Angular band structure of a vortex line in a type-II superconductor

The electronic structure of a vortex line in a type-II superconductor is calculated in the low-magnetic-field limit in the framework of the Bogoliubov--de Gennes theory. It is found that breaking the rotational invariance of a vortex line by means of either an anisotropic crystal potential or by the presence of neighboring vortices induces an angular band structure in the low-energy part of the quasiparticle spectrum. The associated quasiparticle amplitudes become distorted and exhibit a characteristic ``star'' shape whose orientation is energy dependent. This explains recent scanning-tunneling-microscopy experiments by Hess, Robinson, and Waszczak [Phys. Rev. Lett. 64, 2711 (1990)] on NbSe₂.