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

Y. Kitaoka, S. Kawasaki, Y. Kawasaki, T. Mito, G. Q. Zheng

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2 Citations (Scopus)

Abstract

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(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∼0.19GPa) 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 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.

Original languageEnglish
Article number125202
JournalJournal of Physics Condensed Matter
Volume19
Issue number12
DOIs
Publication statusPublished - Mar 6 2007

ASJC Scopus subject areas

  • Materials Science(all)
  • Condensed Matter Physics

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