Superconducting 2D cuprate with a single CuO₂ plane

Atomically thin van der Waals crystals epitomize ideal material systems in the two-dimensional (2D) limit. This reduction in dimensionality often leads to important consequences, best exemplified by the emergence of new physics in graphene and other 2D materials that can be readily tuned by gating^1,2. Vast opportunities arise in extending this top-down approach to other material systems. Recent experiments have demonstrated that the essential physics of high-temperature superconductivity in cuprates is contained within just two CuO₂ planes³. Here we push dimensionality reduction to the extreme by examining a single layer of Bi₂Sr₂CuO_6+δ (Bi-2201), which comprises only one CuO₂ plane. In this ultimate 2D limit, we observe a robust dimensionality effect that manifests as an approximately 10% reduction in the optimal superconducting transition temperature. Moreover, this reduction in dimensionality offers unprecedented tunability-we successfully extended the phase diagram of Bi-2201 into uncharted territories via finely controlled oxygenation of single-monolayer specimens. Leveraging this tunability, we discovered that an anomalous metal state emerges between the insulating and superconducting states as the temperature approaches zero. Concurrently, we observe an anomalous scaling behaviour characterized by a divergent critical exponent. These findings illuminate the nature of the superconductor-to-insulator quantum phase transition in cuprates.