Modulated vortex states in Rashba non-centrosymmetric superconductors

Abstract

Vortex lattice structures to occur in Rashba non-centrosymmetric superconductors under a magnetic field parallel to the basal plane are studied by assuming a singlet Cooper pairing and taking account of both the paramagnetic and orbital depairings. A vortex lattice of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type with modulation perpendicular to the field is expected to occur in the limit of vanishing ${\tilde \eta}$, where ${\tilde \eta}$ is the spin-orbit coupling {\it normalized} by Fermi energy. As ${\tilde \eta}$ increases, however, this state tends to be replaced by another new vortex lattice with a modulation of the gap amplitude parallel to the helical phase modulation induced by the absence of inversion symmetry. Differences of the present results from those in the Pauli limit and a correlation of a structural transition with the $H_{c2}(T)$-curve are discussed in relation to possible experimental realization.