Ordered, Disordered, and Coexistent Stable Vortex Lattices inNbSe2Single Crystals

The peak effect (PE) in the critical current density of type II superconductors has been related to an order-disorder transition in the vortex lattice (VL), but its underlying physics remains a controversial issue. Intrinsic to the PE are strong metastabilities that frequently mask the stationary VL configurations. We follow shaking and thermal protocols in ${\mathrm{NbSe}}_2$ single crystals to access these configurations and examine them by linear ac susceptibility measurements that avoid VL reorganization. We identify three different regions. For $T<T_1(H)$, stable VL configurations are maximally ordered. For $T>T_2(H)$, configurations are fully disordered and no metastability is observed. In the $T_1<T<T_2$ region, we find temperature-dependent stable configurations with an intermediate degree of disorder, possibly associated with the coexistence of ordered and disordered lattices throughout the PE.