Thermodynamic evidence for the formation of a Fulde-Ferrell-Larkin-Ovchinnikov phase in the organic superconductor λ-(BETS)2GaCl4

In this work, the thermodynamic properties of the organic superconductor lambda-(BETS)(2)GaCl4 are investigated to study a high-field superconducting state known as the putative Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase. We observed a small thermodynamic anomaly in the field H-FFLO similar to 10 T, which corresponds to the Pauli limiting field H-P. This anomaly probably originates from a transition from a uniform superconducting state to the FFLO state. H-FFLO does not show a strong-field angular dependence due to a quasi-isotropic paramagnetic effect in lambda-(BETS)(2)GaCl4. The thermodynamic anomaly at H-FFLO is smeared out. and the low-temperature upper critical field H-c2 changes significantly if fields are not parallel to the conducting plane, even for a deviation of similar to 0.5 degrees. This behavior indicates that the high-field state is very unstable, as it is influenced by the strongly anisotropic orbital effect. Our results are consistent with the theoretical predictions on the FFLO state and show that the high-field superconductivity is probably an FFLO state in lambda-(BETS)(2)GaCl4 from a thermodynamic point of view.