Momentum Relaxation Time Research Articles

Spin orientation of electrons by unpolarized light pulses in low-symmetry quantum wells

The spin dynamics of electrons in low-symmetry quantum wells (QWs) under conditions of interband excitation by ultrashort unpolarized light pulses is investigated. It is shown that after the transmission, spin polarization appears in the system after a time comparable with the electron momentum relaxation time for an electron pulse and then vanishes. The microscopic theory of spin orientation of electrons by optical pulses carrying zero angular momentum is developed for asymmetric QWs grown from semiconductors with the zinc blende lattice along the [110] crystallographic direction. Pumping with unpolarized light in such structures in the normal incidence geometry induces a spin in the QW plane along the \([1\bar 10]\) axis.

Quantum Transport in MesoscopicHe3Films: Experimental Study of the Interference of Bulk and Boundary Scattering

We discuss the mass transport of a degenerate Fermi liquid ^{3}He film over a rough surface, and the film momentum relaxation time, in the framework of theoretical predictions. In the mesoscopic regime, the anomalous temperature dependence of the relaxation time is explained in terms of the interference between elastic boundary scattering and inelastic quasiparticle-quasiparticle scattering within the film. We exploit a quasiclassical treatment of quantum size effects in the film in which the surface roughness, whose power spectrum is experimentally determined, is mapped into an effective disorder potential within a film of uniform thickness. Confirmation is provided by the introduction of elastic scattering centers within the film. The improved understanding of surface roughness scattering may impact on enhancing the conductivity in thin metallic films.

Energy-transport and drift-diffusion limits of nonisentropic Euler–Poisson equations

Two relaxation limits in critical spaces for the scaled nonisentropic Euler–Poisson equations with the momentum relaxation time and energy relaxation time are considered. As the first step of this justification, the uniform (global) classical solutions to the Cauchy problem in Chemin–Lernerʼs spaces with critical regularity are constructed. Furthermore, by the compactness argument, it is rigorously justified that the scaled classical solutions converge to the solutions of energy-transport equations and drift-diffusion equations, respectively, with respect to different time scales.

In-plane and growth direction electron cyclotron effective mass in short period InAs/GaSb semiconductor superlattices

In plane and growth direction electron effective mass in short period InAs/GaSb semiconductor superlattices (SL) was measured using cyclotron resonance at different orientations of magnetic field with respect to SL growth direction. It was demonstrated that the electron spectrum near the bottom of the SL subband has 3D character, with the in-plane effective masses ranging from 0.023 m0 to 0.028 m0 and growth direction effective masses of 0.03–0.034 m0 depending on the SL period and growth conditions. The measured effective masses are close to those calculated in the weak coupling limit of the Kronig-Penney model. In this limit the SL electron effective mass is a weighted average of the electron effective masses of corresponding bulk materials. Correlation between the magnitude of cyclotron mobility, amplitude of negative magnetoresistance, and steepness of the long wavelength side of the photoluminescence spectrum indicate that the crystalline structure disorder is a major factor contributing to the momentum relaxation time of the electrons.

Spin relaxation properties in graphene due to its linear dispersion

A spin injection is achieved in a direct-contact cobalt$\ensuremath{-}$single-layer graphene nonlocal spin-valve system, overlaid with a top gate. The spin signal is retained even in bipolar configurations of graphene. Hanle spin-precession analysis demonstrates that proportionality between spin and momentum relaxation times, which supports the Elliot-Yafet-type spin relaxation, holds consistently only when the carrier-density dependence of the density of states is taken into account. The corresponding strong spin-orbit coupling ($\ensuremath{\sim}$10 meV) suggests that covalently bonded adsorbates, rather than charged impurities, govern the spin relaxation in diffusive graphene.

Device Simulation for Evaluating Effects of Inplane Biaxial Mechanical Stress on n-Type Silicon Semiconductor Devices

This paper presents a practical method of drift-diffusion device simulation for evaluating the effects of mechanical stress on n-type silicon semiconductor devices. The device simulation incorporates an electron mobility model for considering the effects of stress. In this paper, we focus on stress effects that are induced by applying inplane biaxial stress to the devices. Therefore, two physical phenomena that are attributed to mechanical stress are modeled in the electron mobility model, i.e., the changes in relative population and the momentum relaxation time (intervalley scattering) of electrons in conduction-band valleys. Stress-induced variations of direct-current characteristics on n-type metal-oxide-semiconductor (MOS) field-effect transistors are evaluated using a device simulation including the proposed electron mobility model. Then, the electron mobility model and the simulation method are verified by comparing with experimental results. It is demonstrated that experimental results can be reasonably estimated using this device simulation method. From discussions regarding the electron mobility model, it is suggested that the comprehensive stress sensitivity of MOS devices is larger than that of lightly doped silicon.

