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Showing papers on "Mean free path published in 2014"


Journal ArticleDOI
TL;DR: Interestingly and in contrast to bulk materials, at 300 K, thermal conductivity keeps increasing and remains logarithmically divergent with sample length even for sample lengths much larger than the average phonon mean free path, which provides fundamental understanding of thermal transport in two-dimensional materials.
Abstract: Graphene exhibits extraordinary electronic and mechanical properties, and extremely high thermal conductivity. Being a very stable atomically thick membrane that can be suspended between two leads, graphene provides a perfect test platform for studying thermal conductivity in two-dimensional systems, which is of primary importance for phonon transport in low-dimensional materials. Here we report experimental measurements and nonequilibrium molecular dynamics simulations of thermal conduction in suspended single-layer graphene as a function of both temperature and sample length. Interestingly and in contrast to bulk materials, at 300 K, thermal conductivity keeps increasing and remains logarithmically divergent with sample length even for sample lengths much larger than the average phonon mean free path. This result is a consequence of the two-dimensional nature of phonons in graphene, and provides fundamental understanding of thermal transport in two-dimensional materials.

791 citations


Journal ArticleDOI
24 Feb 2014-ACS Nano
TL;DR: An experimental investigation on the exciton dynamics of monolayer and bulk WSe2 samples, both of which are studied by femtosecond transient absorption microscopy, resolves the differential reflection signal in both time and space and deduces other parameters characterizing theexciton dynamics such as the diffusion length, the mobility, the mean free path, and themean free length.
Abstract: We present an experimental investigation on the exciton dynamics of monolayer and bulk WSe2 samples, both of which are studied by femtosecond transient absorption microscopy. Under the excitation of a 405 nm pump pulse, the differential reflection signal of a probe pulse (tuned to the A-exciton resonance) reaches a peak rapidly that indicates an ultrafast formation process of excitons. By resolving the differential reflection signal in both time and space, we directly determine the exciton lifetimes of 18 ± 1 and 160 ± 10 ps and the exciton diffusion coefficients of 15 ± 5 and 9 ± 3 cm2/s in the monolayer and bulk samples, respectively. From these values, we deduce other parameters characterizing the exciton dynamics such as the diffusion length, the mobility, the mean free path, and the mean free length. These fundamental parameters are useful for understanding the excitons in monolayer and bulk WSe2 and are important for applications in optoelectronics, photonics, and electronics.

208 citations


Journal ArticleDOI
TL;DR: The magnon contribution to both the thermal conductivity and specific heat at low temperatures has been determined by measuring these properties under an applied magnetic field, which allows to freeze the magnon modes and isolate the phonon contribution relative to the zero-field behavior as discussed by the authors.
Abstract: The magnetothermal properties of monocrystalline yttrium iron garnet (YIG) are reported. The magnon contribution to both the thermal conductivity and specific heat at low temperatures has been determined by measuring these properties under an applied magnetic field, which allows us to freeze the magnon modes and isolate the phonon contribution relative to the zero-field behavior. These results are interpreted within the framework of a simple kinetic gas model for magnon heat conduction that allows us to estimate the magnon thermal mean free path, i.e., the inelastic scattering length scale for thermally driven bulk magnons. We observe this parameter to reach as high as approximately 100 \ensuremath{\mu}m at 2 K. It tracks the acoustic phonon thermal mean free path closely and decreases rapidly as the temperature is increased. This relatively short length scale suggests that magnon modes at thermal energies in YIG are not solely or directly responsible for coherent macroscale thermal spin transport (e.g., in the spin Seebeck effect) at high temperatures. Instead, these results support a growing consensus that subthermal magnons, i.e., those at energies below about 30 \ifmmode\pm\else\textpm\fi{} 10 K, are important for spin transport in YIG at all temperatures. These results also emphasize that magnon effects should be considered wavelength dependent, and that magnon-magnon interactions may be just as important for thermal spin transport as magnon-phonon scattering. This, in turn, has implications for understanding the characteristic temperature and length scales involved in spin caloritronic phenomena.

