d-wave helical superconducting state in tricolor Kondo superlattices revealed by nonreciprocal transport

In two-dimensional (2D) superconductors, strong Rashba spin-orbit interaction (SOI) under broken inversion symmetry gives rise to several fascinating superconducting properties, including the admixture of spin-singlet and triplet pairing, and topoogical superconductivity. Upon the application of a parallel magnetic field to the 2D plane, helical superconducting state characterized by the formation of Cooper pairs with nonzero total momentum is expected to emerge as a result of the shift of Fermi surfaces with the Rashba SOI. However, realization of helical superconductivity has remained largely unclear because of the lack of detection technique. Recently, it has been proposed that nonreciprocal transport can be a promising bulk probe to detect the helical superconductivity.

Here, we report nonreciprocal transport properties of tricolor Kondo superlattices, in which global inversion symmetry breaking is introduced into d-wave superconductor CeCoIn₅ with atomic thickness by an asymmetric sequence YbCoIn₅/CeCoIn₅/YbRhIn₅, where YbCoIn₅ and YbRhIn₅ are nonmagnetic metals. We measured the second harmonic resistance R_2ω under parallel magnetic fields applied perpendicular to the injected electrical current within the 2D plane. At high temperatures near T_c, R_2ω first increases, then peaks at intermediate field regime, and finally vanishes at H_c2. Similar behavior has been reported in previous results on superconductors with s-wave symmetry. On the other hand, at sufficiently low temeratures, strong suppression of R_2ω is observed at H*, showing a double-peak structure. The transition line determined from H* separates low- and high-field superconducting phases, suggesting the emergence of d-wave helical superconducting state in the tricolor Kondo superlattices.