Leonid Y. Gorelik

Quantum shuttle phenomena in a nanoelectromechanical single-electron transistor

An analytical analysis of quantum shuttle phenomena in a nanoelectromechanical single-electron transistor has been performed in the realistic case, when the electron tunneling length is much greater than the amplitude of the zero point oscillations of the central island. It is shown that when the dissipation is below a certain threshold value, the vibrational ground state of the central island is unstable. The steady state into which this instability develops is studied. It is found that if the electric field E between the leads is much greater than a characteristic value E(q), the quasiclassical shuttle picture is recovered, while if E<<E(q) a new quantum regime of shuttle vibrations occurs. We show that in the latter regime small quantum fluctuations result in large (i.e., finite in the limit variant Plancks over 2pi -->0) shuttle vibrations.

Vibrational instability due to coherent tunneling of electrons

Effects of a coupling between the mechanical vibrations of a quantum dot placed between the two leads of a single-electron transistor and coherent tunneling of electrons through a single level in the dot has been studied. We have found that for bias voltages exceeding a certain critical value a dynamical instability occurs and mechanical vibrations of the dot develop into a stable limit cycle. The current-voltage characteristics for such a transistor were calculated and they seem to be in reasonably good agreement with recent experimental results for the single-C60-molecule transistor by Park et al. (Nature 407 (2000) 57).

Cooling of a suspended nanowire by an ac Josephson current flow.

We consider a nanoelectromechanical Josephson junction, where a suspended nanowire serves as a superconducting weak link, and show that an applied dc bias voltage can result in suppression of the flexural vibrations of the wire. This cooling effect is achieved through the transfer of vibronic energy quanta first to voltage-driven Andreev states and then to extended quasiparticle electronic states. Our analysis, which is performed for a nanowire in the form of a metallic carbon nanotube and in the framework of the density matrix formalism, shows that such self-cooling is possible down to the ground state of the flexural vibration mode of the nanowire.

Electromechanical instability in suspended carbon nanotubes

We have theoretically investigated electromechanical properties of freely suspended carbon nanotubes when a current is injected into the tubes using a scanning tunneling microscope. We show that a shuttle-like electromechanical instability can occur if the bias voltage exceeds a dissipation-dependent threshold value. An instability results in large amplitude vibrations of the carbon nanotube bending mode, which modify the current-voltage characteristics of the system.

Coulomb promotion of spin-dependent tunneling.

We study transport of spin-polarized electrons through a magnetic single-electron transistor (SET) in the presence of an external magnetic field. Assuming the SET to have a nanometer size central island with a single-electron level we find that the interplay on the island between coherent spin-flip dynamics and Coulomb interactions can make the Coulomb correlations promote rather than suppress the current through the device. We find the criteria for this new phenomenon--Coulomb promotion of spin-dependent tunneling--to occur.

Spintronics of a Nanoelectromechanical Shuttle

We consider effects of the spin degree of freedom on the nanomechanics of a single-electron transistor (SET) containing a nanometer-sized metallic cluster suspended between two magnetic leads. It is shown that in such a nanoelectromechanical SET (NEM-SET) the onset of an electromechanical instability leading to cluster vibrations and shuttle transport of electrons between the leads can be controlled by an external magnetic field. Different stable regimes of this spintronic NEM-SET operation are analyzed. Two different scenarios for the onset of shuttle vibrations are found.

Electronic Aharonov-Bohm effect induced by quantum vibrations.

Mechanical displacements of a nanoelectromechanical system shift the electron trajectories and hence perturb phase coherent charge transport through the device. We show theoretically that in the presence of a magnetic field such quantum-coherent displacements may give rise to an Aharonov-Bohm-type of effect. In particular, we demonstrate that quantum vibrations of a suspended carbon nanotube result in a positive nanotube magnetoresistance, which decreases slowly with the increase of temperature. This effect may enable one to detect quantum displacement fluctuations of a nanomechanical device.

Electrical Manipulation of Nanomagnets

We demonstrate that it is possible to manipulate the magnetic coupling between two nanomagnets by means of an ac electric field. In the scheme suggested, the magnetic coupling is mediated by a magnetic particle that is in contact with both nanomagnets via tunnel barriers. The time-dependent electric field is applied so that the height of first one barrier then the other is suppressed in an alternating fashion. We show that the result is a pumping of magnetization from one nanomagnet to the other through the mediating particle. The dynamics of the magnetization of the mediating particle allows the coupling to be switched between being ferromagnetic and being antiferromagnetic.

Cooling of nanomechanical resonators by thermally activated single-electron transport.

We show that the vibrations of a nanomechanical resonator can be cooled to near its quantum ground state by tunneling injection of electrons from a scanning tunneling microscope tip. The interplay between two mechanisms for coupling the electronic and mechanical degrees of freedom results in a bias-voltage-dependent difference between the probability amplitudes for vibron emission and absorption during tunneling. For a bias voltage just below the Coulomb blockade threshold, we find that absorption dominates, which leads to cooling corresponding to an average vibron population of the fundamental bending mode of 0.2.

Chiral symmetry breaking and the Josephson current in a ballistic superconductor-quantum wire-superconductor junction

We evaluate the Josephson current through a quasi-1D quantum wire coupled to bulk superconductors. It is shown that the interplay of Rashba spin-orbit interaction and Zeeman splitting results in the appearance of a Josephson current even in the absence of any phase difference between the superconductors. In a transparent junction (Dsimilar or equal to1) at low temperatures this anomalous supercurrent J(an) appears abruptly for a Zeeman splitting of the order of the Andreev level spacing as the magnetic field is varied. In a low-transparency (Dmuch less than1) junction one has J(an)proportional torootD under special (resonance) conditions. In the absence of Zeeman splitting the anomalous supercurrent disappears. We have investigated the influence of dispersion asymmetry induced by the Rashba interaction in quasi-1D quantum wires on the critical Josephson current and have shown that the breakdown of chiral symmetry enhances the supercurrent.