Showing papers on "Mean free path published in 2012"
TL;DR: In this article, it was shown that the high-field magnetoresistance of Bi2Te2Se is linear in field at fields up to B=14T and the slope of this linear-like MR is nearly independent of temperature over the range T=7 to 150K.
Abstract: In addition to the weak antilocalization cusp observed in the magnetoresistance (MR) of topological insulators at low temperatures and low magnetic fields, we find that the high-field MR in Bi2Te2Se is linear in field. At fields up to B=14T the slope of this linear-like MR is nearly independent of temperature over the range T=7 to 150K. We find that the linear MR arises from the competition between a logarithmic phase coherence component and a quadratic component. The quantum phase coherence dominates up to high temperatures, where the coherence length remains longer than the mean free path of electrons.
158 citations
TL;DR: The attenuation coefficients of barium-bismuth-borosilicate glasses have been measured for gammaray photon energies of 662, 1173 and 1332 keV using a narrow beam transmission geometry as discussed by the authors.
152 citations
TL;DR: In this article, the thermal conductivity of PbTe bulk is first studied in the temperature range 300-800 K, where a small vacancy concentration is taken into consideration. And the authors found that the majority of thermal conductivities are contributed by acoustic phonon modes with mean free paths below 100nm.
151 citations
TL;DR: The low-temperature in-plane resistivity has an activated rather than the usual T(5) temperature dependence, suggesting a gapping of effective scattering that is consistent with phonon drag.
Abstract: We present de Haas-van Alphen and resistivity data on single crystals of the delafossite PdCoO(2). At 295 K we measure an in-plane resistivity of 2.6 μΩ cm, making PdCoO(2) the most conductive oxide known. The low-temperature in-plane resistivity has an activated rather than the usual T(5) temperature dependence, suggesting a gapping of effective scattering that is consistent with phonon drag. Below 10 K, the transport mean free path is ∼20 μm, approximately 10(5) lattice spacings and an astoundingly high value for flux-grown crystals. We discuss the origin of these properties in light of our data.
140 citations
TL;DR: In this paper, a non-equilibrium molecular dynamics study on the size-dependent thermal conductivity in single-walled carbon nanotubes with lengths up to micrometers at room temperature was performed.
Abstract: In this paper, we report a non-equilibrium molecular dynamics study on the size-dependent thermal conductivity in single-walled carbon nanotubes with lengths up to micrometers at room temperature. It is found that the size-dependent thermal conductivity of single-walled carbon nanotubes can be described by κ(L,d)≈κg(L)(1−e−0.185d/a0), where L is the tube length, d is the diameter, a0=2.46 A is the graphene lattice constant, and κg(L)∝Lα is the thermal conductivity of a graphene of length L. In the above, α=1 for L l0, independent of the tube chirality (zigzag or armchair), where l0≈200 nm and 300 nm are the effective phonon mean free path for zigzag and armchair tubes, respectively. Physical interpretations of such geometry dependence are provided in the paper by analyzing the spectral energy density, the dispersion relationship, the phonon density of state, and the power spectrum of phonons.
101 citations
TL;DR: In this paper, the effect of pyrochlore structure on the thermophysical properties of rare-earth stannates was investigated by analyzing the variation in the inverse phonon mean free path with temperature in terms of the phonon-scattering theory.
98 citations
TL;DR: In this paper, Monte Carlo sampling of the free paths associated with phononphonon and phonon-boundary scattering is used to predict the thermal conductivity of a nanostructure with arbitrary geometry.
Abstract: We propose a method by which the thermal conductivity of a nanostructure with arbitrary geometry can be predicted through Monte Carlo sampling of the free paths associated with phonon-phonon and phonon-boundary scattering. The required inputs are the nanostructure geometry and the bulk phonon frequencies, group velocities, and mean free paths. The method is applied to a thin film in the in-plane and cross-plane directions and to a polycrystalline bulk material. For the film, a faster approach to the bulk thermal conductivity is found compared to predictions made using the Matthiessen rule with the bulk mean free path and an average phonon-boundary scattering length.
