Superconductivity driven by orbital rearrangement in La$_{2}$CuO$_{4}$

La$_{2}$CuO$_{4}$ is known as the parent compound of hole-doped high temperature superconductors. In La$_{2}$CuO$_{4}$, Cu and O ions form CuO$_{2}$ planes in which superconductivity takes place. It is also known that those Cu ions are octahedrally coordinated with strongly stretched octahedrons along the c-axis of the unit cell owing to the Jahn-Teller effect. Such a system is an antiferromagnetic insulator and superconductivity is induced by hole doping, e.g. Sr or Ba. The arrangement of O around Cu can be altered into a square-plane by state-of-the-art thin film growth techniques thus leaving both of the apical sites vacant. We show that the conversion from La$_{2}$CuO$_{4}$ with octahedral coordinated copper into square-planar coordinated copper triggers an insulator-to-metal transition. This insulator-metal transition is induced via an orbital rearrangement that takes place due to reconfigured oxygen sublattices. More importantly, the metallic La$_{2}$CuO$_{4}$ with square-planar coordinated copper shows a superconducting transition at 28\,K which is essentially identical to Nd$_{2}$CuO$_{4}$ or Pr$_{2}$CuO$_{4}$. These results emphasize that the parent compounds of electron-doped cuprate superconductors are superconducting \textit{per se}.