The fluctuation enhanced conductivity in amorphous superconductors

Considering the effective phonon density of states obtained by tunnel spectroscopic measurements, we conclude that in amorphous metals such as Bi, Ga, and Pb, the electron-phonon collision rate becomes as large as 10¹²/sec at the transition temperature. This is attributed to the collision drag effect which couples low frequency transverse phonons to the electrons. It is shown that such a large collision rate suppresses the Maki contribution to the fluctuation enhanced conductivity.     

Thermal conductivity in superionic conductors

The contribution of mobile ions to the thermal conductivity in superionic conductors is investigated by making use of the lattice gas model with a hopping term. The thermal conductivity is obtained as a function of an ion concentration and a repulsive interaction energy between the nearest neighbors. It is shown that the contribution of mobile ions to the thermal conductivity is of an Arrhenius type in the usual temperature region in which measurements are conducted.     

Thermal conductivity of large-grain niobium and its effect on trapped vortices in the temperature range 1.8–5 K

Experimental investigation of the thermal conductivity of large grain and its dependence on the trapped vortices in parallel magnetic field with respect to the temperature gradient \(\nabla T\) was carried out on four large-grain niobium samples from four different ingots. The zero-field thermal conductivity measurements are in good agreement with the measurements based on the theory of Bardeen–Rickayzen–Tewordt (BRT). The change in thermal conductivity with trapped vortices is analysed with the field dependence of the conductivity results of Vinen et al for low inductions and low-temperature situation. Finally, the dependence of thermal conductivity on the applied magnetic field in the vicinity of the upper critical field H _c2 is fitted with the theory of pure type-II superconductor of Houghton and Maki. Initial remnant magnetization in the sample shows a departure from the Houghton–Maki curve whereas the sample with zero trapped flux qualitatively agrees with the theory. A qualitative discussion is presented explaining the reason for such deviation from the theory. It has also been observed that if the sample with the trapped vortices is cycled through T _c, the subsequent measurement of the thermal conductivity coincides with the zero trapped flux results.     

Field-theoretic approach to the properties of the solid state

The techniques of quantum field theory are used to investigate the thermodynamic ion displacement correlation function—or Green's function of the phonon field—in a crystal and especially in a metal. The structure of thermodynamic Green's functions is outlined and the method for solving for them at finite temperature is fully discussed.The analytic structure of the phonon Green's function is then considered. This function is shown to be bounded and invertible everywhere off the real axis; a spectral form is derived for its inverse. The symmetries imposed by the point group of the crystal are then discussed.Assuming small ionic oscillations, we find the inverse of the phonon Green's function as a linear function of the electronic contribution to the dielectric response function of the metal. This dielectric function is shown to be simply related to the longitudinal part of the conductivity tensor that gives the response of the electrons to the effective electric field in the metal. The assumption of translational invariance then leads to an explicit expression for the phonon Green's function in terms of this conductivity.The deformations in the lattice induced by an arbitrarily time varying external force are calculated in terms of the retarded phonon Green's function. In the static long wavelength limit the phonon Green's function yields the macroscopic elastic constants of the crystal. Their relation to the conductivity is exhibited, and several elastic constants are estimated. We also see that the complete phonon spectrum and the lifetimes of the phonon states may be calculated from this Green's function. A relation between the long wavelength acoustic attenuation in metals and the de conductivity is derived, which is in good agreement with recent experiments. Furthermore, the ions in a metal are shown to have a high-frequency oscillation along with the electrons, at essentially the electron plasma frequency.     

Comments on the equilibrium theory of the quantum hall effect

The relation between the thermal equilibrium Hall conductivity generated by minimal gauge transformation and the isolated Hall conductivity given by the Kubo formula is investigated. The contribution of the edge states and some general questions concerning the definition of the equilibrium Hall conductivity are discussed. It is shown that, in the case of an additive electron-impurity system, the two Hall conductivities coincide as long as the Fermi energy is situated in an energy gap.     

