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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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Infrared-active phonon modes in single-crystal thorium dioxide and uranium dioxide

The infrared-active phonon modes, in single-crystal samples of thorium dioxide (ThO 2 ) and uranium dioxide (UO 2 ), were investigated using spectroscopic ellipsometry and compared with density functional theory. Both ThO 2 and UO 2 are found to have one infrared-active phonon mode pair [consisting of one transverse optic (TO) and one associated longitudinal optic (LO) mode], which is responsible for the dominant features in the ellipsometric data. Furthermore, at room temperature, our results for the mode pair’s resonant frequencies and broadening parameters are comparable with previous reflectance spectroscopy characterizations and density functional theory predictions. For ThO 2 , our ellipsometry and density function theory results both show that the LO mode broadening parameter is larger than the TO mode broadening. This signifies mode anharmonicity, which can be attributed to the intrinsic phonon–phonon interaction. In addition to the main mode pair, a broad low-amplitude impurity-like vibrational mode pair is detected within the reststrahlen band for both ThO 2 and UO 2 . Elevated temperature measurements were performed for ThO 2 in order to study the mechanisms by which the phonon parameters evolve with increased heat. The observed change in the TO resonant frequency is in excellent agreement with previous density functional calculations, which only consider volume expansion of the crystal lattice. This suggests that the temperature-dependent change in the TO frequency is primarily due to volume expansion. The change in the main mode pair’s broadening parameters is nearly linear within the temperature range of this study, which indicates the intrinsic anharmonic scattering (via cubic anharmonicities) as the main decay mechanism.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Connectivity-Dependent Exciton–Phonon Coupling in Cesium Bismuth Halide Quantum Dots

Metal halide octahedra form the fundamental functional building blocks of metal halide perovskites, dictating their structures, optical properties, electronic structures, and dynamics. Here, in this study, we show that the connectivity of bismuth halide octahedra in Cs 3 Bi 2 Br 9 and Cs 3 Bi 2 I 9 quantum dots (QDs) changes with different halide elements. We use first-principles calculations to reveal the key role of the connectivity of bismuth halide octahedra on the wave function symmetry, Huang-Rhys factor, and exciton-phonon interaction strength. Following QD synthesis via a ligand-mediated transport method, the effect of connectivity is verified with transient absorption spectroscopy, where we contrast Cs 3 Bi 2 Br 9 and Cs 3 Bi 2 I 9 QD exciton dynamics. In photoexcited Cs 3 Bi 2 I 9 QDs, phonons related to the vibrational motions of face-sharing [BiI 6 ] 3- bioctahedra couple strongly to the electronic state and drive rapid carrier relaxation. Equivalent signals are not observed for photoexcited Cs 3 Bi 2 Br 9 QDs, implying a lack of phonon involvement in band-edge absorption and subsequent exciton relaxation. Our findings suggest that structural engineering can effectively tune the exciton-phonon coupling and therefore influence exciton relaxation and recombination in perovskite nanomaterials.

TDDFT↗
DOE OSTI2024

Prediction of ambient pressure superconductivity in cubic ternary hydrides with MH 6 octahedra

Exploring high-temperature superconducting (high-T c ) material at ambient pressure holds immense significance for physics, chemistry, and materials science. In this study, we perform a high-throughput screening of strong electron-phonon interactions in X 2 MH 6 compounds (X = Li, Na, Mg, Al, K, Ca, Ga, Rb, Sr, and In; M are 3d, 4d, and 5d transition metals). These compounds have a cubic structure featuring an MH 6 octahedron motif. Our screening calculations suggest that 26 compounds exhibit dynamic stability and strong electron-phonon coupling. Among them, Mg 2 RhH 6 , Mg 2 IrH 6 , Al 2 MnH 6 , and Li 2 CuH 6 show promising energetic stability and T c of more than 50 K at ambient pressure. This study underscores promising high-T c compounds at ambient pressure with distinctive MH 6 motifs.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Carbon trimer as a 2 eV single-photon emitter candidate in hexagonal boron nitride: A first-principles study

