We consider dc-electronic transport through a nanowire suspended between normal- and spin-polarized metal leads in the presence of an external magnetic field. We show that magnetomotive coupling between the electrical current through the nanowire and vibrations of the wire may result in self-excitation of mechanical vibrations. The self-excitation mechanism is based on correlations between the occupancy of the quantized electronic energy levels inside the nanowire and the velocity of the nanowire. We derive conditions for the occurrence of the instability and find stable regimes of mechanical oscillations.

We have theoretically investigated the electromechanical properties of a freely suspended carbon nanotube that is connected to a constant-current source and subjected to an external magnetic field. We show that self-excitation of mechanical vibrations of the nanotube can occur if the magnetic field H exceeds a dissipation-dependent critical value Hc, which we find to be of the order of 10-100 mT for realistic parameters. The instability develops into a stationary regime characterized by time periodic oscillations in the fundamental bending mode amplitude. We find that for nanotubes with large quality factors and a magnetic-field strength just above Hc the frequency of the stationary vibrations is very close to the eigenfrequency of the fundamental mode. We also demonstrate that the magnetic field dependence of the time averaged voltage drop across the nanotube has a singularity at H = Hc. We discuss the possibility of using this phenomenon for the detection of nanotube vibrations.Nanoelectromechanical systems (NEMS) containing a suspended carbon nanotube (CNT) vibrating at radio frequencies (RF) have received increasing attention recently. Advantages of CNT mechanical resonators include their high resonance frequencies -up to the GHz range --their low dissipative losses 1 and the possibility to tune the resonance frequency by adjusting the tension in the tube 2,3 . CNT based NEMS devices have already shown a great potential for a plethora of technological applications including mass sensing 4,5 and tunable high frequency electronics 2,3,6,7 . However, most of the devices which have been realized thus far are passive resonators which perform frequency filtering of the incoming RFsignal 2,3,6,7 . Here we propose an active oscillator based on an current biased doubly-clamped CNT for which the conductance depends monotonically on the nanotube deflection. Such a deflection sensitive resistance has been demonstrated for a semiconducting single-walled CNT suspended over a gate electrode 2,3,8 . The active feedback is provided by a Lorentz force induced by a constant magnetic field directed perpendicular to the direction of the current and parallel to the gate electrode. We show that , by applying a constant external current in a sufficiently high magnetic field, we obtain mechanical instability and self-sustained mechanical oscillations at a frequency close to the mechanical resonance frequency. Furthermore, we show that the mechanical instability results in oscillations of the voltage drop across the nanotube accompanied by a deviation of the time averaged voltage from its static time-independent value.

We demonstrate theoretically the feasibility of selective self-excitation of higher-mode flexural vibrations of graphene nano-ribbons and carbon nanotubes by the means of magnetomotive instability. Apart from the mechanical resonator, the device consists only of a constant voltage source, an inductor, a capacitor, a gate electrode and a constant magnetic field. Numerical simluations were performed on both graphene and carbon nanotubes displaying an overall similar behaviour, but with some differences arising mainly due to the non-linear mechanical bending forces. The advantages and disadvanatges of both materials are discussed.

Abstract:We explore a semi-classical scheme for cooling and excitation of mechanical oscillations of a suspended carbon nanotube which is incorporated as a deflection-sensitive resistor in an LRC-circuit. The active feedback consists of a magnetically induced Lorentz force. We show that, for feasible experimental parameters, we can obtain self-sustained oscillations or cooling by several orders of magnitude. 85.35.Kt, 85.85.+j PACS (2008):