Aluminum scandium nitride alloy (Al1-xScxN) is regarded as a promising material for high-performance acoustic devices used in wireless communication systems. Phonon scattering and heat conduction processes govern the energy dissipation in acoustic resonators, ultimately determining their performance quality. This work reports, for the first time, on phonon scattering processes and thermal conductivity in Al1-xScxN alloys with the Sc content (x) up to 0.26. The thermal conductivity measured presents a descending trend with increasing x. Temperature-dependent measurements show an increase in thermal conductivity as the temperature increases at temperatures below 200K, followed by a plateau at higher temperatures (T> 200K). Application of a virtual crystal phonon conduction model allows us to elucidate the effects of boundary and alloy scattering on the observed thermal conductivity behaviors. We further demonstrate that the alloy scattering is caused mainly by strain-field difference, and less by the atomic mass difference between ScN and AlN, which is in contrast to the well-studied Al1-xGaxN and SixGe1-x alloy systems where atomic mass difference dominates the alloy scattering. This work studies and provides the quantitative knowledge for phonon scattering and the thermal conductivity in Al1-xScxN, paving the way for future investigation of materials and design of acoustic devices.
Understanding microscopic heat conduction in thin films is important for nano/micro heat transfer and thermal management for advanced electronics. As the thickness of thin films is comparable to or shorter than a phonon wavelength, phonon dispersion relations and transport properties are significantly modulated, which should be taken into account for heat conduction in thin films. Although phonon confinement and depletion effects have been considered, it should be emphasized that surface-localized phonons (surface phonons) arise whose influence on heat conduction may not be negligible due to t…
Superlattices provide a great platform for studying coherent transportation of low-frequency phonons, which are the main issues in mastering the manipulation of heat conduction. Studies have shown that the dominating characteristics in the thermal conductivity of superlattice can be adjusted between wave-like and particle-like phonon properties depending on the superlattice period. However, the phonon coherence length and the phonon mean free path from Umklapp processes have not been defined in one superlattice system, and the transition from wave-like and particle-like behavior is not clear t…
Departures in phonon heat conduction from diffusion have been extensively observed in nanostructures through their thermal conductivity reduction and largely explained with classical size effects neglecting phonon's wave nature. Here, we report localization-behavior in phonon heat conduction due to multiple scattering and interference of phonon waves, observed through measurements of the thermal conductivities of GaAs/AlAs superlattices with ErAs nanodots randomly distributed at the interfaces. Near room temperature, the measured thermal conductivities increased with increasing number of SL pe…
We present a dynamical study on the nonlinear conduction behaviour in the commensurate charge-density-wave phase of the quasi-one-dimensional conductor DCNQI$_2$Cu below 75 K. We can accurately simulate magnitude and time-dependence of the measured conductivity in response to large voltage pulses by accounting for the energy exchange between the phononic and electronic subsystems by means of an electrothermal model. Our simulations reveal a distinct non-equilibrium population of optical phonon states with an average energy of E$_{ph}$ = 19 meV being half the activation energy of about $Δ$E$_a$…
Heat conduction mechanisms in superlattices could be different across different types of interfaces. Van der Waals superlattices are structures physically assembled through weak van der Waals interactions by design, and may host properties beyond the traditional limits of lattice matching and processing compatibility, offering new types of interfaces. In this work, natural van der Waals (SnS)1.17(NbS2)n superlattices are synthesized, and their thermal conductivities are measured by time-domain thermoreflectance as a function of interface density. Our results show that heat conduction of (SnS)1…
The coupling between phonon and magnon is ubiquitous in magnetic materials and plays a crucial role in many aspects of magnetic properties, most notably in spintronics. Yet, this academically and technologically interesting problem still poses a severe challenge to a general understanding of the issue in certain materials. We report that Ni3TeO6 exhibits clear evidence of significant magnon-phonon coupling in both longitudinal thermal conductivity ($κ_{xx}$) and thermal Hall coefficient ($κ_{xy}$). The Debye-Callaway model, a phenomenological description for phonon heat conduction, can explain…
Despite recent progress in the first-principles calculations and measurements of phonon mean-free-paths (MFPs), contribution of low-energy phonons to heat conduction in silicon is still inconclusive, as exemplified by the discrepancies between different first-principles calculations. Here we investigate the contribution of low-energy phonons with MFP>0.8 um by accurately measuring the cross-plane thermal conductivity of crystalline silicon films by time-domain thermoreflectance (TDTR), over a wide range of film thickness 1-10 um and temperature 100-300 K. We employ a dual-frequency TDTR approa…
