High-temperature Approximation Research Articles

Revised Shockley–Read–Hall lifetimes for quantum transport modeling

The inclusion of quantization effects on the carrier densities is now the state of the art in modern semiconductor device simulators and yields, for example, quantum-corrected threshold voltages and quantum-mechanical models of the channel mobility. However, the effect of charge quantization on nonradiative thermal generation–recombination has not received much attention. In this article, Shockley–Read–Hall recombination is examined for situations in which electrons and/or holes are confined in semiconductor devices. For the transitions between band states and a single deep level, a previously developed multiphonon description is adopted. It is found that the lifetimes have to be altered due to the same quantized local density of states that also accounts for the carrier distribution. Numerical evaluation of this model for one-dimensional potentials and small phonon energies results in spatially varying lifetime profiles that exhibit two opposite regimes. The additional nonclassical offset of the subband eigenenergies causes an increased lifetime in the limit of strong quantum confinement. For nondegenerate statistics, an analytical high-temperature approximation is presented for this limit, where the activation energy of the lifetime is increased by the lowest-subband offset. In the absence of confinement, however, high electric fields reduce the lifetime due to carrier tunneling into the bandgap.

PHASE BEHAVIOUR OF THE RESTRICTED PRIMITIVE MODEL

The phase behaviour of the restricted primitive model (RPM) is studied using a microscopic approach recently developed for the description of phase transitions in binary uid mixtures. For the model we obtain the explicit expression for the functional of the grand partition function. Based on the functional we calculate the phase diagram of the RPM in the high temperature approximation (HTA) and then we do this calculation taking into account the terms of the higher orders in the effective Hamiltonian. In both cases the phase diagrams demonstrate the gas-liquid (GL) and charge ordering phase instabilities. In the latter case, the obtained value for the GL critical temperature is in good agreement with the MC simulation data whereas the critical density is underestimated.

Variable single electron charging energies and percolation effects in molecularly linked nanoparticle films

We study electrical transport in strongly coupled, molecularly linked, gold nanoparticle (NP) films whose bulk dc conductances are governed by percolation phenomena. Films with fewer NPs exhibit current suppression below a threshold voltage, likely due to single-electron charging of NP clusters. In some cases, the thresholds are very large (∼1 V) and suppression persists to room temperature. The thresholds tend to decrease with increasing amounts of NPs in the film, and eventually, metal-like conductance is observed down to at least 10 K. The observed trend toward metal-like conductance, despite the presence of film disorder, is enabled by strong inter-NP coupling and increasing film connectivity. The latter is an inherent property of molecularly linked NP films due to both robust chemical inter-NP linkages provided by alkane dithiol linker molecules, coupled with the ability to grow chains of connected NPs to arbitrary lengths through cyclical Au/dithol treatments. In the case of small thresholds, our data is well described by a high-temperature approximation of “orthodox” theory for a linear array of tunnel junctions.

Calculation of the temperature dependence of hot electron scattering in heavily p-doped GaAs using a high-temperature approximation to the dielectric function

Using a high-temperature approximation to the dielectric function within the random phase approximation, we calculate hot electron scattering rates, as a function of temperature and doping density, in p-doped GaAs. The dielectric function of the holes contains contributions from intraband excitations and interband excitations. The former reduces to an analytic form within the two pole approximation (which used Boltzmann statistics), whereas the latter was calculated numerically. The collective excitation mode of the holes was defined by intraband excitations, since at very small wavevectors, the interband excitations vanish. However, at low temperature the interband excitations were found to be the dominant Landau damping mechanism, which strongly suppressed the plasmon at moderate doping levels. At high temperature the excitations from the heavy to light band were partially suppressed, and the plasmon was not overwhelmingly Landau damped by either interband or intraband excitations. At room temperature, an analytic dielectric function where the interband excitations have been neglected, may be used to accurately calculate hot electron mean free paths. This approximation was found to become more accurate with lower doping levels, but was not appropriate at low temperature.

Effective Potential on Fuzzy Sphere

The effective potential of quantized scalar field on fuzzy sphere is evaluated to the two-loop level. We see that one-loop potential behaves like that in the commutative sphere and the Coleman-Weinberg mechanism of the radiatively symmetry breaking could be also shown in the fuzzy sphere system. In the two-loop level, we use the heavy-mass approximation and the high-temperature approximation to perform the evaluations. The results show that both of the planar and nonplanar Feynman diagrams have inclinations to restore the symmetry breaking in the tree level. However, the contributions from planar diagrams will dominate over those from nonplanar diagrams by a factor N^2. Thus, at heavy-mass limit or high-temperature system the quantum field on the fuzzy sphere will behave like those on the commutative sphere. We also see that there is a drastic reduction of the degrees of freedom in the nonplanar diagrams when the particle wavelength is smaller than the noncommutativity scale.

