准静态颗粒介质的弹性势能弛豫分析

颗粒体系是典型的多体相互作用体系, 具有多重的能量亚稳态. 对于准静态颗粒体系, 引入构型颗粒温度T_c描述弹性势能涨落. 本文认为平衡的体系具有一定的构型颗粒温度T_a, 其量值反映了其结构特征. 当外界扰动激发的构型颗粒温度超出T_a时, 产生不可逆过程. 通过对应力松弛过程的分析, 发现(T_c-T_a)激发了弹性弛豫, 且(T_c-T_a)越大则松弛过程中应力变化越大, 最终构型颗粒温度T_c→T_a时,宏观应力松弛结束,体系达到新的能量亚稳态.     

Geschwindigkeitsverteilung anisotroper Deuteriumplasmen aus zeitaufgelösten Neutronenspektren

The ion energy in hot deuterium plasmas can be determined by using the Doppler broadening of thed-d-neutron line at 2.45 MeV. A method allowing time resolved measurement of neutron spectra in pulsed discharges is described. It utilizes the known energy dependence of the total neutron cross section of light to medium weight nuclei for energy selection of the neutrons. The neutrons are detected with plastic scintillators which afford both a high detection efficiency and a time resolution in the nanosecond range. The method is applied to a theta pinch for investigating the anisotropy and relaxation of the deuteron velocity distribution. It is shown that, in particular, the ion energy parallel to the magnetic field can readily be measured even at the low neutron yield available at present. Suitable combination of this procedure with known diagnostic methods allows deviation of the velocity distribution from the isotropic Maxwell distribution to be determined with sufficient accuracy. Comparison of the experimental relaxation rate with the theoretical predicted collision relaxation shows that Coulomb collisions are not a sufficient explanation. The short wave-length mirror instabilities also observed probably make a major contribution to the relaxation of the anisotropic velocity distribution.     

Relaxation of the Electron Distribution Function

This paper discusses an analytical technique for calculating the relaxation in time of the electron distribution function f in an environment in which no perturbing forces act on the electrons. For t = 0, f may have any arbitrary form presumed to be caused by perturbing forces which were not zero during t < 0. The technique then allows calculation of the relaxation of f in time for the following types of electron collisions: a) elastic collisions with cold neutrons, b) excitation collisions in which the threshold energy for an elastic excitation collision is small compared to the electron energy, c) ionizing collisions when the energy lost by the electron is small compared to its energy, and d) any combination of the above. In this paper the method is described and simple examples are presented to illustrate the physics of relaxation for the collisional categories listed above. It is pointed out that a number of important problems can be solved by this technique primarily in the area of nuclear EMP: the forrnative lag time problem and the calculation of thermalization time. In addition, the details of the afterglow of extinguished discharges in the monotomic gases can be determined.     

有限元法分析不同形状量子点的应变能及弛豫度变化

利用有限元方法研究了不同形状量子点的应变能量分布和弛豫度随着高宽比变化的规律.分析了量子点间距和量子点形状对量子点应变弛豫的影响,定量地讨论了量子点的弛豫度与量子点形状之间的关系.计算结果表明,在不考虑表面能的情况下,当量子点高宽比增加时,弛豫度上升,并且发现平顶金字塔形量子点最先达到稳定;岛间距增大时,量子点内应变能下降,其中立方体形量子点应变能下降最快.研究表明,量子点的弛豫度可以成为控制量子点成岛形状的重要依据. 关键词： 量子点 弛豫度     

Dynamics of electron beams in plasmas

The characteristic features of the relaxation of the energy and momentum distribution functions of the electrons in a plasma produced by a low-voltage beam discharge in helium are investigated. It is established that, contrary to widely held opinion, the energy of an intense electron beam may relax due to the wave excitation. The critical currents corresponding to a jumplike transition from one relaxation mechanism to another are measured. The density of metastable helium atoms is determined from the comparative analysis of theoretical and experimental results on the structure of the energy spectrum of the electrons of an intense beam. An intense electron beam is found to become more isotropic in the course of its interaction with Langmuir waves in a collisionless plasma. The cross section for quasi-elastic collisions between the electrons and Langmuir plasmons is estimated. The wave nature of the beam-plasma mechanism for the relaxation of the anisotropic electron energy distribution function is demonstrated, and the mechanism itself is shown to come into play when the discharge current exceeds a certain critical level. The experimental threshold criterion for the energy relaxation of an intense monoenergetic beam is obtained for the first time. It is shown that the relaxation occurs in two stages: the isotropization stage, in which the beam energy decreases insignificantly, is followed by the stage in which the beam relaxes to a state with a plateau-like energy distribution function. The threshold criterion for the relaxation of the anisotropic electron energy distribution function is universal in character regardless of the cause of anisotropy.     