95 citations
TL;DR: In this paper, partial coherent treatment of phonons, where phonons are regarded as either wave or particle depending on their frequencies, was considered, and the transport in this regime was modeled by BTE with phonon boundary scattering taken into account.
Abstract: Recent experiments [Yu et al., Nature Nanotech 5, 718 (2010); Tang et al., Nano Lett. 10, 4279 (2010); Hopkins etal., Nano Lett. 11, 107(2011)] on silicon based nanoscale phononic crystals demonstrated substantially reduced thermal conductivity compared to bulk Si, which cannot be explained by incoherent phonon boundary scattering within the Boltzmann Transport Equation (BTE). In this paper, partial coherent treatment of phonons, where phonons are regarded as either wave or particles depending on their frequencies, was considered. Phonons with mean free path smaller than the characteristic size of phononic crystals are treated as particles and the transport in this regime is modeled by BTE with phonon boundary scattering taken into account. On the other hand, phonons with mean free path longer than the characteristic size are treated as waves. In this regime, phonon dispersion relations are computed using the Finite Difference Time Domain (FDTD) method and are found to be modified due to the zone folding ...
94 citations
TL;DR: In this article, a simple universal equation for the computation of attenuation lengths (L) for any material, necessary for quantifying layer thicknesses in XPS, is presented.
Abstract: An analysis is presented for a simple, universal equation for the computation of attenuation lengths (L) for any material, necessary for quantifying layer thicknesses in XPS. Attenuation lengths for selected materials may be computed from the inelastic mean free path (lOpt) computed, in turn, from optical data. The computation of L involves the transport mean free path and gives good L values where values of lOpt are available. However, lOpt values are not available for all materials. Instead, l may be calculated from the TPP 2M relation, but this requires the accurate estimation of a number of materials parameters that vary over a wide range. Although these procedures are all soundly based, they are impractical in many analytical situations. L is therefore simply re-expressed, here, in terms of the average Z of the layer (from XPS analysis), the average atomic size, a (varies in a small range) and the kinetic energy E of the emitted electron. A new equation, "S3", is established with an RMS deviation of 8% compared with the values of attenuation length calculated from lOpt available for elements, inorganic compounds and organic compounds. This excellent result is suitable for practical analysis. In many films, an average value of a of 0.25 nm is appropriate and then L may be expressed only in terms of the average Z and E. Then, L expressed in monolayers, equation "S4", exhibits an RMS deviation of 9% for many elements. These results are valid for the energy range 100 to 30000 eV and for angles of emission up to 65o.
93 citations
TL;DR: In this article, the authors present measurements of S1 exciton transport in (6,5) carbon nanotubes at room temperature in a colloidal environment, and attribute the observed transport to disorder-limited diffusional transport associated with the dynamics of the colloidal interface.
Abstract: We present measurements of S1 exciton transport in (6,5) carbon nanotubes at room temperature in a colloidal environment. Exciton diffusion lengths associated with end quenching paired with photoluminescence lifetimes provide a direct basis for determining a median diffusion constant of approximately 7.5 cm2s–1. Our experimental results are compared to model diffusion constants calculated using a realistic exciton dispersion accounting for a logarithmic correction due to the exchange self-energy and a nonequilibrium distribution between bright and dark excitons. The intrinsic diffusion constant associated with acoustic phonon scattering is too large to explain the observed diffusion length, and as such, we attribute the observed transport to disorder-limited diffusional transport associated with the dynamics of the colloidal interface. In this model an effective surface potential limits the exciton mean free path to the same size as that of the exciton wave function, defined by the strength of the electro...
72 citations
TL;DR: In this paper, the Boltzmann transport equation for charge carriers and phonons is solved for Mg2Si and it is shown that bulk nanostructuring is not an efficient method to enhance the figure-of-merit as the PMFP and CMFP are in the same range.
