Calculation of ballistic conductance through Tamm surface states

Abstract

Ballistic conduction through Tamm surface states is studied theoretically. We calculate the conductance in systems of scanning tunneling microscopy (STM) with double tips as well as a single tip. Conductance spectra of flat surfaces containing surface states are qualitatively proportional to the density of states at the first layer of surfaces if we calculate on boundary conditions that allow net current to flow parallel to surfaces. The potential difference between the first layer and inner layers enhances the conduction through Tamm states. When a STM tip is put on an island of a surface, the conductance varies as a function of the step height of the island. The variation is explained by the difference in the step-height dependence between surface-state and bulk-state conduction. The tunneling between the surface states of islands and substrates is the main path of the surface-state conduction at low step heights. The study of the conduction in islands demonstrates the importance of the conduction path in surfaces to electrodes. The ballistic conductance spectra between two tips in double-tip systems show one-dimensional features, which suggests high conductance when the Fermi energy is located near band edges of surface states.