Macroscopic physical properties and spin dynamics in the layered superconductor Fe1+δTe1-xSex

Macroscopic physical properties of the novel layered superconducting system Fe1+delta Te1-xSex 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 Te-125 and Se-77 NMRs on a single crystal of Fe1.04Te0.67Se0.33 with the highest superconducting transition temperature T-c similar to 14 K. In the superconducting state, the spin part of Knight shifts for both H parallel to a and H parallel to c are suppressed, and the nuclear-spin-lattice relaxation rate 1/T-1 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+delta Te1-xSex is realized in the vicinity of the magnetic quantum phase transition and is possibly mediated by the growth of the antiferromagnetic spin fluctuations.