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...

In LaAlO 3 =SrTiO 3 heterostructures, a still poorly understood phenomenon is that of electron trapping in back-gating experiments. Here, by combining magnetotransport measurements and self-consistent Schrödinger-Poisson calculations, we obtain an empirical relation between the amount of trapped electrons and the gate voltage. The amount of trapped electrons decays exponentially away from the interface. However, contrary to earlier observations, we find that the Fermi level remains well within the quantum well. The enhanced trapping of electrons induced by the gate voltage can therefore not be explained by a thermal escape mechanism. Further gate sweeping experiments strengthen that conclusion. We propose a new mechanism which involves the electromigration and clustering of oxygen vacancies in SrTiO 3 and argue that such electron trapping is a universal phenomenon in SrTiO 3-based two-dimensional electron systems.

By combined top-and backgating, we explore the correlation of superconductivity with band filling and electron confinement at the LaAlO 3 -SrTiO 3 interface. We find that the top-and backgate voltages have distinctly different effects on the superconducting critical temperature, implying that the confining potential well has a profound effect on superconductivity. We investigate the origin of this behavior by comparing the gate-dependence of T c to the corresponding evolution of the band filling with gate voltage. For several backgate voltages, we observe maximum T c to consistently coincide with a kink in tuning the band filling for high topgate voltage. Self-consistent Schrödinger-Poisson calculations relate this kink to a Lifshitz transition of the second d xy subband. These results establish a major role for confinement-induced subbands in the phase diagram of SrTiO 3 surface states, and establish gating as a means to control the relative energy of these states.

The two-dimensional electron system (2DES) formed at the interface of LaAlO 3 (LAO) and SrTiO 3 (STO), both band insulators in bulk, exhibits properties not easily attainable in conventional electronic materials. The extreme shallowness of the 2DES, only a few nanometers below the surface, opens up unique possibilities such as tunneling spectroscopy, local electronic sensing, and in situ patterning by manipulating the surface properties. It is particularly tempting to manipulate the charge carriers with surface acoustic wave (SAW) phonons, which are confined to the surface. However, the absence of intrinsic piezoelectricity in both LAO and STO complicates the electric generation of SAWs, as well as the induction of an acoustoelectric current. Here, we present robust acoustoelectric coupling between SAWs and the LAO/STO 2DES by using electrostriction in STO, induced by a dc electric field. Electromechanical coupling to the carriers is provided by phononinduced modulation of the 2DES potential well, leading to SAW-induced carrier transport. The ability to control charge carriers with SAWs brings the versatile LAO/STO 2DES into reach of quantum acoustics, opening possibilities to study the interplay of nanoscale mechanical waves and the rich physics exhibited by nonpiezoelectric complex oxides, including superconductivity, magnetism, and correlated insulator states.

Surface acoustic waves (SAWs) are capable tools for providing mechanical control over the electronic properties of functional materials. Coupling SAWs with the LaAlO 3 /SrTiO 3 (LAO/STO) conducting interface is particularly interesting as this interface exhibits extraordinary features, such as high mobility at low temperature, ferromagnetism, and superconductivity below 200 mK. For SAW generation, piezoelectricity is indispensable, and due to lack of that in the LAO/STO system, a 200 nm thick Pb(Zr 0.52 Ti 0.48 )O 3 (PZT) film was grown on top of LAO. SAW excitation and propagation was demonstrated on a PZT/LAO/STO multilayer structure. We further employed SAWs in order to transport free electrons confined to the LAO/STO interface, detected as an acoustoelectric voltage at room temperature. Electrical characterization of the interface was carried out by Van der Pauw measurements. We found that having a PZT layer on top of LAO/STO considerably degraded the interfacial conductivity. The degradation became more pronounced at low temperatures. We attribute these effects to the filling of oxygen vacancies due to interlayer oxygen migration, combined with carrier freeze-out at low temperatures.