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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,045 records · Page 58
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Optical conductivity of metal alloys with residual resistivities near or above the Mott-Ioffe-Regel limit

Most interesting examples of violations of the Mott-Ioffe-Regel (MIR) resistivity limit are found in materials with strong electronic correlations that are not well understood by theory. We demonstrate that first principles theory can predict the experimentally-observed frequency dependence of the optical conductivity for a novel class of metals where the residual resistivity is near or above the MIR limit, which we define as a "bad metal". The predicted optical conductivity of a NiCoCr alloy is in good agreement with experiment. It is demonstrated that the width of the Drude peak describing the low frequency part of optical conductivity is comparable to the Fermi energy. The latter, together with a mean free path comparable to the interatomic distance, indicates the absence of well-defined quasiparticles. In contrast to traditional bad meals with strong electron-electron interactions, both the high resistivity and the large width of the Drude peak in these alloys results from strong scattering on disordered atomic potentials that can be understood using modern density functionals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Evolution of structural, magnetic, and transport properties in MnBi 2- x Sb x Te 4

In this work we report the evolution of structural, magnetic, and transport properties in MnBi 2- x Sb x Te 4 (0 ≤ x ≤ 2) single crystals. MnSb 2 Te 4 , isostructural to MnBi 2 Te 4 , is successfully synthesized in single-crystal form. Magnetic measurements suggest an antiferromagnetic order below T N =19K for MnSb 2 Te 4 with the magnetic moments aligned along the crystallographic c axis. With increasing Sb content in MnBi 2- x Sb x Te 4 , the a -lattice parameter decreases linearly following Vegard's law, while the c -lattice parameter shows little compositional dependence. The contraction along a is caused by the reduction of the Mn-Te-Mn bond angle, while the Mn-Te bond length remains nearly constant. The antiferromagnetic ordering temperature slightly decreases from 24 K for MnBi 2 Te 4 to 19 K for MnSb 2 Te 4 . More dramatic change was observed for the critical magnetic fields required for the spin-flop transition and the moment saturation. Both critical fields decrease with increasing Sb content for x ≤ 1.72; a spin-flip transition occurs in MnSb 2 Te 4 at a small field of 3 kOe applied along the c axis. In high magnetic fields, the saturation moment at 2 K shows significant suppression from 3.56 μ B /Mn for MnBi 2 Te 4 to 1.57 μ B /Mn for MnSb 2 Te 4 . Analysis of the magnetization data suggests that both the interlayer magnetic interaction and single-ion anisotropy decrease with increasing Sb content for x ≤ 1.72. The partial substitution of Bi by Sb also dramatically affects the transport properties. A crossover from n -type to p -type conducting behavior is observed around x ≈0.63. Our findings show close correlation between structural, magnetic, and transport properties in MnBi 2- x Sb x Te 4 and that partial substitution of Bi by Sb is an effective approach to fine tuning both the magnetism and transport properties of MnBi 2- x Sb x Te 4 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Reorientation of antiferromagnetism in cobalt doped FeSn

FeSn is an itinerant antiferromagnet that hosts electronic Dirac states and ordered magnetic moments lying within its Fe Kagome-lattice planes. We present magnetization measurements of single crystals of (Fe 1-x Co x )Sn, revealing the evolution and suppression of this magnetic order with Co substitution. We interpret the dramatic changes in magnetic anisotropy to indicate a reorientation of the moments from perpendicular to parallel to the hexagonal c axis and confirm this with neutron diffraction. It has been proposed that the Dirac nodes observed in FeSn should become gapped if the moments rotate as our data suggests. We identify Co-substituted compositions that adopt both antiferromagnetic configurations at different temperatures. This system provides a unique opportunity to study how the details of magnetic order impact Dirac electron states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Flat bands in the CoSn-type compounds

Quantum interference on the kagome lattice generates electronic bands with narrow bandwidth, called flat bands. Crystal structures incorporating this lattice can host strong electron correlations with nonstandard ingredients, but only if these bands lie at the Fermi level. In the six compounds with the CoSn structure type (FeGe, FeSn, CoSn, NiIn, RhPb, and PtTl) the transition metals form a kagome lattice. The two iron variants are robust antiferromagnets so we focus on the latter four and investigate their thermodynamic and transport properties. We consider these results and calculated band structures to locate and characterize the flat bands in these materials. Finally, we propose that CoSn and RhPb deserve the community's attention for exploring flat-band physics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Damped Dirac magnon in the metallic kagome antiferromagnet FeSn

