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Author ORCID ID is 0000000196457993
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  1. BAs was predicted to have an unusually high thermal conductivity with a room temperature value of 2000 W m –1 K –1, comparable to that of diamond. However, the experimentally measured thermal conductivity of BAs single crystals is still lower than this value. To identify the origin of this large inconsistency, we investigate the lattice structure and potential defects in BAs single crystals at the atomic scale using aberration-corrected scanning transmission electron microscopy (STEM). Rather than finding a large concentration of As vacancies ( V As), as widely thought to dominate the thermal resistance in BAs, our STEM results showmore » an enhanced intensity of some B columns and a reduced intensity of some As columns, suggesting the presence of antisite defects with As B (As atom on a B site) and B As (B atom on an As site). Additional calculations show that the antisite pair with As B next to B As is preferred energetically among the different types of point defects investigated and confirm that such defects lower the thermal conductivity for B As. Using a concentration of 1.8(8)% (6.6 ± 3.0 × 10 20 cm –3 in density) for the antisite pairs estimated from STEM images, the thermal conductivity is estimated to be 65–100 W m –1 K –1, in reasonable agreement with our measured value. Our study suggests that As B–B As antisite pairs are the primary lattice defects suppressing thermal conductivity of B As. Possible approaches are proposed for the growth of high-quality crystals or films with high thermal conductivity. In conclusion by employing a combination of state-of-the-art synthesis, STEM characterization, theory, and physical insight, this work models a path toward identifying and understanding defect-limited material functionality.« less
  2. Solids with ultralow thermal conductivity are of great interest as thermal barrier coatings for insulation or thermoelectrics for energy conversion. However, the theoretical limits of lattice thermal conductivity (κ) are unclear. In typical crystals a phonon picture is valid, whereas lowest κ values occur in highly disordered materials where this picture fails and heat is supposedly carried by random walk among uncorrelated oscillators. Here in this paper we identify a simple crystal, Tl 3VSe 4, with a calculated phonon κ [0.16 Watts per meter-Kelvin (W/m-K)] one-half that of our measured κ (0.30 W/m-K) at 300 K, approaching disorder κ values,more » although Raman spectra, specific heat, and temperature dependence of κ reveal typical phonon characteristics. Adding a transport component based on uncorrelated oscillators explains the measured κ and suggests that a two-channel model is necessary for crystals with ultralow κ.« less
  3. The lattice thermal conductivity, kL, of typical metallic and nonmetallic crystals decreases rapidly with increasing temperature because phonons interact more strongly with other phonons than they do with electrons. Using first principles calculations, we show that kL can become nearly independent of temperature in metals that have nested Fermi surfaces and large frequency gaps between acoustic and optic phonons. Then, the interactions between phonons and electrons become much stronger than the mutual interactions between phonons, giving the fundamentally different kL behavior. This striking trend is revealed here in the group V transition metal carbides, vanadium carbide, niobium carbide, and tantalummore » carbide, and it should also occur in several other metal compounds. As a result, this work gives insights into the physics of heat conduction in solids and identifies a new heat flow regime driven by the interplay between Fermi surfaces and phonon dispersions.« less
  4. Here, we study the scattering of phonons from point defects and their effect on lattice thermal conductivity κ using a parameter-free ab initio Green's function methodology. Specifically, we focus on the scattering of phonons by boron (B), nitrogen (N), and phosphorus substitutions as well as single- and double-carbon vacancies in graphene. We show that changes of the atomic structure and harmonic interatomic force constants locally near defects govern the strength and frequency trends of the scattering of out-of-plane acoustic (ZA) phonons, the dominant heat carriers in graphene. ZA scattering rates due to N substitutions are nearly an order of magnitudemore » smaller than those for B defects despite having similar mass perturbations. Furthermore, ZA phonon scattering rates from N defects decrease with increasing frequency in the lower-frequency spectrum in stark contrast to expected trends from simple models. ZA phonon-vacancy scattering rates are found to have a significantly softer frequency dependence (~ω 0) in graphene than typically employed in phenomenological models. The rigorous Green's function calculations demonstrate that typical mass-defect models do not adequately describe ZA phonon-defect scattering rates. Our ab initio calculations capture well the trend of κ vs vacancy density from experiments, though not the magnitudes. In conclusion, this work elucidates important insights into phonon-defect scattering and thermal transport in graphene, and demonstrates the applicability of first-principles methods toward describing these properties in imperfect materials.« less
