Novel superconductivity at the magnetic critical point in heavy-fermion systems: a systematic study of NQR under pressure

We report on the discovery of exotic superconductivity (SC) and novel magnetism in heavy-fermion (HF) compounds, CeCu2Si2, CeRhIn5 and CeIn3, on the verge of antiferromagnetism (AFM) through nuclear-quadrupole-resonance (NQR) measurements under pressure (P). The exotic SC in a homogeneous CeCu2Si2 (Tc = 0.7 K) revealed antiferromagnetic critical fluctuations at the border to AFM or a marginal AFM. Remarkably, it has been found that the application of magnetic field induces a spin-density-wave (SDW) transition by suppressing the SC near the upper critical field. Furthermore, the uniform mixed phase of SC and AFM in CeCu2(Si1-xGex)2 emerges on a microscopic level, once a tiny amount of 1% Ge (x = 0.01) is substituted for Si to expand its lattice. The application of minute pressure (P∼0.19 GPa) suppresses the sudden emergence of the AFM caused by doping Ge. The persistence of the low-lying magnetic excitations at temperatures lower than Tc and TN is ascribed to the uniform mixed phase of SC and AFM. Likewise, the P-induced HF superconductor CeRhIn5 coexists with AFM on a microscopic level in P = 1.5-1.9 GPa. It is demonstrated that SC does not yield any trace of gap opening in low-lying excitations below the onset temperature, presumably associated with an amplitude fluctuation of superconducting order parameter. The unconventional gapless nature of SC in the low-lying excitation spectrum emerges due to the uniform mixed phase of AFM and SC. By contrast, in CeIn3, the P-induced phase separation of AFM and paramagnetism (PM) takes place without any trace for a quantum phase transition. The outstanding finding is that SC sets in at both the phases magnetically separated into AFM and PM in P = 2.28-2.5 GPa. A new type of SC forms the uniform mixed phase with AFM and the HF SC occurs in PM. We propose that the magnetic excitations such as spin-density fluctuations induced by the first-order phase transition from AFM to PM might mediate attractive interaction to form the Cooper pairs in the novel phase of AFM