144 citations


Journal ArticleDOI
TL;DR: In this paper, the mass attenuation coefficients of ZBB glasses have been measured at different energies obtained from a Compton scattering technique, and the results show a decrease of the attenuation coefficient, effective atomic number and effective electron density values with increasing of gamma-ray energies; and good agreements between experimental and theoretical values.

144 citations


Journal ArticleDOI
TL;DR: A review of the theoretical approaches for predicting spectral phonon mean free path and thermal conductivity of solids is given in this article, which can be summarized into two categories: anharmonic lattice dynamics calculation and molecular dynamics simulation.
Abstract: We give a review of the theoretical approaches for predicting spectral phonon mean free path and thermal conductivity of solids. The methods can be summarized into two categories: anharmonic lattice dynamics calculation and molecular dynamics simulation. In the anharmonic lattice dynamics calculation, the anharmonic force constants are used first to calculate the phonon scattering rates, and then the Boltzmann transport equations are solved using either standard single mode relaxation time approximation or the Iterative Scheme method for the thermal conductivity. The MD method involves the time domain or frequency domain normal mode analysis. We present the theoretical frameworks of the methods for the prediction of phonon dispersion, spectral phonon relaxation time, and thermal conductivity of pure bulk materials, layer and tube structures, nanowires, defective materials, and superlattices. Several examples of their applications in thermal management and thermoelectric materials are given. The strength and limitations of these methods are compared in several different aspects. For more efficient and accurate predictions, the improvements of those methods are still needed.

108 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigated the size and edge roughness dependence on thermal conductivity of monolayer MoS2 (MLMoS2) by phonon Boltzmann transport equation combined with relaxation time approximation.
Abstract: We investigate the size and edge roughness dependence on thermal conductivity of monolayer MoS2 (MLMoS2) by phonon Boltzmann transport equation combined with relaxation time approximation. The relative contribution of spectral phonons to thermal conductivity is explored, and we compared the characteristics of phonon transport with those in single layer graphene (SLG), which is a representative two-dimensional material. Quite different from SLG, because of the ultra-short intrinsic phonon mean free path, the thermal conductivity of MLMoS2 ribbons is size and roughness insensitive. The LA phonons have the major contribution to thermal conductivity of MLMoS2, and the ZA phonons in MLMoS2 have high relative contribution to thermal conductivity. The relative contribution to thermal conductivity from both high frequency and low frequency phonons in MLMoS2 is lower than that in SLG. The underlying mechanism of these distinct characteristics results from the different phonon dispersions and anharmonic characteristic between MLMoS2 and SLG.

107 citations


Journal ArticleDOI
TL;DR: In this paper, X-ray diffraction, UV-visible, DSC and ultrasonic techniques have been used to explore the structural properties of PbO-SiO2−Al2O3 and Bi2O-3−SiO 2−Al 2O3 glass systems.

102 citations


Journal ArticleDOI
TL;DR: In this article, the potential of WS2 as a thermoelectric material was assessed using a constant relaxation time approximation from the electronic band structure, whereas the lattice contribution was evaluated using self-consistently calculated phonon lifetimes.
Abstract: The potential of WS2 as a thermoelectric material is assessed. The electronic contribution to the thermoelectric properties is calculated within the constant relaxation time approximation from the electronic band structure, whereas the lattice contribution is evaluated using self-consistently calculated phonon lifetimes. In addition, the dependence of the lattice thermal conductivity on the mean free path of the phonons is determined.