Cluster-components model interpretation of the concentration dependence of electrical conductivity in doped ionic crystals containing Frenkel defects

The cluster-components method is used to study defect behavior and to describe the “property — composition” dependence in binary ionic compounds. A qualitative estimate of the concentration dependence of defects is given, together with the “electrical conductivity — composition” dependence based on the defects. It is shown that when linear approximation for the concentration dependences on a single type of defect are coupled with the experimental data, one can describe the concentration dependence of the electrical conductivity. The calculated results are in good agreement with experiment. The dependence on vacancy and interstitial concentration can be used to calculate other properties.

     

Anisotropy of the electrical conductivity of lithium heptagermanate crystals

The electrical conductivity σ of single crystals of lithium heptagermanate Li₂Ge₇O₁₅ is studied in an electric field in the frequency range 0.5–100 kHz at temperatures ranging from 300 to 700 K. Heating the crystal above 500 K gives rise to a pronounced anisotropy in the electrical conductivity, which differs in magnitude by one to two orders of magnitude for different directions of the measurement field along the crystallographic axes. It is shown that an increase in the electrical conductivity σ with increasing temperature originates from charge transfer with an activation energy U = 1.04 eV. It is assumed that the thermally activated contribution to the electrical conductivity is governed by transport of lithium interstitial ions along channels in the structure of the Li₂Ge₇O₁₅ compound.     

Stress dependence of the chemical potential of adsorbates

It is shown that for a gas—alkali halide adsorption system, pressure applied to the substrate changes the contribution to the dispersion force that arises from the interaction of the gas with the substrate in the infrared region. This results in a corresponding stress-dependent change in the chemical potential of the gas.     

Effect of normal electrons in superconductors on the current-voltage characteristic of a Josephson junction

It is established that the conductivity of normal electrons in superconductors plays a large role in the current-voltage characteristic of a Josephson junction having a high critical current density, described on the basis of a nonlocal electrodynamics. It is shown in the resistive limit that the normal-electron contribution increases the current through the Josephson junction, while in the capacitive limit it decreases the contribution of Cherenkov resonances. Fiz. Tverd. Tela (St. Petersburg) 41, 582–587 (April 1999)     

Impurity pairs and excitation relaxation in doped silicon

The kinetics of photoconductivity is studied in silicon doped with B, Al, Ga, In, P, As, and Sb with concentrations of 10₁₆–10₁₈cm_?3 at 4.2 and 10.5 K placed in an 8-mm microwave electric field under pulsed impurity excitation. It is found that infrared absorption by impurity pairs and a slow component of photoresponse relaxation arise at close impurity concentrations. It is shown that this component is due to an increase in the polarization hopping conductivity in the presence of the optical charge exchange of impurity states—isolated impurities and impurity pairs and dipoles (pairs of the major and compensating impurities). The hopping transfer processes of ion charges in the course of relaxation are analyzed. It is shown that the main contribution to polarization photoconductivity comes from hopping transitions in impurity pairs at relatively small concentrations and from hopping with the participation of isolated ions at increased concentrations.     

The effect of the long-range order in a quantum dot array on the in-plane lattice thermal conductivity

Semiconductor quantum dot superlattices consisting of arrays of quantum dots have shown great promise for a variety of device applications, including thermoelectric power generation and cooling. In this paper we theoretically investigate the effect of long-range order in a quantum dot array on its in-plane lattice thermal conductivity. It is demonstrated that the long-range order in a quantum dot array enhances acoustic phonon scattering and, thus leads to a decrease of its lattice thermal conductivity. The decrease in the ordered quantum dot array, which acts as a phonon grating, is stronger than that in the disordered one due to the contribution of the coherent scattering term. The numerical calculations were carried out for a structure that consists of multiple layers of Si with layers of ordered Ge quantum dots separated by wetting layers and spacers.     

Increasing the sensitivity of resistive and capacitive external-action sensors

The contribution from the relaxation process to the permittivity of a dielectric at high frequencies is shown to depend on the square of the total contribution from this process to the conductivity. The contribution from the relaxation process to the conductivity of a dielectric at low frequencies depends on the square of the total contribution from the relaxation process to the real part of the dielectric permittivity. On this basis, we suggest a method for increasing the sensitivity of resistive and capacitive external-action sensors.     