Microscopic State of Low Doped Manganites La1-xSrxMnO3 Probed by ESR

The electron spin resonance (ESR) measurements were performed in single crystals of La 1-x Sr x MnO 3 (0.05 ≤ x ≤ 0.125) in order to study the interplay of crystal field, Dzyaloshinsky-Moriya interaction, Jahn-Teller (JT) effect, and orbital order. The angular dependence of the ESR linewidth of an untwinned La 0.95 Sr 0.05 MnO 3 single crystal has been analyzed in the high-temperature approximation, which takes into account the microscopic geometry of the four nonequivalent Mn positions in the orthorhombic unit cell. A strong mixing of the |3z 2 - r 2 ) and |x 2 - y 2 ) states for the real orbital configuration was found. Magnetic inhomogeneities observed in the ESR spectra across the composition range 0.075 ≤ x ≤ 0.125 can be attributed to the presence of ferromagnetic clusters (magnetic spin polarons) in the paramagnetic state. New polaronic models are proposed.

High-Temperature Cutoff Approximation of the 3D Kinetic Ising Model**The project supported by the National Basic Research Project “Nonlinear Science”, National Natural Science Foundation of Chinaunder Grant No. 10075025, and Jiangxi Provincial Natural Science Foundation of China under Grant No. 9912001

A single-spin transition critical dynamics is used to investigate the three-dimensional kinetic Ising model on an anisotropic cubic lattice. We first derive the fundamental dynamical equations, and then linearize them by a cutoff approximation. We obtain the approximate solutions of the local magnetization and equal-time pair correlation function in zero field. In which the axial-decoupling terms , and as higher infinitesimal quantity are ignored, where . We think that it is reasonable as the temperature of the system is very high. The result of what we obtain in this paper can go back to the one-dimensional Glauber's theory as long as .

Susceptibility, Magnetization Process and ESR Studies on the Helical Spin System RbCuCl3

The static and dynamic magnetic properties of an $S=1/2$ stacked triangular antiferromagnet RbCuCl$_{3}$ with a helical spin structure due to lattice distortion were investigated by susceptibility, high-field magnetization process and ESR measurements. The susceptibilities were analyzed by high-temperature expansion approximation in terms of ferromagnetic intrachain coupling and the antiferromagnetic interchain coupling. The magnetization saturates at $H_{\rm s}\approx 66.8$ T at 1.5 K, the value of which is twice that of CsCuCl$_3$. A small magnetization jump indicative of a phase transition was observed at $H_{\rm c}=21.2$ T $\approx (1/3)H_{\rm s}$ for $H{\perp}c$. ESR modes observed for $H\parallel c$ are well described by the calculation based on the helical spin structure. From the present measurements, the ferromagnetic intrachain and two kinds of antiferromagnetic interchain exchange interactions, and the planar anisotropy energy were determined as $J_0/k_{\rm B}=25.7$ K, $J_1/k_{\rm B}=-10.6$ K, $J_1'/k_{\rm B}=-17.4$ K, and ${\Delta}J_0/k_{\rm B}=-0.45$ K, respectively.

Interaction of two superhigh-frequency fields with the resonance system of electron spins

Within the framework of the concept of a spin temperature in a steady-state regime, the interaction with a resonance medium of two superhigh-frequency fields, one of which is saturating and the other of which is trial, is considered theoretically in the general case without using a high-temperature approximation. The case where the absorption of a resonance medium vanishes at the trial-field frequency is investigated in detail. This occurs with an intensity of the saturating field lower than in the case of a high-temperature approximation. This intensity is estimated in the second and third orders by the parameter ω0βz/2, where ω0 is the resonance frequency of the transition, βz=ħ/(kTz), Tz is the Zeeman-subsystem temperature, ħ is the Planck constant, and k is the Boltzmann constant. It is shown that the cooling of the dipole-dipole reservoir is more considerable than in the case of a high-temperature approximation.

