The spatially averaged density of states N(0) of an unconventional d-wave superconductor is magnetic field dependent, proportional to H1 /2, owing to the Doppler shift of quasiparticle excitations in a background of vortex supercurrents. This phenomenon, called the Volovik effect, has been predicted to exist for a sign-changing s ± state, 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∞N (0 )2, for an s ± state which will depend on the magnetic field. We have measured the 75As 1/T 1 in a high-quality single crystal of Ba 0.67K 0.33Fe 2As 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 H2, inconsistent with recent theory.
|Original language||English (US)|
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|State||Published - May 11 2012|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics