Showing papers on "Mean free path published in 2017"

First-principles mode-by-mode analysis for electron-phonon scattering channels and mean free path spectra in GaAs

Abstract: We present a first-principles framework to investigate the electron scattering channels and transport properties for polar materials by combining the exact solution of the linearized electron-phonon (e-ph) Boltzmann transport equation in its integral-differential form associated with the e-ph coupling matrices obtained from the polar Wannier interpolation scheme. No ad hoc parameter is required throughout this calculation, and GaAs, a well-studied polar material, is used as an example to demonstrate this method. In this work, the long-range and short-range contributions as well as the intravalley and intervalley transitions in the e-ph interactions (EPIs) have been quantitatively addressed. Promoted by such mode-by-mode analysis, we find that in GaAs, the piezoelectric scattering is comparable to deformation-potential scattering for electron scatterings by acoustic phonons in EPI even at room temperature, and it makes a significant contribution to mobility. Furthermore, we achieved good agreement with experimental data for the mobility, and we identified that electrons with mean free paths between 130 and 210 nm provide the dominant contribution to the electron transport at 300 K. Such information provides a deeper understanding of the electron transport in GaAs, and the presented framework can be readily applied to other polar materials.

Investigation of gamma radiation shielding properties of lithium zinc bismuth borate glasses using XCOM program and MCNP5 code

Abstract: In this work, we examined the usefulness of the lithium zinc bismuth borate glass systems for various radiation shielding applications and for this purpose, the mass attenuation coefficients for the glasses in the composition 50 Bi2O3–15 B2O3–(35-x) ZnO–(x) Li2O (x = 0, 5, 10, 15, 20 mol%) were calculated by both XCOM software and MCNP5 simulation code, respectively, within the energy range 0.015 MeV–10 MeV. The obtained results indicated good agreement between mass attenuation coefficient values derived from XCOM program and MCNP5 code. The obtained mass attenuation coefficients are then used to calculate the effective atomic number (Zeff), half value layer (HVL) and mean free path (MFP) for the glasses. Among the selected glasses, the glass with 35 mol% ZnO was found to possess superior gamma-ray shielding effectiveness due to its higher values of both mass attenuation coefficient and effective atomic number and lower values of both HVL and MFP. The MFP values of the present glasses were compared with different glass systems and ordinary concrete. In addition, the macroscopic effective removal cross-section for fast neutron (ΣR) values was also evaluated. It is found that the ΣR values for the studied glasses lie within the range 0.1286–0.1587 cm− 1.

Thickness dependence of the resistivity of platinum-group metal thin films

Abstract: We report on the thin film resistivity of several platinum-group metals (Ru, Pd, Ir, and Pt). Platinum-group thin films show comparable or lower resistivities than Cu for film thicknesses below about 5 nm due to a weaker thickness dependence of the resistivity. Based on experimentally determined mean linear distances between grain boundaries as well as ab initio calculations of the electron mean free path, the data for Ru, Ir, and Cu were modeled within the semiclassical Mayadas–Shatzkes model [Phys. Rev. B 1, 1382 (1970)] to assess the combined contributions of surface and grain boundary scattering to the resistivity. For Ru, the modeling results indicated that surface scattering was strongly dependent on the surrounding material with nearly specular scattering at interfaces with SiO2 or air but with diffuse scattering at interfaces with TaN. The dependence of the thin film resistivity on the mean free path is also discussed within the Mayadas–Shatzkes model in consideration of the experimental findings.

Thickness dependence of the resistivity of Platinum group metal thin films

Abstract: We report on the thin film resistivity of several platinum-group metals (Ru, Pd, Ir, Pt). Platinum-group thin films show comparable or lower resistivities than Cu for film thicknesses below about 5\,nm due to a weaker thickness dependence of the resistivity. Based on experimentally determined mean linear distances between grain boundaries as well as ab initio calculations of the electron mean free path, the data for Ru, Ir, and Cu were modeled within the semiclassical Mayadas--Shatzkes model [Phys. Rev. B 1, 1382 (1970)] to assess the combined contributions of surface and grain boundary scattering to the resistivity. For Ru, the modeling results indicated that surface scattering was strongly dependent on the surrounding material with nearly specular scattering at interfaces with SiO2 or air but with diffuse scattering at interfaces with TaN. The dependence of the thin film resistivity on the mean free path is also discussed within the Mayadas--Shatzkes model in consideration of the experimental findings.