Heat transfer between two surfaces separated by a nanometre gap is important for a number of applications ranging from spaced head disk systems, scanning thermal microscopy and thermal transport in aerogels. At these separation distances, near field radiative heat transfer competes with heat transfer mediated by phonons. Here we quantify the contribution of phonon assisted heat transfer between apolar solids using lattice dynamics combined with ab-initio calculations. We clearly demonstrate that phonons dominate heat transfer for subnanometre gaps. Strikingly, we conclude that even in the situ…
Phonon boundary scattering is typically treated using the Fuchs-Sondheimer theory, which assumes that phonons are thermalized to the local temperature at the boundary. However, whether such a thermalization process actually occurs and its effect on thermal transport remains unclear. Here we examine thermal transport along thin films with both thermalizing and non-thermalizing walls by solving the spectral Boltzmann transport equation (BTE) for steady state and transient transport. We find that in steady state, the thermal transport is governed by the Fuchs-Sondheimer theory and is insensitive …
In the hydrodynamic regime, phonons drift with a nonzero collective velocity under a temperature gradient, reminiscent of viscous gas and fluid flow. The study of hydrodynamic phonon transport has spanned over half a century but has been mostly limited to cryogenic temperatures (~1 K) and more recently to low-dimensional materials. Here, we identify graphite as a three-dimensional material that supports phonon hydrodynamics at significantly higher temperatures (~100 K) based on first-principles calculations. In particular, by solving the Boltzmann equation for phonon transport in graphite ribb…
The single-layer black phosphorus is characteristic for its puckered structure, which has leaded to highly anisotropy in its optical, electronic, and mechanical properties. We report, using the non-equilibrium Green's function approach and the first-principles method, that the anisotropy in the thermal conduction is very weak in the single-layer black phosphorus -- the difference between two in-plane directions is less than 4%. Our phonon calculations disclose that the two in-plane acoustic phonon branches have lower group velocities in the direction perpendicular to the pucker, as the black p…
In two-dimensional materials, structure difference induces the difference in phonon dispersions, leading to the anisotropy of in-plane thermal transport. Here, we report an exceptional case in layered PdSe2, where the bonding, force constants, and lattice constants are nearly-equal along the in-plane crystallographic axis directions. The phonon dispersions show significant differences between the Gamma-X and Gamma-Y directions, leading to the anisotropy of in-plane thermal conductivity with a ratio up to 1.8. Such anisotropy is not only unexpected in equilaterally structured (in-plane) materia…
In stark contrast with three-dimensional (3D) nanostructures, we show that boundary scattering in two-dimensional (2D) nanoribbons alone does not lead to a finite phonon mean free path. If combined with an intrinsic scattering mechanism, 2D boundary scattering does reduce the overall mean free path, however the latter does not scale proportionally to the ribbon width, unlike the well known Casimir regime occurring in 3D nanowires. We show that boundary scattering can be accounted for by a simple Mathiessen type approach for many different 3D nanowire cross sectional shapes, however this is not…
Understanding and control of thermal transport in solids at nanoscale is crucial to engineer and to enhance properties of a new generation of optoelectronic, thermoelectric, and photonic devices. In this regard, semiconductor superlattice structures provide a unique platform to study phenomena associated with phonon propagations in solids such as heat conduction. Transient X-ray diffraction can directly probe atomic motions and therefore is among the rare techniques sensitive to phonon dynamics in a condensed matter. Here, we study optically induced transient heat conduction in GaAs/AlAs super…
We investigated theoretically the phonon thermal conductivity of single layer graphene. The phonon dispersion for all polarizations and crystallographic directions in graphene lattice was obtained using the valence-force field method. The three-phonon Umklapp processes were treated exactly using an accurate phonon dispersion and Brillouin zone, and accouting for all phonon relaxation channels allowed by the momentum and energy conservation laws. The uniqueness of graphene was reflected in the two-dimensional phonon density of states and restrictions on the phonon Umklapp scattering phase-space…
A solid conducts heat through both transverse and longitudinal acoustic phonons, but a liquid employs only longitudinal vibrations. Here, we report that the crystalline solid AgCrSe2 has liquid-like thermal conduction. In this compound, Ag atoms exhibit a dynamic duality that they are exclusively involved in intense low-lying transverse acoustic phonons while they also undergo local fluctuations inherent in an order-to-disorder transition occurring at 450 K. As a consequence of this extreme disorder-phonon coupling, transverse acoustic phonons become damped as approaching the transition temper…
Selection rules are presented for electron-phonon scattering in GaN with the wurtzite crystal structure. The results are obtained for the interband scattering between the lowest conduction band ($Γ$-valley) and the second conduction band ($U$-valley). These selection rules are derived based on the original group-theoretical analysis of the crystal vibrations in GaN, which included detailed compatibility relations for all phonon modes.
