Spin lattice relaxation rate measurements in Ba$_{0.69}$K$_{0.31}$Fe$_{2}$As$_{2}$ by nuclear magnetic resonance

Magnetic impurities have been a problem with NMR measurements of single crystals in the K doped Ba-122 system because of extremely wide linewidth that can be more than 1MHz at low temperature [1]. We have a high quality single crystal of Ba$_{0.69}$K$_{0.31}$Fe$_2$As$_2$ ($T_c$ = 34K) for which the NMR linewidth does not significantly increase at low temperatures and at very large external magnetic fields. In this sample we measure the spin-lattice relaxation rate, $1/T_1$, from 300 K to 4 K at various magnetic fields 6.4 T, 13 T, and 16 T. The rapid increase of $1/T_1T$ down to $T_c$ on cooling can be attributed to spin fluctuations above $T_c$. In the superconducting state, $1/T_1$ has a kink around 20 K, and below this temperature in a field of 13 T it exhibits a power law dependence, $\backslash$varpropto T$^3$. This behavior can be explained by an impurity effect in a superconductor with extended s-wave symmetry [2]. \\[4pt] [1] S. Mukhopadhyay {\it et al.} New J. Phys. {\bf 11}, 055002 (2009)\\[0pt] [2] Y. Bang {\it et al.} Phys. Rev. B {\bf 79}, 054529 (2009)