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We report single-phase superconducting face-centered cubic (FCC) intermetallic high-entropy alloys (HEAs) synthesized via splat cooling. The technique gives incredibly sharp diffraction peaks and superconducting transitions in resistivity measurements than reported in conventional arc melting. The single-phase materials fall at electron counts in the HEA superconductor alloy family where structural stability and optimal superconducting electron counts clash. The materials' superconducting properties follow the general trends published for metallic alloys. Insights are provided as to why an FCC structure may be stable. [ABSTRACT FROM AUTHOR]

Hexagonal perovskites, in contrast to the more familiar perovskites, when oxides, allow for face-sharing of metal–oxygen octahedra or trigonal prisms within their structural frameworks. This results in dimers, trimers, tetramers, or longer fragments of chains of face-sharing octahedra in the crystal structures, and consequently in much shorter metal–metal distances and lower metal–oxygen–metal bond angles than are seen in the more familiar perovskites. The presence of the face-sharing octahedra can have a dramatic impact on magnetic properties of these compounds, and dimer-based materials, in particular, have been the subjects of many quantum-materials-directed studies in materials physics. Hexagonal oxide perovskites are also of contemporary interest due to their potential for geometrical frustration of the ordering of magnetic moments or orbital occupancies at low temperatures, which is especially relevant to their significance as quantum materials. As such, several hexagonal oxide perovskites have been identified as potential candidates for hosting the quantum-spin-liquid state at low temperatures. In our view, hexagonal oxide perovskites are fertile ground for finding new quantum materials. This review briefly describes the solid state chemistry of many of these materials. [ABSTRACT FROM AUTHOR]

Presents a study which measured the real and imaginary parts of the dielectric function of racemic and chiral CE8. Alignment of the molecules in the samarium carbide phase; Determination of the change in the density of states at low frequencies; Values of the direct current electrical conductivities.

Previously unreported σ-phases in the ternary niobium-based systems Nb–X–Ga for X = Ru, Rh, Pd, Ir, Pt, Au, and Nb–X–Al for X = Ir, and Pt are presented, prepared by arc-melting followed by annealing at 1000 °C for 1 week. The phases are primitive tetragonal (P42/mnm, no. 136) with lattice parameters of a = 9.8336(2)–9.9432(1) Å and c = 5.1098(1)–5.1718(1) Å. Refinements of the single crystal X-ray diffraction data for the Ru and Ir containing materials gave the formulas Nb20Ru5Ga5 and Nb20Ir4Ga6, showing mixing of X/Ga on the crystallographic 8i2 and 2a sites of the structure, a type of disorder frequently found in σ-phases. We present a graphical classification for ternary σ-phases that places our materials in context with previous materials. The materials do not show evidence of bulk superconductivity above 2 K. Synthesis of equivalent Ta-based σ-phases was attempted but they were not found. [ABSTRACT FROM AUTHOR]

Discusses a study which investigated the dielectric properties of polycrystalline ceramics of (Ta[sub2]O[sub5])[sub1-x](SiO[sub2])[subx]. Background on the materials; Methods used; Results.

Reports on the measurement of the temperature dependent dielectric constants in the vicinity of room temperature for bulk ceramics. List of starting materials for ceramic synthesis; Information on the investigations of the Pb-Nb-Mg-Ti-O phase diagram; Summary of dielectric data for pyrochlore based oxides Pb-Nb-Mg-Ti-O.

Presents a study that examined epitaxial thin films of copper-oxide-based isotropic metallic oxide and superconducting heterostructures. Analysis of the crystalline properties and interfaces of the samples; Factors to consider when using copper-oxide-based isotropic metallic oxide materials as a normal-metal barrier; Investigation of the texture of the thin films.