    Cited by 2
  5. 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 [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. Here, to better understand the interfacial scattering of phonons and to test the validity of Ziman'smore » 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/SiO 2 interface roughness was determined to be 0.11±0.04nm 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. The phonons that are specularly transmitted into an amorphous layer will be sufficiently randomized by the time they come back to the crystalline Si layer, the effect of which is practically equivalent to a diffuse reflection at the interface. Finally, we derive a simple expression for the specularity parameter at solid/amorphous interfaces and achieve good agreement between calculations and measurement values.« less
    Cited by 1
  6. We rigorously calculate intrinsic phonon thermal resistance from four-phonon scattering processesusing rst principles Boltzmann transport methods. Fundamental questions concerning the role ofhigher order scattering at high temperature and in systems with otherwise weak intrinsic scatteringare answered. Using diamond and silicon as benchmark materials, the predicted thermal conductiv-ity including intrinsic four-phonon resistance gives signicantly better agreement with measurementsat high temperatures than previous rst principles calculations. In the predicted ultrahigh thermalconductivity material, zincblende BAs, four-phonon scattering is strikingly strong when comparedto three-phonon processes, even at room temperature, as the latter have an extremely limited phasespace for scattering. Including four-phonon thermal resistance reducesmore » the predicted thermal con-ductivity of BAs from 2200 W/m-K to 1400 W/m-K.« less
    Cited by 26Full Text Available
  7. Conventional optical components are limited to size scales much larger than the wavelength of light, as changes to the amplitude, phase and polarization of the electromagnetic fields are accrued gradually along an optical path. However, advances in nanophotonics have produced ultrathin, so-called ‘flat’ optical components that beget abrupt changes in these properties over distances significantly shorter than the free-space wavelength. Although 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 threefold improvement in polariton lifetime through isotopic enrichment of hexagonal boron nitridemore » (hBN). Commensurate increases in the polariton propagation length are demonstrated via direct imaging of polaritonic standing waves by means of infrared nano-optics. Lastly, our results provide the foundation for a materials-growth-directed approach aimed at realizing the loss control necessary for the development of PhP-based nanophotonic devices.« less
    Cited by 21Full Text Available
  8. Strongly anharmonic phonon properties of CuCl are investigated with a combination of first-principles simulations and inelastic neutron scattering measurements. An unusual quasiparticle peak in the spectral functions emerges with increasing temperature, in both simulations and measurements, emanating from exceptionally strong coupling between conventional phonon modes. Associated with this strong anharmonicity, the lattice thermal conductivity (κ) of CuCl is extremely low and exhibits anomalous, non-monotonic pressure dependence. We show how this behavior arises from nested phonon dispersions augmenting the phase space available for anharmonic three-phonon scattering processes, and contrast this mechanism with common arguments based on negative Grüneisen parameters. Here, thesemore » results demonstrate the importance of considering the topology of phonon dispersions toward understanding phonon scattering processes and designing new ultralow thermal conductivity materials for technological applications.« less
  9. We use femtosecond laser pulses to generate coherent longitudinal acoustic phonons at frequencies of 1–1.4 THz and study their propagation in GaN-based structures at room temperature. Two InGaN-GaN multiple-quantum-well (MQW) structures separated by a 2.3 μm-thick GaN spacer are used to simultaneously generate phonon wave packets with a central frequency determined by the period of the MQW and detect them after passing through the spacer. The measurements provide lower bounds for phonon lifetimes in GaN, which are still significantly lower than those from first principles predictions. The material Q-factor at 1 THz is found to be at least as highmore » as 900. The measurements also demonstrate a partial specular reflection from the free surface of GaN at 1.4 THz. This work shows the potential of laser-based methods for THz range phonon spectroscopy and the promise for extending the viable frequency range of GaN-based acousto-electronic devices.« less
  10. Here, the crystallographic and magnetic properties of the cleavable 4d 3 transition metal compound α–MoCl 3 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 diamagneticmore » 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.« less

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