The relaxation-time limit in the compressible Euler–Maxwell equations

The aim of this paper is to study multidimensional Euler–Maxwell equations for plasmas with short momentum relaxation time. The convergence for the smooth solutions to the compressible Euler–Maxwell equations toward the solutions to the smooth solutions to the drift–diffusion equations is proved by means of the Maxwell iteration, as the relaxation time tends to zero. Meanwhile, the formal derivation of the latter from the former is justified.

Diffusive relaxation limits of compressible Euler–Maxwell equations

This work is concerned with compressible Euler–Maxwell equations, which take the form of Euler equations for the conservation laws of mass density, current density and energy density for electrons, coupled to Maxwellʼs equations for self-consistent electromagnetic field. We give a model hierarchy of non-isentropic Euler–Maxwell equations from the point of view of diffusive relaxation limits. More precisely, inspired by Maxwell-type iteration, we construct new approximations and show that periodic initial-value problems of a certain scaled Euler–Maxwell equations have unique smooth solutions in a time interval independent of momentum relaxation time and energy relaxation time. Furthermore, it is proved that smooth solutions converge to solutions of drift-diffusion models and energy-transport models in the process of combined diffusive relaxation limits, and the corresponding convergence rates are also obtained.

Anomalous heat flux inhibition under high frequency radiation pulse absorption in electron–ion collisions

Anomalous heat flux inhibition under high frequency radiation pulse absorption in electron–ion collisions is revealed for plasma with multiply charged ions. The inhibition takes place in time moments greater than momentum relaxation time, but less than energy relaxation time of the electrons, which give main contribution to the heat flux value.

Evaluation of properties of trinitrides in a strong electric field

In this article was made an attempt to systematize the existing information about the group of lll-nitrides and to analyze their behavior in a strong electric field. By the agency of analytical expressions for momentum and energy relaxation times of different scattering mechanisms were estimated the dynamic properties of Ill-Nitrides in high-field as well as the field-temperature dependence, valleys occupation function and field-velocity characteristic for nitrides with different crystal lattice modifications. The calculation results were compared to the existing experimental data and estimation of other authors

Observation of Intrinsic Inverse Spin Hall Effect

We report observation of intrinsic inverse spin Hall effect in undoped GaAs multiple quantum wells with a sample temperature of 10 K. A transient ballistic pure spin current is injected by a pair of laser pulses through quantum interference. By time resolving the dynamics of the pure spin current, the momentum relaxation time is deduced, which sets the lower limit of the scattering time between electrons and holes. The transverse charge current generated by the pure spin current via the inverse spin Hall effect is simultaneously resolved. We find that the charge current is generated well before the first electron-hole scattering event. Generation of the transverse current in the scattering-free ballistic transport regime provides unambiguous evidence for the intrinsic inverse spin Hall effect.

Viscous corrections to the resistance of nanojunctions: A dispersion relation approach

It is well known that the viscosity of a homogeneous electron liquid diverges in the limits of zero frequency and zero temperature. A nanojunction breaks translational invariance and necessarily cuts off this divergence. However, the estimate of the ensuing viscosity is far from trivial. Here, we propose an approach based on a Kramers-Kr\"onig dispersion relation, which connects the zero-frequency viscosity, $\eta(0)$, to the high-frequency shear modulus, $\mu_{\infty}$, of the electron liquid via $\eta(0) =\mu_{\infty} \tau$, with $\tau$ the junction-specific momentum relaxation time. By making use of a simple formula derived from time-dependent current-density functional theory we then estimate the many-body contributions to the resistance for an integrable junction potential and find that these viscous effects may be much larger than previously suggested for junctions of low conductance.

The diffusive relaxation limit of non-isentropic Euler–Maxwell equations for plasmas

This paper concerns the non-isentropic Euler–Maxwell equations for plasmas with short momentum relaxation time. With the help of the Maxwell-type iteration, it is obtained that, as the relaxation time tends to zero, periodic initial-value problem of certain scaled non-isentropic Euler–Maxwell equations has unique smooth solutions existing in the time interval where the corresponding classical drift-diffusion model has smooth solutions. Meanwhile, we justify a formal derivation of the corresponding drift-diffusion model from the non-isentropic Euler–Maxwell equations.

