| speaker: | Rolf Lortz University of Geneva |
| time: | November 17, 15:40 - 16:05 |
The upper critical field Hc2 of superconductors is usually determined by the orbital limiting field at which the current density around vortices reaches a pair breaking value. In certain cases, this limit is particularly high or excluded by an open nature of the Fermi surface. Hc2 reaches then very high values close to or even above the theoretical Pauli limiting field for superconductivity. Superconductivity should disappear at this limiting field due to pair breaking by the Pauli paramagnetism of the electrons. Theory predicts however the possibility of a novel superconducting state, which allows the superconductor to maintain its state above this limiting field: The Fulde-Ferrel-Larkin-Ovchinnikov or FFLO state. Although predicted since a long time, up to now only the heavy Fermion superconductor CeCoIn5 and few layered organic superconductors are suspected to show such a state in high magnetic fields. In the latter case, thermodynamic proofs are currently missing. We studied layered organic superconductors such as kappa-(BEDT)2Cu(NCS)2 in high magnetic fields up to 28 T applied parallel to the metallic layers. We found that the superconducting transition develops a first-order latent heat above a certain field. This is a clear sign that the Pauli limit for superconductivity is reached. In higher fields the Hc2 line shows a sharp upturn and a transition line appears within the superconducting phase. A comparison with a theoretical model gives strong evidence that this phase is a realization of a Fulde-Ferrel-Larkin-Ovchinnikov state.