of the orthorhombic-antiferromagnetic phase transition and its ultimate replacement with the collapsed tetragonal, nonmagnetic phase is similar to what has been observed for CaFe2As2 under hydrostatic pressure. Transmission electron microscopy studies indicate that there is a temperature-dependent width of formation of CaFe2As2 with a decreasing amount of excess Fe and As being soluble in the single crystal at lower annealing temperatures. For samples quenched from 960̂C there is a fine (of order 10 nm) semiuniform distribution of precipitate that can be associated with an average strain field, whereas for samples annealed at 400̂C the excess Fe and As form mesoscopic grains that induce little strain throughout the CaFe2As2 lattice. © 2011 American Physical Society.

We report temperature dependence of low-energy phonons in magnetic nonsuperconducting TbNi2B2C single crystals measured by inelastic neutron scattering. We observed low-temperature softening and broadening of two phonon branches, qualitatively similar to that previously reported for superconducting RNi2B2C (R=rare earth, Y) compounds. This result suggests that superconductivity in TbNi2B 2C compounds is absent not because of weak electron-phonon coupling but as a result of pair breaking due to magnetism. © 2011 American Physical Society.

High-resolution x-ray diffraction measurements reveal an unusually strong response of the lattice to superconductivity in Ba(Fe1-xCox)2As2. The orthorhombic distortion of the lattice is suppressed and, for Co doping near x=0.063, the orthorhombic structure evolves smoothly back to a tetragonal structure. We propose that the coupling between orthorhombicity and superconductivity is indirect and arises due to the magnetoelastic coupling, in the form of emergent nematic order, and the strong competition between magnetism and superconductivity. © 2010 The American Physical Society.

Neutron and x-ray diffraction studies of Ba (Fe1-x Mn x)2 As2 for low doping concentrations (x≤0.176) reveal that at a critical concentration, 0.102&lt;x&lt;0.118, the tetragonal-to- orthorhombic transition abruptly disappears whereas magnetic ordering with a propagation vector of (1 2 1 2 1) persists. Among all of the iron arsenides this observation is unique to Mn doping, and unexpected because all models for "stripelike" antiferromagnetic order anticipate an attendant orthorhombic distortion due to magnetoelastic effects. We discuss these observations and their consequences in terms of previous studies of Ba (Fe 1-x T Mx) 2 As2 compounds (TM=transitionmetal), and models for magnetic ordering in the iron arsenide compounds. © 2010 The American Physical Society.

We describe x-ray resonant magnetic diffraction measurements at the FeK edge of both the parent BaFe2 As2 and superconducting Ba(Fe0.953 Co0.047)2 As2 compounds. From these high-resolution measurements we conclude that the magnetic structure is commensurate for both compositions. The energy spectrum of the resonant scattering is in reasonable agreement with theoretical calculations using the full-potential linear augmented plane-wave method with a local density functional. © 2010 The American Physical Society.

Inelastic neutron-scattering measurements have been performed on underdoped Ba (Fe1-xCox)2As2 (x=4.7%) where superconductivity and long-range antiferromagnetic (AFM) order coexist. The magnetic spectrum found in the normal state is strongly damped and develops into a magnetic resonance feature below Tc that has appreciable dispersion along c axis with a bandwidth of 3-4 meV. This is in contrast to the optimally doped x=8.0% composition, with no long-range AFM order, where the resonance exhibits a much weaker dispersion. [see M. D. Lumsden, Phys. Rev. Lett. 102, 107005 (2009).] The results suggest that the resonance dispersion arises from interlayer spin correlations present in the AFM ordered state. © 2010 The American Physical Society.

CaFe2 As2 single crystals under uniaxial pressure applied along the c axis exhibit the coexistence of several structural phases at low temperatures. We show that the room-temperature tetragonal phase is stabilized at low temperatures for pressures above 0.06 GPa, and its weight fraction attains a maximum in the region where superconductivity is observed under applied uniaxial pressure. Simultaneous resistivity measurements strongly suggest that this phase is responsible for the superconductivity in CaFe 2 As2 found below 10 K in samples subjected to nonhydrostatic pressure conditions. © 2010 The American Physical Society.

Molecular dynamics (MD) simulations were performed of the structural changes occurring through the liquid-glass transition in Cu-Zr alloys. The total scattering functions (TSF), and their associated primary diffuse scattering peak positions (Kp), heights (Kh) and full-widths at half maximum (KFWHM) were used as metrics to compare the simulations to high-energy X-ray scattering data. The residuals of difference between the model and experimental TSFs are ∼0.03 for the liquids and about 0.07 for the glasses. Over the compositional range studied, Zr1-xCux (0.1 ≤ x ≤ 0.9), Kp, Kh and KFWHM show a strong dependence on composition and temperature. The simulation and experimental data correlate well between each other. MD simulation revealed that the Cu-Zr bonds undergo the largest changes during cooling of the liquid, whereas the Cu-Cu bonds change the least. Changes in the partial-pair correlations are more readily seen in the second and third shells. The Voronoi polyhedra (VP) in glasses are dominated by only a few select types that are compositionally dependent. The relative concentrations of the dominant VPs rapidly change in their relative proportion in the deeply undercooled liquid. The experimentally determined region of best glass formability, xCu 65%, shows the largest temperature dependent changes for the deeply undercooled liquid in the MD simulation. This region also exhibits very strong temperature dependence for the diffusivity and the total energy of the system. These data point to a strong topological change in the best glass-forming alloys and a concurrent change in the VP chemistry in the deeply undercooled liquid. © 2010 Taylor &amp; Francis.

The structural and magnetic phase transitions have been studied on NdFeAsO single crystals by neutron and x-ray diffraction complemented by resistivity and specific-heat measurements. Two low-temperature phase transitions have been observed in addition to the tetragonal-to-orthorhombic transition at TS ∼142K and the onset of antiferromagnetic (AFM) Fe order below TN ∼137K. The Fe moments order AFM in the well-known stripelike structure in the (ab) plane but change from AFM to ferromagnetic (FM) arrangement along the c direction below T- ∼15K accompanied by the onset of Nd AFM order below TNd ∼6K with this same AFM configuration. The iron magnetic order-order transition in NdFeAsO accentuates the Nd-Fe interaction and the delicate balance of c -axis exchange couplings that results in AFM in LaFeAsO and FM in CeFeAsO and PrFeAsO. © 2010 The American Physical Society.

Magnetic correlations in the paramagnetic phase of CaFe2As 2 (TN =172K) have been examined by means of inelastic neutron scattering from 180 K (∼1.05 TN) up to 300 K (1.8 TN). Despite the first-order nature of the magnetic ordering, strong but short-ranged antiferromagnetic (AFM) correlations are clearly observed. These correlations, which consist of quasielastic scattering centered at the wave vector QAFM of the low-temperature AFM structure, are observed up to the highest measured temperature of 300 K and at high energy transfer (ω&gt;60meV). The L dependence of the scattering implies rather weak interlayer coupling in the tetragonal c direction corresponding to nearly two-dimensional fluctuations in the (ab) plane. The spin correlation lengths within the Fe layer are found to be anisotropic, consistent with underlying fluctuations of the AFM stripe structure. Similar to the cobalt-doped superconducting BaFe2 As2 compounds, these experimental features can be adequately reproduced by a scattering model that describes short-ranged and anisotropic spin correlations with overdamped dynamics. © 2010 The American Physical Society.

We report the discovery of a binary icosahedral phase in a Sc-Zn alloy obtained through solution-growth, producing millimeter-sized, facetted, single grain quasicrystals that exhibit different growth morphologies, pentagonal dodecahedra, and rhombic triacontahedra, under only marginally different growth conditions. These two morphologies manifest different degrees of quasicrystalline order. The discovery of i -Sc12 Zn88 suggests that a re-examination of binary phase diagrams at compositions close to crystalline approximant structures may reveal other binary quasicrystalline phases. © 2010 The American Physical Society.

Neutron-diffraction and high-resolution x-ray diffraction studies find that, similar to the closely related underdoped Ba (Fe1-x Co x)2As2 superconducting compounds, Ba (Fe 0.961 Rh0.039)2As2 shows strong evidence of competition and coexistence between superconductivity and antiferromagnetic order below the superconducting transition, Tc =14 K. The transition temperatures for both the magnetic order and orthorhombic distortion are in excellent agreement with those inferred from resistivity measurements, and both order parameters manifest a distinct decrease in magnitude below Tc. These data suggest that the strong interaction between magnetism and superconductivity is a general feature of electron-doped Ba (Fe1-x T Mx)2 As2 superconductors (TM=transition metal). © 2010 The American Physical Society.

We have studied the thermal, magnetic, and electrical properties of the ternary intermetallic system CeNiGe3 by means of specific heat, magnetization, and resistivity measurements. The specific heat data, together with the anisotropic magnetic susceptibility, was analyzed on the basis of the point charge model of crystalline electric field. The J=5/2 multiplet of the Ce3+ is split by the crystalline electric field into three Kramers doublets, where the second and third doublets are separated from the first (ground state) doublet by Δ1 ∼100 K and Δ2 ∼170 K, respectively. In zero field CeNiGe3 exhibits an antiferromangeic order below TN =5.0 K. For H||a two metamagnetic transitions are clearly evidenced between 2-4 K from the magnetization isotherm and extended down to 0.4 K from the magnetoresistance measurements. For H||a, TN shifts to lower temperature as magnetic field increases, and ultimately disappears at Hc ∼32.5 kOe. For H&gt; Hc, the electrical resistivity shows the quadratic temperature dependence (Δρ=A T2). For H≫ Hc, an unconventional Tn dependence of Δρ with n&gt;2 emerges, the exponent n becomes larger as magnetic field increases. Although the antiferromagnetic phase transition temperature in CeNiGe3 can be continuously suppressed to zero, it provides an example of field tuning that does not match current simple models of quantum criticality. © 2010 The American Physical Society.

We use magnetic long-range order as a tool to probe the Cooper-pair wave function in the iron arsenide superconductors. We show theoretically that antiferromagnetism and superconductivity can coexist in these materials only if Cooper pairs form an unconventional, sign-changing state. The observation of coexistence in Ba (Fe1-x Cox)2 As2 then demonstrates unconventional pairing in this material. The detailed agreement between theory and neutron-diffraction experiments, in particular, for the unusual behavior of the magnetic order below Tc, demonstrates the robustness of our conclusions. Our findings strongly suggest that superconductivity is unconventional in all members of the iron arsenide family. © 2010 The American Physical Society.

The parent compounds of iron-arsenide superconductors, AFe 2As2 (A=Ca,Sr,Ba), undergo a tetragonal to orthorhombic structural transition at a temperature TTO in the range 135-205 K depending on the alkaline-earth element. Below TTO the free standing crystals split into equally populated structural domains, which mask intrinsic, in-plane, anisotropic properties of the materials. Here we demonstrate a way of mechanically detwinning CaFe2As2 and BaFe 2As2. The detwinning is nearly complete, as demonstrated by polarized light imaging and synchrotron x-ray measurements, and reversible, with twin pattern restored after strain release. Electrical resistivity measurements in the twinned and detwinned states show that resistivity, ρ, decreases along the orthorhombic ao axis but increases along the orthorhombic bo axis in both compounds. Immediately below T TO the ratio ρbo / ρao =1.2 and 1.5 for Ca and Ba compounds, respectively. Contrary to CaFe2As2, BaFe2As2 reveals an anisotropy in the nominally tetragonal phase, suggesting that either fluctuations play a larger role above T TO in BaFe2As2 than in CaFe2As 2 or that there is a higher temperature crossover or phase transition. © 2010 The American Physical Society.

In this study, pure [formula omitted] ([formula omitted] structure) single crystals were grown with no [formula omitted] ([formula omitted] structure) intergrowth, and zero magnetic field transitions were confirmed through specific heat capacity, magnetization, and electric resistivity measurements. The anomalous magnetic transition was found at approximately 6.2 K with long range antiferromagnetic transition at 5.6 K. The difference in field dependence between these two transitions indicates that they do not share a common magnetic origin. In addition, we investigated electrical resistivity down to 20 mK, and found upturn behavior at around 0.8 K. The low temperature upturn anomaly in resistivity was not found for other compounds of investigation for [formula omitted] ([formula omitted] earth elements), which suggests that an [formula omitted] system has a significantly different ground state, compared to a [formula omitted] system. © 2010, American Institute of Physics. All rights reserved.