Anisotropic Spin Fluctuations in Heavy-Fermion Superconductor CeCoIn_5 : In-NQR and Co-NMR Studies

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We report In-NQR and Co-NMR experiments of CeCoLn5 that undergoes a superconducting transition with a record high T_C = 2.3 K to date among heavy-fermion superconductors. At zero magnetic field, an anomalous temperature (T) dependence of nuclear spin-lattice relaxation rate 1/T_1 of ^<115>In is explained by the relation 1/T_1, ∝ T _<XQ>(T)^<3/4> based on the amsotropic spin-fluctuations model in case of the proximity to an antiferromagnetic (AFM) quantum critical point (QCP). The novel behavior of 1/T_1 〜 T^<1/4> over a wide T range of T_C < T < 40 K arises because the staggered susceptibility almost follows the Curie law _<XQ>(T) ∝ l/(T + θ) with θ = 0.6 K and hence 1/T_1 ∝ T/(T + 0.6)^<3/4> 〜 T^<1/4> forθ< T. We highlight that the behavior 1/T_1 〜 T^<1/4> is due to the proximity to the anisotropic AFM QCP relevant with its layered structure, and is not associated with the AFM QCP for isotropic 3D systems. We have also found that the AFM spin fluctuations in CeCoIn_5 are suppressed by small magnetic field so that θ = 0.6 K at H = 0 increases to θ = 2.5 K at H = 1.1 T, reinforcing that CeCoIn_5 is closely located at the QCP.
社団法人日本物理学会の論文
2003-09-15