Recent experiment has demonstrated that two CuO2 planes in a monolayer Bi2Sr2CaCu2O8+δ (Bi2212) contain all essential physics of high-temperature superconductivity . Here, we study cuprate superconductor in the two-dimensional limit—monolayer of Bi2Sr2CuO6+δ (Bi2201) that contains only one CuO2 plane. The extreme thickness brings unprecedented tunability; we succeed in covering the entire phase diagram of Bi2201 with controlled oxygenation in a single monolayer specimen, enabling us to demonstrate an anomalous metallic ground state between the superconducting and insulating region. Although the high-temperature superconductivity, along with various other correlated phenomena, persists in the monolayer, for the first time, we discover the dimensional effect in cuprate superconductors: optimal transition temperature in monolayer Bi2201 is 3~4 K lower than that of bulk samples. Our results establish monolayer Bi2201 as a new two-dimensional material with highly tunable high-temperature superconductivity and a minimal model system of cuprate superconductor.
 Yijun Yu, Liguo Ma, Peng Cai et al., Nature 575, 156–163 (2019).