Abstract: In nanostructured bulk materials, the additional interfaces in the material enhance phonon scattering and reduce the thermal conductivity. However, interfaces also scatter electrons and deteriorate charge carrier transport. In order to benefit from the interfacial effects, the crystallite size in the material must be small compared with phonon mean free path (PMFP) and large compared with the charge carrier mean free path (CMFP). In this paper, we solve the Boltzmann transport equation for charge carriers and phonons. We show that bulk nanostructuring of Mg2Si is not an efficient method to enhance the figure-of-merit as the PMFP and CMFP are in the same range.
TL;DR: This work measures the thermal conductivity along the axial direction of SiNW arrays with varying nanowire diameters, doping concentrations, surface roughness, and internal porosities using nanosecond transient thermoreflectance.
Abstract: The nanoscale features in silicon nanowires (SiNWs) can suppress phonon propagation and strongly reduce their thermal conductivities compared to the bulk value. This work measures the thermal conductivity along the axial direction of SiNW arrays with varying nanowire diameters, doping concentrations, surface roughness, and internal porosities using nanosecond transient thermoreflectance. For SiNWs with diameters larger than the phonon mean free path, porosity substantially reduces the thermal conductivity, yielding thermal conductivities as low as 1 W/m/K in highly porous SiNWs. However, when the SiNW diameter is below the phonon mean free path, both the internal porosity and the diameter significantly contribute to phonon scattering and lead to reduced thermal conductivity of the SiNWs.
TL;DR: In this article, the role of roughness scattering on the electronic properties of graphene nanoribbons is numerically investigated using the nonequilibrium Green function formalism, along with an atomistic tight-binding model.
Abstract: The role of line-edge roughness scattering on the electronic properties of graphene nanoribbons is numerically investigated. The nonequilibrium Green function formalism, along with an atomistic tight-binding model, is employed. Our results indicate that, depending on the geometrical and roughness parameters, the transport of carriers can be in the diffusive or localization regime. We extract the mean free path and the localization length, which characterize the diffusive and localization regimes, respectively. In the diffusive regime, the conductance linearly decreases with length, whereas in the localization regime, it exponentially decreases with length. However, as the localization length depends on the carrier energy, an effective transport gap in this regime can be defined. This gap is evaluated as a function of the geometrical and roughness parameters, and its impact on the device performance is discussed.
TL;DR: Measurements in ballistic graphene mesoscopic wires are reported where the charge carrier mean free path is comparable to the wire width W and the obtained proportionality constant between R(c) and W differs from that of a classical semiconductor two-dimensional electron system.
Abstract: We report magnetotransport measurements in ballistic graphene mesoscopic wires where the charge carrier mean free path is comparable to the wire width W. Magnetoresistance curves show characteristic peak structures where the peak field scales with the ratio of cyclotron radius R(c) and wire width W as W/R(c)=0.9±0.1, due to diffusive boundary scattering. The obtained proportionality constant between R(c) and W differs from that of a classical semiconductor two-dimensional electron system in which W/R(c)=0.55.
TL;DR: A drift-diffusion approach that captures the main features of transverse spin effects in systems with arbitrary spin textures and generalizes the Valet-Fert theory is developed.
Abstract: Spins transverse to the magnetization of a ferromagnet only survive over a short distance. We develop a drift-diffusion approach that captures the main features of transverse spin effects in systems with arbitrary spin textures (e.g., vortices and domain walls) and generalizes the Valet-Fert theory. In addition to the standard characteristic lengths (mean free path for majority and minority electrons, and spin diffusion length), the theory introduces two length scales, the transverse spin coherence length l(⊥) and the (Larmor) spin precession length l(L). We show how l(L) and l(⊥) can be extracted from ab initio calculations or measured with giant magnetoresistance experiments. In long (adiabatic) domain walls, we provide an analytic formula that expresses the so-called "nonadiabatic" (or fieldlike) torque in terms of these length scales. However, this nonadiabatic torque is no longer a simple material parameter but depends on the actual spin texture: in thin (<10 nm) domain walls, we observe very significant deviations from the adiabatic limit.
TL;DR: Surface metallization of SrTiO3(001) by hydrogen adsorption is experimentally confirmed for the first time by photoemission spectroscopy and surface conductivity measurements indicating that the system is in a metallic conduction regime.
