Macroscopic physical properties and spin dynamics in the layered superconductor Fe₁₊Te_1−xSe_x

Macroscopic physical properties of the novel layered superconducting system Fe₁₊Te_1−xSe_x were investigated by means of magnetic-susceptibility, electric resistivity, and heat-capacity measurements. In addition to the substitution effect of the Te site, intercalated excess irons would suppress the bulk superconductivity. We have also performed ¹²⁵Te and ⁷⁷Se NMRs on a single crystal of Fe_1.04Te_0.67Se_0.33 with the highest superconducting transition temperature T_c~14  K. In the superconducting state, the spin part of Knight shifts for both Ha and Hc are suppressed, and the nuclear-spin-lattice relaxation rate 1/T₁ shows the power-law like behavior without any coherent peaks, indicating the nodal spin-singlet superconductivity. In the normal state, 1/T₁T is enhanced by antiferromagnetic spin fluctuations with decreasing temperature. The superconductivity in Fe₁₊Te_1−xSe_x is realized in the vicinity of the magnetic quantum phase transition and is possibly mediated by the growth of the antiferromagnetic spin fluctuations. ©2010 The American Physical Society