We report the magnetic characterization of the frustrated transition metal pyrochlore NaCaCo2F7. This materialhas high spin Co2+ in CoF6 octahedra in a pyrochlore lattice and disordered nonmagnetic Na and Ca on thelarge-atom sites in the structure. Large crystals grown by the floating zone method were studied. The magnetic susceptibility is isotropic; the Co moment is larger than the spin-only value; and in spite of the large CurieWeiss theta (–140 K), freezing of the spin system, as characterized by peaks in the ac and dc susceptibilityand specific heat, does not occur until around 2.4 K. This yields a frustration index of f = –ϑcw/Tf ≈ 56,an indication that the system is highly frustrated. The observed entropy loss at the freezing transition is low,indicating that magnetic entropy remains present in the system at 0.6 K. The compound may be the realization ofa frustrated pyrochlore antiferromagnet with weak bond disorder. The high magnetic interaction strength, strongfrustration, and the availability of large single crystals makes NaCaCo2F7 an interesting alternative to rare earthoxide pyrochlores for the study of geometric magnetic frustration in pyrochlore lattices. [ABSTRACT FROM AUTHOR]

The reactivity of MgB[sub 2] with powdered forms of common substrate and electronic materials is reported. Reaction temperatures between 600 and 800 °C, encompassing the range commonly employed in thin-film fabrication, were studied. The materials tested for reactivity were ZrO[sub 2], yttria stabilized zirconia, MgO, Al[sub 2]O[sub 3], SiO[sub 2], SrTiO[sub 3], TiN, TaN, AlN, Si, and SiC. At 600 °C, MgB[sub 2] reacted only with SiO[sub 2] and Si. At 800 °C, however, reactions were observed for MgB[sub 2] with Al[sub 2]O[sub 3], SiO[sub 2], Si, SiC, and SrTiO[sub 3]. The T[sub c] of MgB[sub 2] decreased in the reactions with SiC and Al[sub 2]O[sub 3]. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report the characterization of (GaSc)2-xFexO3. The composition range where the multinary metal oxide is stable, as well as nonlinear trends in the lattice parameters are described. Select compositions of GaSc1-yFeyO3 are included to compare trends. Fe shows an enhanced solubility when replacing Sc rather than Ga. Field- and temperature-dependent magnetic data are included, and linear trends in the magnetic frustration are discussed alongside a brief comparison to the magnetic properties of GaSc1-yFeyO3. The field-dependent properties, as well as anomalies in the hysteresis loops and relaxation of the remnant magnetization are discussed, as well as indications that long-range order may be present in dilute Fe samples. [ABSTRACT FROM AUTHOR]

The article offers information on a study on quantum spin liquids. Topics discussed include quasiparticles carrying a rational fraction of the elementary charge of an electron; consequence of topological properties of ground state wave functions with a special type of long-range quantum entanglement; and emergent quasiparticles with gauge charge and/or gauge flux, analogous to the electric charge and magnetic flux in electrodynamics.

Presents an abstract of the study 'Synthesis and Physical Properties of RNi[sub2]B[sub2]C and Related Materials,' by R.J. Cava.

KIrO3 in the body-centered cubic variant of the KSbO3-type structure is reported. Black cube-shaped single crystals, obtained from the solid-state reaction in a half-closed silver capsule in a sealed quartz tube, were used for the structural characterization by single-crystal X-ray diffraction. The material, space group Im3¯ (no. 204), exhibits a disordered K array and a three-dimensional (3D) IrO6-based tunnel-like framework. Temperature-dependent magnetization and heat capacity measurements suggest a paramagnetic state for KIrO3, with significant contribution of temperature independent paramagnetism and without any sign of long-range magnetic ordering (down to 1.8 K). The 3D motif of this material, based on the 5d Ir5+ ion, is of interest for investigating unconventional magnetism. [ABSTRACT FROM AUTHOR]

The general magnetic characterisation of a series of materials in the BaNi2V2O8-BaNi2P2O8 layered honeycomb spin-1 system is reported. A change from dominantly 2-dimensional (2D) to dominantly 3-dimentional (3D) magnetic character is observed on progressing from the pure V to the pure P material. The change is continuous with composition in BaNi2(V1−xPx)2O8. But a variety of features in the temperature-dependent magnetic susceptibilities at low temperatures suggest that the magnetic system is complex. Although the Curie–Weiss temperature becomes less strongly negative on increasing the P content, an indication of increased competition between different types of magnetic coupling, the system does not appear to be approaching a quantum spin liquid state at low temperatures. [ABSTRACT FROM AUTHOR]

For electrons living on a triangular lattice, the concept of "geometrical frustration" in the face of strong electric mutual repulsion is a crucial, decisive factor that shapes their behavior. In an insulating material, the coulomb repulsion force is relieved if each electron can point its spin anti-parallel to that of its nearest neighbors. On most lattices, this is readily implemented and leads to a state in which spins alternate up and down along each bond direction. On a triangular lattice, however, the geometric arrangement frustrates such ideal regularity.