The kagome lattice is a fertile platform to explore topological excitations with both Fermi-Dirac and Bose-Einstein statistics. While relativistic Dirac fermions and flat bands have been discovered in the electronic structure of kagome metals, the spin excitations have received less attention. Here, we report inelastic neutron scattering studies of the prototypical kagome magnetic metal FeSn. The spectra display well-defined spin waves extending to 120 meV. Above this energy, the spin waves become progressively broadened, reflecting interactions with the Stoner continuum. Using linear spin-wave theory, we determine an effective spin Hamiltonian that reproduces the measured dispersion. This analysis indicates that the Dirac magnon at the K point remarkably occurs on the brink of a region where well-defined spin waves become unobservable. Furthermore, our results emphasize the influential role of itinerant carriers on the topological spin excitations of metallic kagome magnets.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Thermodynamic insights into the intricate magnetic phase diagram of EuAl 4

The tetragonal intermetallic compound EuAl 4 hosts an exciting variety of low-temperature phases. In addition to a charge density wave below 140 K, four ordered magnetic phases are observed below 15.4 K. Recently, a skyrmion phase was proposed based on Hall effect measurements under a c-axis magnetic field. Here, we present a detailed investigation of the phase transitions in EuAl 4 under c-axis magnetic field. Our dilatometry, heat-capacity, DC magnetometry, AC magnetic susceptibility, and resonant ultrasound spectroscopy measurements reveal three magnetic phase transitions not previously reported. The first key result is a detailed H ∥ [001] magnetic phase diagram mapping the seven phases we observe. Second, we identify a high-field phase, phase VII, which directly corresponds to the region were skyrmions have been previously suggested. Our results provide guidance for future studies exploring the complex magnetic interactions and spin structures in EuAl 4 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Topological Electronic Structure Evolution with Symmetry Breaking Spin Reorientation in (Fe 1–x Co x )Sn

Topological materials hosting kagome lattices have drawn considerable attention due to the interplay between topology, magnetism, and electronic correlations. The (Fe 1–x Co x )Sn system not only hosts a kagome lattice but has a tunable symmetry-breaking magnetic moment with temperature and doping. In this study, angle-resolved photoemission spectroscopy and first-principles calculations are used to investigate the interplay between the topological electronic structure and varying magnetic moment from the planar to axial antiferromagnetic phases. Here, evidence for a theoretically predicted gap at the Dirac point is revealed in the low-temperature axial phase, but no gap opening is observed across a temperature-dependent magnetic phase transition. However, topological surface bands are observed to shift in energy as the surface magnetic moment is reduced or becomes disordered over time during experimental measurements. The shifting surface bands may preclude the determination of a temperature-dependent bulk gap, but this highlights the intricate connections between magnetism and topology with a surface/bulk dichotomy that can affect material properties and their interrogation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

Collapsed tetragonal phase and superconductivity of BaFe[subscript 2]As[subscript 2] under high pressure

High pressure x-ray diffraction and electrical resistance measurements have been carried out on BaFe{sub 2}As{sub 2} to a pressure of 35 GPa and temperature of 10 K using a synchrotron source and designer diamond anvils. At ambient temperature, a phase transition from the tetragonal phase to a collapsed tetragonal (CT) phase is observed at 17 GPa under nonhydrostatic conditions as compared to 22 GPa under hydrostatic conditions. The superconducting transition temperature increases rapidly with pressure up to 34 K at 1 GPa and decreases gradually with a further increase in pressure. Our results suggest that T{sub C} falls below 10 K in the pressure range of 16-30 GPa, where CT phase is expected to be stable under high-pressure and low-temperature conditions.

43 PARTICLE ACCELERATORS↗
DOE OSTI2011

Unusual Phase Transitions and Magnetoelastic Coupling in TlFe1.6Se2

Structural, magnetic, electrical transport, and heat capacity data are reported for single crystals of TlFe{sub 1.6}Se{sub 2}. This compound crystallizes in a tetragonal structure similar to the ThCr{sub 2}Si{sub 2} structure, but with vacancies in the Fe layer. The vacancies can be ordered or disordered depending on temperature and thermal history. If the vacancies are ordered, the basal plane lattice constant increases from a to {radical}5a. Antiferromagnetic order with the Fe spins along the c axis occurs below T{sub N} {approx} 430 K, as shown by single-crystal neutron diffraction, and the magnetic structure is reported. In addition, for the vacancy-ordered crystal, two other phase transitions are found at T{sub 1} {approx} 140 K and T{sub 2} {approx} 100 K. The phase transitions at T{sub 1} and T{sub 2} are evident in heat capacity, magnetic susceptibility, resistivity data, a and c lattice parameters, and the unusual temperature dependence of the magnetic order parameter determined from neutron scattering. The phase transitions at T{sub 1} and T{sub 2} result in significant changes in the magnetic moment per iron, with 1.72(6) {micro}B observed at 300 K, 2.07(9) {micro}B at 140 K, 1.90(9) {micro}B at 115 K, and 1.31(8) {micro}B for 5 K if the same 'block checkerboard' magnetic structure is used at all temperatures. The phase transitions appear to be driven by small changes in the c lattice constant, large magnetoelastic coupling, and the localization of carriers with decreasing temperature.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Pressure effects on the transport of Ba(Fe1-xCox)2As2

