${}^{77}\mathrm{Se}$ NMR Study of the Pairing Symmetry and the Spin Dynamics in ${\mathrm{K}}_y{\mathrm{Fe}}_{2-x}{\mathrm{Se}}_2$

We present a ${}^{77}\mathrm{Se}$ NMR study of the newly discovered iron selenide superconductor ${\mathrm{K}}_y{\mathrm{Fe}}_{2\mathrm{\text{−}}x}{\mathrm{Se}}_2$, in which $T_c=32\mathrm{K}$. Below $T_c$, the Knight shift ${}^{77}K$ drops sharply with temperature, providing strong evidence for singlet pairing. Above $T_c$, Korringa-type relaxation indicates Fermi-liquid behavior. Our experimental results set strict constraints on the nature of possible theories for the mechanism of high-$T_c$ superconductivity in this iron selenide system.

Figure 1

(a) dc magnetization of ${\mathrm{K}}_{0.82}{\mathrm{Fe}}_{1.63}{\mathrm{Se}}_2$ single crystal (sample $S1$), measured in a 10 Oe field under field-cooled (FC) and zero-field-cooled (ZFC) conditions. Inset: Tuning frequency $f$ of the NMR coil in a magnetic field of 11.764 T. The frequency shift below $T_c$ depends on the ac susceptibility, $\Delta f(T)\propto -{\chi }_{\mathrm{ac}}(T)$. Differing results for fields oriented along the $c$ axis or in the $ab$ plane indicate an anisotropic $H_{c2}$. (b) ${}^{77}\mathrm{Se}$ NMR spectra at different temperatures. The spectra broaden below $T_c$ and shift to lower frequencies.

Figure 2

(a) Temperature dependence of the ${}^{77}\mathrm{Se}$ Knight shift for the two samples. The solid line is a guide to the eye. (b) ${}^{77}K$ shown as a function of $T^2$; the solid line is a fit $K(T)=a+b{T}^2$. (c) Low-temperature behavior of ${}^{77}K$; the solid line is a guide to the eye. The dotted line denotes approximately the chemical contribution to the Knight shift (${}^{77}K$, see text).

Figure 3

Temperature dependence of the spin-lattice rate $1/{}^{77}T_1$ for both samples. Inset (a): Data shown as $1/{}^{77}T_{1}T$. Inset (b): $1/{}^{77}T_1$ shown on logarithmic axes at low temperatures. Solid lines indicate different types of power-law dependence ($1/T_1\sim T^n$), while the dotted line is a fit to an activated form $1/{}^{77}T_1=A\exp (-\Delta /k_{B}T)$ (see text).

Figure 4

Korringa plot of $(1/{}^{77}T_{1}T{)}^{0.5}$ against ${}^{77}K(T)$ with temperature as an implicit parameter. The solid line represents the Korringa relation.