Resonance Raman scattering by localized and resonant modes of Ag2 molecules in rare gas matrices

We demonstrate for the first time that excitation of silver containing rare gas matrices at 406.74 nm results in resonance Raman spectra which show low energy localized and resonant modes. As they combine with the Ag₂ stretching vibration and its overtones, they can unambiguously be attributed to the Ag₂ molecule. In Xe-matrices the coupling of lattice phonons and impurity vibrations is documented by side bands which resemble the phonon density of states of the host lattice. Two trapping sites are observed in Kr-matrices.     

Scattering of acoustic phonons by rare earth substitutional atoms in yttrium aluminum garnets

The diffusive maxima of phonon signals, and in particular their arrival timest _m are examined for a number of solid solutions of rare earth atoms in yttrium aluminum garnets. The phonon pulses are generated by metallic films of the characteristic lengthl _h heated by current pulses to the temperatureT _h slightly higher than the ambient temperatureT. The injected phonons travel in wafers of the thicknessL _z. They are scattered by substitutional atoms of rare earth occupying the yttrium dodecahedral sites, rare earth and yttrium atoms occupying the aluminum octahedral sites and by another lattice imperfections generated in the process of sample growing. The qualitative analysis based on our exact formula for the diffusion coefficientD allows us to extract the contribution of rare earth atoms substituting the Y atoms toD. Considering the dependence oft _m/L _z ² on the temperature and the ratiol _h/L_z we conclude thatD∼T _h ⁻⁴ and that the energy of phonons forming the diffusive maxima ranges from 3.2k _BT_H to 4.2k _BT_H, which is in reasonable agreement with the existing estimates.     

Quantum dynamics of charge transfer on the one-dimensional lattice:Wave packet spreading and recurrence

The wave function temporal evolution on the one-dimensional(1D) lattice is considered in the tight-binding approximation. The lattice consists of N equal sites and one impurity site(donor). The donor differs from other lattice sites by the on-site electron energy E and the intersite coupling C. The moving wave packet is formed from the wave function initially localized on the donor. The exact solution for the wave packet velocity and the shape is derived at different values E and C. The velocity has the maximal possible group velocity v = 2. The wave packet width grows with time ～ t1/3and its amplitude decreases ～ t-1/3. The wave packet reflects multiply from the lattice ends. Analytical expressions for the wave packet front propagation and recurrence are in good agreement with numeric simulations.     

Mechanisms of bulk diffusion at “high” and “low” temperatures

A phenomenological model has been proposed for bulk self-diffusion and diffusion of interstitial atoms in the ranges of high (T > T _D) and low (T < T _D) temperatures (where T _D is Debye temperature). It has been shown that the mechanisms of diffusion at high and low temperatures differ significantly. In the high-temperature range, the diffusion is provided by fluctuations, which can be described in terms of local melting, i.e., the formation of a “liquid diffusion channel.” In the low-temperature range, when melting for some reasons is hindered, the diffusion is due to the fluctuation formation of a “hollow diffusion channel.” The calculation of the activation energies of these processes in the case of self-diffusion agrees well with the experiment in the temperature range T > T _D and has demonstrated that the activation energy increases significantly at T < T _D. The calculation of the activation energy for diffusion of interstitial atoms in bcc metals agrees well with the experiment in the entire temperature range and provides an explanation of the decrease in the activation energy of diffusion at low temperatures.     

The role of electron-electron and electronphonon interactions in the processes of destruction of landau quantization in InAs/AlSb nanostructures

A nonmonotonous temperature dependence of the electron quantum relaxation time is found. The results are interpreted, taking into account the competition between the intrasubband and intersubband electron-electron interaction channels. The experimental nonlinear τ_q (T) and T_D (T) dependences are explained by the appearance of an additional channel of two-dimensional (2D) electron scattering by deformation acoustic phonons under interaction with piezoacoustic phonons.     

