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At least 811 records · Page 45
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Magnetism of single crystalline breathing pyrochlore spinel AgInCr 4 ⁢S 8

Single crystals of Ag⁢In⁢Cr 4⁢ S 8 are grown by chemical vapor transport and crystallographic ordering of Ag/In that results in a breathing pyrochlore motif of Cr 3+ is verified by x-ray and neutron diffraction. Long-range antiferromagnetic order is observed below a Néel temperature of 𝑇 N ≈ 9.6 K. The magnetic properties are characterized using ac and dc magnetization, specific heat capacity, and single crystal neutron diffraction measurements. The specific heat data are characterized by a small lambda anomaly near 9.5 K and the estimated magnetic entropy reaches ≈ $\frac{1}{3}$ of the expected value by 3⁢𝑇 N , suggesting significant short-range order in the paramagnetic phase. Single crystal neutron diffraction evidences an incommensurate spin structure with propagation vector 𝒌 = (0,0,𝛿) and 𝛿 = 0.343 at 5 K. The minimal model that accounts for the data consists of ferromagnetic layers of Cr atoms, with magnetic moments lying in the plane of the layers and modulating in the perpendicular direction to form a helical structure propagating along 𝒌. Furthermore, this study represents a rare investigation of single crystals within the family of breathing pyrochlore materials.

Frustrated magnetism↗
DOE OSTI2025

Interplay of superconducting, metallic, and crystalline states of composite fermions at 𝜈 = $\frac{1}{6}$ in wide quantum wells

Evidence for developing fractional quantum Hall effect (FQHE) at filling fraction 𝜈 = 1/6 and 1/8 was recently reported in wide GaAs quantum wells [Wang et al., Phys. Rev. Lett. 134, 046502 (2025)]. In this article, we theoretically investigate the nature of the state at 𝜈 = 1/6 as a function of the quantum well width and the density by considering composite-fermion (CF) crystals, CF Fermi sea, and various kinds of paired CF states. The 𝑓-wave paired state has the lowest energy among the paired CF states. However, for parameters of interest, the energies of the CF crystal, the CF Fermi liquid, and the 𝑓-wave paired CF state are too close to distinguish. We, therefore, predict that 𝑖𝑓 the FQHE at 𝜈 = 1/6 is experimentally confirmed, this state would be an 𝑓-wave paired state of CFs, which can be verified by measurement of its thermal Hall conductance. Exact diagonalization studies on clean systems with up to eight electrons show that the ground states at 𝜈 = 𝑛/(6⁢𝑛 ± 1) are incompressible for all widths and densities we have considered, and are well described by the corresponding Laughlin and Jain states. We propose a phase diagram for large quantum well widths and densities in which at zero disorder, incompressible FQHE states are stabilized at 𝜈 = 𝑛/(6⁢𝑛 ± 1) and 𝜈 = 1/6, but in between these fillings the CF crystal is stabilized. We also present a qualitative discussion on the effects of disorder and propose a schematic phase diagram based on it. With disorder, which creates a spatial variation in the filling factor, two regimes are identified: (i) for small disorder, when the incompressible states percolate at the special fillings, FQHE with quantized Hall plateaus and vanishing longitudinal resistance should occur; and (ii) for larger disorder, when the CF crystal percolates, the longitudinal resistance rises with decreasing temperature but the domains of FQHE liquid produce minima at the special filling factors. Here, experiments are consistent with the latter scenario. We also mention a possible connection of the phase diagram presented here to a puzzling behavior observed for the fractional quantum anomalous Hall effect in pentalayer graphene.

Composite fermions↗
DOE OSTI2025

Deviation from Debye-Waller behavior in single crystalline freestanding Ni⁢ O membranes studied via ultrafast electron diffraction

Here, we realize single-crystal freestanding Ni⁢O films on Si⁢N membranes, enabling thin-film scattering experiments with little or no substrate background signal. These membrane samples provide a critical platform for performing ultrafast electron diffraction (UED) to probe the response of the lattice to laser excitation. We show that epitaxial Ni⁢O films on a water-soluble Sr 2 ⁢Ca⁢Al 2⁢ O 6 buffer layer on Sr⁢Ti⁢O 3 substrates can be transferred as freestanding films to the Si⁢N membranes while maintaining the epitaxial character of the films. An above-gap photoexcitation (𝜆 =266 nm) incident on the sample stimulates an initial lattice response of the order of 1.1 ps, followed by the persistence of this state for more than 20 ps. We observe typical Debye-Waller behavior in peaks with 𝑞 above the (220) family, but a distinct deviation from this behavior is seen in low-𝑞 peaks. We relate this behavior to a dynamic decrease in sample mosaicity in response to the optical pump of the order of 3% that endures for approximately 4 ps. We suspect that this dynamic mosaicity is due to laser-induced strain propagation. This work demonstrates the viability of the thin-film transfer method for UED and paves the way for the study of the dynamic response of antiferromagnetic thin-film oxides.

