Anisotropic Spin Fluctuations in Heavy-Fermion Superconductor CeCoIn₅: In-NQR and Co-NMR Studies

Abstract

We report In-NQR and Co-NMR experiments of CeCoIn5 that undergoes a superconducting transition with a record high Tc = 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 115 In is explained by the relation 1/T1 ∝ T · χQ(T ) 3/4 based on the anisotropic spinfluctuations 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 Tc < T < 40 K arises because the staggered susceptibility almost follows the Curie law χQ(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.

Microscopic investigation of electronic inhomogeneity induced by substitutions in a quantum critical metalCeCoIn5

“…The crossover temperature T * between localized and itinerant regimes is ∼150 K in CeCoIn 5 [29]. As reported earlier [30], well below T * , 1/T 1 shows a characteristic T 1/4 dependence above T c = 2.3 K due to anisotropic AFM spin fluctuations associated with proximity to an AFM QCP. In addition, the field dependence of 1/T 1 T in CeCoIn 5 is well-understood quantitatively by the selfconsistent renormalization (SCR) theory for an itinerant SDW model of criticality [5].…”

Enhancing the Superconducting Transition Temperature of the Heavy Fermion CompoundCeIrIn5in the Absence of Spin Correlations

“…The nuclearspin-lattice-relaxation rate(1/T 1 ) was found to follow the relation of 1/T 1 T ∝ 1/(T + θ) 3/4 with a small value of θ = 8 K [4]. The same analysis was applied to CeCoIn 5 with resulting θ = 0.6 K [9,11]. Note that θ is a measure to what extent the system is close to an antiferromagnetic quantum critical point (QCP) [12].…”

Extremely strong-coupling superconductivity in artificial two-dimensional Kondo lattices

“…Despite its layered structure, the largely corrugated Fermi surface 13 , 3D-like antiferromagnetic fluctuations in the normal state 14 , and small anisotropy of upper critical field 15 , all indicate that the electronic, magnetic and superconducting properties are essentially 3D rather than 2D. Therefore it is still unclear to which extent the 3D nature is essential for the superconductivity of CeCoIn 5 .…”