Showing papers on "Mean free path published in 2007"
TL;DR: An unusually long mean free path at room temperature has been experimentally confirmed and suggests anomalous localization effects in single-walled carbon nanotube devices with channel lengths that vary 4 orders of magnitude.
Abstract: We present an experimental investigation on the scaling of resistance in individual single-walled carbon nanotube devices with channel lengths that vary 4 orders of magnitude on the same sample. The electron mean free path is obtained from the linear scaling of resistance with length at various temperatures. The low temperature mean free path is determined by impurity scattering, while at high temperature, the mean free path decreases with increasing temperature, indicating that it is limited by electron-phonon scattering. An unusually long mean free path at room temperature has been experimentally confirmed. Exponentially increasing resistance with length at extremely long length scales suggests anomalous localization effects.
301 citations
TL;DR: In this paper, a modified effective medium formulation for composites where the characteristic length of the inclusion is on the order of or smaller than the phonon mean free path is introduced.
Abstract: This letter introduces a modified effective medium formulation for composites where the characteristic length of the inclusion is on the order of or smaller than the phonon mean free path. The formulation takes into account the increased interface scattering in the different phases of the nanocomposite and the thermal boundary resistance between the phases. The interface density of inclusions is introduced and is found to be a primary factor in determining the thermal conductivity. The predictions are in good agreement with results from Monte Carlo simulations and solutions to the Boltzmann equation.
249 citations
TL;DR: In this paper, the thermal conductivity of MOF-5 single crystals is measured over a wide temperature range between 6 K and 300 K, using the longitudinal, steady-state heat flow method.
207 citations
TL;DR: In this paper, the resistance of single and multi-wall carbon nanotubes (MWCNs) has been investigated and compared with the model presented in this paper for single and few-wall MCN.
Abstract: Equivalent circuit models are presented for the resistance of single- and multi-wall carbon nanotubes (MWCNs) that capture various electron-phonon scattering mechanisms as well as changes in the number of conduction channels as a function of temperature. For single- and few-wall nanotubes, the temperature coefficient of resistance (TCR) is always positive and increases with length. It reaches 1/(T-200 K) for lengths much larger than the electron mean free path, where T is the temperature in kelvin. For MWCNs with large diameters (>20 nm), TCR varies from -1/T to +0.66/(T-200 K) as the length varies from zero to very large values
161 citations
TL;DR: In this paper, the Boltzmann transport mean free path (LBP) and diffusion constant (DB) were derived in the weak scattering regime in two and in three dimensions.
Abstract: This paper studies multiple scattering of matter waves by a disordered optical potential in two and in three dimensions. We calculate fundamental transport quantities such as the scattering mean free path ls, the Boltzmann transport mean free path lB, and the Boltzmann diffusion constant DB, using a diagrammatic Green functions approach in the weak-scattering regime. Coherent multiple scattering induces interference corrections known as weak localization which entail a reduced diffusion constant. We derive the corresponding expressions for matter wave transport in a correlated speckle potential and provide the relevant parameter values for a possible experimental study of this coherent transport regime, including the critical crossover to the regime of strong or Anderson localization.
131 citations
TL;DR: In this paper, the spatial structure of electron transport in three different GaAs/AlGaAs 2DEGs, whose mobilities range over an order of magnitude, was studied.
Abstract: GaAs-based two-dimensional electron gases (2DEGs) show a wealth of remarkable electronic states1,2,3, and serve as the basis for fast transistors, research on electrons in nanostructures4,5 and prototypes of quantum-computing schemes6. All of these uses depend on the extremely low levels of disorder in GaAs 2DEGs, with low-temperature mean free paths ranging from micrometres to hundreds of micrometres7. Here we study how disorder affects the spatial structure of electron transport by imaging electron flow in three different GaAs/AlGaAs 2DEGs, whose mobilities range over an order of magnitude. As expected, electrons flow along narrow branches that we find remain straight over a distance roughly proportional to the mean free path. We also observe two unanticipated phenomena in high-mobility samples. In our highest-mobility sample we observe an almost complete absence of sharp impurity or defect scattering, indicated by the complete suppression of quantum coherent interference fringes. Also, branched flow through the chaotic potential of a high-mobility sample remains stable to significant changes to the initial conditions of injected electrons.
