Electromechanical Interactions in Shuttle Systems and Carbon Nanotube Relays

In this thesis we theoretically investigate two kinds of nanoscale systems where there is a strong electromechanical coupling, i.e. an intricate relation between the Coulomb forces that can appear in these systems and the displacements of parts of the systems that they cause. We also provide the background and the basic theory needed to understand the models and the results presented here. <p />In the first part of the thesis we focus on properties of a nanoelectromechanical single electron transistor, also known as a shuttle system. First the current-voltage (IV) characteristics of the system in the high dissipation limit is investigated. For voltages above the Coulomb blockade threshold two regimes of charge transfer occur. At low voltages the current is dominated by tunneling through an asymmetric system. For higher voltages an oscillatory motion of the grain appears which shuttles charges across the system. We then investigate the effects of van der Waals (vdW) forces on the shuttle transition. It is shown that the shape of the total potential, which depends on the relative strength of the elastic and vdW forces, is crucial to the grain dynamics and charge transport. For weak elastic forces there is a large hysteresis in the IV-curves. Furthermore, we investigate the mechanisms of energy pumping in a quantum mechanical model of a single-level quantum dot coupled to two leads. Above a threshold voltage energy pumping to the vibrational subsystem occurs. It is shown that higher order processes bring the system to a stationary regime and that this regime is characterized by a lowering of the resistance. <p />In the second part of the thesis we focus on a three-terminal nanorelay based on a conducting carbon nanotube placed on a terraced Si substrate. Investigations are made into how this system can act as a switch, a transistor, and a memory device. We also investigate how short range forces affect the operation of the device