Vortex matter in the type-II superconductor La3Ni2B2N3−x in the light of NMR

A quantitative analysis of spin-echo NMR measurements of the local field distribution and the spin–spin relaxation time T _2 in the field-cooled superconducting state of the type-II superconductor La _3 Ni _2 B _2 N _3− _x shows evidence for two fluctuation modes of the weakly pinned vortex matter dominating T _2 at different temperatures: long wavelength diffusive modes in the vortex solid at low temperature, and plastic deformation of the viscous liquid close to the critical temperature T _c , with a broad crossover region in between where a more complex theory is required. We determine the melting temperature T _m = 11.5 K of the vortex matter as the temperature where the characteristic time of the plastic deformation slows down to the time scale set by the spin-echo experiment. This is preferred to the conventional estimates from a Lindemann criterion for the melting vortex solid since the quantitative description of T _2 near T _m by the viscous vortex liquid appears to be more consistent. It indicates that the plastic deformation mode might be important for T _2 in the mixed state of other type-II superconductors in the weak pinning regime near T _c as well.