Reentrant superconductivity from a phase-fluctuating state in a naturally occurring Josephson junction array tuned by RF current

Correlated electronic systems can exhibit emergent phenomena, challenging to predict from microscopic considerations alone. An exciting, yet not fully understood, example is reentrant superconductivity, which has recently been observed in strongly correlated materials that host competing phases, including UTe₂ and MATTG. Josephson junction arrays (JJAs) provide a versatile platform to simulate a wide range of quantum effects, including theoretically predicted reentrant superconductivity. Despite significant progress in tuning JJAs near the superconductor-insulator transition, experimental evidence has so far been limited to quasi-reentrant behavior, and clear observation of this effect has remained elusive. In this work, we investigate the naturally occurring JJA present in granular aluminum. It features a nanoscale junction size, enabling precise tuning with minimal charge screening. At low temperatures, we drive the system from a phase-coherent regime into a phase-fluctuating regime using RF currents. Under high DC and RF current conditions, we observe giant Shapiro steps, indicating single-junction-like behavior. At elevated temperatures, screening of long-range Coulomb interactions drives the emergence of a reentrant superconducting state from the phase-fluctuating regime. This transition reflects a crossover into a correlated many-body regime. Our results pave the way for using granular superconductors to develop novel RF-controlled quantum devices and explore emergent phenomena in strongly correlated systems.