Stable and metastable vortex states and the first-order transition across the peak-effect region in weakly pinned 2H-NbSe<SUB>2</SUB>

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 Jc, 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 Jc, postulates a stable state of vortex lattice with a critical current density Jcst 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-NbSe2. 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