Modeling phase transition for compressible two-phase flows applied to metastable liquids

The seven-equation model for two-phase flows is a full non-equilibrium model, each phase has its own pressure, velocity, temperature, etc. A single value for each property, an equilibrium value, can be achieved by relaxation methods. This model has better features than other reduced models of equilibrium pressure for the numerical approximations in the presence of non-conservative terms. In this paper we modify this model to include the heat and mass transfer. We insert the heat and mass transfer through temperature and Gibbs free energy relaxation effects. New relaxation terms are modeled and new procedures for the instantaneous temperature and Gibbs free energy relaxation toward equilibrium is proposed. For modeling such relaxation terms, our idea is to make use of the assumptions that the mechanical properties, the pressure and the velocity, relax much faster than the thermal properties, the temperature and the Gibbs free energy, and the ratio of the Gibbs free energy relaxation time to the temperature relaxation time is extremely high. All relaxation processes are assumed to be instantaneous, i.e. the relaxation times are very close to zero. The temperature and the Gibbs free energy relaxation are used only at the interfaces. By these modifications we get a new model which is able to deal with transition fronts, evaporation fronts, where heat and mass transfer occur. These fronts appear as extra waves in the system. We use the same test problems on metastable liquids as in Saurel et al. [R. Saurel, F. Petitpas, R. Abgrall, Modeling phase transition in metastable liquids: application to cavitating and flashing flows, J. Fluid Mech. 607 (2008) 313–350]. We have almost similar results. Computed results are compared to the experimental ones of Simões-Moreira and Shepherd [J.R. Simões-Moreira, J.E. Shepherd, Evaporation waves in superheated dodecane, J. Fluid Mech. 382 (1999) 63–86]. A reasonable agreement is achieved. In addition we consider the six-equation model with a single velocity which is obtained from the seven-equation model in the asymptotic limit of zero velocity relaxation time. The same procedure for the heat and mass transfer is used with the six-equation model and a comparison is made between the results of this model with the results of the seven-equation model.     

Energy relaxation of excitonlike polaritons in semiconductor microcavities: Effect on the parametric scattering of polaritons

Polariton emission in GaAs-based microcavities has been studied under variable conditions, which made it possible to excite (a) polaritons from the upper polariton branch and hot free polaritons and electrons, (b) polaritons from the lower polariton branch (LPB) and localized excitons, and (c) the mixed system. Variation of the excitation conditions leads to substantial differences in the energy distributions of polaritons and in the temperature dependences of polariton emission. It is established that the energy relaxation of resonantly excited LPB polaritons via polariton and localized exciton states at liquid helium temperatures is ineffective. Instead, the relaxation bottleneck effect is suppressed with increasing temperature by means of exciton delocalization (due to thermal excitation by phonons). The most effective mechanism of relaxation to the LPB bottom is via scattering of delocalized excitons on hot free carriers. It is found that the slow energy relaxation of polaritons excited below the free exciton energy can be significantly accelerated at low temperatures by means of additional weak generation of hot excitons and, especially, hot electrons. This acceleration of the energy relaxation of polaritons by means of additional overbarrier photoexcitation sharply decreases the barrier for stimulated parametric scattering of polaritons excited at an LPB inflection point. Therefore, additional illumination can be used to control the polariton-polariton scattering.     

Relaxationserscheinungen in Elektronenstrahlen

In electron beams with high charge density there are observed anomalous shiftings and broadenings as well as symmetrizations of the energy distribution (Boersch 1954). Continuing former work, where these phenomena were ascribed to a relaxation process changing the energy distribution to a new equilibrium distribution, quantitative expressions are derived for the energy spread and the shape of this new energy distribution. Furthermore a relaxation degree is defined which measures the progress of relaxation. Measurements are described with the highest relaxation degrees reached up to now. In addition to the verification of former results up to these new limits the shape of the measured energy-distributions can be fitted to the derived one with suitable choice of a single parameter. Peculiarities of measurements with superposed axial magnetic fields are discussed.     

