arXiv subjects

Lucas Lindsay

Publications and source records attributed to Lucas Lindsay.

31 records · Page 2Linked to original sources

arXiv2020

Phonons, Q-dependent Kondo spin fluctuations, and 4$\textit{f}$/phonon resonance in YbAl$_3$

The intermediate valence (IV) compound YbAl$_3$ exhibits nonintegral valence (Yb 4$f^{14-n_f}$ (5d6s)$^z$ where z = 2+n$_f$ = 2.75) in a moderately heavy (m* = 20-30me) ground state with a large Kondo temperature (T$_K$ ~ 500-600K). We have measured the magnetic fluctuations and the phonon spectra on single crystals of this material by time-of-flight inelastic neutron scattering (INS) and inelastic x-ray scattering (IXS). We find that at low temperature, the Kondo-scale spin fluctuations have a momentum (Q) dependence similar to that seen recently in the IV compound CePd$_3$ and which can be attributed to particle-hole excitations in a coherent itinerant 4$f$ correlated ground state. The Q-dependence disappears as the temperature is raised towards room temperature and the 4$f$ electron band states become increasingly incoherent. The measured phonons can be described adequately by a calculation based on standard DFT+$U$ density functional theory, without recourse to considering 4$f$ correlations dynamically. A low temperature magnetic peak observed in the neutron scattering at ~ 30meV shows dispersion identical to an optic phonon branch. This 4$f$/phonon resonance disappears for T > 150K. The phonons appear to remain unaffected by the resonance. We discuss several possibilities for the origin of this unusual excitation, including the idea that it arises from the large amplitude beating of the light Al atoms against the heavy Yb atoms, resulting in a dynamic 4$f$/3$p$ hybridization.

cond-mat.str-el↗

arXiv2019

Experimental demonstration of a generalized Fourier's Law for non-diffusive thermal transport

Phonon heat conduction over length scales comparable to their mean free paths is a topic of considerable interest for basic science and thermal management technologies. Although the failure of Fourier's law beyond the diffusive regime is well understood, debate exists over the proper physical description of thermal transport in the ballistic to diffusive crossover. Here, we derive a generalized Fourier's law that links the heat flux and temperature fields, valid from ballistic to diffusive regimes and for general geometries, using the Peierls-Boltzmann transport equation within the relaxation time approximation. This generalized Fourier's law predicts that thermal conductivity not only becomes nonlocal at length scales smaller than phonon mean free paths, but also requires the inclusion of an inhomogeneous nonlocal source term that has been previously neglected. We demonstrate the validity of this generalized Fourier's law through direct comparison with time-domain thermoreflectance (TDTR) measurements in the nondiffusive regime without adjustable parameters. Furthermore, we show that interpreting experimental data without this generalized Fourier's law leads to inaccurate measurement of thermal transport properties.

cond-mat.mtrl-sci↗

arXiv2018

Phonon-induced multi-color correlations in hBN single-photon emitters

Color centers in hexagonal boron nitride have shown enormous promise as single-photon sources, but a clear understanding of electron-phonon interaction dynamics is critical to their development for quantum communications or quantum simulations. We demonstrate photon antibunching in the filtered auto- and cross-correlations $g^{(2)}_{lm}(τ)$ between zero-, one- and two-phonon replicas of defect luminescence. Moreover, we combine autocorrelation measurements with a violation of the Cauchy-Schwarz inequality in the filtered cross-correlation measurements to distinguish a low quantum-efficiency defect from phonon replicas of a bright defect. With no background correction, we observe single photon purity of $g^{(2)}(0)=0.20$ in a phonon replica and cross-spectral correlations of $g^{(2)}_{lm}(0)=0.18$ between a phonon replica and the zero phonon line. These results illustrate a coherent interface between visible photons and mid-infrared phonons and provide a clear path toward control of photon-phonon entanglement in 2D materials.