87 citations


Posted Content
TL;DR: In this article, thermal conductivity measurements of thin Si membranes spanning a wide thickness range were used to characterize how bulk thermal conductivities are distributed over phonon mean free paths, and a decrease in the conductivity from 74% to 13% of the bulk value was attributed to diffuse phonon boundary scattering.
Abstract: Knowledge of the mean free path distribution of heat-carrying phonons is key to understanding phonon-mediated thermal transport. We demonstrate that thermal conductivity measurements of thin membranes spanning a wide thickness range can be used to characterize how bulk thermal conductivity is distributed over phonon mean free paths. A non-contact transient thermal grating technique was used to measure the thermal conductivity of suspended Si membranes ranging from 15 to 1500 nm in thickness. A decrease in the thermal conductivity from 74% to 13% of the bulk value is observed over this thickness range, which is attributed to diffuse phonon boundary scattering. Due to the well-defined relation between the membrane thickness and phonon mean free path suppression, combined with the range and accuracy of the measurements, we can reconstruct the bulk thermal conductivity accumulation vs. phonon mean free path, and compare with theoretical models.

82 citations


Journal ArticleDOI
TL;DR: In this article, heat conduction of nanoporous silicon and germanium thin films is studied thanks to a statistical approach and the resolution of phonon Boltzmann transport equation is performed with a Monte Carlo technique in order to assess thermal conductivity.
Abstract: Heat conduction of nanoporous silicon and germanium thin films is studied thanks to a statistical approach. Resolution of phonon Boltzmann transport equation is performed with a Monte Carlo technique in order to assess thermal conductivity. Sensitivity of this latter property with respect to parameters such as phonon mean free path and characteristics of the pores ( distribution, size, porosity) is discussed and compared to predictions from analytical models. Results point out that thermal properties might be tailored through the design of the porosity and more specifically by the adjustment of the phonon-pore mean free path. Finally, an effective medium technique is used to extend our work to multilayered crystalline-nanoporous structures. Results show that ought to pore scattering, a diffusive Fourier regime can be recovered even when the film thickness is below the bulk limit.

80 citations


Journal ArticleDOI
TL;DR: High mean free path associated with low KE electrons, very low or no inelastic scattering, and effective pumping and the design of electrostatic lens regime help to minimize the electron attenuation at NAP conditions.
Abstract: Valence band (VB) changes and hence electronic structure evolution was directly observed with low kinetic energy (KE) electrons at near ambient pressure (NAP) conditions with He I photon source in a custom built laboratory ambient pressure photoelectron spectrometer (Lab-APPES). Polycrystalline Cu surfaces were gradually oxidized in O2 to Cu2O, to a mixture of Cu2O + CuO, and finally to CuO between 300 and 625 K and at NAP. Typical VB features for Cu, Cu2O, and CuO were observed, and the results corroborate well with core level and Auger spectral changes. High mean free path associated with low KE electrons, very low or no inelastic scattering, and effective pumping and the design of electrostatic lens regime help to minimize the electron attenuation at NAP conditions. The present results extend the capabilities of the APPES tool to explore the in situ evolution of electronic structure of materials at NAP and high temperatures.

Journal ArticleDOI
TL;DR: In this article, the phonon dispersion and phonon lifetime of single layer graphene were extracted from a molecular dynamics simulation and the mode dependent thermal conductivity was calculated from phonon kinetic theory, showing that the relative contribution of different mode phonons is not sensitive to the grain size of graphene.
Abstract: Molecular dynamics simulation is performed to extract the phonon dispersion and phonon lifetime of single layer graphene. The mode dependent thermal conductivity is calculated from the phonon kinetic theory. The predicted thermal conductivity at room temperature exhibits important quantum effects due to the high Debye temperature of graphene. But the quantum effects are reduced significantly when the simulated temperature is as high as 1000 K. Our calculations show that out-of-plane modes contribute about 41.1% to the total thermal conductivity at room temperature. The relative contribution of out-of-plane modes has a little decrease with the increase of temperature. Contact with substrate can reduce both the total thermal conductivity of graphene and the relative contribution of out-of-plane modes, in agreement with previous experiments and theories. Increasing the coupling strength between graphene and substrate can further reduce the relative contribution of out-of-plane modes. The present investigations also show that the relative contribution of different mode phonons is not sensitive to the grain size of graphene. The obtained phonon relaxation time provides useful insight for understanding the phonon mean free path and the size effects in graphene.

