Spin correlations for an Anderson impurity in conventional and unconventional superconductors

We present a theoretical study of the spin-spin correlation function C(r) in a superconductor, where C(r) = [S . s(r)]; S is an impurity spin operator, and s(r) is the conduction electron spin density operator. We model the impurity using the Anderson Hamiltonian, and use the U --> infinity, 1/N expansion to do the calculations. bz addition to conventional superconductors, we consider unconventional superconductors, in which the order parameter Delta(k) has a lower rotational symmetry, and has vanishing angular average. Of particular interest is the way that the behavior of C(r) reflects two length scales, the Kondo length xi(K) and the superconducting coherence length, xi(0), and the way that its behavior is affected by the angular dependence of unconventional gaps.