1109.3834 (Sangwon Oh et al.)

Magnetic field dependence of spin-lattice relaxation in the s$\pm$ state
of Ba$_{0.67}$K$_{0.33}$Fe$_{2}$As$_{2}$    [PDF]

Sangwon Oh, A. M. Mounce, W. P. Halperin, C. L. Zhang, Pengcheng Dai, A. P. Reyes, P. L. Kuhns

The spatially averaged density of states, , 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
$, 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]

View original: http://arxiv.org/abs/1109.3834