Possible Fulde-Ferrell-Larkin-Ovchinnikov State in CeCoIn₅

Angular dependent magnetothermal conductivity experiments on CeCoIn_5 indicate that this compound is a d-wave superconductor. In this study, the low-temperature behavior of the upper critical field is measured in a single crystal of CeCoIn_5 along the directions H parallel a and H parallel c. The data is compared with model calculations of the upper critical field in a d-wave superconductor. It is found that the observed H_c2(T) along H parallel a is consistent with a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state at low temperatures, T < 0.7K, whereas for H parallel c the FFLO state appears to be absent in CeCoIn_5. Furthermore, it is predicted that the quasiparticle density of states in the FFLO state exhibits a complex peak structure which should be observable by scanning tunneling microscopy.