Radial modulation enhances critical fields in 2d superconductors
The 1964 reports of Fulde, Ferrell, Larkin, and Ovchinnikov (FFLO or LOFF) on paramagnetic enhancement of superconductivity suggested that superconductivity can persist at applied magnetic fields above both its orbital and paramagnetic limits. By forming spatially alternating superconducting and paramagnetic regions, the increase in local magnetic field in the paramagnetic region allows a reduction in field inside the superconductor. We present an FFLO phase diagram model for layered organic superconductors and confirm it with high magnetic field data from four materials. Our work suggests that paramagnetic and superconducting regions form as radially alternating rings about each vortex rather than plane waves, as FFLO is usually described.