The generation of single-photon emitters in hexagonal boron nitride around 2 eV emission is experimentally well recognized; however, the atomic nature of these emitters is unknown. In this Letter, we use first-principles calculations to demonstrate that carbon trimer substitutional defect (C 2 C N ) is a possible source of 2 eV single-photon emitter in hBN. Here, we showcase the calculations of a complete set of static and dynamical properties related to quantum defects, including exciton-defect couplings and electron-phonon interactions, from both density functional theory and many-body perturbation theory. In particular, we show that it is critical to consider both radiative and nonradiative processes when comparing with experimental lifetime for known 2 eV emitters. We find that C 2 C N has several key physical properties matching the ones of experimentally observed single-photon emitters. These include the zero-phonon line (2.13 eV), Huang-Rhys factor (1.35), photoluminescence lifetime (2.19 ns), phonon-sideband energy (180 meV), and photoluminescence spectrum. The identification of defect candidates for 2 eV emission paves the way for controllable single-photon emission generation.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Comparison of GW band structure to semiempirical approach for an FeSe monolayer

We present the G 0 W 0 band structure, core levels, and deformation potential of monolayer FeSe in the paramagnetic phase based on a starting mean field of the Kohn-Sham density functional theory (DFT) with the Perdew, Burke, and Ernzerhof functional. We find the GW correction increases the bandwidth of the states forming the M pocket near the Fermi energy, while leaving the Γ pocket roughly unchanged. We then compare the G 0 W 0 quasiparticle band energies with the band structure from a simple empirical + A approach, which was recently proposed to capture the renormalization of the electron-phonon interaction going beyond DFT in FeSe, when used as a starting point in density functional perturbation theory. We show that this empirical correction succeeds in approximating the GW nonlocal and dynamical self-energy in monolayer FeSe and reproduces the GW band structure near the Fermi surface, the core energy levels, and the deformation potential (electron-phonon coupling).

36 MATERIALS SCIENCE↗
DOE OSTI2020

Short-Range Order in VI 3

We present a detailed investigation of the crystal structure of VI 3 , a two-dimensional van der Waals material of interest for studies of low-dimensional magnetism. As opposed to the average crystal structure that features $R\bar{3}$ symmetry of the unit cell, our Raman scattering and X-ray atomic pair distribution function analysis supported by density functional theory calculations point to the coexistence of short-range ordered $P\bar{3}$1c and long-range ordered $R\bar{3}$phases. The highest-intensity peak, A 1g 3 , exhibits a moderate asymmetry that might be traced back to the spin–phonon interactions, as in the case of CrI 3 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Softening of a flat phonon mode in the kagome ScV 6 Sn 6

Geometrically frustrated kagome lattices are raising as novel platforms to engineer correlated topological electron flat bands that are prominent to electronic instabilities. Here, we demonstrate a phonon softening at the k z = π plane in ScV 6 Sn 6 . The low energy longitudinal phonon collapses at ~98 K and q = $\tfrac{1}{3}$ $\tfrac{1}{3}$ $\tfrac{1}{2}$ due to the electron-phonon interaction, without the emergence of long-range charge order which sets in at a different propagation vector q CDW = $\tfrac{1}{3}$ $\tfrac{1}{3}$ $\tfrac{1}{3}$. Theoretical calculations corroborate the experimental finding to indicate that the leading instability is located at $\tfrac{1}{3}$ $\tfrac{1}{3}$ $\tfrac{1}{2}$ of a rather flat mode. We relate the phonon renormalization to the orbital-resolved susceptibility of the trigonal Sn atoms and explain the approximately flat phonon dispersion. Our data report the first example of the collapse of a kagome bosonic mode and promote the 166 compounds of kagomes as primary candidates to explore correlated flat phonon-topological flat electron physics.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

Exciton polaron formation and hot-carrier relaxation in rigid Dion–Jacobson-type two-dimensional perovskites

