Yu. M. Galperin

The soft-potential model (an extension of the well-known tunneling model for two-level states in glasses) has been formulated in terms of soft-mode eigenvectors in order to characterize the localization of these modes. The interaction with high-frequency modes explains the absence of very small restoring-force constants. A quantitative comparison to specific-heat and neutron-scattering data from vitreous silica, amorphous selenium, and vitreous boron trioxide shows that both the two-level systems and the low-energy vibrational states can be explained by the same distribution of localized modes.

Abstract The properties of amorphous materials determined by the presence of localized atomic and electronic states are reviewed. Experimental data on atomic dynamics in glasses are described. The two-level-system (TLS) model is presented, and the limits of its applicability analysed. The concept of soft atomic potentials, allowing generalization of the TLS model to higher energies, is introduced. Atomic dynamics in soft potentials, as well as soft-potential coupling with phonons and thermal expansion of glasses, are described. Localized electronic states in glasses are investigated and different theoretical models of such states in glassy semiconductors are reviewed. The connection between localized electronic states and soft potentials is studied. Particular attention is paid to low-relaxation processes in glasses controlled by two-well soft potentials; specifically, low-frequency noise in disordered conductors is considered. The possibility of investigating the localized states in question with the help of point-contact spectroscopy is discussed.

Abstract The purpose of the present review article is to give an outline of a quasiclassical approach to the description of transport in superconductors. The Boltzmann equation for normal excitation in a superconductor is formulated and its various properties are discussed and a full set of equations to describe condensate dynamics in the presence of external fields is given. Applications of this approach are demonstrated with a number of examples.

An analytical expression for shot noise in a correlated sequential tunneling current has been derived by solving the master equation exactly, with the Pauli correlation as a special case. Our theory is applied to the Coulomb-blockade single-electron tunneling system with two tunnel junctions. Given capacitances and resistances of the system, the temperature dependence and the bias-voltage dependence of the noise spectrum in the entire range of frequency have been analyzed in detail. We have demonstrated that the noise spectrum is more sensitive to the bias voltage than is the current itself to the voltage. Therefore, noise spectroscopy is a much more accurate tool to investigate the correlation in tunneling transport through ever smaller multiple-junction structures.