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 T2 in the field-cooled superconducting state of the type-II superconductor La3Ni2B2N3−x shows evidence for two fluctuation modes of the weakly pinned vortex matter dominating T2 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 Tc, with a broad crossover region in between where a more complex theory is required. We determine the melting temperature Tm = 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 T2 near Tm by the viscous vortex liquid appears to be more consistent. It indicates that the plastic deformation mode might be important for T2 in the mixed state of other type-II superconductors in the weak pinning regime near Tc as well. (paper)