Fe_1+δSe_xTe_1-xにおける超伝導と反強磁性量子臨界点近傍の物性

抄録

Macroscopic and microscopic physical properties of Fe_1+δSe_xTe_1-x were investigated through the magnetic susceptibility, electrical resistivity, heat capacity and nuclear magnetic resonance (NMR) measurements. The mother compound Fe_1.14Te shows an antiferromagnetic phase transition accompanied by the structural change at 61.5K. The antiferromagnetic transition temperature of Fe_1+δSe_xTe_1-x decreases with increasing x, then the superconducting transition appears above x = 0.2. The superconductivity with clean limit occurs when δ 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 Fe_1+δSe_xTe_1-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 Fe_1+δSe_xTe_1-x is mediated by the antiferromagnetic spin fluctuations.