Superconductivity and Quantum Criticality in Fe1+SexTe1-x
Chishiro MICHIOKA and Kazuyoshi YOSHIMURA
Abstract:Macroscopic and microscopic physical properties of Fe1+SexTe1-x were investigated through the magnetic susceptibility, electrical resistivity, heat capacity and nuclear magnetic resonance (NMR) measurements. The mother compound Fe1.14Te shows an antiferromagnetic phase transition accompanied by the structural change at 61.5 K. The antiferromagnetic transition temperature of Fe1+SexTe1-x decreases with increasing x, then the superconducting transition appears above x = 0.2. The superconductivity with clean limit occurs when is small. In such a compound, the temperature dependence of the nuclear spin - lattice relaxation rate and the electron contributed specific heat reveals presence of the nodal superconducting gap structure, suggesting that the superconductivity occurs in an unconventional mechanism. From a systematic investigation of the nuclear spin - lattice relaxation rate in Fe1+SexTe1-x, it was found that the antiferromagnetic quantum critical point lies at x `0.03. The superconductivity occurs in the vicinity of the antiferromagnetic quantum critical point, resulting in that the superconductivity in Fe1+SexTe1-x is mediated by the antiferromagnetic spin fluctuations. Key Words:Magnetism, Nuclear magnetic resonance, Itinerant electron system, Superconductivity, Iron chalcogenide