quant-ph↗

arXiv2018

Anisotropic thermal transport in bulk hexagonal boron nitride

Hexagonal boron nitride (h-BN) has received great interest in recent years as a wide bandgap analog of graphene-derived systems. However, the thermal transport properties of h-BN, which can be critical for device reliability and functionality, are little studied both experimentally and theoretically. The primary challenge in the experimental measurements of the anisotropic thermal conductivity of h-BN is that typically sample size of h-BN single crystals is too small for conventional measurement techniques, as state-of-the-art technologies synthesize h-BN single crystals with lateral sizes only up to 2.5 mm and thickness up to 200 μm. Recently developed time-domain thermoreflectance (TDTR) techniques are suitable to measure the anisotropic thermal conductivity of such small samples, as it only requires a small area of 50x50 μm2 for the measurements. Accurate atomistic modeling of thermal transport in bulk h-BN is also challenging due to the highly anisotropic layered structure. Here we conduct an integrated experimental and theoretical study on the anisotropic thermal conductivity of bulk h-BN single crystals over the temperature range of 100 K to 500 K, using TDTR measurements with multiple modulation frequencies and a full-scale numerical calculation of the phonon Boltzmann transport equation starting from the first principles. Our experimental and numerical results compare favorably for both the in-plane and through-plane thermal conductivities. We observe unusual temperature-dependence and phonon-isotope scattering in the through-plane thermal conductivity of h-BN and elucidate their origins. This work not only provides an important benchmark of the anisotropic thermal conductivity of h-BN but also develops fundamental insights into the nature of phonon transport in this highly anisotropic layered material.

cond-mat.mes-hall↗

arXiv2018

Dislocation-induced thermal transport anisotropy in single-crystal group-III nitride films

Dislocations, one-dimensional lattice imperfections, are common to technologically important materials such as III-V semiconductors, and adversely affect heat dissipation in e.g., nitride-based high-power electronic devices. For decades, conventional models based on nonlinear elasticity theory have predicted this thermal resistance is only appreciable when heat flux is perpendicular to the dislocations. However, this dislocation-induced anisotropic thermal transport has yet to be seen experimentally. In this study, we measure strong thermal transport anisotropy governed by highly oriented threading dislocation arrays along the cross-plane direction in micron-thick, single-crystal indium nitride (InN) films. We find that the cross-plane thermal conductivity is more than tenfold higher than the in-plane thermal conductivity at 80 K when the dislocation density is on the order of ~3x10^10 cm^-2. This large anisotropy is not predicted by the conventional models. With enhanced understanding of dislocation-phonon interactions, our results open new regimes for tailoring anisotropic thermal transport with line defects, and will facilitate novel methods for directed heat dissipation in thermal management of diverse device applications.

cond-mat.mtrl-sci↗

arXiv2018

Interfacial Phonon Scattering and Transmission Loss in >1 um Thick Silicon-on-insulator Thin Films

Scattering of phonons at boundaries of a crystal (grains, surfaces, or solid/solid interfaces) is characterized by the phonon wavelength, the angle of incidence, and the interface roughness, as historically evaluated using a specularity parameter p formulated by Ziman [J. M. Ziman, Electrons and Phonons (Clarendon Press, Oxford, 1960)]. This parameter was initially defined to determine the probability of a phonon specularly reflecting or diffusely scattering from the rough surface of a material. The validity of Ziman's theory as extended to solid/solid interfaces has not been previously validated. To better understand the interfacial scattering of phonons and to test the validity of Ziman's theory, we precisely measured the in-plane thermal conductivity of a series of Si films in silicon-on-insulator (SOI) wafers by time-domain thermoreflectance (TDTR) for a Si film thickness range of 1 - 10 μm and a temperature range of 100 - 300 K. The Si/SiO2 interface roughness was determined to be 0.11+/-0.04 nm using transmission electron microscopy (TEM). Furthermore, we compared our in-plane thermal conductivity measurements to theoretical calculations that combine first-principles phonon transport with Ziman's theory. Calculations using Ziman's specularity parameter significantly overestimate values from the TDTR measurements. We attribute this discrepancy to phonon transmission through the solid/solid interface into the substrate, which is not accounted for by Ziman's theory for surfaces. We derive a simple expression for the specularity parameter at solid/amorphous interfaces and achieve good agreement between calculations and measurement values.

cond-mat.mes-hall↗

arXiv2017

Size Dependence and Ballistic Limits of Thermal Transport in Anisotropic Layered Two-Dimensional Materials

