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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 OSTI2021

Electron–Phonon and Spin–Lattice Coupling in Atomically Thin Layers of MnBi 2 Te 4

MnBi 2 Te 4 represents a new class of magnetic topological insulators in which novel quantum phases emerge at temperatures higher than those found in magnetically doped thin films. Here in this paper, we investigate how couplings between electron, spin, and lattice are manifested in the phonon spectra of few-septuple-layer thick MnBi 2 Te 4 . After categorizing phonon modes by their symmetries, we study the systematic changes in frequency, line width, and line shape of a spectrally isolated A1g mode. The electron–phonon coupling increases in thinner flakes as manifested in a broader phonon line width, which is likely due to changes of the electron density of states. In 4- and 5-septuple thick samples, the onset of magnetic order below the Néel temperature is concurrent with a transition to an insulating state. We observe signatures of a reduced electron–phonon scattering across this transition as reflected in the reduced Fano parameter. Finally, spin–lattice coupling is measured and modeled from temperature-dependent phonon frequency.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Vacancy defect control of colossal thermopower in FeSb2

Abstract Iron diantimonide is a material with the highest known thermoelectric power. By combining scanning transmission electron microscopic study with electronic transport neutron, X-ray scattering, and first principle calculation, we identify atomic defects that control colossal thermopower magnitude and nanoprecipitate clusters with Sb vacancy ordering, which induce additional phonon scattering and substantially reduce thermal conductivity. Defects are found to cause rather weak but important monoclinic distortion of the unit cell P n n m → P m . The absence of Sb along [010] for high defect concentration forms conducting path due to Fe d orbital overlap. The connection between atomic defect anisotropy and colossal thermopower in FeSb 2 paves the way for the understanding and tailoring of giant thermopower in related materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Lone-Electron-Pair Micelles Strengthen Bond Anharmonicity in MnPb16Sb14S38 Complex Sulfosalt Leading to Ultra-Low Thermal Conductivity

Designing crystalline solids in which intrinsic extremely low lattice thermal conductivity mainly arises from its unique bonding nature rather than structure complexity and/or atomic disorder could promote thermal energy manipulation and utilization for applications ranging from thermoelectric energy conversion to thermal barrier coatings. Here, we report an extremely low lattice thermal conductivity of ~0.34 W m-1 K-1 at 300 K in the new complex sulfosalt MnPb16Sb14S38. We attribute the ultra-low lattice thermal conductivity to a synergistic combination of scattering mechanisms involving (1) strong bond anharmonicity in various structural building units, owing to the presence of stereoactive lone-electron-pair (LEP) micelles, and (2) phonon scattering at the interfaces between building units of increasing size and complexity. Remarkably, low-temperature heat capacity measurement revealed a Cp value of 0.206 J g-1 K-1 at T > 300 K, which is 22% lower than the Dulong-Petit value (0.274 J g-1 K-1). Further analysis of the Cp data and sound velocity (ν = 1834 m/s) measurement yielded Debye temperature values of 161 K and 187 K, respectively. Here, the resulting Grüneisen parameter, γ = 1.65, further supports strong bond anharmonicity as the dominant mechanism responsible for the observed extremely low lattice thermal conductivity.

36 MATERIALS SCIENCE↗
NASA NTRS2000

Modeling of Optoelectronic Devices

Ultrafast modulation of semiconductor quantum well (QW) laser is of technological importance for information technology. Improvement by order(s) of magnitude in data transfer rate is possible as terahertz (THz) radiation is available for heating the laser at picosecond time scale. Optical gain modulation in the QW is achieved via temperature modulation of electron-hole plasma (EHP). Applications include free-space THz communication, optical switching, and pulse generation. The EHP in the semiconductor QW is described with a two-band model. Semiconductor Bloch equations with many-body effects are used to derive a hydrodynamical model for the active QW region. Because of ultrafast carrier-carrier scatterings in the order of 50 fs, EHP follows quasiequilibrium Fermi-Dirac distributions and THz field interacts incoherently with it. Carrier-longitudinal optical (LO) phonon scatterings and coherent laser-EHP interaction are treated microscopically in our physical model. A set of hydrodynamical equations for plasma density, temperature, and laser envelop amplitude are derived and Runge-Kutta method is adopted for numerical simulation. A typical 8 nm GaAs/Al(0.3)Ga(0.7) As single QW at 300 K is used. Additional information is contained in the original extended abstract.

