Anisotropic spin fluctuations in heavy-fermion superconductor CeCoIn ₅: In-NQR and Co-NMR studies

We report In-NQR and Co-NMR experiments of CeCoIn₅ 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/T₁ of ₁₁₅In is explained by the relation 1/T₁ ∝ T · χ_Q(T)^3/4 based on the anisotropic spin-fluctuations model in case of the proximity to an antiferromagnetic (AFM) quantum critical point (QCP). The novel behavior of 1/T₁ ∼ T ^1/4 over a wide T range of T_c < T < 40 K arises because the staggered susceptibility almost follows the Curie law χ_Q(T) ∝ 1/(T + θ) with θ = 0.6 K and hence 1/T₁ ∝ T/(T + 0.6)^3/4 ∼ T^1/4 for θ < T. We highlight that the behavior 1/T₁ ∼ 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 CeCoIn₅ 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 CeCoIn₅ is closely located at the QCP.