Vortex Glass and Lattice Melting Transitions in a YN${\mathrm{i}}_2$${\mathrm{B}}_2$C Single Crystal

The mixed state of the weakly disordered superconductor ${\mathrm{YNi}}_2{\mathrm{B}}_2\mathrm{C}$ is found to be similar to that of high- $T_c$ materials due to its large fluctuations. The very sharp resistive transition with a kink and the S-shaped $I-V$ curve appear below $H=$ 1 T indicating a lattice melting transition. The scaling behavior of the vortex glass transition in $R-T$ and $I-V$ curves for $H\ge 2$ T is observed with critical exponents $\nu =1.23\mathrm{}\pm 0.02$ and $z=5.46\mathrm{}\pm 0.32$, which are field independent. The vortex solid state changes from the vortex lattice at low fields to the vortex glass at high fields due to a field-enhanced pinning effect. The type of multicriticality is not yet conclusive.