Application of the Coherent Potential Approximation to Impurity Problems in Superconductors. II : Classical Spin

概要

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The effect of the classical spin on superconductivity is investigated by using the CPA. The Ising spin which has only z component is taken as a classical spin, and two cases (I) S=1/2 and (II) S→∞, J→0 (JS=finite) are investigated. The case (I) is shown to be equivalent to the classical limit of the s-d exchange model treated by Shiba in the c-linear approximation. The depression of the transition temperature is shown to be larger if ζ=πρJS/2<1, and smaller if ζ>1 in case (I) compared with the results in the c-linear approximation. In case (II), on the other hand, the depression is shown to be always larger. The correction to the result of c-linear approximation, however, is rather small. The density of states in the energy gap is also calculated and the condition of the gapless superconductor is obtained. In case (I), this condition is more stringent than in the c-linear approximation, and the correction to c-linear approximation is surprisingly large. In case (II), the gapless situation always appears for any concentrations if ζ≧1 which is also the result in c-linear approximation. If ζ<1, the condition of the gapless superconductor is more stringent than in the c-linear approximation. The magnitude of the correction is shown to be rather small in this case.
理論物理学刊行会の論文
1973-05-25