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 (T _c ≤ 0.7K) 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(Si_1-xGe_x) 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∼0.19GPa) suppresses the sudden emergence of the AFM caused by doping Ge. The persistence of the low-lying magnetic excitations at temperatures lower than T_c and T_N is ascribed to the uniform mixed phase of SC and AFM. Likewise, the P-induced HF superconductor CeRhIn ₅ coexists with AFM on a microscopic level in P ≤ 1.5-1.9GPa. 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 ≤ 2.28-2.5GPa. 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.