NaCaNi2F7 is an insulating, frustrated A2B2F7 pyrochlore with magnetic S=1Ni2+ on the pyrochlore B site. Nonmagnetic Na and Ca are disordered on the A site. Magnetic susceptibility measurements made on an oriented single crystal, grown in a floating zone furnace, show isotropic behavior at temperatures between 5 and 300K, with an effective moment of 3.7μB/Ni. Despite displaying a large Curie-Weiss theta (−129K), spin-ordering-related features are not seen in the susceptibility or specific heat until a spin glass transition at 3.6K. This yields an empirical frustration index of f=−θCW/Tf≈36. The spin glass behavior is substantiated by a shift of the freezing temperature with frequency in the ac susceptibility, bifurcation in the dc susceptibility, and by a broad maximum in the magnetic specific heat. The observations as made on large single crystals suggest that NaCaNi2F7 is likely a realization of a frustrated spin 1 pyrochlore antiferromagnet with weak bond disorder. [ABSTRACT FROM AUTHOR]

Na0.85Co2SbO6·1.7H2O and Na0.95Ni2SbO6·1.5H2O are synthesized by oxidation of Na3M2SbO6 (M: Co, Ni) with Br2/MeCN solution at room temperature followed by exposure to ambient air. [ABSTRACT FROM AUTHOR]

The structures of the metallic high-temperature forms of the title compounds are determined by single crystal XRD. [ABSTRACT FROM AUTHOR]

We report the structure and magnetic properties of two new iridium-based honeycomb Delafossite compounds, Cu3NaIr2O6 and Cu3LiIr2O6, formed by a topotactic cation exchange reaction. The starting materials Na2IrO3 and Li2IrO3, which are based on layers of IrO6 octahedra in a honeycomb lattice separated by layers of alkali ions, are transformed to the title compounds by a topotactic exchange reaction through heating with CuCl below 450 °C; higher temperature reactions cause decomposition. The new compounds display dramatically different magnetic behavior from their parent compounds – Cu3NaIr2O6 has a ferromagnetic like magnetic transition at 10 K, while Cu3LiIr2O6 retains the antiferromagnetic transition temperature of its parent compound but displays significantly stronger dominance of antiferromagnetic coupling between spins. These results reveal that a surprising difference in the magnetic interactions between the magnetic Ir ions has been induced by a change in the non-magnetic interlayer species. A combination of neutron and X-ray powder diffraction is used for the structure refinement of Cu3NaIr2O6 and both compounds are compared to their parent materials. [ABSTRACT FROM AUTHOR]

In frustrated quantum magnets, long-range magnetic order fails to develop despite a large exchange coupling between the spins. In contrast to the magnons in conventional magnets, their spin excitations are poorly understood. Here, we show that the thermal Hall conductivity Κxy provides a powerful probe of spin excitations in the "quantum spin ice" pyrochlore Tb2Ti2O7. The thermal Hall response is large, even though the material is transparent. The Hall response arises from spin excitations with specific characteristics that distinguish them from magnons. At low temperature (<1 kelvin), the thermal conductivity resembles that of a dirty metal. Using the Hall angle, we construct a phase diagram showing how the excitations are suppressed by a magnetic field. [ABSTRACT FROM AUTHOR]

When an electron is incident on a superconductor from a metal, it is reflected as a hole in a process called Andreev reflection. If the metal N is sandwiched between two superconductors S in an SNS junction, multiple Andreev reflections (MARs) occur. We have found that, in SNS junctions with high transparency (τ → 1) based on the Dirac semimetal MoTe2, the MAR features are observed with exceptional resolution. By tuning the phase difference φ between the bracketing Al superconductors, we establish that the MARs coexist with a Josephson supercurrent Is=IA sinφ. As we vary the junction voltage V, the supercurrent amplitude IA varies in step with the MAR order n, revealing a direct relation between them. Two successive Andreev reflections serve to shuttle a Cooper pair across the junction. If the pair is shuttled coherently, it contributes to Is. The experiment measures the fraction of pairs shuttled coherently vs. V. Surprisingly, superconductivity in MoTe2 does not affect the MAR features. [ABSTRACT FROM AUTHOR]

