Spontaneous excitation of discrete breathers in crystals with the NaCl structure at elevated temperatures

The density of phonon states at various temperatures has been calculated for crystals with the NaCl structure with equal and strongly differing weights of anions and cations. It has been shown that, in the crystal with a considerable difference in the weights of components, a wide band gap exists in the spectrum of phonon states, which leads to spontaneous excitation of nonlinear localized vibrational modes??gap discrete breathers having frequencies inside the band gap, if the temperature is sufficiently high. It has been found that the peak of the density of phonon states lying above the spectrum of linear vibrations appears at elevated temperatures, which can indicate the existence of discrete breathers with corresponding frequencies. It has been noted that, previously, the existence of gap discrete breathers in the NaI crystal at 555 K was shown experimentally. The presented results bring up the question of the theoretical justification and experimental observation of the breathers with frequencies above the phonon spectrum.     

Thermal conductivity of systems with a gap in the phonon spectrum

An original theoretical model for describing the low-temperature thermal conductivity in systems with a region of forbidden values(a gap) in the phonon spectrum is proposed. The model is based on new experimental results on the temperature dependence of the phonon diffusion coefficient in nanoceramics and dielectric glasses which showed a similar anomalous behavior of the diffusion coefficient in these systems that may be described under the assumption of a gap in the phonon spectrum. In this paper, the role of the gap in low-temperature behavior of the thermal conductivity, κ(T), is analyzed. The plateau in the temperature dependence of the thermal conductivity is shown to correlate with the position and the width of the gap. The temperature dependence of thermal conductivity of such systems when changing the scattering parameters related to various mechanisms is studied. It is found that the umklapp process(U-processes) involving low-frequency short-wavelength phonons below the gap forms the behavior of the temperature dependence of thermal conductivity in the plateau region. A comparison of the calculated and experimental results shows considerable possibilities of the model in describing the low-temperature thermal conductivity in glass-like systems.     

Excitation of gap discrete breathers in an A<Subscript>3</Subscript>B crystal with a flux of particles

The generation of discrete breathers in an A₃B crystal has been modeled by the method of molecular dynamics using Pt₃Al as an example via the application of random unidirectional momenta, which simulate the action of a particle flux, to atoms. Two possible mechanisms of the excitation of gap discrete breathers with a soft type of nonlinearity have been revealed depending on the energy of particles in a flux. If a particle is able to transfer energy of more than 1.4 eV to the Al atom, a discrete breather can be excited by the only particle. Otherwise, a discrete breather is formed upon numerous particle–Al atom collisions, which are possible only at a sufficiently high density of particles, as each following particle must transfer its momentum to the Al atom before its oscillations provoked by previous particles attenuate.     

Head-on and head-off collisions of discrete breathers in two-dimensional anharmonic crystal lattices

The ground-state phase diagram of the extended Falicov-Kimball model with f-f electron hopping is studied numerically in the one-dimensional case. To identify the nature of ground states three complementary numerical methods are used, and namely, (i) the small-cluster exact-diagonalization method, (ii) the density-matrix-renormalization-group method (DMRG) and (iii) an approximate, but very accurate, numerical method based on the reduction of the Hilbert space. It is found that the physics of the Falicov-Kimball model found for the zero value of the f-electron hopping integral t _f (including the existence of the devil’s staircase structure) persists also at finite values of t _f . The critical values of t ^c _f below which the physics of the Falicov-Kimball model dominates are calculated numerically and it is shown that they depend very strongly on the f-electron concentration n _f and only very weakly on the Coulomb interaction. In particular, we have found that for strong Coulomb interactions the value of t ^c _f rapidly increases from t ^c _f ~ 0.003 found for n _f = 1 / 4 up to relatively large t ^c _f ~ 0.4 found for n _f near the half-filled band case n _f = 1 / 2. In addition, the complete picture of valence transitions is presented for non-zero t _f and strong Coulomb interactions.     

