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We report the results of theoretical investigations of the tunneling current noise spectra through a single-level impurity both in the presence and in the absence of electron–phonon interaction based on the nonequilibrium Green’s functions formalism. We show that due to the quantum nature of tunneling, the Fano factor is dramatically different from the Poisson limit both in the presence and in the absence of inelastic processes. The results are demonstrated to be sensitive to the tunneling contact parameters.

In a system of N interacting single-level quantum dots (QDs), we study the relaxation dynamics and the current–voltage characteristics determined by symmetry properties of the QD arrangement. Different numbers of dots, initial charge configurations, and various coupling regimes to reservoirs are considered. We reveal that effective charge trapping occurs for particular regimes of coupling to the reservoir when more than two dots form a ring structure with the C N spatial symmetry. We reveal that the effective charge trapping caused by the C N spatial symmetry of N coupled QDs depends on the number of dots and the way of coupling to the reservoirs. We demonstrate that the charge trapping effect is directly connected with the formation of dark states, which are not coupled to reservoirs due to the system spatial symmetry C N. We also reveal the symmetry blockade of the tunneling current caused by the presence of dark states.

It is shown how the general formulas of the nonequilibrium diagram technique can be used in problems of tunnel planar structures described in the effective mass approach. The relation between such a “continual” approach and the tunneling Hamiltonian method is established, and the applicability conditions for this method are determined. The effects beyond the applicability limits of the tunneling Hamiltonian method, which can be described by the continual approach, are considered.

In the standard Bardeen-Cooper-Schrieffer model, the optical sum rule establishes the relation between the superfluid density and the change in the integral of the real part of the conductivity over the frequency after the transition to the superconducting state. In conventional low-temperature superconductors, these two values become equal if the conductivity is integrated up to a frequency of the order of several widths of the superconducting gap. Optical experiments for high-T c cuprate superconductors performed by many research groups over the world demonstrate that the integration up to a much higher frequency (of the order of the band width) is needed to reproduce the spectral weight of the superfluid component. An interpretation of these experiments has been proposed with allowance for the complex crystal structure of cuprates, in which several orbitals of different symmetries approach the Fermi level.

We show, that in contrast to the free electron model (standard BCS model), a particular gap in the spectrum of multiband superconductors opens at some distance from the Fermi energy, if conduction band is composed of hybridized atomic orbitals of different symmetries. This gap has composite superconducting-hybridization origin, because it exists only if both the superconductivity and the hybridization between different kinds of orbitals are present. So for many classes of superconductors with multiorbital structure such spectrum changes should take place. These particular changes in the spectrum at some distance from the Fermi level result in slow convergence of the spectral weight of the optical conductivity even in quite conventional superconductors with isotropic s-wave pairing mechanism., 6 pages, 4 figures

We found analytical solution for the time evolution of localized electron density in a system of two coupled single-level quantum dots (QDs) connected with continuous spectrum states in the presence of Coulomb interaction. This solution takes into account QD electrons correlation functions of all orders neglecting any correlations between localized and conduction electron filling numbers. We demonstrated that several time scales with the strongly different relaxation rates appear in the system for a wide range of the Coulomb interaction value. We revealed that specific non monotonic behavior of charge relaxation in QD takes place due to Coulomb correlations. We also found out that besides the usual charge oscillations with the period determined by the detuning between the energy levels of the QDs a new effect of period doubling appears in the presence of Coulomb interaction at particular range of the system parameters., 9 pages, 6 figures

An interpolation scheme is proposed for determination of the spectrum of silicon quantum dots 2 to 10 nm in diameter involving the bulk silicon spectrum and the results of our first-principle calculations of silicon clusters 0.7 to 1.5 nm in diameter. In the interpolation we allowed for the discrete character of the electron spectrum of clusters, the law of conservation of the number of electrons in the system, and imperfection of the shape of the quantum dot. Comparison of the results of interpolation with direct first-principle calculations is indicative of sufficiently high accuracy of the scheme, which increases with increasing quantum dot size.

We demonstrate that the tunneling current flowing through a system with Coulomb correlations leads to a charge redistribution between the different localized states. A simple model consisting of two electron levels is analyzed by means of the Heisenberg equations of motion taking correlations of electron filling numbers in localized states into account exactly in all orders. We consider various relations between the Coulomb interaction and localized electron energies. Sudden jumps of the electron density at each level in a certain range of the applied bias are found. We find that for some parameter range, inverse occupation in the two-level system appears due to Coulomb correlations. It is also shown that Coulomb correlations lead to the appearance of negative tunneling conductivity at a certain relation between the values of tunneling rates from the two electron levels.

It is shown how effective Hamiltonians are constructed in the framework of the adiabatic approach to the electron–vibration interaction in electron tunneling through single molecules. Methods for calculating tunneling characteristics are discussed and possible features resulting from the electron–vibration coupling are described. The intensity of vibrations excited by a tunneling current in various systems is examined.