The efficiency of two-dimensional Dion–Jacobson-type materials relies on the complex interplay between electronic and lattice dynamics; however, questions remain about the functional role of exciton–phonon interactions. Here we establish the robust polaronic nature of the excitons in these materials at room temperature by combining ultrafast spectroscopy and electronic structure calculations. We show that polaronic distortion is associated with low-frequency (30–60 cm –1 ) lead iodide octahedral lattice motions. More importantly, we discover how targeted ligand modification of this two-dimensional perovskite structure manipulates exciton–phonon coupling, exciton polaron population and carrier cooling. At high excitation density, stronger exciton–phonon coupling increases the hot-carrier lifetime, forming a hot-phonon bottleneck. Furthermore, our study provides detailed insight into the exciton–phonon coupling and its role in carrier cooling in two-dimensional perovskites relevant for developing emerging hybrid semiconductor materials with tailored properties.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Prediction of superconductivity in metallic boron–carbon compounds from 0 to 100 GPa by high-throughput screening

Boron–carbon compounds have been shown to have feasible superconductivity. In our earlier paper [Zheng et al., Phys. Rev. B, 2023, 107, 014508], we identified a new conventional superconductor of LiB 3 C at 100 GPa. Here, we aim to extend the investigation of possible superconductivity in this structural framework by replacing Li atoms with 27 different cations from periods 3, 4, and 5 under pressures ranging from 0 to 100 GPa. Using the high-throughput screening method of zone-center electron–phonon interaction, we found that ternary compounds like CaB 3 C, SrB 3 C, TiB 3 C, and VB 3 C are promising candidates for superconductivity. The consecutive calculations using the full Brillouin zone confirm that they have a T c of <31 K at moderate pressures. In conclusion, our study demonstrates that fast screening of superconductivity by calculating zone-center electron–phonon coupling strength is an effective strategy for high-throughput identification of new superconductors.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Pressure induced Invar effect in Fe 55 ⁢Ni 45 : An experimental study with nuclear resonant scattering

Pressure-dependent synchrotron x-ray diffraction (XRD), nuclear resonant inelastic x-ray scattering (NRIXS), and nuclear forward scattering (NFS) measurements were made on 57 Fe 55 Ni 45 . XRD measurements were at 298 and 392 K at pressures up to 20 GPa, confirming a pressure-induced Invar effect between 7 and 13 GPa. A decrease of the 57 Fe magnetic moment was found in NFS measurements under pressure, showing an increase in magnetic entropy. The 57 Fe phonon density of states (DOS) was obtained from NRIXS measurements. The low thermal expansion in the high-pressure Invar region originates from a competition between the thermal expansion from spins and phonons as calculated from Maxwell relations. Finally, the longitudinal phonon modes changed their pressure dependence near the Curie transition, which is evidence for a spin-phonon interaction.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Magnetotransport in semiconductors and two-dimensional materials from first principles

Here we demonstrate a first-principles method to study magnetotransport in materials by solving the Boltzmann transport equation (BTE) in the presence of an external magnetic field. Our approach employs ab initio electron-phonon interactions and takes spin-orbit coupling into account. We apply our method to various semiconductors (Si and GaAs) and two-dimensional (2D) materials (graphene) as representative case studies. The magnetoresistance, Hall mobility, and Hall factor in Si and GaAs are in very good agreement with experiments. In graphene, our method predicts a large magnetoresistance, consistent with experiments. Analysis of the steady-state electron occupations in graphene shows the dominant role of optical phonon scattering and the breaking of the relaxation time approximation. Our paper provides a detailed understanding of the microscopic mechanisms governing magnetotransport coefficients, establishing the BTE in a magnetic field as a broadly applicable first-principles tool to investigate transport in semiconductors and 2D materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Interfacial electron-phonon coupling and quantum confinement in ultrathin Yb films on graphite

Interfacial electron-phonon coupling in ultrathin films has attracted much interest recently. Here, by combining angle-resolved photoemission spectroscopy and scanning tunneling microscopy, we report quantized electronic states and strong interfacial electron-phonon coupling in ultrathin Yb films on graphite. We observed clear kinks in the energy-momentum dispersion of quantum well states, and the kink positions agree well with the energies of optical phonons of graphite. The extracted coupling strength λ is largest for the thinnest film with a preferred (“magic”) thickness of four monolayers and exhibits a strong band dependence, which can be qualitatively accounted for by a simple model. The interfacial electron-phonon coupling also gives rise to characteristic steplike structures in the $dI/dV$ spectra, implying dominant coupling with the phonons with zero in-plane momentum. A Lifshitz transition occurs at higher coverage, where quantum well states derived mainly from 5d electrons dominate near the Fermi level and possess large effective mass (up to ~ 19 m e ). Here our results highlight the potentially important role of interfacial electron-phonon interaction for ultrathin films and provide spectroscopic insight to understand this cross-interface fermion-boson interaction.

