We show theoretical framework of various nonreciprocal responses in superconductors. The superconducting diode effect, nonlinear superconducting optics, and nonreciprocal Meissner effect are studied, and observation in parity-breaking superconductors is proposed.
First, stimulated by the recent experiment , we propose an intrinsic mechanism to cause the superconducting diode effect (SDE) . SDE refers to the nonreciprocity of the critical current. We clarify that the SDE is understood by the nonreciprocity of the Landau critical momentum and the change in the nature of the helical superconductivity. The intrinsic SDE unveils the rich phase diagram and functionalities of noncentrosymmetric superconductors.
Second, we show that superconductors with the space-inversion symmetry breaking host giant nonreciprocal optical responses, such as photocurrent generation and second harmonic generation . Divergent behaviors are attributed to the nonreciprocal superfluid density and the Berry curvature derivative, characteristic of parity-breaking superconductors. These indicators quantify the performance of superconductors in nonlinear optics.
The nonreciprocal correction to the superfluid density also leads to the nonreciprocal Meissner effect, namely, asymmetric screening of magnetic fields due to the nonreciprocal magnetic penetration depth. Performing a microscopic analysis of an exotic superconductor UTe2, we show that the nonreciprocal Meissner effect is useful to probing parity-mixing properties and gap structures in superconductors .
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