Unconventional Superconductivity in the Novel Layered Superconductor Fe(Te-Se) Investigated by ${}^{125}$Te NMR on the Single Crystal

Abstract

We have performed ${}^{125}$Te NMR on a single crystal of the novel layered superconductor Fe${}_{1.04}$Te${}_{0.67}$Se${}_{0.33}$ for the first time. The spin parts of the Knight shifts for both $H//a$, $H//c$ are suppressed in the superconducting state, indicating the spin singlet superconductivity. At superconducting state, $1/T_1$ shows the power-law behavior ($\sim T^3$) without any coherent peaks. Observations of the residual density of state and the $T^3$-law indicate the presence of the line node in the superconducting gap. In the normal state, $1/T_{1}T$ which probes the \textbf{q}-summation of spin fluctuations enhances at low temperatures. Our results suggest that the superconductivity in Fe${}_{1+\delta }$Te${}_{1-x}$Se${}_x$ is stabilized by the growth of the antiferromagnetic spin fluctuations as well as in FeSe.

5 pages, 5figures