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 counterparts1. The detailed nature of the orbital reconstruction critically affects the spatial confinement and the physical properties of the electrons occupying the interfacial orbitals2,3,4. Using an example of two-dimensional electron liquids forming at LaAlO3/SrTiO3 interfaces5,6 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 (~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 (~3.3 ± 0.3 nm) with the maximum density located ~2.4 nm away from the interface, in excellent agreement with density functional theory calculations.
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This work was supported by AFOSR Asian Office of Aerospace Research and Development (AOARD) under grant number FA2386-15-1-4046 (S.H.O. and C.B.E.) and AFOSR under grant number FA9550-15-1-0334 (C.B.E.). Research at SKKU was supported by the Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, the ICT and Future Planning (NRF-2015M3D1A1070672) (S.H.O.), an NRF grant funded by the Korean government (NRF-2015R1A2A2A01007904) (S.H.O.) and by the Ministryof Trade, Industry & Energy (MOTIE, Korea) under Industrial Technology Innovation Program (10080654) (S.H.O.). K.S. and S.-Y.C. acknowledge the support of the Fundamental Research of the Korean Institute of Materials Science (KIMS-PNK5260) and the Global Frontier Hybrid Interface Materials of the NRF funded by Korea Government (2013M3A6B1078872). Research at the University of Nebraska was supported by NSF MRSEC (grant no. DMR-420645). C.T.K. acknowledges support by the German Research Foundation (DFG grant KO 2911/12-1).