Li, Jian-Zhong↗
NASA NTRS2005

Length dependence of carbon nanotube thermal conductivity and the "problem of long waves"

We present the first calculations of finite length carbon nanotube thermal conductivity that extend from the ballistic to the diffusive regime, throughout a very wide range of lengths and temperatures. The long standing problem of vanishing scattering of the "long wavelength phonf dramatically here, making the thermal conductivity diverge as the nanotube length increases. We show that the divergence disappears if 3-phonon scattering processes are considered to second or higher order. Nevertheless, for defect free nanotubes, the thermal conductivity keeps increasing up to very large lengths (10 gm at 300 K). Defects in the nanotube are also able to remove the long wavelength divergence.

Evaluation Studies↗
DOE OSTI2022

Study of Carrier Mobilities in 4H-SiC MOSFETS Using Hall Analysis

The channel conduction in 4H-SiC metal–oxide–semiconductor field effect transistors (MOSFETs) are highly impacted by charge trapping and scattering at the interface. Even though nitridation reduces the interface trap density, scattering still plays a crucial role in increasing the channel resistance in these transistors. In this work, the dominant scattering mechanisms are distinguished for inversion layer electrons and holes using temperature and body-bias-dependent Hall measurements on nitrided lateral 4H-SiC MOSFETs. The effect of the transverse electric field (E eƒƒ ) on carrier mobility is analyzed under strong inversion condition where surface roughness scattering becomes prevalent. Power law dependencies of the electron and hole Hall mobility for surface roughness scattering are determined to be E$^{-1.8}_{eƒƒ}$ and E$^{-2.4}_{eƒƒ}$, respectively, analogous to those of silicon MOSFETs. Moreover, for n-channel MOSFETs, the effect of phonon scattering is observed at zero body bias, whereas in p-channel MOSFETs, it is observed only under negative body biases. Along with the identification of regimes governed by different scattering mechanisms, these results highlight the importance of the selection of substrate doping and of E eƒƒ in controlling the value of channel mobility in 4H-SiC MOSFETs.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Photoinduced anisotropic lattice dynamic response and domain formation in thermoelectric SnSe

Identifying and understanding the mechanisms behind strong phonon–phonon scattering in condensed matter systems is critical to maximizing the efficiency of thermoelectric devices. To date, the leading method to address this has been to meticulously survey the full phonon dispersion of the material in order to isolate modes with anomalously large linewidth and temperature-dependence. Here we combine quantitative MeV ultrafast electron diffraction (UED) analysis with Monte Carlo based dynamic diffraction simulation and first-principles calculations to directly unveil the soft, anharmonic lattice distortions of model thermoelectric material SnSe. A small single-crystal sample is photoexcited with ultrafast optical pulses and the soft, anharmonic lattice distortions are isolated using MeV-UED as those associated with long relaxation time and large displacements. We reveal that these modes have interlayer shear strain character, induced mainly by c -axis atomic displacements, resulting in domain formation in the transient state. These findings provide an innovative approach to identify mechanisms for ultralow and anisotropic thermal conductivity and a promising route to optimizing thermoelectric devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Proton Collective Quantum Tunneling Induces Anomalous Thermal Conductivity of Ice under Pressure

Proton tunneling is believed to be nonlocal in ice, but its range has been shown to be limited to only a few molecules. Here, we measured the thermal conductivity of ice under pressure up to 50 GPa and found it increases with pressure until 20 GPa but decreases at higher pressures. We attribute this nonmonotonic thermal conductivity to the collective tunneling of protons at high pressures, supported by large-scale quantum molecular dynamics simulations. The collective tunneling loops span several picoseconds in time and are as large as nanometers in space, which match the phonon periods and wavelengths, leading to strong phonon scattering at high pressures. Finally, our results show direct evidence of global quantum motion existing in high-pressure ice and provide a new perspective to understanding the coupling between phonon propagation and atomic tunneling.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Engineering Electrical Transport by Implantation‐Induced Defects in CrN Films Without Affecting Thermal Conductivity

