Study of upper critical field of superlattices cntaining ferromagnetic layrs

有实验观察表明d_f依赖于超导和铁磁超晶格的转变温度T_c，其中d_f是铁磁层的厚度。对于固定超导层，转变温度T_c单调降低，随着d_f的增加趋于饱和。利用de Gennes发展并由Takahashi和Tachiki推广的微观理论，我们对T_c进行了数值计算，这解释了实验观察（Czech.J.）。物理学报：66 3219，（1997）)。超导超晶格通常用BCS相互作用V、态密度N、扩散常数D和交换势i来表征。在实际体系中，每种金属的德拜频率omega_D的值是不同的。到目前为止，还没有对这一效应的理论研究。我们已经证明，吸引相互作用的大小由BCS相互作用决定，其范围由德拜频率决定。Jpn: 663219,(1997))。从德拜频率差对T_c和H_<c2>的影响与BCS耦合常数的影响在性质上相似的事实出发，我们研究了超导和铁磁超晶格的上临界场H_<c2>作为N、D和I的函数。对于大势相互作用I，转变温度T_c作为d_f的函数表现出非单调行为。我们已经证明，这种特殊的行为是由成对函数的pi相位的出现和计算的成对函数的空间变化引起的，以便定量地解释结果(物理学。Rev B:（1998）出版。

There are experimental observations which show d_f dependence of the transition temperature T_c for the superconducting and ferromagnetic superlattices, where d_f is the thickness of the ferromagnetic layr. The transition temperature T_c, in general, decreases monotonically and saturetes with increasing d_f for the fixed superconducting layr. Using the microscopic theory developed by de Gennes and extended by Takahashi and Tachiki, we have made numerical calculations for T_c, which explain the experimental observations (Czech.J.Phys : 66 3219, (1997) ).The superconducting superlattices are usually characterized by the BCS interaction V,the density of states N,the diffusion constant D and the exchange potential I.In the real systems, the values of the Debye frequency omega_D are different from each metal. Up to now, there is no theoretical investigation of this effect. We have shown that the magnitude of the attractive interaction is given by the BCS interaction and its range by the Debye frequency (J.Phys.Soc.Jpn : 66 3219, (1997) ).From the fact that the difference of the Debye frequency gives the qualitatively similar effect on T_c and H_<c2> as that of the BCS coupling constant, we have investigated the upper critical field H_<c2> for the superconducting and ferromagnetic superlattices as the functions of N,D and I.For the large potential interaction I,the transition temperature T_c shows nonmonotonic behavior as a functon of d_f. We have shown that this peculiar behavior arises from the appearance of the pi phase of the pair function and calculated spatial variation of the pair function in order to interpret quantitatively the results (Phys.Rev B : (1998) in press).