113 citations
TL;DR: The main findings are the main trends can be predicted quantitatively based on the scattering properties of single dopants, and in the diffusive regime the analytical predictions of the Dorokhov-Mello-Pereyra-Kumar theory are in good agreement with the ab initio calculations.
Abstract: We combine the ideas of scaling theory and universal conductance fluctuations with density-functional theory to analyze the conductance properties of doped silicon nanowires. Specifically, we study the crossover from ballistic to diffusive transport in boron or phosphorus doped Si nanowires by computing the mean free path, sample-averaged conductance $⟨G⟩$, and sample-to-sample variations $\mathrm{std}(G)$ as a function of energy, doping density, wire length, and the radial dopant profile. Our main findings are (i) the main trends can be predicted quantitatively based on the scattering properties of single dopants, (ii) the sample-to-sample fluctuations depend on energy but not on doping density, thereby displaying a degree of universality, and (iii) in the diffusive regime the analytical predictions of the Dorokhov-Mello-Pereyra-Kumar theory are in good agreement with our ab initio calculations.
103 citations
TL;DR: In this article, the structural stability and superconducting properties of ultrathin lead-bismuth alloys can be tuned by adding extra electrons via bismuth alloying.
Abstract: Quantum confinement of itinerant electrons in atomically smooth ultrathin lead films produces strong oscillations in the thickness-dependent film energy. By adding extra electrons via bismuth alloying, we showed that both the structural stability and the superconducting properties of such films can be tuned. The phase boundary (upper critical field) between the superconducting vortex state and the normal state indicates an anomalous suppression of superconducting order just below the critical temperature, Tc. This suppression varies systematically with the film thickness and the bismuth content and can be parametrized in terms of a characteristic temperature, Tc* (less than Tc), that is inversely proportional to the scattering mean free path. The results indicate that the isotropic nature of the superconductive pairing in bulk lead-bismuth alloys is altered in the quantum regime.
97 citations
TL;DR: In this article, the Boltzmann transport mean free path (BTF) and diffusion constant (D_B) were derived for coherent multiple scattering of matter waves by a disordered optical potential in two and in three dimensions.
Abstract: This article studies multiple scattering of matter waves by a disordered optical potential in two and in three dimensions. We calculate fundamental transport quantities such as the scattering mean free path $\ell_s$, the Boltzmann transport mean free path $\elltrb$, and the Boltzmann diffusion constant $D_B$, using a diagrammatic Green functions approach. Coherent multiple scattering induces interference corrections known as weak localization which entail a reduced diffusion constant. We derive the corresponding expressions for matter wave transport in an correlated speckle potential and provide the relevant parameter values for a possible experimental study of this coherent transport regime, including the critical crossover to the regime of strong or Anderson localization.
76 citations
TL;DR: Divertors allow reduction of the electron-neutral collision frequency to values where the RMFo coupling indicates full penetration of theRMFo to the major axis.
Abstract: Odd-parity rotating magnetic fields (RMFo) applied to mirror-configuration plasmas have produced average electron energies exceeding 200 eV at line-averaged electron densities of approximately 10(12) cm-3. These plasmas, sustained for over 10(3)tauAlfven, have low Coulomb collisionality, vc* triple bond L/lambdaC approximately 10(-3), where lambdaC is the Coulomb scattering mean free path and L is the plasma's characteristic half length. Divertors allow reduction of the electron-neutral collision frequency to values where the RMFo coupling indicates full penetration of the RMFo to the major axis.
64 citations
TL;DR: In this paper, the quantum transport of nitrogen-doped metallic carbon nanotubes under magnetic field was explored and an accurate modeling of chemical disorder effects was derived from ab initio calculations.
