Z−1 Expansion in Dense Kondo Systems
It is emphasized that the coherent state of dense Kondo systems can be described conveniently by noting the existence of the small expansion parameter Z−1, Z being the mass renormalization factor. This expansion naturally leads to the existence of the heavy quasi-particles at the fermi energy mixing with conduction electrons, but with the mixing matrix element renormalized by Z−1/2 . Such heavy quasi-particles form a band by themselves even without the direct mixing with conduction electrons and have residual repulsive interactions, whose coupling strength is of the order one. Based on this argument the effect of impurity scattering is also examined and it is indicated that the scattering rate due to potential fluctuations is reduced by Z−1 whereas that due to modifications of mixing matrix elements is not. Hence it is concluded that the superconductivity is more easily influenced by alloying which affects the latter.
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