Signatures of the helical phase in the critical fields at twin boundaries of noncentrosymmetric superconductors

Domains in noncentrosymmetric materials represent regions of different crystal structure and spin-orbit coupling. Twin boundaries separating such domains display unusual properties in noncentrosymmetric superconductors (NCSs), where magnetoelectric effects influence the local lower and upper critical magnetic fields. As a model system, we investigate NCSs with tetragonal crystal structure and Rashba spin-orbit coupling (RSOC), and with twin boundaries parallel to their basal planes. There, we report that there are two types of such twin boundaries which separate domains of opposite RSOC. In a magnetic field parallel to the basal plane, magnetoelectric coupling between the spin polarization and supercurrents induces an effective magnetic field at these twin boundaries. We show that this leads to unusual effects in such superconductors, and in particular to the modification of the upper and lower critical fields, in ways that depend on the type of twin boundary, as analyzed in detail, both analytically and numerically. Experimental implications of these effects are discussed.