Quasiparticle spectrum near the gap node directions in the mixed state   of d-wave superconductors
  We report on a calculation of the quantized energy spectrum and quasiparticle eigenfunctions for low lying excitations in the mixed state of clean d-wave superconductors. Our study is based on an approximate analytical solution of the Bogolubov-de Gennes equations for both rectangular and triangular flux lattices with one of the primitive translations chosen parallel to the gap node direction. For excitations with momenta close to a certain gap node we have obtained a set of eigenfunctions which appear to be extended along the chosen gap node direction and localized along the perpendicular one on a scale determined by the intervortex distance. The periodic superfluid velocity field induces a band structure in the spectrum, which depends essentially on the vortex lattice geometry. 
A. S. Mel'nikov