Possible Fulde-Ferrell-Larkin-Ovchinnikov superconducting state in CeCoIn₅

We report observation of the specific heat anomaly within the superconducting state of the heavy fermion CeCoIn₅. It appears in the vicinity of the superconducting critical field H _c2, where the superconducting transition changes from second to first order, above 10 T for H || [1 1 0] and H || [1 0 0], and above 4.7 T for H || [0 0 1], and at temperatures on the order of 0.1Tc. We interpret the anomaly within the superconducting state as a signature of a Fulde–Ferrell–Larkin–Ovchinnikov FFLO inhomogeneous superconducting state.