Wisser, Jacob J. [Stanford Univ., CA (United State↗
DOE OSTI2020

Band gap renormalization, carrier mobilities, and the electron-phonon self-energy in crystalline naphthalene

Organic molecular crystals are expected to feature appreciable electron-phonon interactions that influence their electronic properties at zero and finite temperature. Here, we report first-principles calculations and an analysis of the electron-phonon self-energy in naphthalene crystals. We compute the zero-point renormalization and temperature dependence of the fundamental band gap, and the resulting scattering lifetimes of electronic states near the valence- and conduction-band edges employing density functional theory. Further, our calculated phonon renormalization of the GW-corrected quasiparticle band structure predicts a fundamental band gap of 5 eV for naphthalene at room temperature, in good agreement with experiments. From our calculated phonon-induced electron lifetimes, we obtain the temperature-dependent mobilities of electrons and holes in good agreement with experimental measurements at room temperature. Finally, we show that an approximate energy self-consistent computational scheme for the electron-phonon self-energy leads to the prediction of strong satellite bands in the electronic band structure. We find that a single calculation of the self-energy can reproduce the self-consistent results of the band gap renormalization and electrical mobilities for naphthalene, provided that the on-the-mass-shell approximation is used, i.e., if the self-energy is evaluated at the bare eigenvalues.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Electron-phonon coupling and superconductivity in the doped topological crystalline insulator ( Pb 0.5 Sn 0.5 ) 1 – x In x Te

Here, we present a neutron-scattering study of phonons in single crystals of ( Pb 0.5 Sn 0.5 ) 1 – x In x Te with x = 0 (metallic, but nonsuperconducting) and x = 0.2 (nonmetallic normal state, but superconducting). We map the phonon dispersions (more completely for x = 0 ) and find general consistency with theoretical calculations, except for the transverse and longitudinal optical (LO) modes at the Brillouin-zone center. At low temperature, both modes are strongly damped but sit at a finite energy ( ≈ 4 meV in both samples), shifting to higher energy at room temperature. These modes are soft due to a proximate structural instability driven by the sensitivity of Pb-Te and Sn-Te p -orbital hybridization to off-center displacements of the metal atoms. The impact of the soft optical modes on the low-energy acoustic modes is inferred from the low thermal conductivity, especially at low temperature. Given that the strongest electron-phonon coupling is predicted for the LO mode, which should be similar for both studied compositions, it is intriguing that only the In-doped crystal is superconducting. In addition, we observe elastic diffuse (Huang) scattering that is qualitatively explained by the difference in Pb-Te and Sn-Te bond lengths within the lattice of randomly distributed Pb and Sn sites. We also confirm the presence of anomalous diffuse low-energy atomic vibrations that we speculatively attribute to local fluctuations of individual Pb atoms between off-center sites.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Crystalline solutions of the Kohn-Sham equations in the fractional quantum Hall regime

A Kohn-Sham density functional approach has recently been developed for the fractional quantum Hall effect, which maps the strongly interacting electrons into a system of weakly interacting composite fermions subject to an exchange correlation potential as well as a density dependent gauge field that mimics the "flux quanta" bound to composite fermions. To get a feel for the role of various terms, we study the behavior of the self-consistent solution as a function of the strength of the exchange correlation potential, which is varied through an ad hoc multiplicative factor. Here, we find that a crystal phase is stabilized when the exchange correlation interaction is sufficiently strong relative to the composite-fermion cyclotron energy. Various properties of this crystal are examined.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Small-moment antiferromagnetic ordering in single-crystalline La 2 Ni 7

