We investigate theoretically the dynamics of a spatially symmetric shuttle system subjected to an ac gate voltage. We demonstrate that in such a system parametric excitation gives rise to mechanical vibrations when the frequency of the ac signal is close to the eigenfrequency of the mechanical subsystem. These mechanical oscillations result in a dc shuttle current in a certain direction due to spontaneous symmetry breaking. The direction of the current is determined by the phase shift between the ac gate voltage and the parametrically excited mechanical oscillations. The dependence of the shuttle current on the dc gate voltage is also analyzed. Some years ago, a novel form of electron transport-a shuttle mechanism-based on the mechanical vibrations of a metallic nanoparticle coupled to two electrodes via elastic molecular links was proposed in Ref. 1. Since then, the shuttle phenomenon has been a subject of intensive experimental and theoretical research. [2][3][4][5][6][7] The main feature of the orthodox shuttle phenomenon is that a constant potential difference, applied between two fixed electrodes, leads to a dynamical instability that causes the metal nanoparticle to oscillate. As a consequence, a dc current through the system, induced by the voltage drop between the electrodes, becomes proportional to the frequency of the mechanical oscillations 1 . The idea of shuttle phenomena was also extended to the quantum realm [8][9][10][11] . Nanoelectromechanical shuttle systems have been also studied in the regime of ac excitation and several interesting effects on the transport properties and the dynamics of the shuttle system have been found [12][13][14][15][16] . In particular, a shuttle structure driven by a time-dependent bias voltage has been considered in Refs. 17 and 18. It was shown that in case of asymmetric configuration such a setup can act as a rectifier, where the intensity of the dc current depends on the ratio between the frequency of the external oscillating voltage and the eigenfrequency of the mechanical subsystem. Current rectification was also conjectured by Ahn et al. 19 (and experimentally verified by Kim et al. 20 ) for the case of a symmetric double-shuttle structure. They attributed current-rectification phenomena to spontaneous symmetry breaking in the system caused by parametric instability. One of the conclusions of this work is that dynamical symmetry breaking in single shuttle systems does not lead to a dc current. Parametric excitation of nanoelectromechanical systems (NEMS) has been also considered in Refs. 21-23.In the present work, we investigate the possibility to generate a shuttle dc current, rather than rectifying current, in a completely symmetric single-dot shuttle system. We demonstrate that, in this scheme, despite the lack of a bias voltage, a shuttle dc current can still be detected. This charge transport is achieved by applying an ac voltage to a gate electrode which controls the electronic population of a metallic island and, in this form, also the stiffness of the...

No abstract

In this paper, we propose the mathematical model of a novel device, the nanomechanical transistor, able to control a current through a small drive voltage. The novelty of the device relies in its mechanical working principle where nanopillars vibrate between electrodes under a self-excitation regime which provides a continuative electric charge transportation. The dynamics of the investigated system involves electromechanical phenomena with the addition of quantum effects due to the electron tunneling of charges from pillars to electrodes. The theory here presented is an attempt to build a general model for those multiphysics phenomena (electrical-mechanical with presence of quantum effects) frequently met in nanotechnology that do not fit yet into a systematic frame