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

Y. Kitaoka; S. Kawasaki; Y. Kawasaki; T. Mito; G. -q. Zheng

paper

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

arXiv:cond-mat/0405348

Abstract

We report the discovery of exotic superconductivity (SC) and novel magnetism in heavy-fermion (HF) compounds, CeCu $_{2}$ Si $_{2}$ , CeRhIn $_{5}$ and CeIn $_{3}$ through nuclear-quadrupole-resonance (NQR) measurements under pressure ( $P$ ). The exotic SC in a homogeneous CeCu $_{2}$ Si $_{2}$ revealed antiferromagnetic critical fluctuations at the border to antiferromagnetism (AFM) or marginal AFM. The uniform mixed phase of SC and AFM in CeCu $_{2}$ (Si $_{1 - x}$ Ge $_{x}$ ) $_{2}$ 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 ( $P \sim 0.19$ GPa) suppresses the sudden emergence of the AFM caused by doping Ge. The persistence of the low-lying magnetic excitations at temperatures lower than $T_{c}$ and $T_{N}$ is ascribed due 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 $P = 1.5$ - 1.9 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 CeIn $_{3}$ , 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.

11 pages, 13 figures