Journal ArticleDOI
01 Oct 2014-Carbon
TL;DR: In this article, the in-plane thermal conductivity of monolayer graphene oxide (GO) with randomized surface epoxy and hydroxyl groups, at various degrees of oxidation (O:C ratio), is investigated using non-equilibrium molecular dynamics simulations.

Journal ArticleDOI
TL;DR: In this paper, a non-equilibrium molecular dynamics (MD) simulation of graphite thin films is used to construct the c-axis thermal conductivity as an accumulation function of phonon mean free path (MFP).
Abstract: Phonon transport in the c-axis direction of graphite thin films has been studied using non-equilibrium molecular dynamics (MD) simulation. The simulation results show that the c-axis thermal conductivities for films of thickness ranging from 20 to 500 atomic layers are significantly lower than the bulk value. Based on the MD data, a method is developed to construct the c-axis thermal conductivity as an accumulation function of phonon mean free path (MFP), from which we show that phonons with MFPs from 2 to 2000 nm contribute ∼80% of the graphite c-axis thermal conductivity at room temperature, and phonons with MFPs larger than 100 nm contribute over 40% to the c-axis thermal conductivity. These findings indicate that the commonly believed value of just a few nanometers from the simple kinetic theory drastically underestimates the c-axis phonon MFP of graphite.

Journal ArticleDOI
TL;DR: In this paper, the lattice thermal conductivity of Bi, Sb, and Bi-Sb alloys is calculated using first principles, and the relative contributions from phonons and electrons to the total thermal conductivities as a function of temperature are estimated.
Abstract: Using first principles, we calculate the lattice thermal conductivity of Bi, Sb, and Bi-Sb alloys, which are of great importance for thermoelectric and thermomagnetic cooling applications. Our calculation reveals that the ninth-neighbor harmonic and anharmonic force constants are significant; accordingly, they largely affect the lattice thermal conductivity. Several features of the thermal transport in these materials are studied: (1) the relative contributions from phonons and electrons to the total thermal conductivity as a function of temperature are estimated by comparing the calculated lattice thermal conductivity to the measured total thermal conductivity, (2) the anisotropy of the lattice thermal conductivity is calculated and compared to that of the electronic contribution in Bi, and (3) the phonon mean free path distributions, which are useful for developing nanostructures to reduce the lattice thermal conductivity, are calculated. The phonon mean free paths are found to range from 10 to 100 nm for Bi at 100 K.

Journal ArticleDOI
TL;DR: In this article, the authors used large hybrid (kinetic protons-fluid electrons) simulations to investigate the transport of energetic particles in self-consistent electromagnetic configurations of collisionless shocks.
Abstract: We use large hybrid (kinetic protons-fluid electrons) simulations to investigate the transport of energetic particles in self-consistent electromagnetic configurations of collisionless shocks. In previous papers of this series, we showed that ion acceleration may be very efficient (up to $10-20\%$ in energy), and outlined how the streaming of energetic particles amplifies the upstream magnetic field. Here, we measure particle diffusion around shocks with different strengths, finding that the mean free path for pitch-angle scattering of energetic ions is comparable with their gyroradii calculated in the self-generated turbulence. For moderately-strong shocks, magnetic field amplification proceeds in the quasi-linear regime, and particles diffuse according to the self-generated diffusion coefficient, i.e., the scattering rate depends only on the amount of energy in modes with wavelengths comparable with the particle gyroradius. For very strong shocks, instead, the magnetic field is amplified up to non-linear levels, with most of the energy in modes with wavelengths comparable to the gyroradii of highest-energy ions, and energetic particles experience Bohm-like diffusion in the amplified field. We also show how enhanced diffusion facilitates the return of energetic particles to the shock, thereby determining the maximum energy that can be achieved in a given time via diffusive shock acceleration. The parametrization of the diffusion coefficient that we derive can be used to introduce self-consistent microphysics into large-scale models of cosmic ray acceleration in astrophysical sources, such as supernova remnants and clusters of galaxies.

