Quasi-particle spectrum of the nano-scaled anisotropic superconducting plate

Using the Bogoliubov-de Gennes equation for a tight-binding model of electrons, we studied the superconducting state of nano-structured anisotropic superconductors. Assuming the attractive interaction between nearest-neighbor sites, we obtained mainly d(cos kx − cos ky)-wave pairing state. But the superconducting states are inhomogeneous because of the surface. Especially, square plates with π/4 rotation show suppression of the d(cos kx − cos ky)-wave order parameter and there appear the quasi-particle states along the surface in the superconducting energy gap.