Inelastic scattering of phonons by quadrupole defects with internal degrees of freedom

We solve the quantum mechanical problem of the inelastic scattering of phonons by a quadrupole defect in a crystal lattice for the case of solid parahydrogen whose matrix contains pair complexes of H₂ orthomolecules. By employing the pseudospin approximation for the operator of the energy of quadrupole-quadrupole interaction of the molecules in an orthopair we derive an effective Hamiltonian that describes the interaction of phonons with a pair quadrupole orthodefect in the lattice. We set up the scattering matrix and calculate the effective phonon relaxation time τ(ω, T) as a function of the frequency ω and the crystal temperature T. We also find that a pair quadrupole defect, which has a complicated system of levels, can be replaced by an effective two-level system with temperature-dependent parameters. The fact that a pair quadrupole orthocluster has internal degrees of freedom results in a resonant scattering peak near a certain critical temperature T ₀. Our estimates for H₂ yield T ₀≃ 6–7 K. Finally, we discuss the contribution of this mechanism to the low-temperature thermal conductivity of solid hydrogen. Zh. éksp. Teor. Fiz. 114, 555–569 (August 1998)     

The relationship between chemical and tracer diffusion coefficients of aliovalent ions in an ionic lattice

The theoretical model developed by Lidiard was extended to describe the relationship between the chemical and tracer diffusion coefficients of aliovalent ions in an ionic lattice.It is shown that the relationship between the chemical diffusion coefficient, D, and the tracer diffusion coefficient, D¹, is D = 2D¹ if the migration of dimers is the principal mechanism of transport and for the migration of trimers D = 3D¹ if the concentration of impurity ion is relatively small. These relationships are valid regardless of the charge of the aliovalent or lattice ions.The chemical diffusion coefficients of Cr³⁺ in Cr-doped MgO were determined for three different temperatures, 1656, 1717 and 1768K, and for the concentration region 2.5×10^?2?2.8×10^?1 mole% Cr₂O₃. Using previously determined values for the tracer diffusion coefficient of ⁵¹Cr in Cr-doped MgO it was found that for the temperature and concentration region investigated D = (2.00±0.17)D¹ which indicates that diffusion proceeds primarily by the migration of dimers.     

Near perpendicular propagation of ion cyclotron modes in a deuterium-hydrogen-oxygen fusion plasma

A dispersion relation for the near perpendicular propagation of the electromagnetic ion cyclotron wave has been derived in a fusion plasma that has deuterium as a majority species, hydrogen as a minority species and fully ionized oxygen as an impurity constituent; all being modelled by loss cone distribution functions. The wave has a frequencyω around the deuterium ion gyrofrequency-Ω_D and a wavelength much longer than its Larmor radiusγ _LD(k _⊥ γ _LD<1); the plasma itself being characterized by large ion plasma frequencies (ω _PD ² >Ω _D ² ). Two modes, a low frequency (LF) and a high frequency (HF) mode of opposite electrical energy can propagate in the plasma; the instabilities that arise are thus due to an interaction of modes of opposite energies. We find that while hydrogen tends to destabilize the plasma, the impurity oxygen ions have the reverse effect. Also the plasma is most stable when the ratios of the perpendicular components of oxygen-to-deuterium and hydrogen-to-deuterium temperatures are kept low. Detailed studies of the wave propagation characteristics and energy reveal the close resemblance of a loss cone plasma containing oxygen to a stable Maxwellian plasma in regard to wave stability, propagation and energy.     

Matter-wave propagation in optical lattices

Coherent propagation of atomic-matter waves in a one-dimensional optical lattice is studied. Wave packets of cold two-level atoms propagate simultaneously in two optical potentials in a dressed-state basis. Three regimes of the wave-packet propagation are specified by the quantity Δ² /ω _D , where Δ and ω _D are the dimensionless atom–laser detuning and the Doppler shift, respectively. At Δ² /ω _D ≫ 1, the propagation is essentially adiabatic, at Δ² /ω _D ≪ 1, it is (almost) resonant, and at Δ² ≃ ω _D , the wave packets propagate nonadiabatically, splitting at each node of the standing wave. The latter means that the atom makes a transition from one potential to the other one when crossing each node, and the probability of that transition is given by a Landau–Zener-like formula. All the regimes of propagation are studied with δ-like and Gaussian wave packets in the momentum and position spaces. Varying the control parameters, we can create wave packets trapped in a well of optical potentials and moving ballistically in a given direction in close analogy with point-like atoms. Within some range of the parameters, we force the atom to move in a pure quamtum-mechanical manner in such a way that a part of the packet is trapped in a well, and the other part propagates ballistically. The propagation modes are found to be characterized by different types of time evolution of the uncertainty product and the Wigner function.     