Single crystals of La 2 Ni 7 have been grown out of a binary, La-Ni melt. Temperature dependent, zero magnetic field, specific heat, electrical resistivity, and low field magnetization measurements indicate that there is a series of antiferromagnetic phase transitions at T 1 = 61.0 ± 0.2 K, T 2 = 56.5 ± 0.2 K and T 3 = 42.2 ± 0.2 K. The three specific heat anomalies found at these temperatures qualitatively have very small entropy changes associated with them and the anisotropic M(H) data saturate at ~ 0.12 μ B /Ni; both observations strongly suggesting the AFM order is associated with very small, itinerant, moments. Anisotropic, H ||c and H ⟂c , ρ(H) and M(H) isotherms as well as constant field, ρ(T) and M(T) sweeps manifest signatures of multiple phase lines and result in HT phase diagrams that are clearly anisotropic. Analysis of M(T) and M(H) data allow for the identification of the two lower temperature magnetically ordered states as antiferromagnetically ordered, with the moments aligned along the crystallographic c-axis, and the higher temperature, T 2 < T < T 1 , state as having a finite ferromagnetic component. In addition, the metamagnetic transition at low temperatures, for H applied along the crystallographic c-axis (H ||c ) appears to be a near classic example of a spin-flop transition, resulting in a field stabilized antiferromagnetic state with the moments ordered perpendicular to the c-axis. Finally, although the small moment ordering, and existence of multiple phase transitions in field and temperature, suggesting an energetic proximity of these states, could foretell a degree of pressure sensitivity, our measurements of R(T) for applied pressures up to 2.0 GPa indicate that there is very little pressure dependence of T 1 , T 2 and T 3 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Giant magnetoresistance in the crystalline YCd 6 intermetallic compound

Giant magnetoresistance (GMR) in bulk nonmagnetic compounds has received considerable attention since this phenomenon challenged the classical understanding of electron transport under a magnetic field. Here, we report magnetotransport properties of the diamagnetic intermetallic compound YCd 6 for applied magnetic fields up to 35 T and temperatures down to 0.4 K that reveal GMR. We argue that the large MR value can be explained in terms of field-induced pseudogaps on a Fermi surface. These results are relevant to the understanding of the Fermi-surface topology of quasicrystal approximants.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Importance of nonuniform Brillouin zone sampling for ab initio Bethe-Salpeter equation calculations of exciton binding energies in crystalline solids

Excitons are prevalent in semiconductors and insulators, and their binding energies are critical for optoelectronic applications. The state-of-the-art method for first-principles calculations of excitons in extended systems is the ab initio GW-Bethe-Salpeter equation (BSE) approach, which can require a fine sampling of reciprocal space to accurately resolve solid-state exciton properties. Here, in this study, we show, for a range of semiconductors and insulators, that the commonly employed approach of uniformly sampling the Brillouin zone can lead to underconverged exciton binding energies, as impractical grid sizes are required to achieve adequate convergence. We further show that nonuniform sampling of the Brillouin zone, focused on the region of reciprocal space where the exciton wave function resides, enables efficient rapid numerical convergence of exciton binding energies at a given level of theory. We propose a well-defined convergence procedure, which can be carried out at relatively low computational cost and which in some cases leads to a correction of previous best theoretical estimates by almost a factor of 2, qualitatively changing the predicted exciton physics. These results call for the adoption of nonuniform sampling methods for ab initio GW-BSE calculations and for revisiting previously computed values for exciton binding energies of many systems.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Ion irradiation induced crystalline disorder accelerates interfacial phonon conversion and reduces thermal boundary resistance

Traditional understanding of the thermal boundary resistance (TBR) across solid-solid interfaces posits that the vibrational densities of states overlap between materials dictates interfacial energy transport, with phonon scattering occurring at the interface. Using atomistic simulations, we show a mechanism for control of TBR; point defects near an interface can lead to both short- and midrange disorder, accelerating the conversion of vibrational energy between bulk and interfacial modes, ultimately reducing the TBR. In conclusion, we experimentally demonstrate this reduction through ion irradiation of gallium nitride and subsequently measuring the TBR across Al/GaN interfaces.

Pfeifer, Thomas W.↗
DOE OSTI2024

Enhancement of the Curie temperature in single-crystalline ferromagnetic LaCrGe 3 by electron irradiation-induced disorder