Dynamic polarization of a nuclear zeeman subsystem within the framework of the EPR spin-temperature concept

In a high-temperature approximation, theoretical consideration is given to dynamic polarization of nuclei within the framework of the EPR spin-temperature concept. We show that the maximum polarization of nuclei can be attained at a certain value of the detuning of an SHF-field if the intensity of the latter is fixed. The possibility of determining some relaxation parameters of the system by the value of this detuning at a high value of the saturating field is shown. In the presence of a trial field, the effect of a nuclear Zeeman subsystem on the time of transverse relaxation T2 is evaluated.

Structure and thermodynamics of anisotropic polymer fluids

We investigate the structure and thermodynamics of anisotropic polymer fluids, focusing on the nematic phases of flexible polymers. The chains interact only through monomer–monomer excluded-volume interactions. As a function of an externally provided alignment along a fixed nematic director, we calculate the anisotropic pair correlation function, and demonstrate the existence of two density correlation lengths, ξ⊥ and ξz, controlling transverse and longitudinal density fluctuations, respectively. We allow the possibility that the chains align either along the director (nematic conformations), or are anti-aligned in a “discotic-like” configuration. The cohesive contribution to the free energy is established in a high-temperature approximation, and its sensitivity to the orientation of the chains is probed. Our approach is not limited to homogeneous liquid crystalline phases, but applies in any circumstance when the orientation of otherwise disordered polymers is the physically controlling effect, e.g., confinement in thin films or pores, shear-alignment of flexible polymers, or straining a cross-linked rubber network.

Dipolar relaxation in a many-body system of spins of 1/2

The method utilized in Paper I [J. Chem. Phys. 112, 1413 (2000)] for treating the density matrix equation for a two-spin system in the presence of the dipolar interaction that is randomly modulated by translational diffusion, is extended to a many-body system of identical spins of 1/2. Generalized cumulant expansions are used, which allow one to take full advantage of the statistical independence of the motions of spins. In the high-temperature approximation (appropriate for dilute solutions), for a single nonselective pulse, the symmetry of the problem allows one to obtain a compact ordered binomial expression for the free-induction decay signal that is related to the two-particle solution, and it still contains the two spin-isochromat components. The latter are evaluated by solving the corresponding stochastic Liouville equation, which allows one to recover in a unified way the two limiting cases including Anderson’s result for statistical broadening in a rigid lattice and the classical Torrey–Bloembergen–Redfield expression for the motional narrowing, as corrected by Hwang and Freed. The line shape expression in the thermodynamic limit, i.e., for large numbers of particles in a macroscopic volume, is obtained. It is found that the many-body dipolar line shapes are very close to Lorentzians over the entire motional range studied, with the linewidths proportional to the spin concentration, as predicted earlier for the limiting cases. Linewidths plotted versus the values of the translational diffusion coefficient clearly show the solid-state limit, the motional-narrowing limit, and the intermediate region. The method is extended to describe the behavior of the many-body system in a solid-echo sequence. This enables one to obtain the homogeneous T2’s over the whole range of motions. A minimum in T2 is found at approximately the same value of translational diffusion coefficient as was found for the two-spin case in Paper I.

Manifestation of the Dynamics of Spin Temperature in EPR in the Absence of Interaction with the Lattice

Within the framework of the concept of spin temperature in electron paramagnetic resonance (EPR) in the absence of the interaction with the lattice, the time dependence for spin temperatures of the Zeeman subsystem and dipole–dipole reservoir is theoretically investigated in both high- and low-temperature approximations. It is shown that the gain can be produced in the system by switching the frequency of the saturating field. The gain in the system as a function of the detuning of the frequency of the saturating field is investigated in a high-temperature approximation. In the presence of a test field in a high-temperature approximation, the possibility of determining the magnitude of a local magnetic field, which is associated with the time of transverse relaxation of the system, is discussed.

Intermolecular multiple quantum coherences in liquids

In the early 1990s, the traditional framework of NMR spectroscopy was challenged through a series of simple experiments. The pulse sequences used consisted of a few RF pulses and a few gradient pulses, and the samples were mixtures of simple molecules. The spectra showed unexpected cross peaks between spins in different molecules. () In order to explain these results, two basic assumptions had to be revisited: 1 the () high-temperature approximation to the Boltzmann distribution at equilibrium, and 2 the cancellation of dipolar couplings in solution. A close look at the physics involved showed that correlations between spins in separate molecules exist even after a single pulse, and that dipolar couplings can make these correlations visible in the presence of gradient pulses. A comprehensive description of the effect is given here, and some present and future applications are discussed. 2000 John Wiley & Sons, Inc. Concepts Magn Reson 12: 396409, 2000

