Anisotropic spin fluctuations in heavy-fermion superconductor CeCoIn5:: 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/T-1 of In-115 is explained by the relation 1/T-1 proportional to T (.) chi(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/T-1 similar to T-1/4 over a wide T range of T-c < T < 40 K arises because the staggered susceptibility almost follows the Curie law chi(Q)(T) proportional to 1/(T + theta) with theta = 0.6 K and hence 1/T-1 proportional to T/(T + 0.6)(3/4) similar to T-1/4 for theta < T. We highlight that the behavior 1/T-1 similar to 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 theta = 0.6 K at H = 0 increases to theta = 2.5 K at H = 1.1 T, reinforcing that CeCoIn5 is closely located at the QCP.