The spin glass state in the spinel ZnCr2(1-x)Ga2xO4 is studied with magnetization and specific heat for x <0.05. The freezing temperature is independent of disorder, despite a two-level-like density of states that varies linearly with x. This relationship implies the energy scale for freezing is independent of disorder, in contrast to mean field theories of spin glass. We suggest that the degrees of freedom are shielded spin vacancies, quasispins, which interact via an emergent long-range force mediated by the frustrated spin background. [ABSTRACT FROM AUTHOR]

We report the magnetothermopower measurements of the nonmetallic topological insulator Bi2Te3 in magnetic fields up to 35 T. Quantum oscillations arising from surface states are observed in both thermoelectric and conductivity tensors. The inferred surface thermopower has a peak magnitude ∼1 mV/K possibly as a result of surface electron and bulk phonon interaction. At the n=1 Landau level, we resolve additional quantum oscillations signaling Landau sublevels. [ABSTRACT FROM AUTHOR]

The physical properties and structural evolution of the 122-type solid solution BaNi2(Ge1-xPx)2 are reported. The in-plane X-X (X = Ge1-xPx) dimer formation present in the end member BaNi2Ge2, which results in a structural transition to orthorhombic symmetry, is completely suppressed to zero temperature on P substitution near x =0.7, and a dome-shaped superconducting phase with a maximum Tc = 2.9 K emerges. Clear indications of phonon softening and enhanced electron-phonon coupling are observed at the composition of the structural instability. Our findings show that dimer breaking offers possibilities as a tuning parameter of superconductivity. [ABSTRACT FROM AUTHOR]

WO3-xFx(x ≤ 0.45) perovskite-like oxyfluorides were prepared by a chemically reducing fluorination route using the polymer polytetrafluoroethylene (teflon). The symmetry of the crystal structures of WO3-xFx changes from monoclinic to tetragonal to cubic as the fluorine content increases. Fluorine doping changes insulating WO3 to a metallic conductor, and superconductivity (Tc = 0.4 K) was discovered in the samples with fluorine contents of 0.41 ≤ x ≤ 0.45. This easy fluorination method may be applicable to other systems and presents an opportunity for finding new oxyfluoride superconductors. [ABSTRACT FROM AUTHOR]

In an electric field, the flow of electrons in a solid produces an entropy current in addition to the familiar charge current. This is the Peltier effect, and it underlies all thermoelectric refrigerators. The increased interest in thermoelectric cooling applications has led to a search for more efficient Peltier materials and to renewed theoretical investigation into how electron-electron interaction may enhance the thermopower of materials such as the transition-metal oxides. An important factor in this enhancement is the electronic spin entropy, which is predicted to dominate the entropy current. However, the crucial evidence for the spin-entropy term, namely its complete suppression in a longitudinal magnetic field, has not been reported until now. Here we report evidence for such suppression in the layered oxide NaxCo2O4, from thermopower and magnetization measurements in both longitudinal and transverse magnetic fields. The strong dependence of thermopower on magnetic field provides a rare, unambiguous example of how strong electron-electron interaction effects can qualitatively alter electronic behaviour in a solid. We discuss the implications of our finding-that spin-entropy dominates the enhancement of thermopower in transition-metal oxides-for the search for better Peltier materials. [ABSTRACT FROM AUTHOR]

The structures of five perovskite-related oxides with the general composition ACu3Ru4O12 with A=Na, Ca, Sr, La and Nd, have been examined both by XRD-Rietveld refinements and Ru-K EXAFS-spectroscopy. In addition, the behavior on reduction was investigated by thermogravimetry (TG). The TG measurements revealed that the composition was almost exactly A1Cu3Ru4O12 for all samples. The inter-atomic distances derived from EXAFS- and XRD-Rietveld fits show an excellent agreement with differences smaller than 8 mA˚ even for R>5 A˚. All inter-atomic distances increase in the order Na