Journal ArticleDOI
TL;DR: In this article, the authors derived an analytical solution to the Boltzmann transport equation (BTE) to relate thermal conductivity measurements by thermoreflectance techniques to the bulk thermal conductivities accumulation function, which quantifies cumulative contributions from different mean free path energy carriers (here, phonons).
Abstract: We derive an analytical solution to the Boltzmann transport equation (BTE) to relate nondiffusive thermal conductivity measurements by thermoreflectance techniques to the bulk thermal conductivity accumulation function, which quantifies cumulative contributions to thermal conductivity from different mean free path energy carriers (here, phonons). Our solution incorporates two experimentally defined length scales: thermal penetration depth and heating laser spot radius. We identify two thermal resistances based on the predicted spatial temperature and heat flux profiles. The first resistance is associated with the interaction between energy carriers and the surface of the solution domain. The second resistance accounts for transport of energy carriers through the solution domain and is affected by the experimentally defined length scales. Comparison of the BTE result with that from conventional heat diffusion theory enables a mapping of mean-free-path-specific contributions to the measured thermal conductivity based on the experimental length scales. In general, the measured thermal conductivity will be influenced by the smaller of the two length scales and the surface properties of the system. The result is used to compare nondiffusive thermal conductivity measurements of silicon with first-principles-based calculations of its thermal conductivity accumulation function.

Journal ArticleDOI
TL;DR: In this article, the mass attenuation coefficients, μ / ρ, total interaction cross-section, σ t, and mean free path (MFP) of some Heavy Metal Oxides (HMO) glasses, with potential applications as gamma ray shielding materials, have been investigated using the MCNP-4C code.
Abstract: The mass attenuation coefficients, μ / ρ , total interaction cross-section, σ t , and mean free path ( MFP ) of some Heavy Metal Oxides (HMO) glasses, with potential applications as gamma ray shielding materials, have been investigated using the MCNP-4C code. Appreciable variations are noted for all parameters by changing the photon energy and the chemical composition of HMO glasses. The numerical simulations parameters are compared with experimental data wherever possible. Comparisons are also made with predictions from the XCOM program in the energy region from 1 keV to 100 MeV. Good agreement noticed indicates that the chosen Monte Carlo method may be employed to make additional calculations on the photon attenuation characteristics of different glass systems, a capability particularly useful in cases where no analogous experimental data exist.

Journal ArticleDOI
TL;DR: In this paper, the authors show that the normalized contact thermal conductance per unit area depends linearly on the tube diameter, which is consistent with an unexpected large phonon mean free path in the $c$-axis direction of graphite, phonon reflection at free surfaces, and phonon focusing in highly anisotropic graphitic materials.
Abstract: Measurements of thermal transport through contacts between individual multiwall carbon nanotubes show that, contrary to common expectation, the normalized contact thermal conductance per unit area depends linearly on the tube diameter. The result is corroborated with and extended to multilayer graphene nanoribbons through molecular dynamics simulations. Semiquantitative analyses show that these intriguing observations are consistent with an explanation based on an unexpectedly large phonon mean free path in the $c$-axis direction of graphite, phonon reflection at free surfaces, and phonon focusing in highly anisotropic graphitic materials.

Journal ArticleDOI
TL;DR: In this article, it has been inferred that increase in the composition of PbO leads to the formation of non bridging oxygens which leads to decrease in the rigidity of the glass samples.

