Electron Doping of the Parent CuprateLa2CuO4without Cation Substitution

In the cuprates, carrier doping of the Mott insulating parent state is necessary to realize superconductivity as well as a number of other exotic states involving charge or spin density waves. Cation substitution is the primary method for doping carriers into these compounds, and is the only known method for electron doping in these materials. Here, we report electron doping without cation substitution in epitaxially stabilized thin films of ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$ grown via molecular-beam epitaxy. We use angle-resolved photoemission spectroscopy to directly measure their electronic structure and conclusively determine that these compounds are electron doped with a carrier concentration of $0.09\ifmmode\pm\else\textpm\fi{}0.02\text{ }{e}^{\ensuremath{-}}/\mathrm{Cu}$. We propose that intrinsic defects, most likely oxygen vacancies, are the sources of doped electrons in these materials. Our results suggest a new approach to electron doping in the cuprates, one which could lead to a more detailed experimental understanding of their properties.