LaCrGe 3 has attracted attention as a potential candidate for studies of quantum phase transitions in a ferromagnetic material. The application of pressure avoids a quantum critical point by developing a new magnetic phase. It was suggested that the disorder may provide an alternative route to a quantum critical point. We used low-temperature 2.5 MeV electron irradiation to induce relatively small amounts of pointlike disorder in single crystals of LaCrGe 3 . Irradiation leads to an increase of the resistivity at all temperatures with some deviation from the Matthiessen rule. Hall effect measurements show that electron irradiation does not cause any detectable change in the carrier density. Unexpectedly, the Curie temperature, T FM , increases with the increase of disorder from approximately 90 K in pristine samples up to nearly 100 K in the heavily irradiated sample, with a tendency towards saturation at higher doses. This effect is observed both in resistivity and magnetization measurements. Although the mechanism of this effect is not entirely clear, we conclude that it cannot be caused by effective “doping” or “pressure” due to electron irradiation. Finally, we suggest that disorder-induced broadening of a sharp peak in the density of states, D⁡(E), situated at E p = E F – 0.25 eV below the Fermi energy, E F , causes an increase in D⁡(E F ), leading to an enhancement of T FM in this itinerant ferromagnet.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Large magnetoresistance and first-order phase transition in antiferromagnetic single-crystalline EuAg 4 Sb 2

Here, we present the results of a thorough investigation of the physical properties of EuAg 4 Sb 2 single crystals using magnetization, heat capacity, and electrical resistivity measurements. High-quality single crystals, which crystallize in a trigonal structure with space group 𝑅⁢$\bar{3}$𝑚, were grown using a conventional flux method. Temperature-dependent magnetization measurements along different crystallographic orientations confirm two antiferromagnetic phase transitions around 𝑇 𝑁⁢1 = 10.5K and 𝑇 𝑁⁢2 = 7.5K. Isothermal magnetization data exhibit several metamagnetic transitions below these transition temperatures. Antiferromagnetic phase transitions in EuAg 4 Sb 2 are further confirmed by two sharp peaks in the temperature-dependent heat capacity data at 𝑇 𝑁⁢1 and 𝑇 𝑁⁢2 , which shift to lower temperature in the presence of an external magnetic field. Our systematic heat capacity measurements utilizing a long-pulse and single-slope analysis technique allow us to detect a first-order phase transition in EuAg 4 Sb 2 at 7.5 K. The temperature-dependent electrical resistivity data also manifest two features associated with magnetic order. The magnetoresistance exhibits a broad hump due to a field-induced metamagnetic transition. Remarkably, the magnetoresistance keeps increasing without showing any tendency to saturate as the applied magnetic field increases, and it reaches ∼20 000% at 1.6 K and 60 T. At high magnetic fields, several magnetic quantum oscillations are observed, indicating a complex Fermi surface. A large negative magnetoresistance of about −55% is also observed near 𝑇 𝑁⁢1 . Moreover, the 𝐻−𝑇 phase diagram constructed using magnetization, heat capacity, and magnetotransport data indicates complex magnetic behavior in EuAg 4 Sb 2 .

36 MATERIALS SCIENCE↗
DOE OSTI2020

Particlelike Phonon Propagation Dominates Ultralow Lattice Thermal Conductivity in Crystalline Tl 3 VSe 4

We explore the microscopic mechanisms of ultralow lattice thermal conductivity ($\kappa_{l}$) in Tl$_{3}$VSe$_{4}$~by combining a first-principles density-functional theory (DFT) based framework of anharmonic lattice dynamics with the Peierls-Boltzmann transport equation (PBTE) for phonons. We include contributions of the three- and four-phonon scattering processes to the phonon lifetimes as well as the temperature-dependent anharmonic renormalization of phonon energies arising from an unusually strong quartic anharmonicity in Tl$_{3}$VSe$_{4}$. In contrast to a recent report by Mukhopadhyay~\etal [\textcolor{blue}{Science 360, 1455 (2018)}] which suggested that a significant contribution to $\kappa_{l}$ arises from random walks among uncorrelated oscillators, we show that particle-like propagation of phonon excitations can successfully explain the experimentally observed ultralow $\kappa_{l}$. Our results are also supported by explicit calculations of the off-diagonal terms of the heat-current operator, which are found to be small and indicate that wave-like tunneling of heat-carrying vibrations is of minor importance. Our results (i) resolve the discrepancy between the theoretical and experimental $\kappa_{l}$, (ii) offer new insights into the minimum $\kappa_{l}$ achievable in \TlVSe, and (iii) highlight the importance of high-order anharmonicity in low-$\kappa_{l}$ systems. The methodology demonstrated here may be used to resolve the discrepancies between the experimentally measured and the theoretically calculated $\kappa_{l}$ in skutterides and perovskites, as well as to understand the glasslike $\kappa_{l}$ in complex crystals with strong anharmonicity, leading towards the goal of rational design of new materials.

36 MATERIALS SCIENCE↗