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

Abstract

We report 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 marginal AFM. Remarkably, it has been found that the application of magnetic field induces an 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 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. It is demonstrated that SC does not yield any trace of gap opening in lowlying 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 into 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 the AFM and the HF SC takes place in the PM. We propose that the magnetic excitations such as spin-density fluctuations induced by the first-order phase transition from the AFM to the PM might mediate attractive interaction to form the Cooper pairs in the novel phase of AFM.

Effect of impurities scattering potential on NMR relaxation rate in impure d-wave superconductors

“…(15), (16) and Eqs. (17), (18) are used for numerical calculations of T À1 1 and ðTT 1 Þ À1 in strong scattering potential. The effect of strong scattering potentials are shown in Figs.…”

“…In heavy-fermion superconductors, such as CeCu 2 Si 2 , CeRhIn 5 , CeIn 3 [18], CeCoIn 5 [19] and else, there are the magnetic order in below T c region. The random magnetic orders can be considered as the impurities so they can decrease the critical temperature of superconductors [20].…”