36 MATERIALS SCIENCE↗
DOE OSTI2025

A-Cation-Dependent Structure–Optical Property Relationships of Halide Perovskite Heterostructures with Complex Interfaces

Halide perovskite heterostructures offer promising interfacial interactions for energy conversion, yet challenges in synthesizing structurally well-defined systems limit detailed investigations into structure–property relationships. In this article, we report the synthesis of compositionally controlled 3D/3D and 3D/2D halide perovskite heterostructures using evaporation crystallization-polymer pen lithography (EC-PPL) and a single-particle analysis of their properties. By systematically varying A-site cation combinations and crystal dimensions, we show that heterointerfaces induce local lattice distortions that modulate vibrational dynamics and electron-phonon coupling. These interfacial effects result in significantly extended carrier lifetimes compared to compositionally similar pure phases. Raman spectroscopy, temperature-dependent photoluminescence, and power-dependent emission analysis reveal that localized structural modulations at the interface govern exciton-phonon interactions. These effects are magnified in smaller crystals due to increased interfacial contributions. Our findings highlight the critical role of interface-driven lattice control in tuning the optoelectronic properties of halide perovskites and provide design principles for engineering heterostructures in next-generation optoelectronic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Quantifying Temperature Dependence of Electronic Band Gaps and Optical Properties in SnO 2 and SnO via First-Principles Simulations

Tin metal oxides SnO x ( x = 1, 2) have gained interest as gas-sensing materials. For their applications as high-temperature sensors, however, a better understanding of their temperature dependence sensing responses is needed. Here, we comparatively quantify the temperature-dependent electronic band gaps and optical properties of SnO 2 and SnO using first-principles calculations. Without considering the temperature effect, SnO 2 and SnO are predicted to have direct and indirect band gaps of 2.18 and 1.66 eV, respectively, at the PBE + U -GGA level that we employed. The temperature effect on the electronic and optical properties is captured by taking account of the electron–phonon interaction. Band gap renormalization with temperature is calculated via the Allen–Heine–Cardona theory. For both oxides, we find a monotonic decrease in the electronic band gap such that renormalization at zero point (0 K) is ~–0.17 and ~–0.52 eV at 1000 K. These results are also analyzed by employing an analytical equation that helps characterize the band gap shift with temperature. In addition, the optical properties at finite temperatures are simulated using the frozen-phonon method that combines electron–phonon coupling with the momentum matrix. As temperature increases, the optical property spectra are smoothed because of the smearing effect, which diminishes optical constants at shorter wavelengths. Our results are of interest for high-temperature functional materials in applications of optical detection.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Interfacial Electron-Phonon Coupling Constants Extracted from Intrinsic Replica Bands in Monolayer FeSe/SrTiO 3

The observation of replica bands by angle-resolved photoemission spectroscopy has ignited interest in the study of electron-phonon coupling at low carrier densities, particularly in monolayer FeSe/SrTiO 3 , where the appearance of replica bands has motivated theoretical work suggesting that the interfacial coupling of electrons in the FeSe layer to optical phonons in the SrTiO 3 substrate might contribute to the enhanced superconducting pairing temperature. Alternatively, it has also been recently proposed that such replica bands might instead originate from extrinsic final state losses associated with the photoemission process. Here, we perform a quantitative examination of replica bands in monolayer FeSe/SrTiO 3 , where we are able to conclusively demonstrate that the replica bands are indeed signatures of intrinsic electron-boson coupling, and not associated with final state effects. A detailed analysis of the energy splittings and relative peak intensities between the higher-order replicas, as well as other self-energy effects, allows us to determine that the interfacial electron-phonon coupling in the system corresponds to a value of λ=0.19±0.02, providing valuable insights into the enhancement of superconductivity in monolayer FeSe/SrTiO 3 . The methodology employed here can also serve as a new and general approach for making more rigorous and quantitative comparisons to theoretical calculations of electron-phonon interactions and coupling constants.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Lineshape characterization of excitons in monolayer WS 2 by two-dimensional electronic spectroscopy

