On the low-energy excitations in superconductingYNi2B2CfromB11NMR relaxation around the critical field

Abstract

$^{11}\mathrm{B}$ NMR spectra, spin-echo dephasing and spin-lattice relaxation measurements in a single crystal of superconducting ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ at two temperatures $T⪡{T}_{c}\phantom{\rule{0.3em}{0ex}}(H=0)$, as a function of the external magnetic field $H$ are reported, particularly for $H$ close to the critical field ${H}_{c2}$. For $H⪢{H}_{c1}$ the spin lattice relaxation rate increases linearly with field, consistently with a density of states at the Fermi level going as ${H}^{1∕2}$. It is pointed out how a relaxation process weighted average of the contributions from the intravortex and the intervortex regions also accounts for the field and the temperature dependences of the relaxation rate. For $H\ensuremath{\approx}{H}_{c2}$ a small extra contribution, possibly related to novel excitations in the vortex cores or to the motion of the flux lines at the melting of the flux lattice, is detected. When the field-induced metallic state is attained $(H>{H}_{c2})$ the relaxation rate becomes field independent.