Electro-Thermal Analysis and Monte Carlo Simulation for Thermal Design of Si Devices

Nowadays, precise prediction of heat generation in semiconductor devices is important. An electro-thermal analysis is an attractive method to predict heat generation in devices. In this analysis, momentum and energy relaxation times are taken as the important parameters. Calculated heat generation density is dependent on the energy relaxation time. Conventionally, these relaxation times have been determined by simple models. For a more precise prediction of the relaxation times, Monte Carlo (MC) simulation should be employed. In this research, the impact of these relaxation times on the heat generation in a semiconductor device is evaluated. The results of an electro-thermal analysis using conventional relaxation times (conventional model) and of one using the relaxation times from a MC simulation (MC model) are compared. The calculation results show that the conventional model overestimates the heat generation density especially under a high electric field. The estimated heat generation density of the conventional model is 10% and 25% larger in the case of 100 and 500 kV/cm electric fields, respectively. It can be concluded that, for the thermal design of high-power semiconductor devices in power electronics, appropriate values of the relaxation times, which are obtained from the MC simulation, are required.

Geometric Correlations and Breakdown of Mesoscopic Universality in Spin Transport

We construct a unified semiclassical theory of charge and spin transport in chaotic ballistic and disordered diffusive mesoscopic systems with spin-orbit interaction. Neglecting dynamic effects of spin-orbit interaction, we reproduce the random matrix theory results that the spin conductance fluctuates universally around zero average. Incorporating these effects into the theory, we show that geometric correlations generate finite average spin conductances, but that they do not affect the charge conductance to leading order. The theory, which is confirmed by numerical transport calculations, allows us to investigate the entire range from the weak to the previously unexplored strong spin-orbit regime, where the spin rotation time is shorter than the momentum relaxation time.

Physics of electron mobility independent of channel orientation in n-channel transistors based on (100) silicon wafers and its experimental verification

The six degenerate ellipsoid model can be used to study the effects of channel orientation on electron mobility. However, this approach has two assumptions: (i) the conduction band minimum near the interface between gate dielectric and silicon channel is the same as that of bulk silicon, (ii) the momentum relaxation time is independent of the channel orientation. This letter shows that the effective conductivity electron mass of (100) surface-oriented silicon is independent of channel orientation even though the actual conduction band minimum may be slightly different from the six degenerate ellipsoid model. Experimental data are provided to support our theory.

Electronic phase coherence and relaxation in graphene field effect transistor

Using the low-field magnetoresistance measurement we have studied the electronic phase coherence of the graphene field effect transistor for different carrier types and densities. The characteristic time scales such as phase coherence time ( τ ϕ ), intervalley scattering time ( τ i ), and momentum relaxation time have been deduced by weak localization fit to the magnetoresistance. We found that the magnitude of τ ϕ shows similar magnitudes for both types of charge carriers. In the lower density regime including the Dirac point, τ ϕ increases rapidly as the density of carrier increases. However, τ i shows a weak dependence of carrier type and density. The momentum relaxation time becomes saturated below 3 K regardless of carrier type and density, which is in contrast to the temperature dependence of τ ϕ .

Surface Spin Flip Probability of Mesoscopic Ag Wires

Spin relaxation in mesoscopic Ag wires in the diffusive transport regime is studied via nonlocal spin valve and Hanle effect measurements performed on Permalloy/Ag lateral spin valves. The ratio between momentum and spin relaxation times is not constant at low temperatures. This can be explained with the Elliott-Yafet spin relaxation mechanism by considering the momentum surface relaxation time as being temperature dependent. We present a model to separately determine spin flip probabilities for phonon, impurity and surface scattering and find that the spin flip probability is highest for surface scattering.

ENERGY-TRANSPORT LIMIT OF THE HYDRODYNAMIC MODEL FOR SEMICONDUCTORS

This paper is mainly devoted to study the energy-transport limit of a non-isentropic hydrodynamic model with momentum relaxation time τ and energy relaxation time σ. Inspired by the Maxwell iteration, we construct a new approximation under the assumption τσ = 1, and show that periodic initial-value problems of a certain scaled hydrodynamic model have unique smooth solutions in a finite time interval independent of τ. Furthermore, it is also obtained that as τ tends to zero, the smooth solutions converge to the smooth solutions of energy-transport models at the rate of τ2. The proof of these results is based on a continuation principle.

DYNAMIC APPROACH TO ANALYSIS OF ANGULAR DISTRIBUTIONS OF FISSION AND QUASIFISSION FRAGMENTS

The dynamic model is proposed for description of the angular distributions of quasifission fragments. The model is able to describe experimental anisotropy of the angular distributions of fissionlike fragments for the 32 S , 28 Si +208 Pb reactions. It is also shown that the new model allows the description of the experimental mass-angular correlations in the quasifission fragment yields for the 64 Ni +197 Au reaction at 383 and 418 MeV of incident energies. The analysis was performed with the angular momentum and deformation dependent relaxation time of the tilting mode. Information on the dinuclear system lifetimes were obtained.