We study all of the leading-order contributions to spin relaxation of \textit{conduction} electrons in silicon due to the electron-phonon interaction. Using group theory, $k\cdot p$ perturbation method and rigid-ion model, we derive an extensive set of matrix element expressions for all of the important spin-flip transitions in the multi-valley conduction band. The scattering angle has an explicit dependence on the electron wavevectors, phonon polarization, valley position and spin orientation of the electron. Comparison of the derived analytical expressions with results of empirical pseudopot…
We use ultrafast x-ray diffraction to quantify the transport of energy in laser-excited nanoscale Au/Ni bilayers. Electron transport and efficient electron-phonon coupling in Ni convert the laser-deposited energy in the conduction electrons within a few picoseconds into a strong non-equilibrium between hot Ni and cold Au phonons at the bilayer interface. Modeling of the subsequent equilibration dynamics within various two-temperature models confirms that for ultrathin Au films the thermal transport is dominated by phonons instead of conduction electrons because of the weak electron-phonon coup…
The minimization of electronics makes heat dissipation of related devices an increasing challenge. When the size of materials is smaller than the phonon mean free paths, phonons transport without internal scatterings and laws of diffusive thermal conduction fail, resulting in significant reduction in the effective thermal conductivity. This work reports, for the first time, the temperature dependent thermal conductivity of doped epitaxial 6H-SiC and monocrystalline porous 6H-SiC below room temperature probed by time-domain thermoreflectance. Strong quasi-ballistic thermal transport was observe…
Anharmonic phonon-phonon scattering serves a critical role in heat conduction in solids. Previous studies have identified many selection rules for possible phonon-phonon scattering channels imposed by phonon energy and momentum conservation conditions and crystal symmetry. However, the crystal-symmetry-based selection rules have mostly been \textit{ad hoc} so far in selected materials, and a general formalism that can summarize known selection rules and lead to new ones in any given crystal is still lacking. In this work, we apply a general formalism for symmetry-based scattering selection rul…
Zone-center optical phonon linewidth is a key parameter for infrared and Raman spectra as well as the Lorenz oscillator model. While three-phonon scattering was often assumed to be the leading contribution, in this work we find, surprisingly, that higher-order phonon scattering universally plays a significant or even dominant role over three-phonon scattering at room temperature, and more so at elevated temperatures, for a wide range of materials including diamond, Si, Ge, boron arsenide (BAs), cubic silicon carbide (3C-SiC), and $α$-quartz. This is enabled by the large fourphonon scattering p…
We investigate the spin-dependent electron-phonon scatterings of the $L$ and $Γ$ valleys and the band structure near the conduction band minima in germanium. We first construct a $16\times16$ ${\bm k}\cdot{\bm p}$ Hamiltonian in the vicinity of the $L$ point in germanium, which ensures the correctness of the band structure of the lowest three conduction bands and highest two valence bands. This Hamiltonian facilitates the analysis of the spin-related properties of the conduction electrons. We then demonstrate the phonon-induced electron scatterings of the $L$ and $Γ$ valleys, i.e., the intra-$…
Concerning with the recent experiment of time-resolved two-photon photo-emission spectral measurements on semiconductors (GaAs, InP), we theoretically study real time relaxation dynamics of macroscopically photo-excited electrons, toward the Fermi degeneracy formation in an originally vacant conduction band of these semiconductors. Very soon after the photo-excitation, the whole electrons are shown to exhibit a quite rapid relaxation, like an avalanching phenomenon, mainly due to successive multi-(optical and acoustic) phonon emission from them. Repeating this multi-phonon process, the whole e…
A discovery of the unusual thermal properties of graphene stimulated experimental, theoretical and computational research directed at understanding phonon transport and thermal conduction in two-dimensional material systems. We provide a critical review of recent results in the graphene thermal field focusing on phonon dispersion, specific heat, thermal conductivity, and comparison of different models and computational approaches. The correlation between the phonon spectrum in graphene-based materials and the heat conduction properties is analyzed in details. The effects of the atomic plane ro…
