Stable and metastable vortex states and the first-order transition across the peak-effect region in weakly pinned $2H-{\mathrm{NbSe}}_2$

The peak effect in weakly pinned superconductors is accompanied by metastable vortex states. Each metastable vortex configuration is characterized by a different critical current density $J_c,$ which mainly depends on the past thermomagnetic history of the superconductor. A recent model [G. Ravikumar et al., Phys. Rev. B $61,$ R6479 (2000)] proposed to explain the history-dependent $J_c,$ postulates a stable state of vortex lattice with a critical current density $J_c^{\mathrm{st}}$ determined uniquely by the field and temperature. In this paper, we present evidence for the existence of the stable state of the vortex lattice in the peak-effect region of $2H-{\mathrm{NbSe}}_2.$ It is shown that this stable state can be reached from any metastable vortex state by cycling the applied field by a small amplitude. The minor magnetization loops obtained by repeated field cycling allow us to determine the pinning and “equilibrium” properties of the stable state of the vortex lattice at a given field and temperature unambiguously. The data imply the occurrence of a first-order phase transition from an ordered phase to a disordered vortex phase across the peak effect.