Pressure studies of the thermodynamics of ${\mathrm{CeCoIn}}_5$ under magnetic fields $H\parallel c$ and $H\parallel ab$ have been made up to $P=1.34\mathrm{GPa}$. We recorded the signature of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state for all pressures when $H\parallel ab$. Also remarkably, the FFLO regime suddenly expands for $P=1.34\mathrm{GPa}$. With the help of a microscopic theory for $d$-wave superconductivity, we have extracted the gyromagnetic ratio $g$ and the Fermi velocities $v_a$ and $v_c$. Our study is the first evidence for the existence of the FFLO state away from the influence of the antiferromagnetic fluctuations. We find a close parallel between the $T\mathrm{\text{−}}P$ phase diagram of ${\mathrm{CeCoIn}}_5$ and the $T\mathrm{\text{−}}x$ phase diagram of the high-$T_c$ cuprates, where $x$ is the hole concentration.

• Received 28 November 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.117001