Abstract: In this review, the quantum transport of nitrogen-doped metallic carbon nanotubes under magnetic field are explored. An accurate modeling of chemical disorder effects is derived from ab initio calculations. General properties for low bias Landauer conductance are investigated in the coherent regime, which enlighten the strong interplay between band structure and quantum interference effects. Characteristic transport length scales such as the elastic mean free path and localization length are extracted from phenomenological laws as well as scaling features with nanotube radius and doping level. The statistical analysis of conductance properties allow us to study the transition between weak and strong localization regimes.
TL;DR: In this article, the authors used a nonequilibrium molecular dynamics (MD) method to calculate the temperature distribution and thermal resistance of a nanometer scale constriction formed between two planar silicon substrates of different temperatures.
Abstract: To better understand thermal transport at nanoscale point contacts such as the tip-sample contact of a scanning probe microscope and at the contact between a nanotube and a planar surface, we have used a nonequilibrium molecular dynamics (MD) method to calculate the temperature distribution and thermal resistance of a nanometer scale constriction formed between two planar silicon substrates of different temperatures. Surface reconstruction was observed at the two free silicon surfaces and at the constriction. The radius of the heated zone in the cold substrate was found to approach a limit of about 20 times the average nearest-neighbor distance of boron doping atoms when the constriction radius (a) is reduced below the interdopant distance. The phonon mean free path at the constriction was found to be suppressed by diffuse phonon-surface scattering and phonon-impurity scattering. The MD thermal resistance is close to the ballistic resistance when a is larger than 1nm, suggesting that surface reconstructio...
TL;DR: The influence of disorder on superconducting properties of MgB2 wires was investigated in this paper, where the mean free path of the charge carriers in the σ band was extracted from the Gor'kov-Goodman relation.
Abstract: The influence of disorder on the superconducting properties of MgB2 wires was investigated. Disorder was introduced in three different ways: by the addition of SiC, by neutron irradiation or by a low processing temperature. We find a nearly identical influence of these three methods on the normal state resistivity, on the upper critical field and on the critical currents in all three cases. The residual resistivity turns out to be a useful parameter for disorder, if normalized appropriately. We extract the mean free path of the charge carriers in the σ band from the Gor'kov–Goodman relation. The wires investigated in this study fall in the range from the moderately clean to the dirty limit. The most important change in view of possible applications is the increase of the upper critical field, leading to higher critical currents in high magnetic fields.
TL;DR: A measure of the transport mean free path and a Diffusive Wave Spectroscopy experiment on a packing of glass spheres are proposed that agree with the predictions of this ray tracing approach.
Abstract: We are interested in the propagation of light in a random packing of dielectric spheres within the geometrical optics approximation. Numerical simulations are performed using a ray tracing algorithm. The effective refractive indexes and the transport mean free path are computed for different refractive indexes of spheres and intersticial media. The variations of the optical path length under small deformations of the spheres assembly are also computed and compared to the results of Diffusive Wave Spectroscopy experiments. Finally, we propose a measure of the transport mean free path and a Diffusive Wave Spectroscopy experiment on a packing of glass spheres. The results of those experiments agree with the predictions of this ray tracing approach.
TL;DR: In this article, the authors derived SEA equations from structural ray equations, where rays are assumed to be uncorrelated leading to the additivity of energy, and the net exchanged power between two adjacent subsystems is then proportional to the difference of energy densities.
TL;DR: In this paper, the authors analyzed the time response of a gas confined in a small-scale gap to an instantaneous jump in the temperature of its boundaries and found that the characteristic time scale for arriving at the new equilibrium state is of the order of several acoustic time scales (the ratio of the gap width to the most probable molecular speed of gas molecules).