Journal ArticleDOI
TL;DR: In this paper, the authors explore the possibility that the thermal diffusivity of an electrical insulator could include both a contribution of lattice phonons (the FT−G term) and a contribution from diffusive bulk phonon-polaritons (BPP) at infrared (IR) frequencies (the HT term).
Abstract: We show that laser-flash analysis measurements of the temperature (T) dependence of thermal diffusivity (D) for diverse non-metallic (e.g., silicates) single-crystals is consistently represented by D(T) = FT−G + HT above 298 K, with G ranging from 0.3 to 2, depending on structure, and H being ∼10−4 K−1 for 51 single-crystals, 3 polycrystals, and two glasses unaffected by disorder or reconstructive phase transitions. Materials exhibiting this behavior include complex silicates with variable amounts of cation disorder, perovskite structured materials, and graphite. The high-temperature term HT becomes important by ∼1300 K, above which temperature its contribution to D(T) exceeds that of the FT−G term. The combination of the FT−G and HT terms produces the nearly temperature independent high-temperature region of D previously interpreted as the minimal phonon mean free path being limited by the finite interatomic spacing. Based on the simplicity of the fit and large number of materials it represents, this finding has repercussions for high-temperature models of heat transport. One explanation is that the two terms describing D(T) are associated with two distinct microscopic mechanisms; here, we explore the possibility that the thermal diffusivity of an electrical insulator could include both a contribution of lattice phonons (the FT−G term) and a contribution of diffusive bulk phonon-polaritons (BPP) at infrared (IR) frequencies (the HT term). The proposed BPP diffusion exists over length scales smaller than the laboratory sample sizes, and transfers mixed light and vibrational energy at a speed significantly smaller than the speed of light. Our diffusive IR-BPP hypothesis is consistent with other experimental observations such as polarization behavior, dependence of D on the number of IR peaks, and H = 0 for Ge and Si, which lack IR fundamentals. A simple quasi-particle thermal diffusion model is presented to begin understanding the contribution from bulk phonon-polaritons to overall heat conduction.

Journal ArticleDOI
TL;DR: In this paper, the authors reformulated the linearized phonon Boltzmann transport equation by incorporating the direction-dependent phonon-boundary scattering, and derived their phonon mean free path spectrum.
Abstract: We reformulate the linearized phonon Boltzmann transport equation by incorporating the direction-dependent phonon-boundary scattering, and based on this equation, we study the thermal conductivity of Si1−xGex nanowires and derive their phonon mean free path spectrum. Due to the severe suppression of high-frequency phonons by alloy scattering, the low frequency phonons in Si1−xGex nanowires have a much higher contribution to the thermal conductivity than pure silicon nanowires. We also find that Si1−xGex nanowires possess a stronger length-dependent, weaker diameter-dependent, and weaker surface roughness-dependent thermal conductivity than silicon nanowires. These findings are potentially useful for engineering Si1−xGex nanowires for thermoelectric applications.

Journal ArticleDOI
TL;DR: In this article, the Anderson localization in graphene with short-range disorder using the real-space Kubo-Greenwood method implemented on graphics processing units was studied and the applicability of the one-parameter scaling theory of localization length was demonstrated.
Abstract: We study Anderson localization in graphene with short-range disorder using the real-space Kubo-Greenwood method implemented on graphics processing units. Two models of short-range disorder, namely, the Anderson on-site disorder model and the vacancy defect model, are considered. For graphene with Anderson disorder, localization lengths of quasi-one-dimensional systems with various disorder strengths, edge symmetries, and boundary conditions are calculated using the real-space Kubo-Greenwood formalism, showing excellent agreement with independent transfer matrix calculations and superior computational efficiency. Using these data, we demonstrate the applicability of the one-parameter scaling theory of localization length and propose an analytical expression for the scaling function, which provides a reliable method of computing the two-dimensional localization length. This method is found to be consistent with another widely used method which relates the two-dimensional localization length to the elastic mean free path and the semiclassical conductivity. Abnormal behavior at the charge neutrality point is identified and interpreted to be caused by finite-size effects when the system width is comparable to or smaller than the elastic mean free path. We also demonstrate the finite-size effect when calculating the two-dimensional conductivity in the localized regime and show that a renormalization group $\ensuremath{\beta}$ function consistent with the one-parameter scaling theory can be extracted numerically. For graphene with vacancy disorder, we show that the proposed scaling function of localization length also applies. Last, we discuss some ambiguities in calculating the semiclassical conductivity around the charge neutrality point due to the presence of resonant states.

