Influence of Antiferromagnetic Fluctuations on the Fulde–Ferrell–Larkin–Ovchinnikov State in CeCoIn5

The Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state is a spatially inhomogeneous superconducting (SC) phase with a periodically modulated order parameter predicted to appear in sufficiently clean type-II superconductors, close to the upper critical field, if the orbital pair breaking is small relative to the Pauli-limiting effect. The heavy-fermion superconductor CeCoIn₅ is the first material, where different physical probes show strong experimental evidence pointing to the realization of the FFLO state, even though strong antiferromagnetic (AFM) spin-fluctuations (SF) are present at atmospheric pressure. To study the influence of the AFM-SF on the FFLO state we performed heat-capacity experiments under pressure. We utilized a newly developed miniature piston-cylinder type pressure cell specially suited for measuring small samples at high-magnetic fields and low temperatures (0 GPa ≤ P ≤ 1.5 GPa, 0 kOe ≤ H ≤ 140 kOe, and 100 mK ≤ T ≤ 4 K). We found the second anomaly inside the SC state in CeCoIn5 can still be observed with pressure, which suppress the strong AFM-SF. The FFLO phase extends to higher fields and temperatures on applying pressure while the Pauli-limiting effect is becoming weaker and the SF are suppressed. This reveals the detrimental effect of the AFM-SF on the FFLO phase stability.