Theoretical Study of Quantum Transport Phenomena through an Atom Sized Contact.

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When the tip of a scanning tunneling microscope is brought into contact with a metal surface and then lifted up slowly, an atom-sized contact is formed between the tip and the surface. An atom-sized bridge is thus formed and its cross section consists of a few atoms to a few tens of atoms. Conductance of the bridge is measured as a function of the stretching length of the bridge, while the tip is lifted up. A stepwise variation of the conductance is experimentally observed roughly in units of 2e^<>h. In order to satisfy requirements for downsizing from mesoscopic scale to atomic scale in electronics, it is necessary to examine various phenomena occurring in such atom-sized systems. With this in mind, we examine the effects of the constricted structure of the atom-sized bridge and the effects of the Coulomb interaction between electrons on the quantized conductance through an atom-sized bridge. The obtained results are summarized as follows.1. The effects of the constricted structure of … More the atom-sized bridge :Stepwise variation of the conductance is disturbed by the broken symmetry of the shape of the atom-sized bridge.2. The effects of the intra-site Coulomb interaction between electrons :At finite temperatures, the Coulomb interaction causes various types of electron-electron scattering processes, inter-subband as well as intra-subband scattering processes and Umklapp as well as normal scattering processes, and strongly affects the quantum transport property of an atom-sized bridge. As a result, the following are derived.(1) At the conductance plateau, the deviation of the conductance value from multiples of the universal value of 2e2/h grows as the temperature is decreased. This temperature dependence is stronger for stronger Coulomb interactions.(2) Near the step, a valley structure appears as a function of the stretching length of the bridge. As the temperature is decreased, the valley gets narrower. The depth of the valley grows significantly, as the Coulomb interaction becomes stronger. Less