The conducting interface of LaAlO3/SrTiO3 heterostructures has been studied by hard X-ray photoelectron spectroscopy. From the Ti 2p signal and its angle-dependence we derive that the thickness of the electron gas is much smaller than the probing depth of 4 nm and that the carrier densities vary with increasing number of LaAlO3 overlayers. Our results point to an electronic reconstruction in the LaAlO3 overlayer as the driving mechanism for the conducting interface and corroborate the recent interpretation of the superconducting ground state as being of the BerezinskiiKosterlitz-Thouless type.

The interface between LaAlO(3) and SrTiO(3) hosts a two-dimensional electron system of itinerant carriers, although both oxides are band insulators. Interface ferromagnetism coexisting with superconductivity has been found and attributed to local moments. Experimentally, it has been established that Ti 3d electrons are confined to the interface. Using soft x-ray angle-resolved resonant photoelectron spectroscopy we have directly mapped the interface states in k space. Our data demonstrate a charge dichotomy. A mobile fraction contributes to Fermi surface sheets, whereas a localized portion at higher binding energies is tentatively attributed to electrons trapped by O vacancies in the SrTiO(3). While photovoltage effects in the polar LaAlO(3) layers cannot be excluded, the apparent absence of surface-related Fermi surface sheets could also be fully reconciled in a recently proposed electronic reconstruction picture where the built-in potential in the LaAlO(3) is compensated by surface O vacancies serving also as a charge reservoir.

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We report the application of resonant inelastic x-ray scattering to explore the nature of the single conducting interface in the oxide heterostructure LaAlO 3 / SrTiO 3 . From the Ti 3d crystal-field excitations measured at the Ti L 3 resonance we not only derive information on the local geometry at the interface but are also able to follow the evolution of the sheet carrier density with the thickness of the LaAlO 3 overlayer. These findings confirm after calibration to previous hard x-ray photoelectron spectroscopy measurements that the charge density from spectroscopy exceeds the one derived from Hall-effect measurements, indicating the coexistence of itinerant and localized Ti 3d electrons at the interface. On the other hand, we observe a saturation of the charge-carrier concentration above a LaAlO 3 thickness of 6 unit cells at ϳ1 ϫ 10 14 cm −2 , well below the canonical value for ideal electronic reconstruction.

We present a detailed study of the electronic structure and band alignment in LaAlO 3 /SrTiO 3 oxide heterostructures by hard x-ray photoelectron spectroscopy. Our spectroscopic measurements find no evidence for the strong potential gradient within the polar LaAlO 3 film predicted by band theory. Due to the high interface sensitivity of the method, we are further able to determine the valence band offset between the LaAlO 3 film and the SrTiO 3 substrate, which is found to be independent of the number of LAO overlayers. Finally, we discuss several explanations for the apparent absence of the built-in field in ex situ prepared LaAlO 3 /SrTiO 3 heterostructures.