Direct $k$-Space Mapping of the Electronic Structure in an Oxide-Oxide Interface

The interface between ${\mathrm{LaAlO}}_3$ and ${\mathrm{SrTiO}}_3$ 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 ${\mathrm{SrTiO}}_3$. While photovoltage effects in the polar ${\mathrm{LaAlO}}_3$ 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 ${\mathrm{LaAlO}}_3$ is compensated by surface O vacancies serving also as a charge reservoir.

Figure 1

(a) Angle-integrated resonant photoemission spectra upon tuning the photon energy through the Ti $L$ absorption edge. (b) Ti $L$ absorption spectra, recorded in total electron yield mode, of LAO/STO (red open circles) and ${\mathrm{LaTiO}}_3$ (green open triangles, taken from Ref. 7). The photon energies used in the measurements displayed in (a) are indicated by dashed and solid lines. (c) On-resonance photoemission spectrum, displaying the complete valence band.

Figure 2

(a) Band map along the $\Gamma \mathrm{\text{−}}X\mathrm{\text{−}}\Gamma$ line of the Brillouin zone (BZ). (b) Same data as in (a) depicted as energy distribution curves. (c) Band map along a cut close to the $M\mathrm{\text{−}}\Gamma \mathrm{\text{−}}M$ line of the BZ (hence denoted by “$M\mathrm{\text{−}}\Gamma \mathrm{\text{−}}M$”). (d) Same data as in (c), depicted as energy distribution curves. All data were taken at $h\nu =460.20\mathrm{eV}$. White and red lines represent theoretical band dispersions (for details see text).

Figure 3

(a) FS map recorded at $h\nu =460.20\mathrm{eV}$. (b) Same map as (b) but with the cuts in the BZ corresponding to the data in Figs. 2a, 2b, 2c, 2d indicated and the FS sheets from DFT calculations overlaid.

Figure 4

(a) Schematic band diagram for the standard electronic reconstruction scenario in LAO/STO. (b) Possible situation where the polarization field in the LAO is screened by the separation of electron-hole pairs created upon illumination with x rays. (c) Tentative band situation with the inclusion of O vacancies at the LAO surface as a charge reservoir for electronic reconstruction.