Photogalvanic transport of nonreciprocal Cooper-pair fluctuations

Abstract: We develop a theory of the nonlinear optical and transport responses of two-dimensional noncentrosymmetric superconductors in the fluctuation regime above the transition temperature, encompassing the photogalvanic effect, second-harmonic generation, and the photovoltaic Hall effect. In the vicinity of the transition these responses are strongly enhanced by preformed Cooper pairs, whose nonreciprocity enters the time-dependent Ginzburg-Landau description in two physically distinct ways: through thermodynamic Lifshitz invariants, which encode an asymmetric pair spectrum, and through kinetic Lifshitz invariants, which encode an asymmetric pair relaxation and are locked to the Langevin noise by the fluctuation-dissipation theorem. We derive generalized master formulas for the paraconductivity (Aslamazov-Larkin) and the quantum-interference (Maki-Thompson) channels of the nonlinear current, valid at arbitrary drive frequency and to linear order in the nonreciprocal perturbations, and reduce them to closed-form dimensionless functions. Circular polarization discriminates sharply between the mechanisms: for reciprocal momentum-structureless noise the Aslamazov-Larkin channel is polarization insensitive and its circular photogalvanic response vanishes for any pair spectrum, whereas the Maki-Thompson channel and the nonreciprocal noise support helicity-odd rectified currents, including a fluctuation photovoltaic Hall current flowing transverse to the strain axis. Applications to Rashba-type ($C_{3v}$) and Ising-type ($D_{3h}$) superconductors demonstrate how the point-group symmetry dictates the allowed vector structures of the nonlinear currents, and how polarization analysis together with the frequency and dephasing dependences can be used to separate the individual channels experimentally.