Layered materials have uncommonly anisotropic thermal properties due to their strong in-plane covalent bonds and weak out-of-plane van der Waals interactions. Here we examine heat flow in graphene (graphite), h-BN, MoS2, and WS2 monolayers and bulk films, from diffusive to ballistic limits. We determine the ballistic thermal conductance limit (Gball) both in-plane and out-of-plane, based on full phonon dispersions from first-principles calculations. An overall phonon mean free path (λ) is expressed in terms of Gball and the diffusive thermal conductivity, consistent with kinetic theory if proper averaging of phonon group velocity is used. We obtain a size-dependent thermal conductivity k(L) in agreement with available experiments, and find that k(L) only converges to >90% of the diffusive thermal conductivity for sample sizes L > 16λ, which ranges from ~140 nm for MoS2 cross-plane to ~10 um for suspended graphene in-plane. These results provide a deeper understanding of microscopic thermal transport, revealing that device scales below which thermal size effects should be taken into account are generally larger than previously thought.

cond-mat.mes-hall↗

arXiv2017

High-temperature magnetostructural transition in van der Waals-layered a-MoCl3

The crystallographic and magnetic properties of the cleavable 4d3 transition metal compound a-MoCl3 are reported, with a focus on the behavior above room temperature. Crystals were grown by chemical vapor transport and characterized using temperature dependent x-ray diffraction, Raman spectroscopy, and magnetization measurements. A structural phase transition occurs near 585 K, at which the Mo-Mo dimers present at room temperature are broken. A nearly regular honeycomb net of Mo is observed above the transition, and an optical phonon associated with the dimerization instability is identified in the Raman data and in first-principles calculations. The crystals are diamagnetic at room temperature in the dimerized state, and the magnetic susceptibility increases sharply at the structural transition. Moderately strong paramagnetism in the high-temperature structure indicates the presence of local moments on Mo. This is consistent with results of spin-polarized density functional theory calculations using the low- and high-temperature structures. Above the magnetostructural phase transition the magnetic susceptibility continues to increase gradually up to the maximum measurement temperature of 780 K, with a temperature dependence that suggests two-dimensional antiferromagnetic correlations.

cond-mat.mtrl-sci↗

arXiv2017

Phonon thermal transport in 2H, 4H and 6H silicon carbide from first principles

Silicon carbide (SiC) is a wide band gap semiconductor with a variety of industrial applications. Among its many useful properties is its high thermal conductivity, which makes it advantageous for thermal management applications. In this paper we present \textit{ab initio} calculations of the in-plane and cross-plane thermal conductivities, $κ_{\text{in}}$ and $κ_{\text{out}}$, of three common hexagonal polytypes of SiC: 2H, 4H and 6H. The phonon Boltzmann transport equation is solved iteratively using as input interatomic force constants determined from density functional theory. Both $κ_{\text{in}}$ and $κ_{\text{out}}$ decrease with increasing $n$ in $n$H SiC because of additional low-lying optic phonon branches. These optic branches are characterized by low phonon group velocities, and they increase the phase space for phonon-phonon scattering of acoustic modes. Also, for all $n$, $κ_{\text{in}}$ is found to be larger than $κ_{\text{out}}$ in the temperature range considered. At electron concentrations present in experimental samples, scattering of phonons by electrons is shown to be negligible except well below room temperature where it can lead to a significant reduction of the lattice thermal conductivity. This work highlights the power of \textit{ab initio} approaches in giving quantitative, predictive descriptions of thermal transport in materials. It helps explain the qualitative disagreement that exists among different sets of measured thermal conductivity data and provides information of the relative quality of samples from which measured data was obtained.

cond-mat.mtrl-sci↗

arXiv2017

Ultra-low-loss Polaritons in Isotopically Pure Materials: A New Approach

Conventional optical components are limited to size-scales much larger than the wavelength of light, as changes in the amplitude, phase and polarization of the electromagnetic fields are accrued gradually along an optical path. However, advances in nanophotonics have produced ultra-thin, co-called "flat" optical components that beget abrupt changes in these properties over distances significantly shorter than the free space wavelength. While high optical losses still plague many approaches, phonon polariton (PhP) materials have demonstrated long lifetimes for sub-diffractional modes in comparison to plasmon-polariton-based nanophotonics. We experimentally observe a three-fold improvement in polariton lifetime through isotopic enrichment of hexagonal boron nitride (hBN). Commensurate increases in the polariton propagation length are demonstrated via direct imaging of polaritonic standing waves by means of infrared nano-optics. Our results provide the foundation for a materials-growth-directed approach towards realizing the loss control necessary for the development of PhP-based nanophotonic devices.