We report the temperature dependence of the resistivity and thermoelectric power (TEP) under hydrostatic pressure of the itinerant antiferromagnet BaFe{sub 2}As{sub 2} and the electron-doped superconductor Ba(Fe{sub 0.9}Co{sub 0.1}){sub 2}As{sub 2}. We observe a hole-like contribution to the thermopower below the structural/magnetic (S-M) transition in the parent compound that is suppressed in magnitude and temperature with pressure. Pressure increases the contribution of electrons to transport in both the doped and undoped compound. In the 10% Co-doped sample we used a two-band model for thermopower to estimate the carrier concentrations and determine the effect of pressure on the band structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Effect of molybdenum 4d hole substitution in BaFe2As2

We investigate the thermodynamic and transport properties of molybdenum-doped BaFe2As2 (122) crystals, the first report of hole doping using a 4d element. The chemical substitution of Mo in place of Fe is possible up to {approx} 7%. For Ba(Fe1-xMox)2As2, the suppression rate of the magnetic transition temperature with x is the same as in 3d Cr-doped 122 and is independent of the unit cell changes. This illustrates that the temperature-composition phase diagram for hole-doped 122 can be simply parameterized by x, similar to the electron-doped 122 systems found in the literature. Compared to 122 with a coupled antiferromagnetic order (TN) and orthorhombic structural transition (T0) at {approx}132 K, 1.3% Mo-doped 122 (x=0.013) gives TN=T0=125(1) K according to neutron diffraction results and features in specific heat, magnetic susceptibility, and electrical resistivity. The cell volume expands by {approx}1% with maximum Mo doping and TN is reduced to {approx}90 K. There is a T* feature that is identified for lightly Cr- or Mo-doped 122 crystals, which is x dependent. This low-temperature transition may be a trace of superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Direct probe of the variability of Coulomb correlation in iron pnictide superconductors

We use core-valence-valence Auger spectra to probe the Coulomb repulsion between holes in the valence band of Fe pnictide superconductors. By comparing the two-hole final-state spectra to density functional theory calculations of the single-particle density of states, we extract a measure of the electron correlations that exist in these systems. Our results show that the Coulomb repulsion is highly screened and can definitively be considered as weak. We also find that there are differences between the 1111 and 122 families and even a small variation as a function of the doping x in Ba(Fe{sub 1-x}Co{sub x}){sub 2}As{sub 2}. We discuss how the values of the hole-hole Coulomb repulsion obtained from our study relate to the onsite Coulomb parameter U used in model and first-principles calculations based on dynamical mean field theory and establish an upper bound for its effective value. Our results impose stringent constraints on model-based phase diagrams that vary with the quantity U or U/W by restricting the latter to a rather small range of values.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Elastic Properties of the Zintl Ferromagnet Yb14MnSb11

We report measurements of the elastic moduli as a function of temperature (5-300) K and magnetic field (0-2 T) for the Zintl ferromagnet Yb{sub 14}MnSb{sub 11}, which is believed to be a rare example of an underscreened Kondo lattice. The elastic moduli measured below the Curie temperature in this complex ferromagnet exhibit unusual lattice stiffening that is independent of the magnetic field and can be adequately modeled using the Landau theory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Physical properties of Ce3-xTe4 below room temperature

The physical properties of polycrystalline Ce{sub 3-x}Te{sub 4} were investigated by measurements of the thermoelectric properties, Hall coefficient, heat capacity, and magnetization. The fully filled, metallic x = 0 compound displays a soft ferromagnetic transition near 4 K, and analysis of the corresponding heat capacity anomaly suggests a doublet ground state for Ce{sup 3+}. The transition is suppressed to below 2 K in the insulating x = 0.33 composition, revealing that magnetic order in Ce{sub 3-x}Te{sub 4} is driven by a Ruderman-Kittel-Kasuya-Yosida (RKKY)-type interaction. The thermoelectric properties trend with composition as expected from simple electron counting, and the transport properties in Ce{sub 3}Te{sub 4} are observed to be similar to those in La{sub 3}Te{sub 4}. Trends in the low-temperature thermal conductivity data reveal that the phonons are efficiently scattered by electrons, while all compositions examined have a lattice thermal conductivity near 1.2 W/m K{sup -1} at 200 K.

36 MATERIALS SCIENCE↗