Takayuki NISHIO (Tohoku Univ.), Kenji Yonemitsu and Hiromichi EBISAWA (Tohoku Univ.)
[J. Phys. Soc. Jpn. 66, 953 (1997)]
The quasi-particle spectrum for s-wave superconductors in the vortex-lattice state remains gapless in magnetic fields much below the upper critical field Hc2 where the cyclotron frequency is smaller than the amplitude of the pair potential. This is shown through self-consistent solutions of the Bogoliubov-de Gennes equations for the attractive Hubbard model on the square lattice. It has not been predicted in earlier theoretical works based on a continuous model because it is due to the dispersion of Landau subbands in the lattice model.