Spin susceptibility of Andreev bound states

Abstract

We calculate the electronic spin susceptibility and spin-lattice relaxation rate in a singlet superconductor near a pair-breaking surface, or in a domain wall of the order parameter. We directly link the presence of high-density Andreev bound states in the inhomogeneous region, combined with coherence factors, to the enhancement of the susceptibility above the normal state\'s value for certain q vectors. Besides the dominant peak at ferromagnetic vector q = 0, we find significant enhancement of antiferromagnetic correlations at vectors q less than or similar to 2k(f), with q along the domain wall in an S-wave superconductor, and across the domain wall in D-wave (nodes along the wall). These features are destroyed by applying a moderate Zeeman field that splits the zero-energy peak. We solve Bogoliubov-de Gennes equations in momentum space and discuss the deviation of our results from the lattice models investigated previously. Large enhancement of the spin-lattice relaxation rate T-1(-1) at the domain wall provides a clear signature of the quasiparticle bound states, and is in good agreement with recent experiment in organic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2).