Novel superconductivity on the magnetic criticality in heavy-fermion systems: a systematic study of NQR under pressure

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We report the discovery of exotic superconductivity (SC) and novel magnetism in heavy-fermion (HF) compounds, CeCu₂Si₂, CeRhIn₅ and CeIn₃ through nuclear-quadrupole-resonance (NQR) measurements under pressure (P). The exotic SC in a homogeneous CeCu₂Si₂ revealed antiferromagnetic critical fluctuations at the border to antiferromagnetism (AFM) or marginal AFM. 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 due 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. 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, we propose that the magnetic excitations such as spin-density fluctuations induced by the first-order phase transition from the AFM to the paramagnetism (PM) might mediate attractive interaction to form the Cooper pairs in the novel phase of AFM.