Electronic states and mixing of different symmetry or antiferromagnetism in vortex cores in dx2-y2-wave superconductivity

The microscopic structure of vortices in high-T-c superconductors is studied using the two-dimensional t-J model. The spatial dependences of order parameters are determined self-consistently using the Gutzwiller approximation, in which the effect of correlation is taken into account. In the high-doping region, the pair potential has d(x2-y2)-wave nature and the local density of states in the vicinity of the core shows a zero-energy peak. However, in the low-doping region, a spatially oscillating (extended) s-wave-type order parameter is locally induced around the vortex core. As a result, the local density of states near the core shows a splitting of the zero-energy peak. This gives a possible explanation for the experimental data of scanning tunneling spectroscopy for YBCO. The possibility of the antiferromagnetic vortex core is also studied by modifying the Gutzwiller approximation. It is found that, close to the boundary to the antiferromagnetically ordered state, the antiferromagnetic correlation develops inside the vortex core, even if the bulk state is the pure d-wave state. In this case the local density of states near the core does not have a zero-energy peak but instead shows a gap-like feature, which can be observed experimentally. Finally the sign change of the charge of the vortex core as a function of the doping rate is found.