Abstract: We analyse the time response of a gas confined in a small-scale gap (of the order of or smaller than the mean free path) to an instantaneous jump in the temperature of its boundaries. The problem is formulated for a collisionless gas in the case where the relative temperature jump at each wall is small and independent of the other. An analytic solution for the probability density function is obtained and the respective hydrodynamic fields are calculated. It is found that the characteristic time scale for arriving at the new equilibrium state is of the order of several acoustic time scales (the ratio of the gap width to the most probable molecular speed of gas molecules). The results are compared with direct Monte Carlo simulations of the Boltzmann equation and good agreement is found for non-dimensional times (scaled by the acoustic time) not exceeding the system Knudsen number. Thus, the present analysis describes the early-time behaviour of systems of arbitrary size and may be useful for prescribing the initial system behaviour in counterpart continuum-limit analyses.
TL;DR: A novel parametrization of the energy dependence of the attenuation coefficient that allows for closed form evaluation of certain spectral integrals is based on, and a simple extension of this model gives a rather good description of beam hardening for x-rays traveling through water.
Abstract: Polychromatic x-ray beams traveling though material are prone to beam hardening, i.e., the high energy part of the incident spectrum gets over represented when traveling farther into the material. This study discusses the concept of a mean attenuation coefficient in a formal way. The total energy fluence is one-to-one related to the traveled distance in case of a polychromatic beam moving through a given, inhomogeneous material. On the basis of this one-to-one relation, it is useful to define a mean attenuation coefficient and study its decrease with depth. Our results are based on a novel parametrization of the energy dependence of the attenuation coefficient that allows for closed form evaluation of certain spectral integrals. This approach underpins the ad hoc semianalytical expressions given in the literature. An analytical model for the average attenuation coefficient is proposed that uses a simple fit of the attenuation coefficient as a function of the photon energy as input. It is shown that a simple extension of this model gives a rather good description of beam hardening for x-rays traveling through water.
TL;DR: In this paper, a theory of photoelectron energy down-conversion in superconductors in the vicinity of interfaces was developed, where the process takes place closer to an interface than the mean free path of pair-breaking phonons.
Abstract: We have developed a theory of photoelectron energy down-conversion in superconductors in the vicinity of interfaces. Significant differences from the situation in bulk materials arise when the process takes place closer to an interface than the mean free path of pair-breaking phonons. Then some of the energetic phonons generated in the down-conversion cascade can escape from the superconducting film, giving rise to a decrease in the mean number of quasiparticles generated and to statistical fluctuations in that quantity. An additional source of variability is the spatial distribution of the initial photoabsorption sites, giving rise to vertical inhomogeneity. Both effects can be observed in photoabsorption experiments on thin film superconducting tunnel junctions, the former at optical energies and the latter primarily with x rays.
TL;DR: In this article, the results of heat transfer measurements in polycrystalline Y3Al5O12 garnets as well as Y2O3 and Lu2O 3 sesquioxide materials obtained by self-energy-driven sintering of nano-particles were reviewed.
Abstract: Grain boundaries play a key role in determining several key properties of polycrystalline laser ceramics. Heat transfer measurements at low temperature constitute a good tool to probe grain boundaries. We review the results of heat transfer measurements in polycrystalline Y3Al5O12 garnets as well as Y2O3 and Lu2O3 sesquioxide materials obtained by self-energy-driven sintering of nano-particles. The average phonon mean free path in Y3Al5O12 was found to be significantly larger than the average grain size and to scale with temperature as T−2 at low temperature. Existing models describing the interaction between phonons and grain boundaries are reviewed. Correct temperature dependence of the mean free path and order of magnitude of scattering rates were found by assuming the existence of a grain boundary layer having acoustic properties different from those of the bulk. A different temperature dependence of phonon mean free path was found for the sesquioxides and was ascribed to the stronger elastic anisotropy of these materials. The thermal resistance associated to the grain boundaries of laser ceramics was found to be lower than in other dense polycrystalline ceramic materials reported in the literature.
TL;DR: In this article, the authors studied the radial dependence of peak intensities and fluences of solar energetic particle (SEP) events in the framework of the focused transport theory and deduced functional forms to extrapolate peak fluxes and event fluences with radial distance that depend on the energy of the particles.
