Spin-susceptibility of spin-orbit coupled Fermi superfluids.

Under the self-consistent mean-field approach for the BCS-BEC crossover problem, we derive a closed-form analytical expression for the general spin response of noncentrosymmetric Fermi superfluids with arbitrary spin-orbit coupling and Zeeman fields. In addition to the paramagnetic, i.e., the Pauli intra-helicity and Van Vleck type inter-helicity, contributions to the spin-susceptibility tensor that have normal-state counterparts, we identify a diamagnetic inter-helicity contribution that is unique to the superfluid state. Our extensive numerical calculations for the Weyl, Rashba and equal Rashba-Dresselhaus spin-orbit couplings illustrate that it is this diamagnetic contribution that grows gradually with pairing and cancels precisely the Van Vleck contribution away from the BCS regime in general.