Interface Fermi states of LaAlO3/SrTiO3 and related heterostructures.

The interfaces of LaAlO3/SrTiO3 and (LaAlO3)(x)(SrTiO3)(1-x)/SrTiO3 heterostructures have been investigated by soft x-ray photoelectron spectroscopy for different layer thicknesses across the insulator-to-metal interface transition. The valence band and Fermi edge were probed using resonant photoemission across the Ti L(2,3) absorption edge. The presence of a Fermi-edge signal originating from the partially filled Ti 3d orbitals is only found in the conducting samples. No Fermi-edge signal could be detected for insulating samples below the critical thickness. Furthermore, the angular dependence of the Fermi intensity allows the determination of the spatial extent of the conducting electron density perpendicular to the interface.