The preparation and physical characterization of non-stoichiometric Ru2Ge3+x (0≤x≤1) are reported for the first time. The defect TiSi2-type chimney-ladder structure is maintained for the full stoichiometry range. The resistivity of Ru2Ge3+x increases systematically with x from 300 mΩ cm, x=0 –3 Ω cm, x=1 at 300 K. The temperature dependence is consistent with a variable range-hopping mechanism for x≥0.6. The Seebeck coefficients of samples do not evolve simply with x. A low thermal conductivity (κ300 K=0.03 W/K cm) suggests that Ru2Ge3 has some of the properties of a phonon-glass–electron-crystal. The low value of the thermoelectric figure of merit ZT=3.2×10−3 (T=300 K) calculated for Ru2Ge3 is due primarily to a low conductivity. [Copyright &y& Elsevier]

A comparative study of the properties of topological insulator Bi2Te2Se (BTS) crystals grown by the vertical Bridgeman method is described. Two defect mechanisms that create acceptor impurities to compensate for the native n-type carriers are compared: Bi excess, and light Sn doping. Both methods yield low carrier concentrations and an n-p crossover over the length of the grown crystal boules, but lower carrier concentrations and higher resistivities are obtained for the Sn-doped crystals, which reach carrier concentrations as low as 8×1014 cm-3. Further, the temperature dependent resistivities for the Sn-doped crystals display strongly activated behavior at high temperatures, with a characteristic energy of half the bulk band gap. The (001) cleaved Sn-doped BTS crystals display high quality Shubnikov de Haas (SdH) quantum oscillations due to the topological surface state electrons. Angle resolved photoelectron spectroscopy (ARPES) characterization shows that the Fermi energy (EF) for the Sn-doped crystals falls cleanly in the surface states with no interference from the bulk bands, which the Dirac point for the surface states lies approximately 60meV below the top of the bulk valence band maximum, and allows for a determination of the bulk and surface state carrier concentrations as a function of Energy near EF. Electronic structure calculations that compare Bi excess and Sn dopants in BTS demonstrate that Sn acts as a special impurity, with a localized impurity band that acts as a charge buffer occurring inside the bulk band gap. We propose that the special resonant level character of Sn in BTS gives rise to the exceptionally low carrier concentrations and activated resistivities observed. [ABSTRACT FROM AUTHOR]

We report the fabrication of field-effect transistors on single crystals of the highly anisotropic organic semiconductor tetramethyltetraselenafulvalene. We find maximum mobilities of 0.2 cm[sup 2]/V s at room temperature. We observe an exponential decrease of mobility with decreasing temperature with an activation energy 160±30 meV, independent of sample quality or fabrication method. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

After the recent finding that CrI3, displays ferromagnetic order down to its monolayer, extensive studies have followed to pursue new two-dimensional (2D) magnetic materials. In this article, we report on the growth of single crystal CrCl3 in the layered monoclinic phase. The system after mechanical exfoliation exhibits stability in ambient air (the degradation occurs on a time scale at least four orders of magnitude longer than is observed for CrI3). By means of mechanical cleavage and atomic force microscopy (AFM) combined with optical identification, we demonstrate the systematic isolation of single and few layer flakes onto 270 nm and 285 nm SiO2/Si (100) substrates with lateral size larger than graphene flakes isolated with the same method. The layer number identification has been carried with statistically significant data, quantifying the optical contrast as a function of the number of layers for up to six layers. Layer dependent optical contrast data have been fitted within the Fresnel equation formalism determining the real and imaginary part of the wavelength dependent refractive index of the material. A layer dependent (532 nm) micro-Raman study has been carried out down to two layers with no detectable spectral shifts as a function of the layer number and with respect to the bulk. [ABSTRACT FROM AUTHOR]

