We examine the interplay between anisotropy of the in-plane magnetization and the nodal gap structure on the basis of the approximate analytic solution in the quasiclassical formalism. We show that a four-fold oscillation appears in the magnetization, and its amplitude changes sign at an intermediate field. The high-field oscillation originates from the anisotropy of the upper critical field, while the low-field behavior can be understood by the thermally activated quasiparticles near nodes depending on the applied field angles. The temperature dependence of the magnetization also shows a similar sign change. The anisotropy of the magnetization offers a possible measurement to identify the gap structure directly for a wide class of type II superconductors.
@article{arxiv.cond-mat/0411540,
title = {Anisotropy of in-plane magnetization due to nodal gap structure in the vortex state},
author = {Hiroaki Kusunose},
journal= {arXiv preprint arXiv:cond-mat/0411540},
year = {2009}
}