Journal ArticleDOI
TL;DR: In this article, an enhanced Fourier law was proposed to account for the effect of low-frequency phonon modes of long mean-free path that propagate concomitantly to the dominant high-frequency modes.
Abstract: An enhanced Fourier law that we term the unified nondiffusive-diffusive (UND) phonon transport model is proposed in order to account for the effect of low-frequency phonon modes of long mean-free path that propagate concomitantly to the dominant high-frequency modes. The theory is based on spherical harmonic expansions of the phonon distribution functions, wherein the high-frequency mode distribution function is truncated at the first order in the expansion, while the low-frequency mode distribution function, which is farther out of thermal equilibrium, is truncated at the second order. As an illustrative application, the predictions of the proposed model are compared with data from a recent experiment that utilized the transient gratings method to investigate the deviation of thermal transport in a silicon membrane from the predictions of the Fourier law. The good fit of the experimental effective thermal conductivity (ETC) with the analytical solution derived in this work yields quantitative information about the mean-free path of the dominant low-frequency heat-transfer mode in silicon.

Journal ArticleDOI
TL;DR: In this paper, the effect of grain size on thermal conductivity of thin film barium titanate over temperatures ranging from 200 to 500 K was studied and it was shown that the thermal conductivities of Barium Titanate (BaTiO3) decreases with decreasing grain size as a result of increased phonon scattering from grain boundaries.
Abstract: We study the effect of grain size on thermal conductivity of thin film barium titanate over temperatures ranging from 200 to 500 K. We show that the thermal conductivity of Barium Titanate (BaTiO3) decreases with decreasing grain size as a result of increased phonon scattering from grain boundaries. We analyze our results with a model for thermal conductivity that incorporates a spectrum of mean free paths in BaTiO3. In contrast to the common gray mean free path assumption, our findings suggest that the thermal conductivity of complex oxide perovskites is driven by a spectrum of phonons with varying mean free paths.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the behavior of carrier populations generated at the interface of an n-Si wafer to an Si quantum dot (QD) array embedded in SiO2 with a photon flux ranging from 1.24 to 2.48
Abstract: We investigated the behavior of carrier populations generated at the interface of an n-Si wafer to an Si quantum dot (QD) array embedded in SiO2 with a photon flux ranging from 1.24 to 2.48 eV (1000 to 500 nm). The optically assisted IV method was used with the Si wafer as hot carrier (HC) absorber and the Si QD array as energy selective contact (ESC). Charge carriers obtain excess energy from photons with energies significantly exceeding the band gap, resulting in an HC population. This reduces the bias field required to provide kinetic energy by field emission. The ESC can collect HCs at lower bias voltages. Tunneling resonances show the energy selective behavior under illumination at 80 K and room temperature (295 K). The data at 80 K can arguably be interpreted as an HC population in Si within the range of the ballistic mean free path from the QD array. We discovered a correlation between the energetic shift of the average hot hole temperature near the valence band edge and the energy of the photons impinging on the mesa structures. The optically assisted IV technique delivers a proof of principle for operation of an Si QD array as an ESC at room temperature, and furthermore for an HC solar cell with one ESC at 80 K. Copyright © 2013 John Wiley & Sons, Ltd.