Pressure and Isotropic-Nematic Transition Temperature of Model Liquid Crystals

The pressure in the gaseous, the isotropic liquid and nematic liquid crystalline states, as well as the isotropic-nematic transition temperature are calculated for a model system composed of non-spherical particles. The potential is a generalization of the Lennard-Jones interaction where the attractive part depends on the relative orientations of the particles and the vector joining their centers of mass. Point of departure is an augmented van der Waals approach. It involves a modified Carnahan-Starling expression associated with the repulsive part of the interaction, and an orientation dependent second virial coefficient, as well as the orientational distribution functions of a pair of particles, linked with the attractive part of the potential. In a high temperature approximation, and for a special choice of model parameters, results are presented and displayed graphically.

Multiple Echoes, Multiple Quantum Coherence, and the Dipolar Field: Demonstrating the Significance of Higher Order Terms in the Equilibrium Density Matrix

It is well known that dipolar field effects lead to multiple spin echoes in a simple two-RF pulse experiment (the MSE experiment). We show here that coherence transfer echoes (which identify the existence of multiple quantum coherences in liquid NMR) and multiple spin echoes have a common origin. Using density matrix theory we have calculated the phase and timing of multiple spin echoes from all quadrature phase combinations of RF pulses. We show for the MSE experiment that there is a one-to-one correspondence between the time domain echo order and the multiple quantum coherence order. The experimental confirmation of these phase predictions shows that multiple spin echoes provide independent evidence for the breakdown of the high temperature approximation as proposed by Warren et al. (Science 262, 2005 (1993)).

Damping rates in the MSSM and electroweak baryogenesis

We present an analysis of the thermalization rate of Higgsinos and winos based on the imaginary part of the two-point Green function in the unbroken phase of the MSSM. We use improved propagators including resummation of hard thermal loops and the thermalization rate is computed at the one-loop level in the high temperature approximation. We find that the damping is typically dominated by scattering with gauge bosons, resulting in a damping rate of about γ H ̃ ≃0.025T , γ W ̃ ≃0.065T . The contribution from scattering with scalars is relatively small. Implications for baryogenesis are also discussed.

Investigation of the possibilities of measuring relaxation parameters by the EPR method within the framework of the concept of spin temperature

Procedures for measuring some relaxation parameters of uniformly and nonuniformly broadened systems by the EPR method in a high-temperature approximation are proposed. The theoretical consideration is based on the concept of spin temperature and Provotorov's equations. A trial field is used along with a saturating superhigh-frequency field. The measuring methods for the time of the transverse relaxation T2 in the case of uniform broadening and some relaxation parameters or their combinations in the case of nonuniform broadening are based on measuring the frequency characteristics of the system when the absorption becomes zero at the frequency of the trial field.

Intermolecular multiple-quantum coherence transfer echoes and multiple echoes in nuclear magnetic resonance

In recent years evidence was presented that two radio frequency pulses in combination with suitably matched field gradient pulses produce cross peaks in two-dimensional NMR spectroscopy that reveal all the features of multiple-quantum/single-quantum coherence transfer signals. This experiment was termed CRAZED by Warren et al. Quite amazingly, identical signals were predicted on a purely classical basis. Thus the intriguing question arises whether this means that a classical analog to multiple-quantum coherences exists. In this study we show that this is clearly not the case. Coherence pathways of a purely single-quantum basis are excited anyway and lead independently to superimposed identical signals in parallel. Moreover, the observation of CRAZED signals per se neither disproves the adequacy of the high-temperature approximation, which was questioned in this context, nor proves a strong influence of intermolecular multiple-quantum coherences. However, the indispensable prerequisite for the description of the manifold CRAZED phenomena is that long-range dipolar interactions are taken into account either explicitly or in the continuous-limit form.

Baryogenesis and the thermalization rate of stop

We take the first steps towards the complete computation of the thermalization rate of the supersymmetric particles involved in electroweak baryogenesis by computing the thermalization rate of the right-handed stop from the imainary part of the two-point Green function. We use improved propagators including resummation of hard thermal loops. The thermalization rate is computed at the one-loop level in the high temperature approximation as a function of M t ̃ R (T) . We also give an estimate for the magnitude of the two-loop contributions which dominate the rate for small M t ̃ R (T) . If the stop is non-relativistic with M t ̃ R (T)≫T , thermalization takes place by decay and is very fast.