Gamma-ray shielding properties of zinc oxide soda lime silica glasses

Abstract: In the present work, the gamma ray shielding properties of zinc oxide soda lime silica, (ZnO)x(SLS)1−x glasses with 0 ≥ x ≥ 50 wt% have been investigated. By using WinXCom computer software, the mass attenuation coefficient (µ/ρ) and half value layer (HVL) for total photon interaction in the energy range of 1 keV–100 GeV were calculated. Furthermore and by Geometric Progression method exposure buildup factor values were calculated for incident photon energy 0.015–15 MeV up to penetration depths of 40 mfp (mean free path). The addition of zinc oxide (ZnO) into soda lime silica (SLS) glass resulted in an increase the mass attenuation coefficient and decreases both the half value layer and exposure buildup factor. The obtained results of the selected glass series have been compared, in terms of mass attenuation coefficient, half value layer and exposure buildup factor with some common shielding materials. The shielding effectiveness of the selected glasses is found comparable to that of common ones; which indicates that the SLS glasses with suitable ZnO content may be developed as gamma ray shielding materials.

Mesoscopic Free Path of Nonthermalized Photogenerated Carriers in a Ferroelectric Insulator

Abstract: We show how finite-size scaling of a bulk photovoltaic effect-generated electric field in epitaxial ferroelectric insulating BaTiO_{3}(001) films and a photo-Hall response involving the bulk photovoltaic current reveal a large room-temperature mean free path of photogenerated nonthermalized electrons. Experimental determination of mesoscopic ballistic optically generated carrier transport opens a new paradigm for hot electron-based solar energy conversion, and for facile control of ballistic transport distinct from existing low-dimensional semiconductor interfaces, surfaces, layers, or other structures.

Kinetic effects in dynamic wetting

Abstract: The maximum speed at which a liquid can wet a solid is limited by the need to displace gas lubrication films in front of the moving contact line. The characteristic height of these films is often comparable to the mean free path in the gas so that hydrodynamic models do not adequately describe the flow physics. This Letter develops a model which incorporates kinetic effects in the gas, via the Boltzmann equation, and can predict experimentally observed increases in the maximum speed of wetting when (a) the liquid's viscosity is varied, (b) the ambient gas pressure is reduced, or (c) the meniscus is confined.

Phonon-boundary scattering in nanoporous silicon films: Comparison of Monte Carlo techniques

Abstract: The thermal conductivities of silicon thin films with periodic pore arrays (ie, nanoporous films) and square silicon nanowires are predicted at a temperature of 300 K The bulk phonon properties are obtained from lattice dynamics calculations driven by first-principles calculations Phonon-boundary scattering is included by applying three Monte Carlo-based techniques that treat phonons as particles The first is a path sampling technique that modifies the intrinsic bulk mean free paths without using the Matthiessen rule The second uses ray-tracing under an isotropic assumption to calculate a single, mode-independent boundary scattering mean free path that is combined with the intrinsic bulk mean free paths using the Matthiessen rule The third modifies the ray-tracing technique to calculate the boundary scattering mean free path on a modal basis For the square nanowire modeled using isotropic ray-tracing, the maximum mean free path is comparable to the wire width, an unphysical result that is a consequence of the isotropic approximation Free path sampling and modal ray-tracing produce physically meaningful mean free path distributions The nanoporous film thermal conductivity predictions match a previously measured trend, suggesting that coherent effects are not relevant to thermal transport at room temperature A line-of-sight for phonons in the nanoporous films is found to change how thermal conductivity scales with porosity

