Upper critical field and Fulde-Ferrell-Larkin-Ovchinnikov state inCeCoIn5

Abstract

Angle dependent magnetothermal conductivity experiments on ${\mathrm{CeCoIn}}_{5}$ indicate that this compound is a ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave superconductor. In this study, the low-temperature behavior of the upper critical field is measured in a single crystal of ${\mathrm{CeCoIn}}_{5}$ along the directions $\stackrel{\ensuremath{\rightarrow}}{H}\ensuremath{\Vert}\stackrel{\ensuremath{\rightarrow}}{a}$ and $\stackrel{\ensuremath{\rightarrow}}{H}\ensuremath{\Vert}\stackrel{\ensuremath{\rightarrow}}{c}.$ The data are compared with model calculations of the upper critical field in a ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave superconductor. It is found that the observed ${H}_{c2}(T)$ along $\stackrel{\ensuremath{\rightarrow}}{H}\ensuremath{\Vert}\stackrel{\ensuremath{\rightarrow}}{a}$ is consistent with a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state at low temperatures, $T<0.7\mathrm{K},$ whereas for $\stackrel{\ensuremath{\rightarrow}}{H}\ensuremath{\Vert}\stackrel{\ensuremath{\rightarrow}}{c}$ the FFLO state appears to be absent in ${\mathrm{CeCoIn}}_{5}.$ 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.