Vortex effects as an indication of a transition to the FFLO state*

The FFLO state is an exotic superconducting state that allows materials to remain in a superconducting state at higher fields than the paramagnetic limit where a magnetic field would normally break the singlet state of the Cooper pairs. The angular dependence of the rf penetration depth of the sample relative to the field is sensitive to distinct data signatures such as vortex lock-in where the Josephson vortices are very lightly pinned and increase the penetration depth. This presentation will discuss the process by which we were able to analyze these vortex data signatures as an indication of vortex lock-in and the transition to the FFLO state. The analysis of angular and field sweeps as well as the transformation from angular sweeps to field sweeps and vice versa make evident the validity of the features we are seeing as well as the connections between the features and vortex motion. This presentation will focus on the results of measurements of λ-(BETS)2GaCl4 and κ-(ET)2Cu(NCS)4 superconductors.