Thermal conductivity of Bi2Te3 nanowires: how size affects phonon scattering

Abstract: This work provides an in-depth study of how the thermal conductivity of stoichiometric [110] Bi2Te3 nanowires becomes affected when reducing its diameter from an experimental and theoretical point of view. The thermal conductivity was observed to decrease more than 70% (from 1.78 ± 0.46 W K-1 m-1 to 0.52 ± 0.35 W K-1 m-1) when the diameter of the nanowire was reduced one order of magnitude (from 300 nm to 25 nm). The Kinetic-Collective model was used to understand such a reduction, which can be explained by the impact that surface scattering has in acoustic phonons. The smaller the diameter of the nanowires is, the larger the alteration in the mean free path of the low-frequency phonons is. The model agrees well with the experimental data, and the reduction in the thermal conductivity of the nanowires can be explained in terms of an increment of phonon scattering.

Mean path length invariance in multiple light scattering

Abstract: Our everyday experience teaches us that the structure of a medium strongly influences how light propagates through it. A disordered medium, e.g., appears transparent or opaque, depending on whether its structure features a mean free path that is larger or smaller than the medium thickness. While the microstructure of the medium uniquely determines the shape of all penetrating light paths, recent theoretical insights indicate that the mean length of these paths is entirely independent of any structural medium property and thus also invariant with respect to a change in the mean free path. Here, we report an experiment that demonstrates this surprising property explicitly. Using colloidal solutions with varying concentration and particle size, we establish an invariance of the mean path length spanning nearly two orders of magnitude in scattering strength, from almost transparent to very opaque media. This very general, fundamental and counterintuitive result can be extended to a wide range of systems, however ordered, correlated or disordered, and has important consequences for many fields, including light trapping and harvesting for solar cells and more generally in photonic structure design.

Resistivity bound for hydrodynamic bad metals.

Abstract: We obtain a rigorous upper bound on the resistivity [Formula: see text] of an electron fluid whose electronic mean free path is short compared with the scale of spatial inhomogeneities. When such a hydrodynamic electron fluid supports a nonthermal diffusion process-such as an imbalance mode between different bands-we show that the resistivity bound becomes [Formula: see text] The coefficient [Formula: see text] is independent of temperature and inhomogeneity lengthscale, and [Formula: see text] is a microscopic momentum-preserving scattering rate. In this way, we obtain a unified mechanism-without umklapp-for [Formula: see text] in a Fermi liquid and the crossover to [Formula: see text] in quantum critical regimes. This behavior is widely observed in transition metal oxides, organic metals, pnictides, and heavy fermion compounds and has presented a long-standing challenge to transport theory. Our hydrodynamic bound allows phonon contributions to diffusion constants, including thermal diffusion, to directly affect the electrical resistivity.

Surface roughness dependence of the electrical resistivity of W(001) layers

Abstract: The resistivity ρ of epitaxial W(001) layers grown on MgO(001) at 900 °C increases from 5.63 ± 0.05 to 27.6 ± 0.6 μΩ-cm with decreasing thickness d = 390 to 4.5 nm. This increase is due to electron-surface scattering but is less pronounced after in situ annealing at 1050 °C, leading to a 7%–13% lower ρ for d < 20 nm. The ρ(d) data from in situ and ex situ transport measurements at 295 and 77 K cannot be satisfactorily described using the existing Fuchs-Sondheimer (FS) model for surface scattering, as ρ for d < 9 nm is larger than the FS prediction and the annealing effects are inconsistent with a change in either the bulk mean free path or the surface scattering specularity. In contrast, introducing an additive resistivity term ρmound which accounts for surface roughness resolves both shortcomings. The new term is due to electron reflection at surface mounds and is, therefore, proportional to the ballistic resistance times the average surface roughness slope, divided by the layer thickness. This is confir...

The importance of the electron mean free path for superconducting radio-frequency cavities

Abstract: Impurity-doping of niobium is an exciting new technology in the field of superconducting radio-frequency accelerators, producing cavities with record-high quality factor Q0 and Bardeen-Cooper-Schrieffer surface resistance that decreases with increasing radio-frequency field. Recent theoretical work has offered a promising explanation for this so-called “anti-Q-slope,” but the link between the decreasing surface resistance and the shortened electron mean free path of doped cavities has remained elusive. In this work, we investigate this link, finding that the magnitude of this decrease varies directly with the mean free path: shorter mean free paths correspond to stronger anti-Q-slopes. We draw a theoretical connection between the mean free path and the overheating of the quasiparticles, which leads to the reduction of the anti-Q-slope towards the normal Q-slope of long-mean-free-path cavities. We also investigate the sensitivity of the residual resistance to trapped magnetic flux, a property that is great...

