Artificial quantum confinement in $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures

Heterostructures of transition metal oxides perovskites represent an ideal platform to explore exotic phenomena involving the complex interplay between the spin, charge, orbital and lattice degrees of freedom available in these compounds. At the interface between such materials, this interplay can lead to phenomena that are present in none of the original constituents such as the formation of the interfacial two-dimensional electron system (2DES) discovered at the $\mathrm{LA}{\mathrm{O}}_3/\mathrm{ST}{\mathrm{O}}_3$ (LAO/STO) interface. In samples prepared by growing a LAO layer onto a STO substrate, the 2DES is confined in a band bending potential well, whose width is set by the interface charge density and the STO dielectric properties, and determines the electronic band structure. Growing LAO (2 nm) /STO $(x$ nm)/LAO (2 nm) heterostructures on STO substrates allows us to control the extension of the confining potential of the top 2DES via the thickness of the STO layer. In such samples, we explore the electronic structure trend under an increase of the confining potential with using soft x-ray angle-resolved photoemission spectroscopy combined with ab initio calculations. The results indicate that varying the thickness of the STO film modifies the quantization of the $3d{t}_{2\mathrm{g}}$ bands and, interestingly, redistributes the charge between the $d_{xy}$ and $d_{xz}/d_{yz}$ bands.

Figure 1

(a) Scheme of the samples, and an AFM image showing the typical topography of the topmost layer. (b) X-ray diffraction data of an example LAO (10 u.c.)/STO (35 u.c.)/LAO (5 u.c.) sample, showing the 002 reflection of the substrate together with the fringes originated from the ultrathin films. Data are compared with a calculated pattern using the following out-of-plane axis values: c(LAO, 5 u.c.) = 3.773 A, c(STO, 35 u.c.) = 3.91 Å, c(LAO, 10 u.c.) = 3.766 A.

Figure 2

(a) Shows a schematic of the electronic structure along the $\mathrm{\Gamma }\text{−}\mathrm{X}$ direction of the 2DES of a “standard” bulk LAO/STO interface. (b)–(d) are the experimental electronic structures for the $n=20$, 10, and 5 u.c. samples (marked on the panel).

Figure 3

Detail of the electronic structure (left panel) of the $n=5\mathrm{u}.\mathrm{c}.$ sample along the $\mathrm{\Gamma }\text{−}\mathrm{X}$ direction. The blue dashed lines represent the integration window of the EDC (right panel) plotted together with the fitting results. The horizontal dashed lines highlight the double-peak structure (see text for details).

Figure 4

(a) Band structure calculated for LAO/9.5 u.c. STO slab (see Supplemental Material). (b) Decomposition in the Mulliken scheme electron charge for different 2 u.c. LAO/ $n$ u.c. STO superlattices $(n=2$, 4, 8, 12, 30, see Supplemental Material) in the different Ti sites. The labeling of the Ti sites starts at 0 for the first $\mathrm{Ti}{\mathrm{O}}_2$ next to the LAO plane. Data are interpolated by the mean of cubic splines as a guide to the eye.