The transport properties of CrN thin films deposited on sapphire have been tailored through structural modifications induced by cumulative argon implantation. As-grown samples experience the typical structural transition in CrN films from orthorhombic at low temperature to cubic above the Néel temperature (≈280 K) and exhibit a metallic-like conduction in both phases. With increasing implantation dose, the conduction mode shifts to a semiconductor-like behavior in both phases, albeit at different damage levels. Analysis of the results suggests that hopping conduction becomes dominant beyond a given damage threshold. The results highlight a promising correlation between defect engineering and conduction mechanisms, offering valuable insights into the versatile electrical properties of CrN films. These implantation-induced defects scatter carriers, leading to a decrease in their mobility. As the implantation dose increases, the defect landscape evolves, modifying the density of states. However, up to a dose of 0.050 dpa, no significant influence on phonon scattering is observed. This approach demonstrates that ion implantation enables precise tuning of CrN's electrical properties without affecting thermal conductivity, offering valuable insights into defect engineering in transition metal nitrides and underscoring its potential for transport properties decorrelation.

defects↗
DOE OSTI2026

Tracking Optical Phonon Dynamics in InP Nanocrystals via Transient Absorption and Femtosecond Stimulated Raman Spectroscopy

Semiconductor nanocrystals (NCs) offer actualized and prospective utility in optoelectronic technologies, yet key aspects of their thermal and vibrational behaviors remain unresolved. Compared with bulk crystals, heat dissipation involving NCs can differ substantially owing to pervasiveness of interfacial scattering, phonon confinement, and relaxed phonon momentum selection rules, as well as influences of ligands. Here, in addition to transient absorption, we use femtosecond stimulated Raman spectroscopy to track nonequilibrium optical-phonon dynamics in InP NCs capped with myristic acid ligands and compare the same particles coated with a ZnS shell. We identify distinct phonon decay pathways in ligand-capped versus ZnS-shelled NCs. Increasing the excitation density and introducing the ZnS shell measurably modify optical-phonon lifetimes, whereas phonon formation times remain largely unchanged. In addition, we observe phonon mode softening in ligand-capped nanocrystals, consistent with lattice expansion. Together, these results demonstrate that core–shell structures can strongly govern nanocrystal thermal dissipation pathways and should be considered a key design parameter for optoelectronic operation.

dissipation↗
DOE OSTI2021

$\mathrm{Perturbo}$: A software package for ab initio electron–phonon interactions, charge transport and ultrafast dynamics

We report Perturbo is a software package for first-principles calculations of charge transport and ultrafast carrier dynamics in materials. The current version focuses on electron–phonon interactions and can compute phonon-limited transport properties such as the conductivity, carrier mobility and Seebeck coefficient. It can also simulate the ultrafast nonequilibrium electron dynamics in the presence of electron–phonon scattering. Perturbo uses results from density functional theory and density functional perturbation theory calculations as input, and employs Wannier interpolation to reduce the computational cost. It supports norm-conserving and ultrasoft pseudopotentials, spin–orbit coupling, and polar electron–phonon interactions for bulk and 2D materials. Hybrid MPI plus OpenMP parallelization is implemented to enable efficient calculations on large systems (up to at least 50 atoms) using high-performance computing. Taken together, Perturbo provides efficient and broadly applicable ab initio tools to investigate electron–phonon interactions and carrier dynamics quantitatively in metals, semiconductors, insulators, and 2D materials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Phonon softening near topological phase transitions

Topological phase transitions occur when the electronic bands change their topological properties, typically featuring the closing of the band gap. While the influence of topological phase transitions on electronic and optical properties has been extensively studied, its implication on phononic properties and thermal transport remains unexplored. In this paper, we use first-principles simulations to show that certain phonon modes are significantly softened near topological phase transitions, leading to increased phonon-phonon scattering and reduced lattice thermal conductivity. We demonstrate this effect using two model systems: pressure induced topological phase transition in ZrTe 5 and chemical composition induced topological phase transition in Hg 1-x Cd x Te. We attribute the phonon softening to emergent Kohn anomalies associated with the closing of the band gap. Our paper reveals the strong connection between electronic band structures and lattice instabilities, and opens up a potential direction towards controlling heat conduction in solids.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Dynamics of nonequilibrium magnons in gapped Heisenberg antiferromagnets

