Simple models suffice for the single dot quantum shuttle

A quantum shuttle is an archetypical nanoelectromechanical device, where the mechanical degree of freedom is quantized. Using a full-scale numerical solution of the generalized master equation describing the shuttle, we have recently shown [Novotn\'{y} {\it et al.}, Phys. Rev. Lett. {\bf 92}, 248302 (2004)] that for certain limits of the shuttle parameters one can distinguish three distinct charge transport mechanisms: (i) an incoherent tunneling regime, (ii) a shuttling regime, where the charge transport is synchronous with the mechanical motion, and (iii) a coexistence regime, where the device switches between the tunneling and shuttling regimes. While a study of the cross-over between these three regimes requires the full numerics, we show here that by identifying the appropriate time-scales it is possible to derive vastly simpler equations for each of the three regimes. The simplified equations allow a clear physical interpretation, are easily solved, and are in good agreement with the full numerics in their respective domains of validity.Comment: 23 pages, 14 figures, invited paper for the Focus issue of the New Journal of Physics on Nano-electromechanical system

Shuttle instabilities: semiclassical phase analysis

We present a semiclassical analysis of the instability of an electron shuttle composed of three quantum dots: two are fixed and coupled via leads to electron resevoirs at different chemical potentials, while the central dot is mounted on a classical harmonic oscillator. The semiclassical analysis, which is valid if the central dot oscillation amplitude is larger than the quantum mechanical zero point motion, can be used to gain additional insight about the relationship of resonances and instabilities of the device.Comment: 4 pages, 3 figures, presented at EP2DS-15, Nara, July 200

Quantum pumping in deformable quantum dots

The charge current pumped adiabatically through a deformable quantum dot is studied within the Green's function approach. Differently from the non-deformable case, the current shows an undefined parity with respect to the pumping phase \phi. The unconventional current-phase relation, analyzed in the weak pumping regime, is due to a dynamical phase shift \phi_D caused by the elastic deformations of the central region (classical phonons). The role of the quality factor Q of the oscillator, the effects induced by a mechanical resonance and the implications for current experiments on molecular systems are also discussed

Current and current fluctuations in quantum shuttles

We review the properties of electron shuttles, i.e. nanoelectromechanical devices that transport electrons one-by-one by utilizing a combination of electronic and mechanical degrees of freedom. We focus on the extreme quantum limit, where the mechanical motion is quantized. We introduce the main theoretical tools needed for the analysis, e.g. generalized master equations and Wigner functions, and we outline the methods how the resulting large numerical problems can be handled. Illustrative results are given for current, noise, and full counting statistics for a number of model systems. Throughout the review we focus on the physics behind the various approximations, and some simple examples are given to illustrate the theoretical concepts. We also comment on the experimental situation.Comment: Minireview; technical level aimed at general audience, based on an invited talk at "Transport Phenomena in Micro and Nanodevices", October 17-21 Kona, Hawai

Quantum theory of shuttling instability in a movable quantum dot array

We study the shuttling instability in an array of three quantum dots the central one of which is movable. We extend the results by Armour and MacKinnon on this problem to a broader parameter regime. The results obtained by an efficient numerical method are interpreted directly using the Wigner distributions. We emphasize that the instability should be viewed as a crossover phenomenon rather than a clear-cut transition.Comment: 4 pages, 2 figures, presented at HCIS-13, Modena, July 200

Thermally induced subgap features in the cotunneling spectroscopy of a carbon nanotube

We report on nonlinear cotunneling spectroscopy of a carbon nanotube quantum dot coupled to Nb superconducting contacts. Our measurements show rich subgap features in the stability diagram which become more pronounced as the temperature is increased. Applying a transport theory based on the Liouville-von Neumann equation for the density matrix, we show that the transport properties can be attributed to processes involving sequential as well as elastic and inelastic cotunneling of quasiparticles thermally excited across the gap. In particular, we predict thermal replicas of the elastic and inelastic cotunneling peaks, in agreement with our experimental results.Comment: 21 pages, 9 figures, submitted to New Journal of Physic

Sub-gap spectroscopy of thermally excited quasiparticles in a Nb contacted carbon nanotube quantum dot

We present electronic transport measurements of a single wall carbon nanotube quantum dot coupled to Nb superconducting contacts. For temperatures comparable to the superconducting gap peculiar transport features are observed inside the Coulomb blockade and superconducting energy gap regions. The observed temperature dependence can be explained in terms of sequential tunneling processes involving thermally excited quasiparticles. In particular, these new channels give rise to two unusual conductance peaks at zero bias in the vicinity of the charge degeneracy point and allow to determine the degeneracy of the ground states involved in transport. The measurements are in good agreement with model calculations.Comment: 5 pages, 4 figure

Virtual Photon Contribution to Frictional Drag in double-layer Devices

The first order contribution to frictional drag in bi-layered fermion gas is examined. We discuss the relevance of single photon exchange in the evaluation of transresistance, which is usually explained by second order effects such as Coulomb and phonon drag. Since the effective e.m. interaction is unscreened, in the d.c. limit we obtain a finite (and large) contribution to transconductivity.Comment: 13 pages, 1 figur

Full counting statistics of nano-electromechanical systems

We develop a theory for the full counting statistics (FCS) for a class of nanoelectromechanical systems (NEMS), describable by a Markovian generalized master equation. The theory is applied to two specific examples of current interest: vibrating C60 molecules and quantum shuttles. We report a numerical evaluation of the first three cumulants for the C60-setup; for the quantum shuttle we use the third cumulant to substantiate that the giant enhancement in noise observed at the shuttling transition is due to a slow switching between two competing conduction channels. Especially the last example illustrates the power of the FCS.Comment: 7 pages, 3 figures; minor changes - final version as published in Europhys. Let

Current without bias and diode effect in shuttling transport of nanoshafts

A row of parallely ordered and coupled molecular nanoshafts is shown to develop a shuttling transport of charges at finite temperature. The appearance of a cu rrent without applying an external bias voltage is reported as well as a natura l diode effect allowing unidirectional charge transport along one field directi on while blocking the opposite direction. The zero-bias voltage current appears above a threshold of initial thermal and/or dislocation energy