The two-dimensional electron system at the interface between LaAlO 3 and SrTiO 3 has several unique properties that can be tuned by an externally applied gate voltage. In this work, we show that this gate tunability extends to the effective band structure of the system. We combine a magnetotransport study on top-gated Hall bars with self-consistent Schrödinger-Poisson calculations and observe a Lifshitz transition at a density of 2.9 × 10 13 cm −2 . Above the transition, the carrier density of one of the conducting bands decreases with increasing gate voltage. This surprising decrease is accurately reproduced in the calculations if electronic correlations are included. These results provide a clear, intuitive picture of the physics governing the electronic structure at complex-oxide interfaces. DOI: 10.1103/PhysRevLett.118.106401 The two-dimensional electron system (2DES) at the interface between the band insulators LaAlO 3 (LAO) and SrTiO 3 (STO) displays many intriguing phenomena, which may be harnessed for novel electronic devices [1][2][3][4][5]. The discovery of superconductivity [6], magnetic signatures [7][8][9][10], and their apparent coexistence [11] sparked growing interest in this material system. These properties can be tuned by varying parameters during growth [4,7], as well as by an externally applied electric field after growth [12]. Using this field effect, control of superconductivity [13][14][15][16][17], of spin-orbit coupling [17][18][19][20], and of carrier mobility [21,22] has been reported. Recent progress on local control of superconductivity [16] opened a route towards electrically controlled oxide Josephson junctions [23,24], providing new opportunities for superconducting electronic devices. Because these phenomena are related to the interfacial band structure, a fundamental understanding of the band structure is vital for the understanding of these phenomena and their exploitation in electronic devices.The interface band structure is formed by the conduction band of STO, which is bent down at the interface and crosses the Fermi level [25]. The origin of the band bending is still an open question [26][27][28]. This band bending creates a potential well, confining the carriers to a few nanometers in the out-of-plane direction [29][30][31][32]. In the well, the effective band structure is formed by the Ti t 2g orbitals. For interfaces grown along the [001] direction, the in-plane oriented d xy bands lie below the out-of-plane oriented d yz;xz bands in energy due to the confinement, as measured using x-ray absorption [33].By backgating the interface through the STO substrate, an additional conduction channel was observed to emerge above a carrier density of ð1.7 AE 0.1Þ × 10 13 cm −2 [34]. This observation was linked to tuning the Fermi level across the bottom of the d yz;xz bands, making additional electron pockets available for conduction. As a result, the Fermi surface topology changes, which is the characteristic feature of a Lifshitz transition [35].The model proposed by Joshua et...