${\text{LaAlO}}_3/{\text{SrTiO}}_3$ oxide heterostructures studied by resonant inelastic x-ray scattering

We report the application of resonant inelastic x-ray scattering to explore the nature of the single conducting interface in the oxide heterostructure ${\text{LaAlO}}_3/{\text{SrTiO}}_3$. From the $\text{Ti}3d$ crystal-field excitations measured at the $\text{Ti}{L}_3$ 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 ${\text{LaAlO}}_3$ 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 $\text{Ti}3d$ electrons at the interface. On the other hand, we observe a saturation of the charge-carrier concentration above a ${\text{LaAlO}}_3$ thickness of 6 unit cells at $\sim 1\times {10}^{14}{\text{cm}}^{-2}$, well below the canonical value for ideal electronic reconstruction.

Figure 1

(Color online) X-ray absorption spectra at the $\text{Ti}{L}_3$ edge of undoped STO and of a LAO/STO heterostructure with a 5-uc-thick LAO overlayer. The energy axis has been calibrated with respect to the electron energy-loss data of Ref. 18.

Figure 2

(Color online) (a) RIXS overview spectra of STO and a LAO/STO (5 uc overlayer thickness) measured at high energy $\left(h\nu =459.4\text{eV}\right)$. (b) RIXS spectra of STO and LAO/STO (5 uc) measured at low energy $\left(h\nu =457.1\text{eV}\right)$, covering the range of the STO band gap. (c) The same for high energy. The peak at 2.35 eV results from the $t_{2g}-e_g$ crystal-field excitation of the $\text{Ti}3d$ electrons in the interfacial 2DES.

Figure 3

(Color online) RIXS spectra from several samples with LAO overlayers of different thickness, normalized to the integrated intensity of the $3d^1\underset{̱}{L}$ excitation [not shown here, see Fig. 2a]. The shaded peaks represent Gaussian fits to the $e_g$ peak. For reference, the spectrum of bare STO is also shown.

Figure 4

(Color online) Comparison of the normalized weights of the inelastic $e_g$ peak and the sheet carrier density ${\text{n}}_{2D}$ resulting from HAXPES and Hall measurements, taken from Refs. 5, 6, respectively.