Electron Doping by Charge Transfer at LaFeO$_3$/Sm$_2$CuO$_4$ Epitaxial Interfaces

We examine the interfacial charge transfer in epitaxial heterostructures formed between Mott insulating Sm$_2$CuO$_4$ (SCO) and charge transfer insulator LaFeO$_3$ (LFO) in LFO/SCO superlattices. High resolution EELS measurements at the O-K edge have provided evidence for 0.09$+$/-0.01 extra electrons in the SCO d- band as revealed by a reduction of the Cu oxidation state. The transfer of electrons from LFO to SCO is further supported by the spectroscopic signature of Cu$^{1+}$ as obtained from XAS measurements. Transport measurements have evidenced a metallic state at the interface between two nominally insulating materials. Dielectric spectroscopy measurements have allowed ascribing the metallic state to the LFO/SCO interfaces, consistent with DC measurements. When lowering the temperature a metal to insulator transition occurs at 120 K, indicating, in accordance with the phase diagram, an insufficient doping level to enter a superconducting state.