Abstract: Surface metallization of SrTiO3(001) by hydrogen adsorption is experimentally confirmed for the first time by photoemission spectroscopy and surface conductivity measurements The metallic state is assigned to a quantized state in the space-charge layer induced by electron doping from hydrogen atoms The measured two-dimensional (2D) conductivity is well above the 2D Ioffe-Regel limit indicating that the system is in a metallic conduction regime The mean free path of the surface electron is estimated to be several nanometers at room temperature
TL;DR: In this article, lead Selenide thin films were prepared by vacuum evaporation technique with different thickness ranges from 50 to 200nm on glass substrates. And the structural studies revealed that the prepared films are strongly oriented on (2 0 0) plane with rock-salt crystal structure.
TL;DR: It is shown that with an appropriate choice of the inner step size, the time-step restriction on the outer time step is similar to the stability condition for the diffusion equation, whereas the required number of inner steps does not depend on the mean free path.
Abstract: We investigate a projective integration scheme for a kinetic equation in the limit of vanishing mean free path in which the kinetic description approaches a diffusion phenomenon. The scheme first takes a few small steps with a simple, explicit method, such as a spatial centered flux/forward Euler time integration, and subsequently projects the results forward in time over a large time step on the diffusion time scale. We show that with an appropriate choice of the inner step size, the time-step restriction on the outer time step is similar to the stability condition for the diffusion equation, whereas the required number of inner steps does not depend on the mean free path. We also provide a consistency result. The presented method is asymptotic-preserving in the sense that the method converges to a standard finite volume scheme for the diffusion equation in the limit of vanishing mean free path. The analysis is illustrated with numerical results, and we present an application to the Su-Olson test.
TL;DR: It has been observed that among the selected concretes, steel magnetite offers maximum value for linear attenuation coefficient, mass attenuation coefficients, equivalent atomic number and least values in terms of penetration depth equivalent to mean free path and exposure buildup factors.
TL;DR: In this article, the effects of surface roughness and the nanowire-contact interface scattering on phonon thermal conductivity were studied at low temperatures using a Green's function method based on an elastic wave equation.
Abstract: Using a Green’s function method based on an elastic wave equation, the effects of surface roughness and the nanowire-contact interface scattering on phonon thermal conductivity are studied at low temperatures. It is found that the interface geometry between a nanowire and its contacts affects the transmission function at small energies related to the gapless modes and it gives rise to deviated results from the universal conductance. It is also shown that the surface roughness is crucial in the suppression of phonon thermal conductivity with reducing the nanowire size by averaging the transmission function over the rough-surface configurations. Furthermore, the phonon mean free path is proportional to the ratio of the correlation length and roughness heights quadratically as well as the cross-section area of the nanowire.
TL;DR: In this paper, a relationship between the optical force from a plane wave on small electric and magnetic dipolar particles, the transport cross section, and the scattering asymmetry parameter was established, and negative $g$ was predicted for a dilute suspension of both perfectly reflecting spheres as well as of lossless dielectric nanospheres made of moderate permittivity materials.
Abstract: Lossless dielectric nanospheres (made of nonmagnetic materials) with relatively low refraction index may present strong electric and magnetic dipolar resonances. We establish a relationship between the optical force from a plane wave on small electric and magnetic dipolar particles, the transport cross section, and the scattering asymmetry parameter $g$. In this way we predict negative $g$ (that minimize the transport mean free path below values of the scattering mean free path) for a dilute suspension of both perfectly reflecting spheres as well as of lossless dielectric nanospheres made of moderate permittivity materials, e.g., silicon or germanium nanospheres in the infrared region. Lossless dielectric Mie spheres of relatively low refraction index (as low as 2.2) are shown to present negative $g$ in specific spectral ranges.
TL;DR: In this article, the skeleton expansion is applied to pions in chiral perturbation theory to approximate the correlated correlation of the viscous part of the energy-momentum tensor.
