Electronic structure of the system ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4+\mathrm{\delta }}$

The electronic structure of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_{4+\mathrm{\delta }}$ has been studied by measuring O 1s absorption edges using high-energy electron-energy-loss spectroscopy in transmission. Upon doping the insulating compound the conduction-band states are reduced and states in the gap are formed. When going from insulating to conducting compounds there is a continuous increase of states at the Fermi level, which is at the bottom of the gap. The insulator-metal transition is probably driven by delocalization of these states.