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≲2kf, 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 T1−1 at the domain wall provides a clear signature of the quasiparticle bound states, and is in good agreement with recent experiment in organic superconductor κ−(BEDT−TTF)2Cu(NCS)2.

Source-reported materials — not catalogue approval

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