Nonequilibrium dynamics in spin systems is a topic currently under intense investigation as it provides fundamental insights into thermalization, universality, and exotic transport phenomena. While most of the studies have been focused on ideal closed quantum many-body systems such as ultracold atomic quantum gases and one-dimensional spin chains, driven-dissipative Bose gases in steady states away from equilibrium in classical systems also lead to intriguing nonequilibrium physics. Here, in this work, we theoretically investigate out-of-equilibrium dynamics of magnons in a gapped Heisenberg quantum antiferromagnet based on Boltzmann transport theory. We show that, by treating scattering terms beyond the relaxation-time approximation in the Boltzmann transport equation, energy and particle number conservation mandate that nonequilibrium magnons cannot relax to equilibrium, but decay to other nonequilibrium stationary states. The only decay channel for these stationary states back to equilibrium is through the nonconserving interactions (i.e., changing particle number and/or energy within the magnon system) such as boundary or magnon-phonon scattering. At low temperatures, these nonconserving interactions are much slower processes than intrinsic magnon-magnon interaction in a gapped spin system. Using magnon-phonon interaction as a quintessential type of nonconserving interaction, we then propose that nonequilibrium steady states of magnons can be maintained and tailored using periodic driving at frequencies faster than relaxation due to phonon interactions. These findings reveal a class of classical material systems that are suitable platforms to study nonequilibrium statistical physics and macroscopic phenomena such as classical Bose-Einstein condensation of quasiparticles and magnon supercurrents that are relevant for spintronic applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS2020

Desorption Kinetics of O and CO from Graphitic Carbon Surfaces

The desorption of O/CO from graphitic carbon surfaces is investigated using a one-dimensional model describing the adsorbate interactions with the surface phonon bath. The kinetics of desorption are described through the solution of a master equation for the time-dependent population of the adsorbate in an oscillator state, which is modified through thermal fluctuations at the surface. The interaction of the adsorbate with the surface phonons is explicitly captured by using the computed phonon density Of states (PDOS) of the surface. The coupling of the adsorbate with the phonon bath results in the transition of the adsorbate up and down a vibrational ladder. The adsorbate-surface interaction is represented in the model using a Morse potential, which allows for the desorption process to be directly modeled as a transition from bound to free (continuum) state. The PDOS is a property of the material and the lattice; and is highly sensitive to the presence of defects. The effect of etch pits along with random surface defects on the PDOS is considered in the present work. The presence of defects causes a redshift and broadening of the PDOS, which in turn changes the phonon frequency modes available for adsorbate coupling at the surface. Using the realistic PDOS distributions, the phonon-induced desorption (PID) model was used to compute the transition and desorption rates for both pristine and defective systems. Mathissen’s rule is used to compute the phonon relaxation time for pristine and defective systems based on the phonon scattering times for each of the different scattering processes. First, the desorption rates of the pristine system is fitted against the experimental values to obtain the Morse potential parameters for each of the observed adatoms. These Morse potential parameters are used along with the defective PDOS and phonon relaxation time to compute the desorption rates for the defective system. The defective system rates (both transition and desorption) were consistently lower in comparison with the pristine system. The difference between the transition rates is more significant at lower initial states due to higher energy spacing between the levels. In the case of the desorption rates, the difference between the defective and pristine system is more significant at higher temperatures. The desorption rates for each of the system shows an order of magnitude decrease with the strongly bound systems exhibiting the greatest reduction in the desorption rates.

Swaminathan-Gopalan, Krishnan↗
DOE OSTI2022

High thermal conductivity in wafer-scale cubic silicon carbide crystals

High thermal conductivity electronic materials are critical components for high-performance electronic and photonic devices as both active functional materials and thermal management materials. We report an isotropic high thermal conductivity exceeding 500 W m –1 K –1 at room temperature in high-quality wafer-scale cubic silicon carbide (3C-SiC) crystals, which is the second highest among large crystals (only surpassed by diamond). Furthermore, the corresponding 3C-SiC thin films are found to have record-high in-plane and cross-plane thermal conductivity, even higher than diamond thin films with equivalent thicknesses. Our results resolve a long-standing puzzle that the literature values of thermal conductivity for 3C-SiC are lower than the structurally more complex 6H-SiC. We show that the observed high thermal conductivity in this work arises from the high purity and high crystal quality of 3C-SiC crystals which avoids the exceptionally strong defect-phonon scatterings. Moreover, 3C-SiC is a SiC polytype which can be epitaxially grown on Si. We show that the measured 3C-SiC-Si thermal boundary conductance is among the highest for semiconductor interfaces. These findings provide insights for fundamental phonon transport mechanisms, and suggest that 3C-SiC is an excellent wide-bandgap semiconductor for applications of next-generation power electronics as both active components and substrates.