Edge supercurrents in superconductors have long been an elusive target. Interest in them has reappeared in the context of topological superconductivity. We report evidence for the existence of a robust edge supercurrent in the Weyl superconductor molybdenum ditelluride (MoTe2). In a magnetic field B, fluxoid quantization generates a periodic modulation of the edge condensate observable as a “fast-mode” oscillation of the critical current Ic versus B. The fast-mode frequency is distinct from the conventional Fraunhofer oscillation displayed by the bulk supercurrent. We confirm that the fast-mode frequency increases with crystal area as expected for an edge supercurrent. In addition, weak excitation branches are resolved that display an unusual broken symmetry. [ABSTRACT FROM AUTHOR]

We report the synthesis of MgB[sub 2] with bulk superconducting properties by conventional solid state methods at temperatures as low as 550 °C. Mg deficiencies of the type Mg[sub 1-x]B[sub 2] were tested. T[sub c] was found to decrease by about 1 K at large x, though the amount of nonstoichiometry, if any, is likely to be very small. For specific processing conditions, indications of the 25–30 K transition often seen in thin films were seen in the bulk materials. The lower temperature transition may be associated with the grain boundaries. These results indicate that it should be possible to fabricate MgB[sub 2] with bulk properties in in situ thin films at temperatures of 600 °C or less. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Examines thermoelectric properties of semiconductor materials. Room-temperature Seebeck coefficient; Thermal conductivity; Sensitivity of transport properties to chemical doping and nonstoichiometry.

Presents research which studied macroscopic degeneracy in a magnet with the pyrochlore structure, or spin ice. How common water ice is an unusual solid; Previous research; Corresponding behavior between water ice and spin ice in degeneracy; Data for spin entropy for Dy2Ti2O7; Behavior in spin ice not accessible in water ice.

The Mössbauer effect of the 37.2 keV γ transition of 121Sb has been employed to study the electronic configuration of Sb ions in BaPb1-xSbxO3 superconducting materials. The isomer shift is constant as a function of Sb content and falls in the region correspoding to the Sb(V) valence state. The isomer shift of 7 mm/s is the lowest value yet observed for an Sb(V)O6 chromophore and indicates a high degree of covalency, corresponding to significant 5sm 5pn hybridization. The mixed valence state of an equal amount of Sb(III) and Sb(V) has not been observed. [ABSTRACT FROM AUTHOR]

Presents an abstract of the study 'Easy-Axis Transition in SmMn[sub2]Ge[sub2],' by R.B. van Dover, E.M. Gyorgy et al., published in the 'Journal of Applied Physics.'

Presents an abstract of a study on magnetoresistance in layered magnetic compounds. Factors that consist the structural phase transition that accompanies the magnetic transition; Function of arc-grown single crystals.

Presents a study which examined copper oxide superconductors using nuclear magnetic resonance and nuclear quadrupole resonance. Experimental procedures; Results of spin-lattice relaxation; Conclusion.

The high-pressure behavior of germanates is of interest as these compounds serve as analogs for silicates of the deep Earth. Current theoretical and experimental studies of iron germanate, FeGeO3, are limited. Here, we have examined the behavior of FeGeO3 to 127 GPa using the laser-heated diamond anvil cell combined with in situ synchrotron X-ray diffraction. Upon compression at room temperature, the ambient-pressure clinopyroxene phase transforms to a disordered triclinic phase [FeGeO3 (II)] at ~ 18 GPa in agreement with earlier studies. An additional phase transition to FeGeO3 (III) occurs above 54 GPa at room temperature. Laser-heating experiments (~ 1200-2200 K) were conducted at three pressures (33, 54, and 123 GPa) chosen to cover the stability regions of different GeO2 polymorphs. In all cases, we observe that FeGeO3 dissociates into GeO2 + FeO at high pressure and temperature conditions. Neither the perovskite nor the post-perovskite phase was observed up to 127 GPa at ambient or high temperatures. The results are consistent with the behavior of FeSiO3, which also dissociates into a mixture of the oxides (FeO + SiO2) at least up to 149 GPa. [ABSTRACT FROM AUTHOR]

Neutron time-of-flight spectroscopy has been employed to study the crystal-field interaction in the pyrochlore titanate Ho[sub 2]Ti[sub 2]O[sub 7]. The crystal-field parameters and corresponding energy-level scheme have been determined from a profile fit to the observed neutron spectra. The ground state is a well separated E[sub g] doublet with a strong Ising-like anisotropy, which can give rise to frustration in the pyrochlore lattice. Using the crystal-field parameters determined for the Ho compound as an estimate of the crystal-field potential in other pyrochlore magnets, we also find the Ising type behavior for Dy. In contrast, the almost planar anisotropy found for Er and Yb prevents frustration, because of the continuous range of possible spin orientations in this case. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The antiferromagnetic pyrochlore material NaCaCo2F7 is a thermal spin liquid over a broad temperature range (≈140 K down to TF=2.4 K), in which magnetic correlations between Co2+ dipole moments explore a continuous manifold of antiferromagnetic XY states [K. A. Ross et al., Phys. Rev. B 93, 014433 (2016)]. The thermal spin liquid is interrupted by spin freezing at a temperature that is ~2% of the mean-field interaction strength, leading to short-range static XY clusters with distinctive relaxation dynamics. Here we report the low-energy inelastic neutron scattering response from the related compound NaSrCo2F7, confirming that it hosts the same static and dynamic spin correlations as NaCaCo2F7. We then present the single-ion levels of Co2+ in these materials as measured by inelastic neutron scattering. An intermediate spin-orbit coupling model applied to an ensemble of trigonally distorted octahedral crystal fields accounts for the observed transitions. The single-ion ground state of Co2+ is a Kramers doublet with a strongly XY-like g tensor (gxy/gz~3). The local disorder inherent from the mixed pyrochlore A sites (Na+/Ca2+ and Na+/Sr2+) is evident in these measurements as exaggerated broadening of some of the levels. A simple model that reproduces the salient features of the single-ion spectrum produces approximately 8.4% and 4.1% variation in the z and xy components of the g tensor, respectively. This study confirms that an Seff=½ model with XY antiferromagnetic exchange and weak exchange disorder serves as a basic starting point in understanding the low-temperature magnetic behavior of these strongly frustrated magnets. [ABSTRACT FROM AUTHOR]

The anharmonic effect in thermoelectrics has been a central topic for decades in both condensed matter physics and material science. However, despite the long-believed strong and complex anharmonicity in the Bi2 Te3-x Sex series, experimental verification of anharmonicity and its evolution with doping remains elusive. We fill this important gap with high-resolution, temperature-dependent Raman spectroscopy in high-quality single crystals of Bi2 Te3, Bi2 Te2 Se, and Bi2 Se3 over the temperature range from 4 to 293K. Klemens's model was employed to explain the renormalization of their phonon linewidths. The phonon energies of Bi2 Se3 and Bi2 Te3 are analyzed in detail from three aspects: lattice expansion, cubic anharmonicity, and quartic anharmonicity. For the first time, we explain the evolution of anharmonicity in various phonon modes and across the series. In particular, we find that the interplay between cubic and quartic anharmonicity is governed by their distinct dependence on the phonon density of states, providing insights into anomalous anharmonicity designing of new thermoelectrics. [ABSTRACT FROM AUTHOR]

Following the predictions of total energy calculations, a tetragonal SrMn2P2 phase is proposed and successfully formed under high pressure. At ambient pressure, SrMn2P2 adopts the primitive trigonal La2O3 structure type (space group P3̅m1). However, the results of total energy calculations indicate that SrMn2P2 should be more stable in the tetragonal ThCr2Si2 structure type (space group I4/mmm) than in its known trigonal structure, thus motivating our synthetic experiments. Guided by these calculations, a new tetragonal polymorph of SrMn2P2 was found under the relatively mild conditions of 5 GPa applied pressure at a temperature of 900 °C through the transformation of the ambient pressure trigonal form. The new polymorph has the body centered tetragonal ThCr2Si2 structure type, as predicted. The electronic structure calculations indicate the likelihood of antiferromagnetic, semiconducting properties for the high pressure SrMn2P2 phase. [ABSTRACT FROM AUTHOR]

Studies the structure and magnetic properties of epitaxial ferrite multilayer films. Determination of crystal orientation and rocking curve widths; Structural characterization of buffer films; Grain sizes of ferrite films grown on top of buffered substrates.