The optical properties of monolayer transition metal dichalcogenides (TMDCs), an important family of two-dimensional (2D) semiconductors for optoelectronic applications, are dominated by two excitons A (XA) and B (XB) located at K/K's valleys. The lineshape of the excitons is an indicator of the interaction of the excitons with other particles and also largely determines the performance of TMDC-based optoelectronic devices. In this work, we apply 2D electronic spectroscopy (2DES), which enables separation of the intrinsic homogeneous linewidth and the extrinsic inhomogeneous linewidth, to dissect the lineshape of XA in monolayer WS 2 . With a home-built broadband optical parametric amplifier, the 2D spectra give the exciton linewidth values for extensive ranges of excitation densities and temperatures, reflecting inter-exciton and exciton–phonon interactions. Meanwhile, the time-domain evolution of the lineshape reveals a similar rate of spectral diffusion to that in quantum wells (QWs) based on III–V semiconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Charge density waves on a half-filled decorated honeycomb lattice

Tight binding models like the Hubbard Hamiltonian are most often explored in the context of uniform intersite hopping $\textit{t}$. The electron-electron interactions, if sufficiently large compared to this translationally invariant $\textit{t}$, can give rise to ordered magnetic phases and Mott insulator transitions, especially at commensurate filling. The more complex situation of nonuniform $\textit{t}$ has been studied within a number of situations, perhaps most prominently in multiband geometries where there is a natural distinction of hopping between orbitals of different degree of overlap. In this paper we explore related questions arising from the interplay of multiple kinetic energy scales and electron-phonon interactions. Specifically, we use determinant quantum Monte Carlo (DQMC) to solve the half-filled Holstein Hamiltonian on a “decorated honeycomb lattice,” consisting of hexagons with internal hopping $\textit{t}$ coupled together by $\textit{t'}$. This modulation of the hopping introduces a gap in the Dirac spectrum and affects the nature of the topological phases. Here, we determine the range of $\textit{t/t'}$ values which support a charge density wave phase about the Dirac point of uniform hopping $\textit{t = t'}$, as well as the critical transition temperature $T_c$. The QMC simulations are compared with the results of mean field theory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Violation of the T –1 Relationship in the Lattice Thermal Conductivity of Mg 3 Sb 2 with Locally Asymmetric Vibrations

Most crystalline materials follow the guidelines of T –1 temperature-dependent lattice thermal conductivity (κ L ) at elevated temperatures. Here, we observe a weak temperature dependence of κ L in Mg 3 Sb 2 , T –0.48 from theory and T –0.57 from measurements, based on a comprehensive study combining ab initio molecular dynamics calculations and experimental measurements on single crystal Mg 3 Sb 2 . These results can be understood in terms of the so-called “phonon renormalization” effects due to the strong temperature dependence of the interatomic force constants (IFCs). The increasing temperature leads to the frequency upshifting for those low-frequency phonons dominating heat transport, and more importantly, the phonon-phonon interactions are weakened. In-depth analysis reveals that the phenomenon is closely related to the temperature-induced asymmetric movements of Mg atoms within MgSb 4 tetrahedron. With increasing temperature, these Mg atoms tend to locate at the areas with relatively low force in the force profile, leading to reduced effective 3 rd -order IFCs. The locally asymmetrical atomic movements at elevated temperatures can be further treated as an indicator of temperature-induced variations of IFCs and thus relatively strong phonon renormalization. The present work sheds light on the fundamental origins of anomalous temperature dependence of κ L in thermoelectrics.

36 MATERIALS SCIENCE↗