Wave packet splitting and zitterbewegung in two-dimensional semiconductor structures with spin-orbit coupling

Dynamics of electron wave packets in an asymmetric quantum well in the presence of Rashba spinorbit coupling was analytically and numerically studied. Electron Green’s functions were introduced and the evolution of 1D and 2D wave packets was studied. The effect of packet splitting caused by the presence of two branches with different chiralities in the Rashba Hamiltonian spectrum and zitterbewegung, i.e., packet center’s jitter, was studied. Spatial components of the spin density were calculated. It was shown that the component of the spin density S _y in split parts of the wave packet has opposite signs, and two other spin density components oscillate in space between scattering packets.     

Bound polaron in a spherical quantum dot under an electric field

A variational approach is used to study the ground state of a bound polaron in a spherical quantum dot under an external electric field. The binding energy of the hydrogenic impurity state is calculated by taking the interaction of an electron with both the confined longitudinal optical phonons and the surface optical phonons into account. The interaction between impurity and longitudinal optical phonons has also been considered to obtain the binding energy of a bound polaron. It shows that the polaron effects give significant corrections to the binding energy and its Stark energy shift. The external electric field increases the phonon contributions to the binding energy.     

Selection rules for multiphonon transitions at tetrahedral lattice sites

Selection rules for the involvement of phonons of the host crystal in multiphonon transitions associated with substitutional impurities having either a |s> or |p> localized state are derived. It is shown that for P₁ → P₁ transitions only LA and LO phonons are important, and also that a P₄ localized state couples with all phonons. The latter, plus the lack of a selection bar, suggests that such impurity states are more strongly coupled to the lattice that P₁ states.     

Polar optical phonon limited energy and momentum relaxation of hot electrons in a GaAs-quantum well (QW)

The average energy loss rate, the energy- as well as the momentum relaxation time of hot electrons confined in a GaAs-square quantum well are calculated as a function of the external controllable parametersn _s (electron density),T (lattice temperature) andT _e (electron temperature) for the interaction of the charge carriers with bulk- and surface polar optical phonons. Analytical expressions are derived in the limit of vanishing quantum well width at non-degeneracy and degeneracy of the electron system. Both energy-and momentum relaxation time are found to be complicated functions of the ratiosT _D /T _e andT _D /T withT _D being the Debye-temperature of the polar optical phonon involved in the scattering. In a thick (very thin) QW the energy loss rate to bulk PO-phonons is found to be larger (smaller) than the corresponding loss rate to surface modes. The energy- (momentum-) relaxation times are found to be constant (increasing) functions ofn _s at non-degeneracy (degeneracy) of the electron system. Dedicated to Professor Karlheinz Seeger on the occasion of his 60th birthday     

Comparison of methods for calculating turbulent diffusion coefficients

Different methods for calculating the turbulent diffusion coefficient D _T of a passive scalar impurity in an infinite homogeneous isotropic stationary turbulent medium are examined. The values of D _T calculated by these methods are compared for two limiting types of turbulence, viz., turbulence with a δ-function spectrum and turbulence with a Kolmogorov-type spectrum. The temporal dependence of the velocity correlators is assumed to be exponential. It is shown that the most accurate method is based on the use of the solution of the nonlinear equation for the averaged Green’s function with consideration of the contribution from the four-point turbulent velocity correlators. A comparison with the results of other methods that are simpler from the mathematical standpoint shows that some of them also permit the calculation of D _T with relatively good accuracy. Zh. éksp. Teor. Fiz. 111, 871–881 (March 1997)     

Dynamics of Dirac quasi-particles in lattice vibration and anomalous phonon frequency shift of graphene

By exact solution of time-dependent Schrödinger equation of electron in graphene under interaction with E_2g phonons, we investigate the dynamical behavior of Dirac quasi-particle in the process of lattice vibration. Due to the global geometric phases acquired by electron during lattice vibration, an anomalous shift of the vibration frequency is obtained. We calculate the Fermi energy dependence of frequency shift which is in consistence with experiment in case of small doping density.     

