Light induced magnetization in d-wave superconductors

Abstract

We develop a microscopic theory of the inverse Faraday effect in $s$- and $d$-wave superconductors. An extended version of the Keldysh--Nambu quasiclassical formalism, which retains the particle-hole asymmetric terms responsible for the branch population imbalance, is used to compute the dc component of the nonlinear current density induced by an external monochromatic radiation. We demonstrate how the branch population imbalance produces a nonvanishing nonlinear and nonlocal dc response, evaluate the magnitude of the induced current, and obtain estimates for the induced static magnetization. For $d$-wave pairing we identify a qualitatively new contribution: the radiation induces a linear-in-field oscillation of the order-parameter amplitude -- the Schmid-Higgs mode -- in the $B_{1g}$ channel, which feeds the rectified response with a weight proportional to the pair susceptibility and is therefore resonantly enhanced at the pair-breaking threshold. This contribution is symmetry-forbidden for an isotropic $s$-wave gap, so the light-induced magnetization serves both as a dc-channel probe of the Higgs mode and as a discriminator of the pairing symmetry. Experimental implications of our theory and future extensions of our work are briefly discussed.