Femtosecond optical diagnostics of interlevel energy relaxation in the band gap of silver iodide nanocrystals

The kinetics of the relaxation processes in silver iodide nanocrystals synthesized in a nanoporous silicate glass matrix and excited by femtosecond laser pulses has been experimentally studied. A short-lived characteristic transmission band is found to be formed in the spectral range of exciton absorption of nanocrystals. The specific features of photoexcited-carrier relaxation, which are due to the energy redistribution of captured carriers over levels in the band gap of silver iodide nanocrystals, are considered. The energy transfer relaxation rates are estimated and possible mechanisms of the energy transfer are discussed.     

Exciton energy relaxation on acoustic phonons in double-quantum-well structures

The paper analyzes the rate of energy relaxation involving acoustic phonon emission between exciton states in a double quantum well. A theoretical study is made of the part played by two mechanisms, one of which is a one-step transition with emission of an acoustic phonon and the other is a two-step transition, which includes elastic exciton scattering from interface nonuniformities followed by energy relaxation within an exciton subband. The rate of the two-step transition in real double quantum wells is shown to be higher than that of the one-step transition. As follows from calculations, the fast energy relaxation between exciton states is determined by the elastic scattering of phonons from the interface.     

Anisotropic local stress and particle hopping in a deeply supercooled liquid

The origin of the microscopic motions that lead to stress relaxation in deeply supercooled liquid remains unclear. We show that in such a liquid the stress relaxation is locally anisotropic which can serve as the driving force for the hopping of the system on its free energy surface. However, not all hoppings are equally effective in relaxing the local stress, suggesting that diffusion can decouple from viscosity even at the local level. On the other hand, orientational relaxation is found to be always coupled to stress relaxation.     

Stoßbestimmte Relaxation des Elektronenensembles im Plasma bei zusätzlicher Aufheizung durch ein elektrisches Feld I. Die charakteristischen Zeiten für den Übergang in stationäre Zustände

Collision Dominated Relaxation of the Electron Ensemble in a Plasma with Additional Heating by an Electric Field. I. Characteristic Times for the Transition to Stationary States Starting from time dependent Boltzmann equation for electrons the time development of the isotropic part of the distribution function and of macroscopic quantities as mean energy, mobility, excitation frequency and energy transfer quotients during transition between two stationary states are determined. The computation is referred to weak ionized neon plasmas which are typical for low pressure and for medium pressure discharges. As a result of this investigations we get informations about the characteristic relaxation times which are different in order of magnitude, and about their dependence of the processes of energy transfer. The energy transfer quotients which determine the energy loss by different collision processes in consideration are found to be suitable quantities to characterize the relaxation times.     

Thermoactivated molecular motions in solids and determination of the energy of their activation by NQR spectroscopy methods

The activation energy of thermoactivated molecular motions in solids is determined by examining the influence of these motions on the temperature dependence of the nuclear quadrupole spin-lattice relaxation rate and quadrupole resonance (NQR) signal intensity for chlorine-35. In the latter case, the heating of the crystals is accompanied by the fading of resonance signals, which is analyzed using a linear correlation between the activation energy of this motion and the fade-out temperature. The correlation parameters are demonstrated to be dependant on the type of molecular motion. The relaxation method is shown to be more effective in studying molecular motion and evaluating its activation energy as compared to the NQR signal fading technique.     

Effects of Screening and Finite Phonon Energy on the Transport in Quantized Layers in Compound Semiconductors

Analytical expressions for the momentum relaxation times of the conduction electrons in a non-degenerate two dimensional electron gas in the surface of a compound semiconductor have been obtained for interactions with the piezoelectric and deformation potential acoustic phonons taking due account of the screening of the perturbing potential under the the condition of low lattice temperature when the phonon energy cannot be neglected in comparison to the average thermal energy of the electrons and for that matter the equipartition approximation for the phonon distribution is hardly valid. The relaxation times calculated for inversion layers in GaAs and ZnO are found to depend upon the carrier energy, the lattice temperature and the impurity concentration in rather complex manners which are significantly different from what follows from the traditional approach of either neglecting the phonon energy or disregarding the process of screening. It is seen how the finite value of the phonon energy and the screening of the perturbing potential change the mobility characteristics significantly at the low lattice temperatures. The temperature dependence of the zero field mobility that one obtains using the relaxation times calculated here is quite different from the traditional laws.     

