Rectification and nonlinear Hall effect by fluctuating helical Cooper pairs

Recent studies have revealed various nonreciprocal phenomena in noncentrosymmetric materials, including directional resistance (magnetochiral anisotropy), nonlinear Hall effect, and superconducting diode effect. They are appealing not only for engineering purposes but also as novel probes of electronic states and are among the hallmarks of modern condensed matter physics. Further study of nonreciprocal phenomena would shed light on unprecedented phenomena in noncentrosymmetric materials.

In this talk, we discuss the intrinsic nonreciprocity of the charge transport in noncentrosymmetric superconductors under magnetic fields. In such systems, Cooper pairs spontaneously acquire a finite center-of-mass momentum in equilibrium. Such a state is known as helical superconductivity and originates from the magnetoelectric coupling of electrons due to the Rashba spin-orbit coupling. We discuss the fingerprint of finite-momentum Cooper pairs by focusing on the directional resistance and nonlinear Hall effect in the paraconductivity regime and show that the development of helical Cooper pairs gives rise to a strong enhancement of nonreciprocity [1]. The results uncover the novel aspect of finite-momentum superconductors and show that nonreciprocal charge transport offers a promising probe of helical superconductivity.

[1] A. Daido and Y. Yanase, arXiv:2302.10677.