Abstract: The shear viscosity of an interacting pion gas is studied using the Kubo formalism as a microscopic description of thermal systems close to global equilibrium. We implement the skeleton expansion in order to approximate the retarded correlator of the viscous part of the energy-momentum tensor. After exploring this in gφ
4 theory we show how the skeleton expansion can be consistently applied to pions in chiral perturbation theory. The shear viscosity η is determined by the spectral width, or equivalently, the mean free path of pions in the heat bath. We derive a new analytical result for the mean free path which is well conditioned for numerical evaluation and discuss the temperature and pion-mass dependence of the mean free path and the shear viscosity. The ratio η/s of the interacting pion gas exceeds the lower bound 1/4π from AdS/CFT correspondence.
TL;DR: In this paper, the effective thermal conductivity of porous Si by means of the phonon Boltzmann transport equation is computed, and it is shown that the roughness of the pore walls plays an important role.
Abstract: In this work we compute the effective thermal conductivity of porous Si by means of the phonon Boltzmann transport equation. Simulations of heat transport across aligned square pores reveal that the thermal conductivity can be decreased either by increasing the pore size or decreasing the pore spacing. Furthermore, by including the surface specularity parameter we show that the roughness of the pore walls plays an important role when the pore size is comparable with the phonon mean free path, because to the increase in the surface-to-volume ratio. Thanks to these results, in qualitatively agreement with those obtained with Molecular Dynamics simulations, we gained insights into the scaling of thermal properties of porous materials and interplay between disorder at different length scales. The model, being based on a flexible multiscale finite element context, can be easily integrated with electrical transport models, in order to optimize the figure of merit ZT of thermoelectric devices.
TL;DR: In this article, the charges of μm-size particles in the quasineutral bulk plasma of a dc discharge are determined experimentally in a pressure range between 100 and 500 Pa, spanning the transition between the weakly collisional and highly collisional regimes, where the ion mean free path drops below the plasma screening length.
Abstract: The charges of μm-size particles in the quasineutral bulk plasma of a dc discharge are determined experimentally in a pressure range between 100 and 500 Pa, spanning the transition between the weakly collisional and highly collisional (hydrodynamic) regimes, where the ion mean free path drops below the plasma screening length. The charge is determined using the force balance condition from the measured particle drift velocities in stable particle flows. A simple interpolation formula for the ion flux to the grain in the transitional regime is shown to fit quite well the experimental results.
TL;DR: In this article, spatial profiles of the plasma potential and electron energy distribution function (EEDF) were measured in inductively coupled plasma (ICP) under weakly collisional and electron nonlocal kinetic regimes.
Abstract: Spatial profiles of the plasma potential and electron energy distribution function (EEDF) were measured in inductively coupled plasma (ICP) under weakly collisional and electron nonlocal kinetic regimes The measured EEDF at the discharge center was a bi-Maxwellain distribution with low (T1) and high (T2) electron temperature groups, while the EEDF at the radial boundary was closely Maxwellian distribution due to cutting of the low energy electrons by relatively large ambipolar potential in this discharge regime The ambipolar potential in the entire radial region was in the scale of Teff − 15 Teff, where Teff is the effective electron temperature At the boundary region with the ion mean free path scale, the ambipolar potential increased abruptly and was about Teff,edge/2, where the Teff,edge is the effective electron temperature at the boundary, which corresponds to the presheath scale These results of the ICP, which are contrary to the ambipolar potential of capacitively coupled plasma in a nearly fr
TL;DR: In this paper, a direct analytical formula as a function of parameters concerning the physical properties of SEPs and solar wind was provided to directly and quickly determine the parallel mean free path with adiabatic focusing.
