Theoretical and experiental study of electronic transport and structure in atomic-sized contacts

Resumen

We can summarize the main achievements in this thesis work as follows: a) The first stage of the formation of an atomic-sized contact and the process of its breakage have been studied and modeled for different materials. These two phenomena are shown to undergo respectively through a Jump to contact and a Jump out of contact process. Our studies provide information on the influence on both processes of the geometries of the nanocontacts before the contact and before breaking the contact. Moreover, a relationship between these atomic scale processes and the macroscopic properties of the materials have been provided through a systematic study for different materials. b) We show that it is possible to create stable and atomically well-defined tips through a ``mechanical annealing'' process involving the repeated indentation of an electrode into a surface as long as the indentation is limited to a few conductance quanta (approx 5G0) in the case of gold). Simulations provide an explanation showing how, under repeated indentations, two pyramidal tips with (111) faces form. c) We provide experimental evidence for the formation of diatomic long chains in gold. These diatomic chains have been confirmed in MD simulations and their conductance, as obtained by DFT, is in agreement with the experimentally measured values. Moreover, we show that there is a relationship between the formation of long monoatomic chains and diatomic chains in gold since normally these double stranded chains evolve to form the monoatomic ones d) Finally, our experiments show that a nanocontact between two Bismuth electrodes at room conditions presents an almost constant conductance close to 1G0 when stretched. We can understand this phenomena as a result of pulling a single bilayer of Bi(111) when such bilayer conducts a constant conductance of a quantum. Tight-Binding calculations on these structures show that these results can be understood as a manifestation of topological insulator behavior. This result provides a possible path to create topological insulators at room temperature. Therefore, we can say that this work has improved our understanding on how two metals connect at the atomic scale, as well as how a nanocontact breaks also at an atomic level.