42 ENGINEERING↗
DOE OSTI2024

High-Temperature Observation of Intralayer, Interlayer, and Rydberg Excitons in Bulk Van Der Waals Alloy Single Crystals

Transition metal dichalcogenides exhibit remarkable optical properties due to the diverse number of strongly bound excitons, which can be fine-tuned by alloying. Despite a flurry of research activity in characterizing these excitons, a comprehensive and profound understanding of their behavior with temperature is lacking. Here, we report the rich spectrum of excitonic features within bulk van der Waals alloy Mo0.5 W0.5 S2 and Mo0.5 W0.5 Se2 single crystals through temperature-dependent reflectance spectroscopy and first-principles calculations. We observed Rydberg excitons and interlayer excitons in both the single crystals. Notably, we provide the first experimental evidence of highly energetic A' and B' excitons in Mo0.5 W0.5 S2 at room temperature. The strong carrier-phonon scattering significantly broadens the A', B', and interlayer excitons at room temperature in bulk Mo0.5 W0.5 S2 single crystal compared to its selenide. Our findings, supported by density functional theory and Bethe-Salpeter equation calculations, signify the crucial role of carrier-phonon interactions. These results open pathways for next-generation optoelectronic devices and quantum technologies operating at high temperature.

excitons↗
DOE OSTI2020

Molybdenum Contacts to MoS 2 Field‐Effect Transistors: Schottky Barrier Extraction, Electrical Transport, and Low‐Frequency Noise

MoS 2 has great interest for nanoscale electronic devices, including transistors and sensors. Defect‐free structure, clean interface with the gate dielectric, and metals with suitable metal–semiconductor junctions are key for reliable devices. Molybdenum is shown as an excellent electrode for MoS 2 field‐effect transistors (FETs), but the correlation among device current, field‐effect mobility, and the low‐frequency noise (LFN) with number of atomic layers is not studied. Furthermore, the impact of Mo/MoS 2 Schottky barrier on electronic injection and scattering in the device needs to be understood. Herein, by studying the FETs with channels of different numbers of atomic‐layer thicknesses, it is shown that a molybdenum metal contact to MoS 2 provides reliable current injection with low Schottky barrier height, low contact resistance, low electron–phonon scattering, and negligible impurity scattering. The mobility and LFN are optimal at a channel thickness of ≈12 atomic layers, which is consistent with the “mobility maximum window” in MoS 2 transistors with prior studies using other contact metals.

Kwon, Jiseok↗
DOE OSTI2022

Carrier Mobility Modulation in Cu 2 Se Composites Using Coherent Cu 4 TiSe 4 Inclusions Leads to Enhanced Thermoelectric Performance

Carrier transport engineering in bulk semiconductors using inclusion phases often results in the deterioration of carrier mobility (μ) owing to enhanced carrier scattering at phase boundaries. Here, we show by leveraging the temperature-induced structural transition between the α-Cu 2 Se and β-Cu 2 Se polymorphs that the incorporation of Cu 4 TiSe 4 inclusions within the Cu 2 Se matrix results in a gradual large drop in the carrier mobility at temperatures below 400 K (α-Cu 2 Se), whereas the carrier mobility remains unchanged at higher temperatures, where the β-Cu 2 Se polymorph dominates. The sharp discrepancy in the electronic transport within the α-Cu 2 Se and β-Cu 2 Se matrices is associated with the formation of incoherent α-Cu 2 Se/Cu 4 TiSe 4 interfaces, owing to the difference in their atomic structures and lattice parameters, which results in enhanced carrier scattering. In contrast, the similarity of the Se sublattices between β-Cu 2 Se and Cu 4 TiSe 4 gives rise to coherent phase boundaries and good band alignment, which promote carrier transport across the interfaces. Interestingly, the different cation arrangements in Cu 4 TiSe 4 and β-Cu 2 Se contribute to enhanced phonon scattering at the interfaces, which leads to a reduction in the lattice thermal conductivity. The large reduction in the total thermal conductivity while preserving the high power factor of β-Cu 2 Se in the (1–x)Cu 2 Se/(x)Cu 4 TiSe 4 composites results in an improved ZT of 1.2 at 850 K, with an average ZT of 0.84 (500–850 K) for the composite with x = 0.01. Furthermore, this work highlights the importance of structural similarity between the matrix and inclusions when designing thermoelectric materials with improved energy conversion efficiency.

36 MATERIALS SCIENCE↗