Nonlinear optical responses in noncentrosymmetric superconductors

TL;DR

This paper investigates the mechanisms and conditions for second-order nonlinear optical responses in noncentrosymmetric, time-reversal symmetric superconductors, highlighting the roles of pairing types and Berry curvature.

Contribution

It provides a microscopic analysis of the conditions necessary for second-order optical responses in superconductors, emphasizing the coexistence of pairing types and the superconducting Berry curvature.

Findings

Photocurrent and second harmonic generation observed in superconducting state.

Nonlinear responses vanish under certain pair potential conditions.

Coexistence of intraband and interband pairing is essential for nonlinear responses.

Abstract

The unique nonreciprocal responses of superconductors, which stem from the Cooper pairs' quantum condensation, have been attracting attention. Recently, theories of the second-order nonlinear response in noncentrosymmetric superconductors were formulated based on the Bogoliubov-de Gennes theory. In this paper, we study the mechanism and condition for second-order optical responses of time-reversal symmetric superconductors. The numerical results show the characteristic photocurrent and second harmonic generation in the superconducting state. However, the superconductivity-induced nonlinear optical responses disappear under some conditions on pair potential. We show that the coexistence of intraband and interband pairing is necessary for the second-order superconducting optical responses. In addition, the superconducting Berry curvature factor, which is related to a component of Berry curvature in the superconducting state, is essential for the nonlinear responses. Thus, we derived the microscopic conditions where the superconducting nonlinear response appears.