cond-mat.mtrl-sci↗

arXiv2016

Physically founded phonon dispersions of few-layer materials, and the case of borophene

An increasing number of theoretical calculations on few-layer materials have been reporting a non-zero sound velocity for all three acoustic phonon modes. In contrast with these reports, here we show that the lowest phonon dispersion branch of atomistically described few-layer materials should be quadratic, and this can have dramatic consequencies on calculated properties, such as the thermal conductivity. By reformulating the interatomic force constants (IFC) in terms of internal coordinates, we find that a delicate balance between the IFCs is responsible for this quadraticity. This balance is hard to obtain in ab-initio calculations even if all the symmetries are numerically enforced a posteriori, but it arises naturally in our approach. We demonstrate the phenomenon in the case of borophene, where a very subtle correction to the ab-initio IFCs yields the physically correct quadratic dispersion, while leaving the rest of the spectrum virtually unmodified. Such quadraticity nevertheless has a major effect on the computed lattice thermal conductivity, which in the case of borophene changes by more than a factor 2, and reverses its anisotropy, when the subtle IFC correction is put in place.

cond-mat.mtrl-sci↗

arXiv2015

Tensile Strains Give Rise to Strong Size Effects for Thermal Conductivities of Silicene, Germanene and Stanene

Based on first principles calculations and self-consistent solution of linearized Boltzmann-Peierls equation for phonon transport approach within a three-phonon scattering framework, we characterize lattice thermal conductivities k of freestanding silicene, germanene and stanene under different isotropic tensile strains and temperatures. We find a strong size dependence of k for silicene with tensile strain, i.e., divergent k with increasing system size, in contrast, the intrinsic room temperature k for unstrained silicene converges with system size to 19.34 W/m-K by 178 nm. The room temperature k of strained silicene becomes as large as that of bulk silicon by 84 um, indicating the possibility of using strain in silicene to manipulate k for thermal management. The relative contribution to the intrinsic k from out-of-plane acoustic modes is largest for unstrained silicene, about 39% at room temperature. The single mode relaxation time approximation, which works reasonably well for bulk silicon, fails to appropriately describe phonon thermal transport in silicene, germanene and stanene within the temperature range considered. For large samples of silicene, k increases with tensile strain, peaks at about 7% strain and then decreases with further strain. In germanene and stanene increasing strain hardens and stabilizes long wavelength out-of-plane acoustic phonons, and leads to similar k behaviors to those of silicene. These findings further our understanding of phonon dynamics in group-IV buckled monolayers and may guide transfer and fabrication techniques of these freestanding samples and engineering k by size and strain for applications of thermal management and thermoelectricity.

cond-mat.mes-hall↗

arXiv2015

Thermal conductivity of graphene mediated by strain and size

Based on first-principles calculations and full iterative solution of the linearized Boltzmann-Peierls transport equation for phonons within three-phonon scattering framework, we characterize the lattice thermal conductivities $κ$ of strained and unstrained graphene. We find $κ$ converges to 5450 W/m-K for infinite unstrained graphene, while $κ$ diverges for strained graphene with increasing system size at room temperature. The different $κ$ behaviors for these systems are further validated mathematically through phonon lifetime analysis. Flexural acoustic phonons are the dominant heat carriers in both unstrained and strained graphene within the temperature considered. Ultralong mean free paths of flexural phonons contribute to finite size effects on $κ$ for samples as large as 8 cm at room temperature. The calculated size-dependent and temperature-dependent $κ$ for finite samples agree well with experimental data, demonstrating the ability of the present approach to predict $κ$ of larger graphene sample. Tensile strain hardens the flexural modes and increases their lifetimes, causing interesting dependence of $κ$ on sample size and strain due to the competition between boundary scattering and intrinsic phonon-phonon scattering. These findings shed light on the nature of thermal transport in two-dimensional materials and may guide predicting and engineering $κ$ of graphene by varying strain and size.

cond-mat.mtrl-sci↗