Resonant transport and near-field effects in photonic glasses

Abstract: A fundamental quantity in multiple scattering is the transport mean free path the inverse of which describes the scattering strength of a sample. In this paper, we emphasize the importance of an appropriate description of the effective refractive index ${n}_{\mathrm{eff}}$ in multiple light scattering to accurately describe the light transport in dense photonic glasses. Using ${n}_{\mathrm{eff}}$ as calculated by the energy-density coherent-potential approximation we are able to predict the transport mean free path of monodisperse photonic glasses. This model without any fit parameter is in qualitative agreement with numerical simulations and in fair quantitative agreement with spectrally resolved coherent backscattering measurements on new specially synthesized polystyrene photonic glasses. These materials exhibit resonant light scattering perturbed by strong near-field coupling, all captured within the model. Our model might be used to maximize the scattering strength of high index photonic glasses, which are a key in the search for Anderson localization of light in three dimensions.

Transport regimes spanning magnetization-coupling phase space

Abstract: The manner in which transport properties vary over the entire parameter-space of coupling and magnetization strength is explored. Four regimes are identified based on the relative size of the gyroradius compared to other fundamental length scales: the collision mean free path, Debye length, distance of closest approach, and interparticle spacing. Molecular dynamics simulations of self-diffusion and temperature anisotropy relaxation spanning the parameter space are found to agree well with the predicted boundaries. Comparison with existing theories reveals regimes where they succeed, where they fail, and where no theory has yet been developed.

Length Scale of Diffusive Phonon Transport in Suspended Thin Silicon Nanowires.

Abstract: Recent experimental advances have revealed that the mean free path (mfp) of phonons contributing significantly to thermal transport in crystalline semiconductors can be several microns long. Almost all of these experiments are based on bulk and thin film materials and use techniques that are not directly applicable to nanowires. By developing a process with which we could fabricate multiple electrically contacted and suspended segments on individual heavily doped smooth Silicon nanowires, we measured phonon transport across varying length scales using a DC self-heating technique. Our measurements show that diffusive thermal transport is still valid across O(100) nm length scales, supporting the diffuse nature of phonon-boundary scattering even on smooth nanowire surfaces. Our work also showcases the self-heating technique as an important alternative to the thermal bridge technique to measure phonon transport across short length scales relevant to mapping the phonon mfp spectrum in nanowires.

Cyclotron resonant scattering feature simulations I. Thermally averaged cyclotron scattering cross sections, mean free photon-path tables, and electron momentum sampling

Abstract: Context. Electron cyclotron resonant scattering features (CRSFs) are observed as absorption-like lines in the spectra of X-ray pulsars. A significant fraction of the computing time for Monte Carlo simulations of these quantum mechanical features is spent on the calculation of the mean free path for each individual photon before scattering, since it involves a complex numerical integration over the scattering cross section and the (thermal) velocity distribution of the scattering electrons. Aims. We aim to numerically calculate interpolation tables which can be used in CRSF simulations to sample the mean free path of the scattering photon and the momentum of the scattering electron. The tables also contain all the information required for sampling the scattering electron’s final spin. Methods. The tables were calculated using an adaptive Simpson integration scheme. The energy and angle grids were refined until a prescribed accuracy is reached. The tables are used by our simulation code to produce artificial CRSF spectra. The electron momenta sampled during these simulations were analyzed and justified using theoretically determined boundaries. Results. We present a complete set of tables suited for mean free path calculations of Monte Carlo simulations of the cyclotron scattering process for conditions expected in typical X-ray pulsar accretion columns (0.01 ≤ B/B_(crit) ≤ 0.12, where B_(crit) = 4.413 × 10^(13) G, and 3 keV ≤ k_BT ≤ 15 keV). The sampling of the tables is chosen such that the results have an estimated relative error of at most 1/15 for all points in the grid. The tables are available online (see link in footnote, page 1).

