L. Y. Gorelik

SUPERCONDUCTING SINGLE-MODE CONTACT AS A MICROWAVE-ACTIVATED QUANTUM INTERFEROMETER

The dynamics of a superconducting quantum point contact biased at subgap voltages is shown to be strongly affected by a microwave electromagnetic field. Interference among a sequence of temporally localized, microwave-induced Landau-Zener transitions between current carrying Andreev levels results in energy absorption and in an increase of the subgap current by several orders of magnitude. The contact is an interferometer in the sense that the current is an oscillatory function of the inverse bias voltage. Possible applications to Andreev-level spectroscopy and microwave detection are discussed.

Microwave-induced "Somersault effect" in flow of Josephson current through a quantum constriction.

We consider the supercurrent flow through gated mesoscopic semiconductor heterostructures in which a two-dimensional normal constriction is confined between superconducting electrodes. Photon-assisted Landau-Zener transitions between Andreev bound states in the constriction are shown to give rise to (i) a series of reversals in the direction of supercurrent flow (somersault effect) if the phase difference between the superconducting elements is fixed or (ii) a series of voltage spikes if the junction is current biased. We discuss necessary conditions for the described phenomena to be experimentally observable.

Spatial structure in mode population induced by coherent pumping in a ballistic quantum channel

We predict a spatially varying mode population to appear in a ballistic quantum channel formed in the two-dimensional electron gas of a gated GaAs/AlGaAs heterostructure due to pumping by a THz field. If a resonant coupling between two modes is suddenly switched on at the entrance, Rabi oscillations in the mode population will arise. We propose to use an array of gates in order to simulate a moving quantum point contact for detecting mode population oscillations since they discriminate between different modes. By consecutively activating them we expect to see both photovoltaic effects and photoconductive effects that can easily be distinguished from noise.

MICRO-MECHANICAL CHARGE TRANSFER MECHANISM IN SOFT COULOMB BLOCKADE NANOSTRUCTURES

Abstract Room-temperature Coulomb blockade of charge transport through nanostructures containing organic inter-links has recently been observed. A pronounced charging effect in combination with the softness of the molecular links implies that charge transfer gives rise to a significant deformation of these structures. For a simple model system containing one nanoscale metallic cluster connected by molecular links to two bulk metallic electrodes, we estimate the characteristic frequency of tunneling charge transport to be of the same order or lower than the frequency of elastic cluster vibrations determined by the elasticity of the molecular links. We show that in this case periodic oscillations of the cluster in conjunction with sequential processes of cluster charging and decharging may appear for a sufficiently large bias voltage. This new “electron shuttle” mechanism of discrete charge transfer gives rise to a current through the nanostructure, which is proportional to the cluster vibration frequency.

FRACTIONAL PUMPING OF ENERGY INTO A BALLISTIC QUANTUM RING

We consider the energy stored in a one-dimensional ballistic ring with a barrier subject to a linearly time-dependent magnetic flux. An exact analytic solution for the quantum dynamics of electrons in the ring is found for the case when the electromotive force E is much smaller than the level spacing, D. Electron states exponentially localized in energy are found for irrational values of the ratio A ; Dy2eE . Because of relaxation localization does not develop if A is close to a rational number. As a result the accumulated energy as a function of A contains sharp peaks at rational values (fractional pumping). [S0031-9007(97)02728-2] PACS numbers: 73.23.Ad Physical properties of mesoscopic systems are governed by quantum interference. Several phenomena of such a nature have been discussed for systems close to equilibrium. Persistent currents in multiply connected systems [1] as well as universal fluctuations of the conductance are important examples [2]. Coherent dynamics remains crucially important in situations far from equilibrium provided the energy associated with the phase breaking rate is less than the characteristic rate of redistributing electrons in energy space. Consequently, one can expect pronounced mesoscopic behavior even in strongly biased mesoscopic devices, where the dynamics can be effectively tuned by external electric or magnetic fields. In this paper, we consider an example of such a system, namely, a single-channel mesoscopic ring subjected to a nonstationary perpendicular magnetic field, linearly dependent on time. We concentrate on the energy accumulation in such a system. To investigate the role of interference, we take into account electron backscattering from a single potential barrier, embedded in the ring. Tuning the transmission through the barrier by gate potentials one can influence the interference pattern and in this

