Direct imaging of the electron liquid at oxide interfaces

The breaking of symmetry across an oxide heterostructure causes the electronic orbitals to be reconstructed at the interface into energy states that are different from their bulk counterparts(1). The detailed nature of the orbital reconstruction critically affects the spatial confinement and the physical properties of the electrons occupying the interfacial orbitals(2-4). Using an example of two-dimensional electron liquids forming at LaAlO3/SrTiO3 interfaces(5,6) with different crystal symmetry, we show that the selective orbital occupation and spatial quantum confinement of electrons can be resolved with subnanometre resolution using inline electron holography. For the standard (001) interface, the charge density map obtained by inline electron holography shows that the two-dimensional electron liquid is confined to the interface with narrow spatial extension (similar to 1.0 +/- 0.3 nm in the half width). On the other hand, the two-dimensional electron liquid formed at the (111) interface shows a much broader spatial extension (similar to 3.3 +/- 0.3 nm) with the maximum density located similar to 2.4 nm away from the interface, in excellent agreement with density functional theory calculations.