We show the theoretical framework of various nonreciprocal responses in superconductors. The superconducting diode effect, nonlinear superconducting optics, and nonreciprocal Meissner effect are discussed, and observation in noncentrosymmetric superconductors and parity-breaking superconductors is proposed.
First, stimulated by the recent discovery of the superconducting diode effect (SDE) , we propose an intrinsic mechanism to cause the SDE . The SDE refers to the nonreciprocity of the critical current. We show that the SDE is caused by the nonreciprocity of the Landau critical momentum and propose that the crossover phenomenon in the helical superconducting state can be proved by the SDE. 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 the 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. We also show the microscopic conditions for the nonreciprocal optical responses in noncentrosymmetric superconductors .
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 for probing parity-mixing properties and gap structures in superconductors .
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Keywords: Nonreciprocal responses, Noncentrosymmetric superconductors, Parity-breaking superconductors, Superconducting diode effect