Resonant tunneling through Andreev levels

Abstract We use a semiclassical approach for analyzing the tunneling transport through a normal conductor in contact with superconducting mirrors. Our analysis of the electron–hole propagation along semiclassical trajectories shows that resonant transmission through Andreev levels is possible resulting in an excess, low-energy quasiparticle contribution to the conductance. The excess conductance oscillates with the phase difference between the superconductors having maxima at odd multiples of π for temperatures much below the Thouless temperature.

Pumping of energy into a mesoscopic ring: Exactly solvable model

We consider the energy stored in a one-dimensional ballistic ring with a barrier subjected to a linearly time-dependent magnetic flux. An exact analytical solution for the quantum dynamics of electrons in the ring is found for the case when the electromotive force multiplied by the electron charge, ee, is much smaller than the interlevel spacing, Δ. Electron states exponentially localized in energy space are found for irrational values of the dimensionless ratio A≡Δ/2ee. Relaxation limits the dynamic evolution and the localization does not develop if A is sufficiently close to a rational number. As a result the accumulated energy becomes a regular function of A containing a set of sharp peaks at rational values with small enough denominators (fractional pumping). The shape of the peaks and the distances between them are governed by the interplay between the strength of backscattering and the relaxation rate.

Far infrared induced multi-mode pumping reveals deviations from parabolicity in a ballistic quantum channel

We consider, theoretically, resonant multi-mode pumping induced by a coherent far infrared field, in a ballistic quantum channel, created in a gated GaAs/AlGaAs heterostructure. For frequencies that enable momentum conservation in a transition between the lowest and a higher mode, we predict complete depopulation of the lowest mode, due to Rabi oscillations. These frequencies are characteristic of the mode spectrum and are therefore closely related to the confining potential. If all but the lowest modes are filtered out by a quantum point contact, we expect the depopulation effect to completely block the transport of current. In addition to providing a spectroscopic tool we believe that the resonance effect may find use as a THz frequency demodulator or as a transistor device.

Coulomb correlations and coherent charge tunneling in mesoscopic coupled rings

We study the effect of a strong electron-electron (e-e) interaction in a system of two concentric one-dimensional rings with incommensurate areas A1 and A2, coupled by a tunnel amplitude. For noninteracting particles the magnetic moment (persistent current) m of the many-body ground state and first excited states is an irregular function of the external magnetic field. In contrast, we show that when strong e-e interactions are present the magnetic-field dependence of m becomes periodic. In such a strongly correlated system disorder can only be caused by inter-ring charge fluctuations, controllable by a gate voltage. The oscillation period of m is 1/(A1 + A2) if fluctuations are suppressed. Coherent inter-ring tunneling doubles the period when charge fluctuations are allowed.

Ordering effect of Coulomb interaction in ballistic double-ring systems

Abstract We study a model of two concentric onedimensional rings with incommensurate areas A1 and A2, in a constant magnetic field. The two rings are coupled by an nonhomogeneous inter-ring tunneling amplitude, which makes the one-particle spectrum chaotic. For noninteracting particles the energy of the many-body ground state and the first excited state exhibit random fluctuations characterized by the Winger-Dyson statistics. In contrast, we show that the electron-electron interaction orders the magnetic field dependence of these quantities, forcing them to become periodic functions, with period ∝ 1 (A 1 + A 2 ) . In such a strongly correlated system the only possible source of disorder comes from charge fluctuations, which can be controlled by a tunable inter-ring gate voltage.