High thermal conductivity dominated by thermal phonons with mean free paths exceeding hundred nanometer in three-dimensional covalent organic framework derivatives: a molecular dynamics study
A thorough understanding of the microscopic picture of heat conduction in solids is critical to a broad range of applications, from thermal management of microelectronics to more efficient thermoelectric materials. The transport properties of phonons, the major microscopic heat carriers in semiconductors and insulators, particularly their scattering mechanisms, have been a central theme in microscale heat conduction research. In the past two decades, significant advancements have been made in computational and experimental efforts to probe phonon-phonon, phonon-impurity, and phonon-boundary sc…
We have measured the heat conduction between 0.05 K and 1.0 K of high purity silicon wafers carrying on their polished faces a variety of thin dielectric films and polycrystalline thin metallic films. Using a Monte Carlo simulation to analyze the conduction measurements, we have determined the phonon mean free path within the films, and found all of them to be much shorter even than in typical bulk amorphous solids, with no exceptions. We have also measured the internal friction of these films below 10 K and found, however, their internal friction at low temperatures strikingly close to that o…
We propose two possible quantum microscopic mechanisms for intervalley transitions just at the Franck-Condon state in GaAs. The first is the simultaneously combined coulombic elastic transitions of two electrons,from the originally photo-excited $Γ$ valley to the mutually opposite L and -L valleys in the Brillouin zone. The second is the "elastic" scattering of an electron from the $Γ$ valley to the L one by the frozen phonon, which is not newly created in the excited state, but already exists, being inherited from the starting ground state, according to the Franck-Condon principle of the phot…
Engineering of thermoelectric materials requires an understanding of thermal conduction by lattice and electronic degrees of freedom. Filled skutterudites denote a large family of materials suitable for thermoelectric applications where reduced lattice thermal conduction attributed to localized low-frequency vibrations (rattling) of filler cations inside large cages of the structure. In this work, a multi-wavelength method of exploiting X-ray dynamical diffraction in single crystals of CeFe$_4$P$_{12}$ is presented and applied to resolve the atomic amplitudes of vibrations. The results suggest…
Heat conduction in graphite has been studied for decades because of its exceptionally large thermal anisotropy. While the bulk thermal conductivities along the in-plane and cross-plane directions are well known, less understood are the microscopic properties of the thermal phonons responsible for heat conduction. In particular, recent experimental and computational works indicate that the average phonon mean free path (MFP) along the c-axis is considerably larger than that estimated by kinetic theory, but the distribution of MFPs remains unknown. Here, we report the first quantitative measurem…
We investigate the electronic and vibrational spectra of SrTiO$_3$, as well as the coupling between them, using first-principles calculations. We compute electron-phonon scattering rates for the three lowest-energy conduction bands and use Boltzmann transport theory to calculate the room-temperature mobility of SrTiO$_3$. The results agree with experiment and highlight the strong impact of longitudinal optical phonon scattering. Our analysis provides important insights into the key factors that determine room temperature mobility, such as the number of conduction bands and the nature and frequ…
We measure the temperature-dependent carrier density and resistivity of the topological surface state of thin exfoliated Bi2Se3 in the absence of bulk conduction. When the gate-tuned chemical potential is near or below the Dirac point the carrier density is strongly temperature dependent reflecting thermal activation from the nearby bulk valence band, while above the Dirac point, unipolar n-type surface conduction is observed with negligible thermal activation of bulk carriers. In this regime linear resistivity vs. temperature reflects intrinsic electron-acoustic phonon scattering. Quantitativ…
The theory of Raman scattering in half-metals by optical phonons interacting with conduction electrons is developed. We evaluate the effect of electron-phonon interactions at ferromagnetic ordering in terms of the Boltzmann equation for carriers. The chemical potential is found to decrease with temperature decreasing. Both the linewidth and frequency shift exhibit a dependence on temperature.
