Stable and metastable vortex states and the first-order transition across the peak-effect region in weakly pinned 2H-NbSe₂

Abstract

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^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-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.