Theory of quantum transport through molecular and atomic bridges

Quantum transport of atom and molecular bridges are theoretically investigated based on Recursion-Transfer Matrix (RTM) Method and Eigen Channel Analyses. The concept of eigen-channels affords clear images of the physics of quantum transport through nano-structures. Detailed relationship between atomic orbital and quantum transport is clarified for atomic/molecular bridges formed between the metallic surfaces, For a straight Al atom bridge, the value of the conductance is close to 2 in the unit of the quantization, but this is due to an incidental situation. Thus, depending on the relative energy of the onset of the 2nd channel to the Fermi level, the value of the conductance changes sensitively. When the bridge is bent, the conductance is remarkably reduced and this explains some experimental features observed experimentally. For the case of Au atom bridge, the Fermi level is located slightly above the d channel offset. Therefore remarkable non-linear feature is expected. We report the results of eigen channel analyses showing the strong bias effect of the conductance. For the case of bridges formed by molecules with loop parts of the bond network, we found an interesting feature of the internal current distribution. Namely we found a significantly strong loop current induced by a small source-drain current. Such loop current appears close to a degenerate level of the molecule and is caused by the interaction between the component molecular states. The direction of the loop current is sharply reversed when the electron energy changes across the molecular level. We show several examples of the loop current for various planer molecules as well as C/sub 60/.