Commensurability pinning of fluctuation conductivity in quasi-one-dimensional charge density wave systems

In incommensurate charge-density wave systems fluctuations give a contribution to the electrical conductivity above the phase transition temperature. We show that this contribution is drastically reduced if the system is commensurate. On the other hand, the effects of commensurability on the single-particle conductivity are shown to be quite small. Thus, commensurability pinning of fluctuation conductivity is a reasonable explanation of recent experimental results in organic conductors.     

Topological bulk-edge effects in quantum graph transport

We examine quantum transport in periodic quantum graphs with a vertex coupling non-invariant with respect to time reversal. It is shown that the graph topology may play a decisive role in the conductivity properties illustrating this claim with two examples. In the first, the transport is possible at high energies in the bulk only being suppressed at the sample edges, while in the second one the situation is opposite, the transport is possible at the edge only.     

Pair breaking parameter of two-dimensional dirty superconductors

Intrinsic mechanism of pair breaking is given for dirty superconducting films. Dynamical interaction between electrons such as Coulomb interaction plays an essential role. The diffusion processes of both density- and pair-fluctuations are also important. The problem of the Thompson's parameter for the Maki term of fluctuation conductivity is solved and the correct dependence of the parameter of the sheet resistance R_sq is given.     

The origin of the isotropic EPR spectrum of a Jahn-Teller impurity in crystals

The motional narrowing processes and their contribution to the origine of the isotropic EPR spectrum of a cubic ²E — term in the cases of strong linear and both weak and strong second-order vibronic interaction, are investigated. It is shown that these relaxation processes are nonefficient at low temperature, and the isotropic spectrum is associated with the nearest exited vibronic singlet level.     

Normal phonon-phonon scattering processes and the thermal conductivity of germanium crystals with isotope disorder

The influence of the normal phonon-phonon scattering processes on the thermal conductivity was theoretically studied for germanium crystals with various degrees of the isotope disorder. The theory takes into account redistribution of the phonon momentum in the normal scattering processes both inside each oscillation branch (Simons mechanism) and between various phonon oscillation branches (Herring mechanism). Contributions to the thermal conductivity due to the drift mobility of the longitudinal and transverse phonons are analyzed. It is shown that the momentum redistribution between longitudinal and transverse phonons according to the Herring relaxation mechanism leads to a significant suppression of the drift motions (and to the corresponding drop in contribution to the thermal conductivity) of the longitudinal phonons in isotopically pure germanium crystals. The results of the thermal conductivity calculations involving the Herring relaxation mechanism agree well with the experimental data available for germanium crystals with various degrees of the isotope disorder.     

Molecular dynamics study of phonon-mediated thermal transport in a Ni50Al50 melt: case analysis of the influence of the process on the kinetics of solidification

The phonon-mediated contribution to the thermal transport properties of liquid NiAl alloy is investigated in detail over a wide temperature range. The calculations are performed in the framework of equilibrium molecular dynamics making use of the Green–Kubo formalism and one of the most reliable embedded-atom method potentials for the intermetallic alloy. The phonon-mediated contribution to the thermal conductivity of the liquid alloy is calculated at equilibrium as well as for the steady state. The relative magnitude of the thermal conductivity decrease induced by the transition to the steady state is estimated to be less than 2% below 2000 K and less than 1% at 3000 and 4000 K. It is also found that the phonon-mediated contribution to the thermal conductivity of the liquid alloy can be accurately estimated (well within 1%) on the basis of an approximation which invokes the straightforwardly accessible microscopic expression for the total heat flux without demanding calculations of the partial enthalpies needed for the precise evolution of the reduced heat flux (pure heat conduction). On the basis of these calculations, the correspondence between the experimentally observed and modelled kinetics of solidification due to a difference in thermal conductivity is discussed.     

Computer simulation of the thermodynamics of the B → Z-DNA transition: effect of the ionic size and charge

The B- to Z-DNA transition free energy as a function of the size and charge of added ions is investigated by Monte Carlo simulation for simple grooved B- and Z-DNA models. It is shown that the electrostatic contribution to the free energy depends almost linearly on the logarithm of the salt concentration for all the systems. The effect of increasing the size of the ions is to make the curves steeper, although its influence on the transition midpoint is more complex. Divalent cations markedly reduce both the slope and the transition midpoint with respect to monovalent ions. These conclusions are in agreement with the experimental findings. The effect of increasing the charge of the anions—not yet experimentally studied—is less pronounced.