The Role of Diffusion in the Transport of Energetic Electrons during Solar Flares

Abstract: The transport of the energy contained in suprathermal electrons in solar flares plays a key role in our understanding of many aspects of flare physics, from the spatial distributions of hard X-ray emission and energy deposition in the ambient atmosphere to global energetics. Historically the transport of these particles has been largely treated through a deterministic approach, in which first-order secular energy loss to electrons in the ambient target is treated as the dominant effect, with second-order diffusive terms (in both energy and angle) generally being either treated as a small correction or even neglected. Here, we critically analyze this approach, and we show that spatial diffusion through pitch-angle scattering necessarily plays a very significant role in the transport of electrons. We further show that a satisfactory treatment of the diffusion process requires consideration of non-local effects, so that the electron flux depends not just on the local gradient of the electron distribution function but on the value of this gradient within an extended region encompassing a significant fraction of a mean free path. Our analysis applies generally to pitch-angle scattering by a variety of mechanisms, from Coulomb collisions to turbulent scattering. We further show that the spatial transport of electrons along the magnetic field of a flaring loop can be modeled rather effectively as a Continuous Time Random Walk with velocity-dependent probability distribution functions of jump sizes and occurrences, both of which can be expressed in terms of the scattering mean free path.

Disorder-dominated linear magnetoresistance in topological insulator Bi2Se3 thin films

Abstract: The linear magnetoresistance (MR) effect is an interesting topic due to its potential applications. In topological insulator Bi2Se3, this effect has been reported to be dominated by the carrier mobility (μ) and hence has a classical origin. Here, we study the magnetotransport properties of Bi2Se3 thin films and observe the linear MR effect, which cannot be attributed to the quantum model. Unexpectedly, the linear MR does not show the linear dependence on μ, in conflict with the reported results. However, we find that the observed linear MR is dominated by the inverse disorder parameter 1/kFl, where kF and l are the Fermi wave vector and the mean free path, respectively. This suggests that its origin is also classical and that no μ-dominated linear MR effect is observed which may be due to the very small μ values in our samples.

Amorphous two-dimensional black phosphorus with exceptional photocarrier transport properties

Abstract: Recently, two-dimensional materials have been extensively studied. Due to the reduced dielectric screening and confinement of electrons in two dimensions, these materials show dramatically different electronic and optical properties from their bulk counterparts. So far, studies on two-dimensional materials have mainly focused on crystalline materials. Here we report studies of atomically thin amorphous black phosphorus, as the first two-dimensional amorphous material. Spatially and temporally resolved pump-probe measurements show that large-area and uniform atomic layers of amorphous black phosphorus, synthesized at low temperature, possess a long exciton lifetime of about 400 ps, a room-temperature exciton diffusion coefficient of 5 cm2 s−1, which is at least two orders of magnitude larger than amorphous silicon, and an exciton mobility of about 200 cm2 V−1 s−1. We also deduce from these values an exciton mean free time of 50 fs, a mean free path of 5 nm, and a diffusion length of 450 nm. These results suggest that amorphous black phosphorus can be potentially used in low-cost optoelectronic devices.

Acoustic phonon softening and reduced thermal conductivity in Mg2Si1−xSnx solid solutions

Abstract: The measured thermal conductivity κph of Mg2Si1−xSnx solid solutions exhibits a dramatic decrease compared with Mg2Si and Mg2Sn. By solving the full Boltzmann equation, the significantly reduced κph was quantitatively reproduced by our calculations. Besides the expected increase in phonon-phonon scattering, we also observed acoustic phonon softening in Mg2Si1−xSnx, despite their smaller average atomic mass relative to Mg2Sn. In agreement with measurements, the lowest κph appears in Mg2Si0.375Sn0.625, which has the shortest mean free path and lowest group velocity. According to our calculated cumulative curve, nanoinclusions of tens of nanometers are suggested for the further reduction of κph in Mg2Si1−xSnx solid solution.