We show that the vibrations of a nanomechanical resonator can be cooled to near its quantum ground state by tunnelling injection of electrons from an STM 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.PACS numbers: 85.35. Kt, 85.85.+j Remarkable steps are now being taken towards achieving conditions under which quantum effects are experimentally accessible in nano-electromechanical systems (NEMS) [1]. This is encouraging both for the prospects of realizing a plethora of applications that depend on our ability to control and monitor the coherent dynamics of nanometer-scale mechanical systems and for shedding light on purely fundamental issues, such as the nature of the crossover from classical to quantum physics [2].A number of proposals have been put forward in order to reach the extremely low temperatures T of order ω/k B , where quantum effects become observable in mechanical resonators of eigenfrequency ω. Their aim has been to replace the conventional dilution refrigerators by more efficient active cooling methods.Most of them are based on well-established principles for laser cooling of atoms and molecules [3], but alternative approaches have been proposed by several authors who suggest that the coupling between mechanical and electronic degrees of freedom can be exploited for inducing energy to flow from the former to the latter [4]. All these schemes for ground state cooling are based on quantum resolved sideband transitions between discrete quantum levels of the refrigerant, a cooling mechanism that relies on energy conservation in order to suppress processes which involve emission of vibrational energy quanta (vibrons) with respect to those that lead to absorption of such quanta.In this Letter we suggest a new mechanism for ground state cooling of a nanomechanical resonator, which is based on passing a current through the resonator under conditions such that the probability amplitude for tunneling electrons to emit vibrons is much lower than it is to absorb them. As such it has the advantage that it does not require the refrigerant to have a discrete energy spectrum, which puts fewer constraints on the experimental design.To be specific we consider the model system sketched in Fig. 1, where electrons are injected from the tip of a Figure 1. Sketch of the model system considered. A metallic carbon nanotube is suspended over a trench between two grounded metallic leads, while an STM tip placed a distance h above the nanotube is biased at a negative voltage −V .scanning tunneling microscope (STM) into a suspended metallic carbon nanotube. Low-temperature tunneling spectroscopy studies on a similar device [5] have s...

Strong coupling between electronic and mechanical degrees of freedom is a basic requirement for the operation of any nanoelectromechanical device. In this Review we consider such devices and in particular investigate the properties of small tunnel-junction nanostructures that contain a movable element in the form of a suspended nanowire. In these systems, electrical current and charge can be concentrated to small spatial volumes resulting in strong coupling between the mechanics and the charge transport. As a result, a variety of mesoscopic phenomena appear, which can be used for the transduction of electrical currents into mechanical operation. Here we will in particular consider nanoelectromechanical dynamics far from equilibrium and the effect of quantum coherence in both the electronic and mechanical degrees of freedom in the context of both normal and superconducting nanostructures.

We investigate theoretically multimode electromechanical "shuttle" instabilities in dc voltage-biased nanoelectromechanical single-electron tunneling devices. We show that initially irregular (quasiperiodic) oscillations that occur as a result of the simultaneous self-excitation of several mechanical modes with incommensurable frequencies self-organize into periodic oscillations with a frequency corresponding to the eigenfrequency of one of the unstable modes. This effect demonstrates that a local probe can selectively excite global vibrations of extended objects.

Abstract. We theoretically investigate the electromechanical properties of freely suspended nanowires that are in tunneling contact with the tip of a scanning tunneling microscope (STM) and two supporting metallic leads. The aim of our analysis is to characterize the fluctuations of the dynamical variables of the nanowire when a temperature drop is maintained between the STM tip and the leads, which are all assumed to be electrically grounded. By solving a quantum master equation that describes the coupled dynamics of electronic and mechanical degrees of freedom, we found that the stationary state of the mechanical oscillator has a Gaussian character, but that the amplitude of its rootmean square center-of-mass fluctuations is smaller than would be expected if the system were coupled only to the leads at thermal equilibrium.

We study the dynamics of transverse oscillations of a suspended carbon nanotube into which current is injected from the tip of a scanning tunneling microscope (STM). In this case the correlations between the displacement of the nanotube and its charge state, determined by the positiondependent electron tunneling rate, can lead to a "shuttle-like" instability for the transverse vibrational modes. We find that selective excitation of a specific mode can be achieved by an accurate positioning of the STM tip. This result suggests a feasible way to control the dynamics of this nano-electromechanical system (NEMS) based on the "shuttle instability". PACS numbers: 85.35.Kt, 85.85.+j There are several reasons for the considerable current interest in nano-electromechanical systems (NEMS), both for technological applications and fundamental research. The peculiar combination of several features such as high vibrational frequencies and small masses which characterize most NEMS makes these systems very suitable for the realization of new measurement tools with extremely high sensitivity in mass sensing and force microscopy applications [1,2]. Furthermore, the mechanical elements of the NEMS (typically cantilevers or beams) are considered the most promising structures where quantum features of motion could be experimentally detected [3].The physical basis for many of the interesting functionalities of NEMS is the strong interplay between mechanical and electronic degrees of freedom [4,5,6,7]. In the particular case of a nano-electromechanical single-electron transistor device having a metallic dot as movable part, the equilibrium position of the dot can become unstable as a consequence of the electromechanical coupling. In this case the dominant mechanism for the transport of charge is based on the oscillations of the dot which can "shuttle" the tunneling electrons across the system [8,9].The typical set-up for many NEMS includes a spatially extended movable element such as a suspended carbon nanotube, whose dynamics has been demonstrated to be characterized by a number of different vibrational modes [10]. The relevance of many mechanical modes in the transport of charge suggests that the variety of effects due to the electromechanical coupling in suspended carbon nanotube-based NEMS may be even richer than in the ordinary "shuttle" system. Jonsson et al. have shown [11,12,13] that if extra charge is injected into the movable part of the device from the tip of a scanning tunneling microscope (STM) a nano-electromechanical "shuttle-like" instability can be induced for the transverse vibrational modes of the nanotube.The selective promotion of the electromechanical instability for different vibrational modes provides an interesting perspective for probing the dynamics of NEMS. Here we show that such selective excitation can be achieved by means of local injection of electric charge. The main Figure 1: Sketch of the model system considered. The distance of the suspended carbon nanotube from the STM tip affects the electron tu...