Evidence for Unconventional Strong-coupling Superconductivity in PrOs_4Sb_12 : An Sb Nuclear Quadrupole Resonance (NQR) Study

Authors:
  • HKH. Kotegawa
    H. Kotegawa
  • MYM. Yogi
    M. Yogi
  • YIY. Imamura
    Y. Imamura
  • YKY. Kawasaki
    Y. Kawasaki
  • GZG. -Q. Zheng
    G. -Q. Zheng
  • YKY. Kitaoka
    Y. Kitaoka
  • SOS. Ohsaki
    S. Ohsaki
  • HSH. Sugawara
    H. Sugawara
  • YAY. Aoki
    Y. Aoki
  • HSH. Sato
    H. Sato
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Abstract
We report Sb-NQR results which evidence a heavy-fermion (HF) behavior and an unconventional superconducting (SC) property in the filled-skutterudite compound PrOs_4Sb_12 revealing a SC transition temperature T_c=1.85 K. The temperature (T) dependence of nuclear-spin-lattice-relaxation rate 1/T_1 and NQR frequency unravel a low-lying crystal-electric-field splitting below T_0~10 K, associated with Pr^3+ (4f^2)-derived ground state. The emergence of T_1T=const. behavior below T_F~4 K points to the formation of heavy-quasiparticle state. In the SC state, 1/T_1 shows neither a coherence peak nor a T^3like power-law behavior observed for HF superconductors to date. The isotropic energy-gap with a size of gap Delta/k_B=4.8 K begins to already open up at T^*~2.3 K without any coherence effect just below T_c=1.85 K. We highlight that the superconductivity in PrOs_4Sb_12, which is in an unconventional strong-coupling regime, differs from neither a conventional s-wave type nor any unconventional ones with the line-node gap.