The strongly correlated superconductor CeCoIn₅: Evidence for the realization of a FFLO state

I will be reporting on specific-heat and thermal-conductivity measurements at low temperatures on the strongly correlated superconductor CeCoIn5 in magnetic fields close to the upper critical field Hc2. This system is a prime example for illustrating how in a strongly correlated electron system superconductivity emerges near a so-called quantum critical point (QCP). Close to this point the system fluctuates between a paramagnetic heavy-fermion state and an uncorrelated antiferromagnetically ordered metal. What makes CeCoIn5 special is the fact that just before the magnetic order is established at the QCP, superconductivity supersedes it. Several aspects of the superconductivity in CeCoIn5 are unconventional: On the one hand, the temperature dependence of the specific heat and thermal conductivity well below the critical temperature suggests lines of zeroes in the energy gap of the superconducting state. On the other hand, we observed a second phase inside the superconducting part of the temperature-magnetic field phase diagram close to Hc2. The limiting upper critical field to superconductivity in CeCoIn5 is set by the Pauli criterion, suggesting that this second phase is a Fulde-Ferrell-Larkin-Ovchinnikov state (FFLO). In the FFLO state, the superconducting wave function acquires an additional modulation. The FFLO phase is also of great interest to the astrophysics community, as it could describe the dense quark matter inside neutron stars, or quasars.