By a polaronic energy shift, the effective charging energy of molecules can become negative, favoring ground states with even numbers of electrons. Here we show that charge transport through such molecules near ground-state degeneracies is dominated by tunneling of electron pairs which coexists with (featureless) single-electron cotunneling. Because of the restricted phase space for pair tunneling, the current-voltage characteristics exhibit striking differences from the conventional Coulomb blockade. In asymmetric junctions, pair tunneling can be used for gate-controlled current rectification and switching.

We study theoretically the exciton absorption on a ring threaded by a magnetic flux. For the case when the attraction between electron and hole is short-ranged we get an exact solution of the problem. We demonstrate that, despite the electrical neutrality of the exciton, both the spectral position of the exciton peak in the absorption, and the corresponding oscillator strength oscillate with magnetic flux with a period Φ 0 -the universal flux quantum.The origin of the effect is the finite probability for electron and hole, created by a photon at the same point, to tunnel in the opposite directions and meet each other on the opposite side of the ring. 71.35.Cc, 03.65.Bz Typeset using REVT E X 1

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Areal density of disorder-induced resonators with a high quality factor, Q ≫ 1, in a film with fluctuating refraction index is calculated theoretically. We demonstrate that for a given kl > 1, where k is the light wave vector, and l is the transport mean free path, when on average the light propagation is diffusive, the likelihood for finding a random resonator increases dramatically with increasing the correlation radius of the disorder. Parameters of most probable resonators as functions of Q and kl are found.

Magnetoconductivity of the two-dimensional electron gas occupying two size-quantization subbands is studied theoretically. When the bottoms of subbands are separated by an integer number of Landau levels, the staircases of Landau levels in both subbands are completely aligned. For such values of magnetic field the intersubband scattering is enhanced. As it was pointed out by Polyanovsky, this results in additional Shubnikovde Haas oscillations of conductivity with magnetic field, with period depending on subband separation, and amplitude depending weakly on temperature, provided that a large number of Landau levels in each subband are occupied. In the calculation of conductivity we make use of the self-consistent Born approximation generalized to the case of two subbands. An analytical theory for the case of strong disorder and numerical results for the case of weak disorder are presented.

We study theoretically the renormalization of the spin-orbit coupling constant of two-dimensional electrons by electron-electron interactions. We demonstrate that, similarly to the g factor, the renormalization corresponds to the enhancement, although the magnitude of the enhancement is weaker than that for the g factor. For high electron concentrations (small interaction parameter r s ) the enhancement factor is evaluated analytically within the static random phase approximation. For large r s ∼ 10 we use an approximate expression for effective electron-electron interaction, which takes into account the local field factor, and calculate the enhancement numerically. We also study the interplay between the interaction-enhanced Zeeman splitting and interaction-enhanced spin-orbit coupling.

A classical model for dc transport of two dimensional electrons in a perpendicular magnetic field and under strong irradiation is considered. We demonstrate that, near the cyclotron resonance condition, and for linear polarization of the ac field, a strong change of the diagonal component, σ d , of the dc conductivity occurs in the presence of a weak nonparabolicity of the electron spectrum. Small change in the electron effective mass due to irradiation can lead to negative σ d , while the Hall component of the dc conductivity remains practically unchanged. Within the model considered, the sign of σ d depends on the relative orientation of the dc and ac fields, the sign of the detuning of the ac frequency from the cyclotron resonance, and the sign of nonparabolic term in the energy spectrum.1. Introduction. Recently reported observation [1,2] of a zero-resistance state, that emerges upon microwave irradiation of a high-mobility 2D electron gas in a weak magnetic field, was immediately followed by a number of theoretical papers [3][4][5][6], in which the origin of this state was discussed. The only microscopic calculation to date [4] indicates that, for strong enough radiation intensity, the diagonal component, σ d , of the dc conductivity tensor changes sign from the dark value σ d > 0 to σ d < 0 within certain frequency intervals of the ac field, away from the cyclotron frequency and its harmonics. Negative local value of σ d results in the instability of the homogeneous current distribution. In Ref.[5] the scenario of how the instability might develop into the zero-resistance state was proposed.In this situation it seems important to trace the emergence of negative σ d in an ac-driven system from the simplest possible model. Such a model is considered in the present paper. Obviously, σ d is sensitive to the illumination only if the Kohn theorem is violated. It is commonly assumed that the reason for this violation is a random impurity potential. Here we consider a model, in which the Kohn theorem is violated due to an intrinsic reason, namely, due to a weak nonparabolicity of the electron spectrum. More specifically, we adopt the following form of the dispersion relation for the conduction band electrons