Two-dimensional cuprate high-temperature superconductivity

Dimensionality is of fundamental importance in cuprate high-temperature superconductivity: all cuprate superconductors adopt layered structure, and most high-temperature superconductivity theories are based on two-dimensional (2D) models. A monolayer Bi2201 contains only a single layer of CuO₂ plane, and therefore represents a cuprate superconductor in the ultimate 2D limit. Here, we probe the high-temperature superconductivity in La doped Bi2201 (Bi₂Sr_2-xLa_xCuO_6+δ) single crystals down to monolayer (i.e. half unit cell). We show that monolayer Bi2201 has a transition temperature T_c approximately 3 K lower than the bulk T_c, and the transport in the vortex liquid regime is also distinctively different from that in the bulk. The extreme thickness brings unprecedented tunability. We succeed in covering the entire phase diagram of Bi2201 with controlled oxygenation in a single monolayer sample. Taking advantages of such tunability, we discover an anomalous metal phase in the superconductor-insulator transition. Our results establish monolayer Bi2201 as a new two-dimensional material with highly tunable high-temperature superconductivity.