Journal ArticleDOI
TL;DR: In this paper, a method for measuring the mean free path and extracting the momentum relaxation time of hot electrons in GaN using the hot electron transistor (HET) was presented, where electrons are injected over a high energy emitter barrier into the base where they experience quasi-ballistic transport well above the conduction band edge.
Abstract: We present a method for measuring the mean free path and extracting the momentum relaxation time of hot electrons in GaN using the hot electron transistor (HET). In this device, electrons are injected over a high energy emitter barrier into the base where they experience quasi-ballistic transport well above the conduction band edge. After traversing the base, high energy electrons either surmount the base-collector barrier and become collector current or reflect off the barrier and become base current. We fabricate HETs with various base thicknesses and measure the common emitter transfer ratio (α) for each device. The mean free path is extracted by fitting α to a decaying exponential as a function of base width and the relaxation time is computed using a suitable injection velocity. For devices with an injection energy of ∼1 eV, we measure a hot electron mean free path of 14 nm and calculate a momentum relaxation time of 16 fs. These values are in agreement with theoretical calculations where longitudinal optical phonon scattering is the dominant momentum relaxation mechanism.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the conditions for producing rapid variations of solar energetic particle (SEP) intensity commonly known as "dropouts" using numerical model simulations based on solving the focused transport equation in the three-dimensional Parker interplanetary magnetic field.
Abstract: This paper investigates the conditions for producing rapid variations of solar energetic particle (SEP) intensity commonly known as "dropouts." In particular, we use numerical model simulations based on solving the focused transport equation in the three-dimensional Parker interplanetary magnetic field to put constraints on the properties of particle transport coefficients in both directions perpendicular and parallel to the magnetic field. Our calculations of the temporal intensity profile of 0.5 and 5 MeV protons at the Earth show that the perpendicular diffusion must be small while the parallel mean free path is long in order to reproduce the phenomenon of SEP dropouts. When the parallel mean free path is a fraction of 1 AU and the observer is located at 1 AU, the perpendicular to parallel diffusion ratio must be below 10(-5) if we want to see the particle flux dropping by at least several times within 3 hr. When the observer is located at a larger solar radial distance, the perpendicular to parallel diffusion ratio for reproducing the dropouts should be even lower than that in the case of 1 AU distance. A shorter parallel mean free path or a larger radial distance from the source to observer will cause the particles to arrive later, making the effects of perpendicular diffusion more prominent and SEP dropouts disappear. All of these effects require the magnetic turbulence that resonates with the particles to be low everywhere in the inner heliosphere.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the relative contribution of different branches of the phonon Boltzmann transport equation to the thermal conductivity of graphene ribbons, focusing on the effects of their size and temperature.
Abstract: Although graphene holds great promise in thermal applications owing to its superior thermal conductivity, an intriguing question remains as to which polarizations and frequencies are dominant in its heat conduction. In this work, by incorporating the direction-dependent phonon-boundary scattering and the special selection rule for three-phonon scattering into the linearized phonon Boltzmann transport equation, we systematically investigate the relative contributions from longitudinal-acoustic, transverse-acoustic, and out-of-plane acoustic (ZA) branches to the thermal conductivity of graphene ribbons, focusing on the effects of their size and temperature. We find that the relative contribution from ZA branch to heat conduction increases with decreasing the size, specularity parameter, and temperature of graphene ribbons. Our analysis reveals that this change arises from the huge difference in the phonon dispersion and in the phonon mean free path of Umklapp process between in-plane and out-of-plane branches.

Journal ArticleDOI
TL;DR: In this article, the Boltzmann transport equation (BTE) is solved within a frequency-dependent relaxation time approximation, and the calculated lattice thermal conductivities in the BTE are found to be in good agreement with the values obtained in the NEMD.
Abstract: Thermal conductivity of a material is an important physical parameter in electronic and thermal devices, and as the device size shrinks down, its length-dependence becomes unable to be neglected. Even in micrometer scale devices, materials having a long mean free path of phonons, such as crystalline silicon (Si), exhibit a strong length dependence of the thermal conductivities that spans from the ballistic to diffusive thermal transport regime. In this work, through non-equilibrium molecular-dynamics (NEMD) simulations up to 17 μm in length, the lattice thermal conductivities are explicitly calculated for crystalline Si and up to 2 μm for amorphous Si. The Boltzmann transport equation (BTE) is solved within a frequency-dependent relaxation time approximation, and the calculated lattice thermal conductivities in the BTE are found to be in good agreement with the values obtained in the NEMD. The isotopic effects on the length-dependent lattice thermal conductivities are also investigated both in the crystalline and amorphous Si.