Transient interband light absorption by quantum dots: Degenerate pump-probe spectroscopy

The optical pump-probe method, which makes it possible to determine the energy relaxation rate for excited electron-hole pairs and excitons in semiconductor quantum dots (QDs), is theoretically described. A scheme in which the carrier frequencies of optical pump and probe pulses are close to resonance with the same interband transition in the QD electron subsystem (degenerate case) is considered. The pump-induced probe energy absorption is analyzed as a function of the delay time between the pump and probe pulses. It is shown that under certain conditions this dependence is reduced to monoexponential, whose exponent is proportional to the energy relaxation rate for the considered state of electron-hole pairs and excitons. The size dependence of the energy relaxation rate of the electron-hole pair states is modeled by the example of PbSe-based QDs, whose electron subsystem is in the strong-confinement regime.     

Relaxation effects during core ionization in metal—metal-bonded complexes

Using the ligand group shift model to calculate metal binding energies (BE's), comparisons of BE(calc) and BE(obs) values reveal a correlation between relaxation energy and the extent of metal—metal bonding. No significant difference in relaxation energy is detected between monometal complexes and the dipalladium complexes which contain a single MM bond. However, substantially larger relaxation energies (1–2 eV) relative to monometal complexes are found for multiply MM-bonded complexes. The magnitude of the additional relaxation energy is in the range expected on the basis of comparisons with other systems.     

PulsedμSR and the measurement of Zeeman and Dipolar relaxation times

Pulsed muon sources are ideally suited to experiments which require a repetitive or periodic excitation of the sample. Various possibilities are discussed. Magnetic relaxation is considered in terms of the various thermodynamic reservoirs which make up a spin system, both for nuclear magnetism and for electronic paramagnetism.A synchronous pulse sequence is presented which, in a single experiment, allows the following distinct relaxation functions of the samples host spin system to be determined: that of its Zeeman energy, that of its secular dipolar components, and that of the total dipolar energy in zero field. The timescale of the high field measurements overlaps with and substantially extends the range accessible to NMR and ESR.     

Relaxation phenomena in nucleon-induced reactions and preequilibrium angular distributions

The intranuclear relaxation process in nucleon-induced reactions is investigated in a Fermi gas model utilizing a statistical operator for the non-equilibrium state, which contains energy, particle number, and linear momentum as relevant observables. In the hydrodynamic stage the phase space is subdivided into subspaces, assumed to be in a quasi-equilibrium, which is characterized by a Fermi distribution function with time-dependent thermodynamic parameters. A set of coupled non-linear equations for the time development of the thermodynamic parameters is derived. For the case of two sybsystems, a numerical solution of these generalized transport equations is provided, with kinetic coefficients calculated microscopically. The initial conditions are fixed in accordance with the energy and angle distribution of both particles occupying states above the Fermi surface after the first collision event. From the results, a fast and a slow stage are established in the relaxation process, with equal relaxation times for all physical quantities under consideration. The dependence of the relaxation time on particle number and excitation energy is estimated. The particle emission from the precompound stage of the reaction is taken into account by using the principle of detailed balance to find expressions for the mean fluxes from the compound system to the outer space, which are included in the equations for the relaxation. From the time evolution of the occupation number for states above the nucleon binding energy, the precompound double-differential cross section cumulating up to a certain time is calculated. The results for the angular distributions in nucleon-induced precompound reactions are compared with measured data as well as with previous predictions from generalized exciton models. Satisfactory agreement with experimental data is obtained. Following the time development of the compound nucleus the consistency of the present model with the evaporation theory is demonstrated by investigating the mean nucleon decay width.     

On the relaxation of the order parameter in the BCS model

The relaxation of the nonequilibrium order parameter (wave function of pairs) of a “pure” superconductor is considered for the homogeneous case. The relaxation is due to the electron-phonon interaction. The orderparameter relaxation time is shown to be much longer than the time interval between electron-electron collisions. This relation is explained by the smallness of the superconducting transition temperature compared to both the Fermi energy and the Debye energy in the BCS model.