Probing Re-entrant Vortex Melting Using STM and Low Frequency Electrodynamics

Re-entrant melting in superconductors, characterized by a vortex liquid–solid–liquid transition, has been theoretically proposed but lacks direct experimental evidence. We report the observation of re-entrant vortex melting in a 20 nm amorphous Re₆Zr thin film using scanning tunneling spectroscopy and magnetotransport measurements. At a low field of 3 kOe, the vortex ensemble evolves from a spatially inhomogeneous vortex liquid to an ordered vortex solid near 3 K, followed by re-melting into an isotropic liquid around 4 K. This re-entrant behavior vanishes at higher fields (≥20 kOe), where temperature only enhances vortex disorder. Mutual inductance measurements reveal two distinct dissipation peaks below 28 kOe, associated respectively with vortex melting and solidification. The low-temperature peak broadens with decreasing field and corresponds to a secondary maximum in the resistivity–temperature curve, indicative of the peak effect and an underlying order–disorder transition. The constructed H–T phase diagram delineates the evolution of re-entrant melting with pinning strength, showing suppression in strongly pinned (5 nm) films and pronounced behavior in weak-pinning regimes, suggesting the emergence of a hexatic-like vortex state.