电子密度对等离子体光子晶体禁带特性的影响

从Maxwell方程出发,采用类似于量子力学Kronig-Penney模型求解周期势的方法,结合双水电极介质阻挡放电的实验结果,研究了电子密度ne对一维等离子体光子晶体禁带特性的影响。研究发现:电子密度对等离子体光子晶体光子禁带的位置和宽度均有重要的影响;等离子体光子晶体的禁带宽度随电子密度的增加而增大,增长速率为电子密度的函数;等离子体光子晶体的截止频率、光子禁带边缘频率随电子密度的增大而增大。给出了当等离子体光子晶体具有显著禁带宽度时的电子密度的理论临界值。     

Influence of phonon dispersion and exciton spectrum on the energy spectrum of magnetopolarons in a quantum well

An analysis is made of the influence of the spatial dispersion of LO phonons and the exciton effect on the energy spectrum of magnetopolarons in a quantum well. It is shown that in optical experiments where light is incident normally on the plane of the quantum well, a discrete spectrum of magnetopolarons is observed. Both the phonon dispersion and the Coulomb attraction of an electron and a hole may lead to a shift of the discrete magnetopolaron energy levels and additional contributions to the broadening of various levels.     

The effect of elastic strain on the dispersion characteristics of a polariton 2D structure

The dispersion characteristics of a two-dimensional lattice of tunnel-coupled micropores containing single-level quantum dots are numerically simulated. It is shown that, as a result of elastic strain of the system under study, it is possible to achieve necessary changes in its energy structure and optical properties determined by the rearrangement of the polariton spectrum.     

Influence of quasi-local levels on the phonon spectrum of semimetals in a strong magnetic field

The influence of quantizing magnetic field-induced quasi-local levels, due to diamagnetic impurities, on the velocity of ultrasound in semimetals is discussed.     

Influence of zero-point anomalies in the electron density of states of electrodes on the inelastic tunneling spectrum

It is shown that a minimum in the electron density of states near the Fermi surface of one of the electrodes shifts the peaks in inelastic tunneling spectrum toward higher voltages. The shift depends on the correlation parameter and increases with temperature. It is argued that the observation of the shift in the local singularities of inelastic tunneling, in conjunction with the presence of large-scale zero-point anomaly in the differential conductivity, can serve as a firm evidence of the presence of the corresponding singularity in the electron density of metal oxides and magnetoresistive materials.     

Critical temperature and energy gap in superconductors with singular density of states

The influence of-functional (symmetric as well as asymmetric) singularities in density of states (DOS) on the critical temperature and zero temperature energy gap is calculated. Surprisingly, we have obtained the same function for the off-symmetry of the peak position in DOS on the corresponding critical temperature as for the temperature dependence of the energy gap in the strong-coupling limit. The enhancement of the critical temperature due to the singularity (compared with the constant DOS near the Fermi surface) is much lower for strong-coupling superconductors than in the weak-coupling limit. Hence, the singularity in DOS cannot be the exclusive reason for large values of critical temperatures in highTc superconductors.This work was supported by the grant GA SAV 188/1991.     

Stable and metastable states and the formation and destruction of breathers in the discrete nonlinear Schrödinger equation

Statistical mechanics explains many localization phenomena of lattices such as the discrete nonlinear Schrödinger equation. However, numerical simulations show that the complete thermalization is rarely achieved. Instead, one observes metastable statistical states that are robust when excited locally. This paper investigates thermodynamically metastable states where the trajectory is confined to some part of the energy shell. The partition function and the entropy are computed with a perturbation method. This method is applicable to stable and metastable states, and it allows us to give approximative analytic expressions for the entropy in the complete thermodynamic state space.     

Effect of the dimension of the phonon spectrum on the stability of condensed media states

The effect of dimension index d _f of the phonon spectrum, which is a structural characteristic in continual models, on the stability of states of condensed media is considered in the Einstein and Debye approximations. The estimate of the phase state stability is based on the Lindemann criterion generalized to arbitrary values of 0 ≤ d _f ≤ ∞. The problem of variation of physical characteristic of a substance by controlling the structure of its phonon spectrum is considered by analyzing the possibility of obtaining molecular hydrogen in the superfluid state. The Einstein and Debye models as applied to the problem on the dynamics of atomic oscillations are compared, and the divergence of the latter model for fractal dimensions d _f < 2 of the phonon spectrum is demonstrated, as well as the incompatibility of the Debye model at high temperatures and the model of a classical oscillator for all dimensions except d _f → ∞.     

Trap statistics and the gap density of states in amorphous materials

We present a one-electron theory of non-equilibrium trap occupancy in amorphous materials, with simple results for electrons in states above the equilibrium Fermi level. The density of these states can then be obtained by analysis of transient photoconductivity. Specific values are given for a-Si: H samples for energies in the range 0.55–0.40 eV below the conduction band mobility edge. We conclude that the state density there can be low, as suggested by the results of deep level transient spectroscopy.