Quantum diffusion of atomic species in metals

The coefficient of quantum diffusion D(T) is calculated using exact states of the localized impurity in the deformed vibrating lattice. Without resorting to the Born-Oppenheimer approximation an explicit general expression for D(T), valid for impurities of arbitrary mass and holding in the whole range of temperature, is obtained in terms of the crystal properties. Interesting new features arise from retaining some terms of the Hamiltonian which were left aside in previous theories.     

Scattering of acoustic phonons by rare earth substitutional atoms in yttrium aluminum garnets

The diffusive maxima of phonon signals, and in particular their arrival timest _m are examined for a number of solid solutions of rare earth atoms in yttrium aluminum garnets. The phonon pulses are generated by metallic films of the characteristic lengthl _h heated by current pulses to the temperatureT _h slightly higher than the ambient temperatureT. The injected phonons travel in wafers of the thicknessL _z. They are scattered by substitutional atoms of rare earth occupying the yttrium dodecahedral sites, rare earth and yttrium atoms occupying the aluminum octahedral sites and by another lattice imperfections generated in the process of sample growing. The qualitative analysis based on our exact formula for the diffusion coefficientD allows us to extract the contribution of rare earth atoms substituting the Y atoms toD. Considering the dependence oft _m/L _z ² on the temperature and the ratiol _h/L_z we conclude thatD～T _h ^?4 and that the energy of phonons forming the diffusive maxima ranges from 3.2k _BT_H to 4.2k _BT_H, which is in reasonable agreement with the existing estimates.     

旋声晶体NaBrO3与NaClO3的弹性研究

用中子相干非弹性散射、沿三个主晶轴方向测量了同构异质晶体NaBrO₃和NaClO₃的声子色散关系。从小波矢区的实验结果获得了沿晶体这些方向的弹性波速及三个独立的弹性常数C₁₁,C₁₂,C₄₄。实验结果表明,尽管这两种晶体在旋声方面表现出十分不同的特性,但是,它们的左旋和右旋圆偏振声子的声速都是在一个特定的平均速度v_(TD)上下均衡的分布,这个平均速度可由 关键词：     

Cathodoluminescence of ZnSe(Bi):Al crystals

The cathodoluminescence (CL) in ZnSe crystals annealed at T=1200 K in a Bi melt containing an aluminum impurity is investigated. The spectra are recorded for different excitation levels, temperatures, and detection delay times t ₀. As t ₀ is increased, the intensity of the orange band at λ _max=630 nm (1.968 eV) in the CL spectrum decreases in comparison to the intensity of the dominant yellow-green band at λ _max=550 nm (2.254 eV), whose half-width increases in the temperature range 6–120 K and then decreases as the temperature increases further. It is shown that such behavior of the yellow-green band is caused by the competition between two processes: recombination of donor-acceptor pairs and of free electrons with holes trapped on acceptors. The former mechanism is dominant at low temperatures, and the latter mechanism is dominant at high temperatures. At T∼120 mK the contributions of the two mechanisms to the luminescence are comparable. The resultant structureless band then achieves its greatest half-width, which is dictated by the interaction of the recombining charge carriers with longitudinal-optical and longitudinal-acoustic phonons and with the free-electron plasma. The mean number of longitudinal-optical phonons emitted per photon is determined mainly by their interaction with holes trapped on deep acceptors in the form of Al atoms replacing Se. The donor in the pair under consideration is an interstitial Al atom. Fiz. Tverd. Tela (St. Petersburg) 39, 1526–1531 (September 1997)     

晶格振动对激子运动的影响

在本文中用Haga研究极化子时提出的微扰法讨论晶格振动对激子运动的影响。把作者过去的工作推广到电子和空穴质量不相等的普遍情形。在忽略反冲效应中不同波矢的声子之间的相互作用时,导出了激子的基态能量、有效质量、约化质量和内部势能。指出:激子的有效质量和电子、空穴与声子的相互作用有关;激子的约化质量不仅和电子、空穴与声子的相互作用有关,而且还和电子空穴质量比有关;激子形成自陷态的条件并非由电子、空穴与声子相互作用的大小,而是由电子、空穴质量比决定。形成激子自陷的条件是电子、空穴质量比0.261<μ_e/μ_h<3.83。在μ_e/μ_h<0.261或μ_e/μ_h>3.83的情形,激子极化晶体中并不形成自陷态。 关键词：