Switching and memory effects in electron-vibron systems: from single-site junctions to chains and networks

The general framework in which this thesis is embedded is called Molecular Electronics. In this field the dream is to be able to produce stable junctions in which a given molecule is in contact with a certain number of electrodes. Those allow to apply voltages and to perform specific tasks, exploiting the functionality of the molecule itself. Different kinds of molecules have specific electronic, structural and vibrational properties, but there is something that can be thought as a general property: the typical dimension of a molecule is in general very small (of the order of nanometers or smaller). Molecules can undergo structural changes when additional charges are inserted through electron-tunneling in transport setups. Because of that, the electronic and the vibrational degrees of free- dom are strongly related in molecules and their mutual interaction plays a fundamental role in the investigation of a molecular junction and in view of possible applications. In general we can consider a molecule as a very tiny object that is flexible and has localized vibrations. This property is peculiar of molecules and is absent in semiconductor devices like quantum-dots, two dimensional electron gases and bulk materials. In those systems the vibrational properties are associated to the phonon structure, i.e. to the lattice structure of the material one considers. The flexibility of the molecules make them interesting and different from semiconductors devices, opening new perspectives and bringing new effects into the game. The idea of using single molecule junctions in order to obtain functional devices like switches, rectifiers and memory elements, dates back to 1974 when Aviram and Ratner proposed to use a single organic molecule as a rectifier.