Theory of quantum conductance of atomic and molecular bridges

Recent topics on theoretical analyses and prediction of the nano-scale structures connected between the electrodes are presented with discussions on several fundamental issues of these systems. Theoretical approaches used are the first-principles recursion transfer matrix method implemented with non-local pseudo-potentials, as well as non-equilibrium Green's function technique with tight binding bases. The former method is utilized for the analyses of atom wires and field emission process. As for the atom wire bridges, a very large effect of the terminal impurity and localized bias field distribution is elucidated. On the other hand, the latter method is mainly used for the analyses of the molecular bridges. The significant terminal effect is also found for the molecular bridge systems, if the transport is caused by the resonant tunneling. In addition, the effect of the conformational change inside the molecule is found to be sufficiently large for the STM tunneling current. Next, the remarkable feature of the large loop current inside the molecule is discussed in detail, as well as its relationship with the magnetically induced intramolecular current. Finally, the effect of the electron phonon couplings on the electronic states and the electron transfer processes are discussed in some detail, and the transition from the coherent to the dissipative hopping is considered.