Within the two-band superconductor model, which is a generalization of the standard BCS model to the case of a complicated crystal structure, an expression has been obtained for the conductivity of the superconductor at an arbitrary frequency of the external electromagnetic field. This expression has been derived using the microscopic theory in the framework of the diagram technique for nonequilibrium processes. The σ(ω) dependencies calculated for T = 0 are compared with the results of single-band models with the s and d symmetries of the order parameter. It has been shown that the behavior of the optical conductivity as a function of the frequency depends strongly on the doping level.

Within the adiabatic approximation, it is shown that the effective electron-phonon interaction Hamiltonian for tunneling through a single molecular electronic level contains two different contributions. The interference of the two interaction channels can lead to either the enhancement or suppression of phonon generation. Conditions determining the intensity of excitation of vibrational modes of the molecule are found.

The results of UHV STM investigations of tunneling current noise spectra in the vicinity of individual impurity atoms on the InAs(110) surface are reported. It was found that the power law exponent of 1/f α noise depends on the presence of an impurity atom in the tunneling junction area. This is consistent with the proposed theoretical model considering tunneling current through a two-state impurity complex model system taking into account many-particle interaction.

We report on the direct observation by low-temperature scanning tunneling microscopy and scanning tunneling spectroscopy of the d orbitals of a Mn p-type impurity appearing on a cleaved InAs(110) surface. We show that the crucial interplay between nonequilibrium charging effects and many-particle interaction leading to Coulomb singularities provides a consistent description of the experimental results.

Nonequilibrium interaction effects of two Hubbard-Anderson impurities have been experimentally studied by means of STM/STS methods and theoretically analyzed using a self-consistent approach based on the Keldysh formalism.

Interaction of two localized impurity states of Si atoms at a GaAs surface was studied by scanning tunneling microscopy and spectroscopy. The effects of a twofold “switching” on and off of the states of each of the interacting atoms, the tunneling-interaction-induced mutual level pulling of these states, and the level stabilization near EF were observed. These effects are explained in terms of the extended Anderson model.

We have performed voltage dependent imaging and spatially resolved spectroscopy on the (110) surface of Te doped GaAs single crystals with a low temperature scanning tunneling microscope (STM). A large fraction of the observed defects are identified as Te dopant atoms which can be observed down to the fifth subsurface layer. For negative sample voltages, the dopant atoms are surrounded by Friedel charge density oscillations. Spatially resolved spectroscopy above the dopant atoms and above defect free areas of the GaAs (110) surface reveals the presence of conductance peaks inside the semiconductor band gap. The appearance of the peaks can be linked to charges residing on states which are localized within the tunnel junction area. We show that these localized states can be present on the doped GaAs surface as well as at the STM tip apex., 8 pages, 8 figures, accepted for publication in PRB

The paper considers the effect of a magnetic field B on the transport of neutral composite particles, namely excitons, in weakly disordered two-dimensional (2D) systems. In the case of classical transport (when the interference of different paths is neglected), the magnetic field suppresses exciton transport, and the static diffusion constant D(B) monotonically drops with B. When quantum-mechanical corrections due to weak localization are taken into account, D(B) becomes a nonmonotonic function of B. In weak magnetic fields, where the magnetic length is much larger than the exciton Bohr radius, lB=(ℏc/eB)1/2≫aB=e ℏ2/μe2,a positive magnetodiffusion effect is predicted, i.e., the exciton mobility should increase with B.

The current-voltage $IV$ characteristics of $SIS$ junctions are calculated in the framework of a multiband model with an anisotropic effective order parameter of HTSC. The results of calculations show that the shape of $IV$ characteristic and the density of electron states changes significantly depending on the parameters of the model. A theoretical explanation is proposed for the experimentally observed $s$-like behavior of the $IV$ characteristics of $SIN$ and $SIS$ junctions with BSCCO-type superconductors. The dependence of the superconducting peaks asymmetry on the mutual arrangement of the bands is analyzed. The difference between the obtained results and the results of single-band models with the $s$ and $d$ symmetries of the order parameter is discussed., 11 pages, 4 figures

The influence of many-particle interaction on tunneling characteristics in low dimensional structures is analyzed theoretically and investigated experimentally by means of STM/STS methods. It is shown that indirect interaction (trough band states of semiconductors) can often lead to increased tunneling conductivity for bias range where the direct interaction between impurity states is not significant. New method for preparation of electronic states with definite spin configuration on neighboring atoms is suggested. The energy splitting of opposite spin electrons can be about 0.5 - 1 eV. The different types of tunneling conductivity behavior in ultra small junction with superconductors is analyzed.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A consistent method of calculating the linear response to an external magnetic field that allows obtaining the result in a manifestly gauge-invariant form is proposed. Within the diagram technique for nonequilibrium processes, the self-consistency equations for the order parameter in an arbitrary-gauge field allow deriving an equation that determines the phase of the order parameter as a function of the external field. Such a method automatically accounts for the existence of collective excitations in superconductors, which must be taken into consideration in accordance with the continuity equation. The possible types of collective excitations in pure superconductors at different temperatures are considered. The authors present a microscopic theory that explains the possibility of observing collective modes in superconducting tunnel junctions.