  Rich functionalities of transition-metal oxides and their interfaces bear an
enormous technological potential. Its realization in practical devices
requires, however, a significant improvement of yet relatively low electron
mobility in oxide materials. Recently, a mobility boost of about two orders of
magnitude has been demonstrated at the spinel/perovskite {\gamma}-Al2O3/SrTiO3
interface compared to the paradigm perovskite/perovskite LaAlO3/SrTiO3. We
explore the fundamental physics behind this phenomenon from direct measurements
of the momentum-resolved electronic structure of this interface using resonant
soft-X-ray angle-resolved photoemission. We find an anomaly in orbital ordering
of the mobile electrons in {\gamma}-Al2O3/SrTiO3 which depopulates electron
states in the top STO layer. This rearrangement of the mobile electron system
pushes the electron density away from the interface that reduces its overlap
with the interfacial defects and weakens the electron-phonon interaction, both
effects contributing to the mobility boost. A crystal-field analysis shows that
the band order alters owing to the symmetry breaking between the spinel
{\gamma}-Al2O3 and perovskite SrTiO3. The band-order engineering exploiting the
fundamental symmetry properties emerges as another route to boost the
performance of oxide devices.
