Atomic tunneling states and low-temperature anomalies of thermal properties in amorphous materials

A general theory of local atomic non-single-well potential in amorphous materials is presented. It is shown that the potentials practically are critical double-well potentials with low spring constants. Two essentially different types of two-level tunneling states in such potentials are found. The spectral density of the states is shown to be smooth under realistic conditions. The respective specific heat logarithmically deviates from the linear temperature dependence and logarithmically or more weakly depends on the measurement time. Both types of the tunneling states can essentially contribute to the specific heat, whereas one of the types is rather more strongly coupled with phonons and determines the phononic thermal conductivity. Some other questions are also discussed.     

Atomic tunneling states with dissipation in glasses: temperature-dependent two-level systems

By taking account of the influence of dissipation on atomic tunneling states in glasses, we introduce a density of states of two-level systems which is non-linearly temperature dependent. Based on this model, we can explain the “excess T³” anomaly in the specific heat and the “plateau” in the thermal conductivity of glasses.     

Correlation between low-temperature tunneling density of states in glasses and the glass transition temperature

The density of states of the low-temperature tunneling levels which contribute to the excess specific heat and other anomalous properties of amorphous materials is shown to be proportional to the reciprocal glass transition temperature. This result agrees with recent experimental observations for a number of glass formers.     

Atomic collisions in semiconductors

Abstract

Energetic particle bombardment of semi-conductors (Si and GaAs) is studied by means of Molecular Dynamics simulations using many-body potentials. The simulations show that the diamond lattice structures can lead to the trajectories of particles within the crystal being channelled even at low energies. Some results concerning damage production, low energy implantation profiles and angular distributions of ejected particles are presented.     

Zener tunneling in one-dimensional organic semiconductors

Tunneling effect in one-dimensional organic semiconductors in the presence of an external electric field is studied within the framework of a tight-binding model and a nonadiabatic dynamical method. It is found that under a high electric field, electrons can transit from the valence band (VB) to the conduction band (CB), which is demonstrated to be Zener tunneling in organic semiconductors. The results also indicate a field-induced insulator-metal transition accompanied by the vanishing of the energy gap. It is found that, after the field is turned off, the Peierls phase cannot be recovered.     

Paramagnetic centers in neutron irradiated glassy semiconductors

EPR was used for investigation of neutron irradiated glassy semiconductors (E_n = 2 MeV). It was shown that the neutron irradiation causes an increased number of unpaired spins: EPR signal was detected in glassy semiconductors which have not exhibited any signal before irradiation. A possible interpretation of the formation of paramagnetic centers due to neutron irradiation is suggested.     

Chaos-assisted tunneling in dielectric microcavities

We demonstrate that the lifetimes and emission patterns of the optical modes in generic (asymmetric) microresonators are strongly affected by the phenomenon of chaos-assisted tunneling and develop a theory of the effect.     

Electronic transitions in mobility gap in glassy semiconductors

Electronic transitions in mobility gap in glassy semiconductors are considered. Energies characteristics of the transitions, including de-trapping and luminescence energies, are in a simple way related to both the gap width and each other. Basic types of luminescence centres and some types of radiationless centres, including centres of photo-structural changes, are revealed. Correlations of the luminescence and its fatigue with some other photoinduced transitions are discussed. Some features of the luminescence are presented, including those associated with the inverse- Arrhenius law.     

Crystallization of chalcogenide glassy semiconductors with admixtures

Crystallization of chalcogenide glassy semiconductors with different contents of admixtures were investigated. It was shown that photoluminescence studies can reveal a lack of the centers responsible for radiation, i.e., possessing negative correlation energy, and the onset of crystallization in chalcogenide glassy semiconductors.     

Polarization-dependent laser-induced giant mass transport in glassy semiconductors

Direct experimental evidence of the validity of the two-phase model of chalcogenide glass has been obtained. It has been shown that the polarization-light irradiation of glassy semiconductor films induces giant mass transport in the direction perpendicular to the polarization of the incident light. It has been revealed that a surface relief appears in the irradiated films. The shape of the relief depends on the laser polarization state. A macroscopic model qualitatively describing the observed phenomenon is presented.     

Internal friction in glassy semiconductors of the Ge-As-Se system

Relaxation processes in glasses of the Ge-As-Se system have been investigated by the internal friction technique. The process that can be classified as the β relaxation process has been revealed in the temperature range 180–280 K. The assumption has been made that a change in the magnitude of the internal friction maximum in the above temperature range with a change in the chemical composition is associated with the change in the concentration of structural units responsible for the β relaxation in these materials. It has been established that the activation energy for the α relaxation process in the glasses of the Ge-As-Se system under investigation increases considerably in the range of the average coordination number Z = 2.6.     

Band-gap tunneling states in DNA

An important issue regarding DNA electrical conductivity is the electron (hole) transfer rate. Experiments have found that this transfer rate involves quantum mechanical tunneling for short distances and thermally activated hopping over large distances. The electron (or hole) tunneling probability through a molecule depends on the length of molecule L, as e(-beta(E)L), where the tunneling betaE factor is strongly energy dependent. We have calculated betaE in DNA for poly(dA)-poly(dT) and poly(dG)-poly(dC) for the first time using a complex band structure approach. Although the DNA band gap is not exceptionally large, we find that the very large beta value near midgap makes DNA a poor tunneling conductor. The tunneling decay in DNA is more rapid than many other organic molecules, including those with a far wider gap.     

Impurity states in narrow-gap and gapless semiconductors

The influence of interband interactions on the location of impurity stages in narrow-gap and gapless semiconductors of A^IVB^VI type is considered in the T-matrix approximation. It is shown that taking account of the interband interaction results in diminution of the activation energy of the impurity level.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 108–111, January, 1977.The authors are grateful to B. L. Gel'mont for discussing the research and the remarks made.     

Nematic liquid crystals with positive dielectric and negative diamagnetic anisotropy

p-Alkyl-p′-cyanocyclohexylcyclohexanes CCH represent a new class of thermotropic nematic liquid crystals which no longer exhibit UV absorption abobe 200 nm, are distinguised by an optical anisotropy of less than 0.1, and in contrast to all previously known “dielectric positive”, thermotropic nematic liquid crystals possess a negative diamagnetic anisotropy.     

Theory of dielectric losses in ionic crystals

The orientation mechanism of associated structural point defects (dipoles, A- and K-dipoles) in an external electric field is considered. The kinetic equation for centrosymmetric crystals is obtained and solved. The buildup of polarization with time allows to investigate the temperature-dependence of the dielectric loss angle, and also the frequencies and defects of the structure. Comparison of the theory with available experimental data on alkali-halide crystal shows complete agreement.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 7–14, August, 1972.