In itinerant ferromagnets, the quenched disorder is predicted to dramatically affect the ferromagnetic to paramagnetic quantum phase transition driven by external control parameters at zero temperature. Here we report a study on Fe-doped Cr2B, which, starting from the paramagnetic parent, orders ferromagnetically for Fe-doping concentrations x larger than xc=2.5%. In parent Cr2B, 11B nuclear magnetic resonance data reveal the presence of both ferromagnetic and antiferromagnetic fluctuations. The latter are suppressed with Fe doping, before the ferromagnetic ones finally prevail for x>xc. Indications for non-Fermi-liquid behavior, usually associated with the proximity of a quantum critical point, were found for all samples, including undoped Cr2B. The sharpness of the ferromagneticlike transition changes on moving away from xc, indicating significant changes in the nature of the magnetic transitions in the vicinity of the quantum critical point. Our data provide some constraints for understanding quantum phase transitions in itinerant ferromagnets in the limit of weak quenched disorder. [ABSTRACT FROM AUTHOR]

Nematic quantum fluids with wave functions that break the underlying crystalline symmetry can form in interacting electronic systems. We examined the quantum Hall states that arise in high magnetic fields from anisotropic hole pockets on the Bi(111) surface. Spectroscopy performed with a scanning tunneling microscope showed that a combination of single-particle effects and many-body Coulomb interactions lift the six-fold Landau level (LL) degeneracy to form three valley-polarized quantum Hall states. We imaged the resulting anisotropic LL wave functions and found that they have a different orientation for each broken-symmetry state. The wave functions correspond to those expected from pairs of hole valleys and provide a direct spatial signature of a nematic electronic phase. [ABSTRACT FROM AUTHOR]

The crystal structures and magnetic properties of RE3Sb3Mg2O14 (RE = La, Pr, Sm, Eu, Tb, Ho) with a perfect kagome lattice are presented and compared to RE3Sb3Zn2O14. Rietveld structure refinements were performed using X-ray diffraction data, indicating that the layered compounds are fully structurally ordered. The compounds crystallize in a rhombohedral supercell of the cubic pyrochlore structure, in the space group R-3 m. Magnetic susceptibility measurements show no signs of magnetic ordering above 2 K. The RE3Sb3Mg2O14 family is similar to that of RE3Sb3Zn2O14; however, the series reported here features a fully ordered distribution of cations in both the nonmagnetic antimony and magnetic rare earth kagome lattices. Unlike the offsite disorder that Zn2+ experiences in RE3Sb3Zn2O14, the magnesium sites in RE3Sb3Mg2O14 are completely ordered. Here we compare the magnetic properties in both series of kagome compounds to determine how significant Zn2+'s positional ordering is within this structure type. The compounds reported here appear to be relatively defect-free and are therefore model systems for investigating magnetic frustration on an ideal 2D rare earth kagome lattice. [ABSTRACT FROM AUTHOR]

In quantum field theory, we learn that fermions come in three varieties: Majorana, Weyl, and Dirac. Here, we show that in solid-state systems this classification is incomplete, and we find several additional types of crystal symmetry–protected free fermionic excitations. We exhaustively classify linear and quadratic three-, six-, and eight-band crossings stabilized by space group symmetries in solid-state systems with spin-orbit coupling and time-reversal symmetry. Several distinct types of fermions arise, differentiated by their degeneracies at and along high-symmetry points, lines, and surfaces. Some notable consequences of these fermions are the presence of Fermi arcs in non-Weyl systems and the existence of Dirac lines. Ab initio calculations identify a number of materials that realize these exotic fermions close to the Fermi level. [ABSTRACT FROM AUTHOR]

The optical properties of layered tungsten ditelluride have been measured over a wide temperature and frequency range for light polarized in the a-b planes. A striking low-frequency plasma edge develops in the reflectance at low temperature where this material is a perfectly compensated semimetal. The optical conductivity is described using a two-Drude model which treats the electron and hole pockets as separate electronic subsystems. At low temperature, one scattering rate collapses by over two orders of magnitude, while the other also undergoes a significant, but less dramatic, decrease; both scattering rates appear to display the quadratic temperature dependence expected for a Fermi liquid. First principles electronic structure calculations reveal that the low-lying optical excitations are due to direct transitions between the bands associated with the electron and hole pockets. [ABSTRACT FROM AUTHOR]