The heavy-fermion superconductor CeCoIn5 is the first material, where different experimental probes show strong evidence pointing to the realization of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. The inhomogeneous superconducting FFLO state wi
th a periodically modulated order parameter was predicted to appear in Pauli-limited, sufficiently clean type-II superconductors already more than 40 years ago. On the other hand, CeCoIn5 is supposed to be close to a magnetic quantum critical point (QCP) showing strong antiferromagnetic (AFM) spin fluctuations (SF) at atmospheric pressure. We studied the evolution of the FFLO phase away from the influence of the strong AFM-SF by heat capacity experiments under pressure (0 GPa <= P <= 1.5 GPa, 0 T <= mu_0 H <= 14 T, and 100 mK <= T <= 4 K). Our results prove the stability of the the FFLO phase under pressure. It even expands, while the Pauli-limiting becomes weaker and the AFM-SF are suppressed. This shows the intriguing influence of the AFM-SF on the FFLO state.
Our previous point-contact Andreev reflection studies of the heavy-fermion superconductor CeCoIn$_5$ using Au tips have shown two clear features: reduced Andreev signal and asymmetric background conductance [1]. To explore their physical origins, we
have extended our measurements to point-contact junctions between single crystalline heavy-fermion metals and superconducting Nb tips. Differential conductance spectra are taken on junctions with three heavy-fermion metals, CeCoIn$_5$, CeRhIn$_5$, and YbAl$_3$, each with different electron mass. In contrast with Au/CeCoIn$_5$ junctions, Andreev signal is not reduced and no dependence on effective mass is observed. A possible explanation based on a two-fluid picture for heavy fermions is proposed. [1] W. K. Park et al., Phys. Rev. B 72 052509 (2005); W. K. Park et al., Proc. SPIE-Int. Soc. Opt. Eng. 5932 59321Q (2005); W. K. Park et al., Physica C (in press) (cond-mat/0606535).
