摘要
In the experiment of upper critical field H_c2 in Na_xCoO_2. yH_2O, it showed that there existed a change of slope near T_c. In the thesis we study how to solve the anomalous phenomenon under macroscopic and microscopic descriptions.Since it is a second order phase transition, we apply the Ginzburg-Landau approach and propose two allowed pairing symmetries, f-p mixing, to treat the problem. Under the macroscopic description, we obtain the result which explains the change of slope well. Secondly, we adopt the method of microscopic calculations in one-band consideration and, however, it does not support the result in the Ginzburg-Landau approach. We find the reason why the two approaches conflict with each other. Moreover we consider the influence of two-band system on the upper critical field Hc2(T). In the result we discover that two-band system containing some microscopic characteristics which dominate the slope of each pairing symmetry may cause the upward curvature in Na_xCoO_2 yH_2O.