Magnetic field dependence of spin-lattice relaxation in the s$\pm$ state   of Ba$_{0.67}$K$_{0.33}$Fe$_{2}$As$_{2}$
  The spatially averaged density of states, <N(0)>, of an unconventional d-wave superconductor is magnetic field dependent, proportional to $H^{1/2}$, owing to the Doppler shift of quasiparticle excitations in a background of vortex supercurrents[1,2]. This phenomenon, called the Volovik effect, has been predicted to exist for a sign changing $s\pm$ state [3], although it is absent in a single band s-wave superconductor. Consequently, we expect there to be Doppler contributions to the NMR spin-lattice relaxation rate, $1/T_1 \propto <N(0)^2>$, for an $s\pm$ state which will depend on magnetic field. We have measured the $^{75}$As $1/T_1$ in a high-quality, single crystal of Ba$_{0.67}$K$_{0.33}$Fe$_{2}$As$_{2}$ over a wide range of field up to 28 T. Our spatially resolved measurements show that indeed there are Doppler contributions to $1/T_1$ which increase closer to the vortex core, with a spatial average proportional to $H^2$, inconsistent with recent theory [4] 
Sangwon Oh, A. M. Mounce, W. P. Halperin, C. L. Zhang, Pengcheng Dai,   A. P. Reyes, P. L. Kuhns
cond-mat.supr-con cond-mat.str-el