Theory for tunneling spectroscopy of anisotropic superconductors

A theory for the tunneling spectroscopy of a normal-metal-insulator-anisotropic-superconductor (N-I-S) junction is presented. In anisotropic superconductors, the effective pair potential felt by the quasiparticles depends on their wave vectors in contrast to the case of isotropic s-wave superconductors. By introducing the effect into the Blonder-Tinkham-Klapwidjk formula, a conductance formula for N-I-S junctions is obtained. It is shown that the conductance spectra are a function not only of the amplitudes of pair potentials but also of their phases. Tunneling conductance spectra calculated for various symmetries strongly depend on the relation between the tunneling direction and crystalline axes. In some crystalline angle regions of d-wave superconductors, a zero-energy peak in the conductance spectra is calculated. This conductance peak reflects the existence of anomalous bound states around the insulator-anisotropic-superconductor interface. The relation between the tunneling conductance spectra and the local density of states of superconductors is discussed.