Quasiparticle excitation in and around the vortex core of underdoped YBa2Cu4O8 studied by site-selective NMR -: art. no. 060503

We report a site-selective O-17 spin-lattice relaxation rate T-1(-1) in the vortex state of underdoped YBa2Cu4O8, We found that T-1(-1) at the planar sites exhibits an unusual nonmonotonic NMR frequency dependence. In the region well outside the vortex core, T-1(-1) cannot be simply explained by the density of states of the Doppler-shifted quasiparticles in the d-wave superconductor. Based on T-1(-1) in the vortex core region, we establish strong evidence that the local density of states within the vortex core is strongly reduced.