Solid-state precipitation can be used to tailor materials properties, ranging from ferromagnets and catalysts to mechanical strengthening and energy storage. Thermoelectric properties can be modified by precipitation to enhance phonon scattering while retaining charge-carrier transmission. Here, we uncover unconventional dual Janus-type nanoprecipitates in Mg3Sb1.5Bi0.5 formed by side-by-side Bi- and Ge-rich appendages, in contrast to separate nanoprecipitate formation. These Janus nanoprecipitates result from local co-melting of Bi and Ge during sintering, enabling an amorphous-like lattice t…
A realistic pseudopotential model is introduced to investigate the phonon-induced spin relaxation of conduction electrons in bulk silicon. We find a surprisingly subtle interference of the Elliott and Yafet processes affecting the spin relaxation over a wide temperature range, suppressing the significance of the intravalley spin-flip scattering, previously considered dominant, above roughly 120 K. The calculated spin relaxation times $T_1$ agree with the spin resonance and spin injection data, following a $T^{-3}$ temperature dependence. The valley anisotropy of $T_1$ and the spin relaxation r…
We study the effect of nanoscale precipitates on lattice thermal conduction in thermoelectric PbTe using a combination of ab-initio phonon calculations and molecular dynamics. We take into account the effects of mass difference and change in force constants, and find an enhanced influence of the latter with increased precipitate concentration. As a consequence, our inclusion of the change in force constants in the calculation affords a smaller predicted optimal nano-precipitate size that minimizes the thermal conductivity. These results suggest that the phonon scattering by nanoprecipitates in…
Rutile TiO2 is a paradigmatic transition metal oxide with applications in optics, electronics, photocatalysis, etc., that are subject to pervasive electron-phonon interaction. To understand how energies of its electronic bands, and in general semiconductors or metals where the frontier orbitals have a strong d-band character, depend on temperature, we perform a comprehensive theoretical and experimental study of the effects of electron-phonon (e-p) interactions. In a two-photon photoemission (2PP) spectroscopy study we observe an unusual temperature dependence of electronic band energies withi…
High-entropy alloys (HEAs) are exceptional candidates for radiation-resistant materials due to their complex local chemical environment and slow defect migration. Despite commonly overlooked, electronic effects on defects evolution in radiation environments also play a crucial role by dissipating excess energy through electron-phonon coupling and electronic heat conduction during cascade events. We present a systematic study on electronic properties in random-solid solutions (RSS) in four and five principal elements HEAs and their effect on defect formation, clustering, and recombination. Elec…
We have utilized time-domain magneto-terahertz spectroscopy to investigate the low frequency optical response of topological insulator Cu$_{0.02}$Bi$_2$Se$_3$ and Bi$_2$Se$_3$ films. With both field and frequency dependence, such experiments give sufficient information to measure the mobility and carrier density of multiple conduction channels simultaneously. We observe sharp cyclotron resonances (CRs) in both materials. The small amount of Cu incorporated into the Cu$_{0.02}$Bi$_2$Se$_3$ induces a true bulk insulator with only a \textit{single} type of conduction with total sheet carrier dens…
An approach is presented for the atomistic study of phonon transport in real dielectric nanowires via Green functions. The formalism is applied to investigate the phonon flow through nanowires coated by an amorphous material. Examples for a simple model system, and for real Si nanowires coated by silica are given. New physical results emerge for these systems, regarding the character of the transition from ballistic to diffusive transport, the low temperature thermal conductance, and the influence of the wire-coating interface on the thermal transport. An efficient treatment of phonon scatteri…
Semiconductor alloys exhibit a strong dependence of effective thermal conductivity on measurement frequency. So far this quasi-ballistic behaviour has only been interpreted phenomenologically, providing limited insight into the underlying thermal transport dynamics. Here, we show that quasi-ballistic heat conduction in semiconductor alloys is governed by Lévy superdiffusion. By solving the Boltzmann transport equation (BTE) with ab initio phonon dispersions and scattering rates, we reveal a transport regime with fractal space dimension $1 < α< 2$ and superlinear time evolution of mean square e…
We study the heat relaxation in current biased metallic films in the regime of strong electron-phonon coupling. A thermal gradient in the direction normal to the film is predicted, with a spatial temperature profile determined by the temperature-dependent heat conduction. In the case of strong phonon scattering the heat conduction occurs predominantly via the electronic system and the profile is parabolic. This regime leads to the linear dependence of the noise temperature as a function of voltage bias, in spite of the fact that all the dimensions of the film are large compared to the electron…
The thermal conductivity of polycrystalline semiconductors with type-I clathrate hydrate crystal structure is reported. Ge clathrates (doped with Sr and/or Eu) exhibit lattice thermal conductivities typical of amorphous materials. Remarkably, this behavior occurs in spite of the well-defined crystalline structure and relatively high electron mobility ($\sim 100 cm^2/Vs$). The dynamics of dopant ions and their interaction with the polyhedral cages of the structure are a likely source of the strong phonon scattering.
A distinct thermal Hall signal is observed in a quantum spin liquid candidate Ba$_3$CuSb$_2$O$_9$. The transverse thermal conduction shows a power-law temperature dependence below 40 K where a spin gap opens. We suggest that, through the very low longitudinal thermal conductivity and the thermal Hall signals, a phonon Hall effect is induced by strong phonon scatterings by orphan Cu$^{2+}$ spins formed in random domains of Cu$^{2+}$-Sb$^{5+}$ dumbbells.
Understanding the mechanisms of thermal conduction in graphene is a long-lasting research topic, due to its high thermal conductivity. Peierls-Boltzmann transport equation (PBTE) based studies have revealed many unique phonon transport properties in graphene, but most previous works only considered three-phonon scatterings and relied on interatomic force constants (IFCs) extracted at 0 K. In this paper, we explore the roles of four-phonon scatterings and the temperature dependent IFCs on phonon transport in graphene through our PBTE calculations. We demonstrate that the strength of four-phonon…
Silver nanowires have great application potential in fields like flexible electronic devices, solar cells and transparent electrodes. It is critical and fundamental to study the thermal and electrical transport properties in a single silver nanowire. In this work, the thermal and electrical transport in an individual silver nanowire is characterized down to 35 K with the steady state electro-thermal technique. The results indicate that, at room temperature, the electrical resistivity increases by around 4 folds compared from that of its bulk counterpart. After fitting the temperature dependent…
Structural, magnetic, and dielectric properties of Sm$_2$CoMnO$_6$ have been studied. X-ray diffraction and Rietveld analysis show that the sample crystallizes in the monoclinic structure with \textit{P2$_1$/n} space group. Magnetic study shows that the sample undergoes a paramagnetic to ferromagnetic phase transition around $T_c$ $\sim$148 K and a low-temperature ordering of Sm$^{3+}$ moments. Temperature-dependent Raman study shows spin-phonon coupling present in this material. Dielectric response of Sm$_2$CoMnO$_6$ shows strong frequency dependence with large dispersion and large dielectric…
We have developed a new theoretical formalism for phonon transport in nanostructures using the nonequilibrium phonon Green's function technique and have applied it to thermal conduction in defective carbon nanotubes. The universal quantization of low-temperature thermal conductance in carbon nanotubes can be observed even in the presence of local structural defects such as vacancies and Stone-Wales defects, since the long wavelength acoustic phonons are not scattered by local defects. At room temperature, however, thermal conductance is critically affected by defect scattering since incident p…
The strain effects on the spin-orbit induced splitting of the valence band maximum and conduction band minimum in monolayer MoS2 and the gap in graphene are calculated using first-principles calculations. The dependence of these splittings on the various symmetry types of strain is described by means of an effective Hamiltonian based on the method of invariants and the parameters in the model are extracted by fitting to the theory. These splittings are related to acoustic phonon deformation potentials, or electron-phonon coupling matrix elements which enter the spin-dependent scattering theory…
Thermal conductivity measurements over variable lengths on nanostructures such as nanowires provide important information about the mean free paths (MFPs) of the phonons responsible for heat conduction. However, nearly all of these measurements have been interpreted using an average MFP even though phonons in many crystals possess a broad MFP spectrum. Here, we present a reconstruction method to obtain MFP spectra of nanostructures from variable-length thermal conductivity measurements. Using this method, we investigate recently reported length-dependent thermal conductivity measurements on Si…