Abstract: The parallel mean free path of solar energetic particles (SEPs), which is determined by physical properties of SEPs as well as those of solar wind, is a very important parameter in space physics to study the transport of charged energetic particles in the heliosphere, especially for space weather forecasting. In space weather practice, it is necessary to find a quick approach to obtain the parallel mean free path of SEPs for a solar event. In addition, the adiabatic focusing effect caused by a spatially varying mean magnetic field in the solar system is important to the transport processes of SEPs. Recently, Shalchi presented an analytical description of the parallel diffusion coefficient with adiabatic focusing. Based on Shalchi's results, in this paper we provide a direct analytical formula as a function of parameters concerning the physical properties of SEPs and solar wind to directly and quickly determine the parallel mean free path of SEPs with adiabatic focusing. Since all of the quantities in the analytical formula can be directly observed by spacecraft, this direct method would be a very useful tool in space weather research. As applications of the direct method, we investigate the inherent relations between the parallel mean free path and various parameters concerning physical properties of SEPs and solar wind. Comparisons of parallel mean free paths with and without adiabatic focusing are also presented.
TL;DR: In this paper, an experimental and theoretical study of the electric field distribution in the cathode-fall (CF) region of an obstructed abnormal glow discharge in hydrogen is presented.
Abstract: We present the results of an experimental and theoretical study of the electric field distribution in the cathode-fall (CF) region of an obstructed abnormal glow discharge in hydrogen. The distribution of electric field strength was measured using an improved Stark polarization spectroscopy technique of the hydrogen Balmer beta line profile. The developed technique extends field measurements to the low-field region while remaining in good agreement at larger field strengths with the well established Stark polarization peak separation electric field measurement technique. The experimental electric field distribution is modeled by a simple analytical formula, adopted after consideration of elementary processes relevant in the CF. It fits the experimental distribution within error bars and consequently enables determination of the CF parameters such as its thickness and the mean free path for electrons in the CF region.
TL;DR: In this article, the luminescence properties of CaF2 nanoparticles with various sizes (20-140nm) were studied upon the excitation by VUV and x-ray quanta in order to reveal the influence of ratio of mean free path and thermalization length of charge carriers and nanoparticle size on the self-trapped exciton luminecence.
Abstract: The luminescence properties of CaF2 nanoparticles with various sizes (20–140 nm) are studied upon the excitation by VUV and x-ray quanta in order to reveal the influence of ratio of mean free path and thermalization length of charge carriers and nanoparticle size on the self-trapped exciton luminescence. The luminescence intensity for exciting quantum energies corresponding to optical creation of exciton and to the range of electronic excitation multiplication is not so sensitive to nanoparticle size as for quanta with energy of Eg < hν < 2Eg. The dependences of luminescence intensity on nanoparticle size at the excitation by quanta of various energies are discussed in terms of electron-phonon and electron-electron scattering lengths and energy losses on surface defects.
TL;DR: The mechanism leading to gas damping in microelectro-mechanical systems (MEMS) devices vibrating at high frequencies is investigated by using the linearized Boltzmann equation based on simplified kinetic models and diffuse reflection boundary conditions as mentioned in this paper.
Abstract: The mechanism leading to gas damping in micro-electro-mechanical systems (MEMS) devices vibrating at high frequencies is investigated by using the linearized Boltzmann equation based on simplified kinetic models and diffuse reflection boundary conditions. Above a certain frequency of oscillation, the sound waves propagating through the gas are trapped in the gaps between the moving elements and the fixed boundaries of the microdevice. In particular, we found a scaling law, valid for all Knudsen numbers Kn (defined as the ratio between the gas mean free path and a characteristic length of the gas flow), that predicts a resonant response of the system. This response enables a minimization of the damping force exerted by the gas on the oscillating wall of the microdevice.
TL;DR: In this article, the authors measured the dependence of the conductivity of graphene as a function of magnetic field, temperature, and carrier density and discovered a saturation of the dephasing length at low temperatures that they ascribe to spin memory effects.
Abstract: We measure the dependence of the conductivity of graphene as a function of magnetic field, temperature, and carrier density and discover a saturation of the dephasing length at low temperatures that we ascribe to spin memory effects. Values of the spin coherence length up to eight microns are found to scale with the mean free path. We consider different origins of this effect and suggest that it is controlled by resonant states that act as magneticlike defects. By varying the level of disorder, we demonstrate that the spin coherence length can be tuned over an order of magnitude.