TL;DR: In this article, the surface roughness is modelled by an array of triangular modules and the boundary conditions, generally including velocity slip, no-slip and negative slip, depend not only on the Knudsen number but also on the surface rougheness.
Abstract: Rarefied gas flows in rough microchannels are investigated by non-equilibrium molecular dynamics simulations. The surface roughness is modelled by an array of triangular modules. The Maxwell slip model is found to break down due to the surface roughness for gas flows in microchannels with large surface roughness. Non-Maxwell slippage shows that the slip length is smaller than that predicted by the Maxwell model and is nonlinearly related to the mean free path. For larger surface roughness and smaller Knudsen number, the non-Maxwell effect becomes more pronounced. The boundary conditions, generally including velocity slip, no-slip and negative slip, depend not only on the Knudsen number but also on the surface roughness. Simulation results show that A/λ ≈ 1 is a good criterion to validate the no-slip boundary condition and A/λ > 0.3 can be a criterion to judge the occurrence of non-Maxwell slippage, where A is the surface roughness size and λ is the mean free path of gas molecules. The permeability enhance...
TL;DR: In this paper, the average phonon mean-free path for the zinc-blende phase is approximately four times that for the wurtzite phase, suggesting that AlN will exhibit far better high thermal conductivity behavior in its cubic phase.
Abstract: We present theoretical investigations of the anharmonic phonon mean-free path in cubic and hexagonal AlN. The cubic anharmonicity in crystal potential has been modeled within an anharmonic elastic continuum model. Numerical calculations have been carried out within the Fermi's golden rule scheme and by using the phonon dispersion and group velocity results from a full lattice dynamical model. The calculated mean-free path results for both crystal phases are compared with estimates made previously by Watari et al. [J. Mater. Res. 17, 2940 (2002)] for the hexagonal phase, and a discussion on the level of agreement is provided. Our work predicts that at room temperature and above, the average phonon mean-free path for the zinc-blende phase is approximately four times that for the wurtzite phase, suggesting that AlN will exhibit far better high thermal conductivity behavior in its cubic phase.
TL;DR: In this article, the authors investigate the effects of collisions between ion and neutral gas atoms on the characteristics of a magnetized plasma sheath in an external magnetic field, where the ion cross section depends on the ion flow velocity.
Abstract: Recently some correlative works have been done to investigate the effects of collisions between ion and neutral gas atoms on the characteristics of plasma sheath in an external magnetic field. In general, the momentum transferring cross section (and thus the ion collision frequency) has a power law dependence on the ion flow velocity in the depth direction. Usually in the literature, constant collision frequency and constant mean free path are treated. Here, by using a collisional fluid model, we investigate the characteristics of a magnetized plasma sheath where the ion cross section depends on the ion flow velocity. The numerical calculations show that the effects of collisions on ion characteristics are more powerful when the momentum transferring cross section is constant. However, some parameters such as the electron density distribution are independent of this dependence.
TL;DR: In this paper, the authors derived expressions for the ion perpendicular viscosity as well as for the electron and ion gyroviscosities, parallel viscosities and heat fluxes for arbitrary mean-free path plasmas, in which the lowest order distribution function is a Maxwellian, by assuming the gyroradius is small compared with the shortest perpendicular scale length.
Abstract: Expressions for the ion perpendicular viscosity as well as for the electron and ion gyroviscosities, parallel viscosities and heat fluxes are derived for arbitrary mean-free path plasmas, in which the lowest order distribution function is a Maxwellian, by assuming the gyroradius is small compared with the shortest perpendicular scale length. The results are given in terms of a few velocity space integrals of the gyrophase independent correction to the Maxwellian and correctly reproduce known results in the collisional limit.
TL;DR: In this paper, the transition from 0 to π states in Nb/Fe/Nb Josephson junctions was investigated by varying the Fe barrier thickness from 0.5 nm to 5.1 nm, and an exchange energy of 256 meV, a Fermi velocity of 1.98 ×105 m/s and an electron mean free path of 6.2 nm were derived.
Abstract: The physics of the π phase shift in ferromagnetic Josephson junctions may enable a range of applications for spin-electronic devices and quantum computing. We investigate transitions from “0” to “π” states in Nb/Fe/Nb Josephson junctions by varying the Fe barrier thickness from 0.5 nm to 5.5 nm. From magnetic measurements we estimate for Fe a magnetic dead layer of about 1.1 nm. By fitting the characteristic voltage oscillations with existing theoretical models we extrapolate an exchange energy of 256 meV, a Fermi velocity of 1.98 ×105 m/s and an electron mean free path of 6.2 nm, in agreement with other reported values. From the temperature dependence of the ICRN product we show that its decay rate exhibits a nonmonotonic oscillatory behavior with the Fe barrier thickness.
Abstract: Motivated by experimental investigations of electrical discharges in N2/CO2/H2O, Monte Carlo (MC) electron dynamics simulations in atmospheric
N2/CO2 mixtures were performed The goal was to obtain electron energy distribution functions (EEDFs), mean free path, drift velocity, collision frequency and mean energy of electrons,
rate coefficients of electron-impact reactions, ionisation and attachment coefficients, as functions of the reduced electric field strength (E/N) and of the concentration of individual gas components The results obtained by MC simulations were fitted with polynomials of up to the 3rd order with reasonable accuracy for E/N above 80 Td
The studied parameters below 80 Td were strongly non-linear as functions of E/N This is mostly due to the influence of elastic collisions of electrons with CO2 molecules prevailing in CO2-dominant mixtures for E/N < 40 Td, and vibrational excitation collisions of N2 species prevailing in N2-dominant mixtures for E/N from 40 to 80 Td The effect of these electron-impact processes was specific for each of the studied parameters
TL;DR: In this article, the effects of particle distribution pattern, particle size, shape and volume fraction on the dislocation mean free path length and average obstacle distance are analyzed in two-and three-dimensional models.
TL;DR: In this paper, the thermal conductivity of LaB6 and SmB6 was measured from 6 to 300 K. The results indicate strong phonon scattering by structural defects, presumably those typical of hexaborides.
Abstract: The thermal conductivity of LaB6 and SmB6 crystals has been measured from 6 to 300 K. The electronic and lattice contributions to the thermal conductivity of SmB6 and the temperature-dependent phonon mean free path in this hexaboride have been evaluated. The results indicate strong phonon scattering by structural defects, presumably those typical of hexaborides.
01 Jan 2007
TL;DR: In this article, the low-temperature mean-free path of phonon modes in single-wall carbon nanotubes is calculated within the relaxation time scheme using analytic expressions for the phonon dispersion relations and the specific heat capacity.
Abstract: The low-temperature mean-free path of phonon modes in single-wall carbon nanotubes is calculated within the relaxation time scheme using analytic expressions for the phonon dispersion relations and the specific heat capacity. We resolve the discrepancy presented by Yu et al. [Nanoletters, 5, 1842] between the length of the nanotube and their estimated mean-free path. This is explained to arise from the kinks and bends present in their sample. An analysis of our calculated radius shows that Yu et al. have studied a (9,9) single-wall nanotube.
01 Dec 2007
TL;DR: In this paper, the low-temperature mean-free path of phonon modes in single-wall carbon nanotubes is calculated within the relaxation time scheme using analytic expressions for the phonon dispersion relations and the specific heat capacity.
Abstract: The low-temperature mean-free path of phonon modes in single-wall carbon nanotubes is calculated within the relaxation time scheme using analytic expressions for the phonon dispersion relations and the specific heat capacity. We resolve the discrepancy presented by Yu et al. [Nanoletters, 5, 1842] between the length of the nanotube and their estimated mean-free path. This is explained to arise from the kinks and bends present in their sample. An analysis of our calculated radius shows that Yu et al. have studied a (9,9) single-wall nanotube.