Novel two-dimensional electron systems at the interfaces of oxide heterostructures, such as LaAlO3/SrTiO3, have attracted much attention as they open a new route to harness the rich quantum phases of transition-metal oxides (TMOs) for potentially useful functionalities not available in conventional semiconductor electronics. For such applications, the controllability of these interface properties is key. For LaAlO3/SrTiO3, previous theoretical and experimental investigations of the band offset and the potential profile near the interface have yielded not only quantitatively different but sometimes even contradictory results, e.g., the absence vs presence of a potential gradient in the LaAlO3 film. By analyzing angle-dependent hard x-ray photoelectron spectroscopy (HAXPES) data with a Poisson-Schrodinger model, we determine the charge carrier distribution and the valence band edge profile across the LaAlO3/SrTiO3 interface self-consistently. By systematically controlling the oxygen vacancy concentration, i.e., the doping level, during the photoemission experiments, we derive a comprehensive picture of the band scheme and show that the two-dimensional electron system is always narrowly confined to the interface. We observe a crossover of the band alignment from type II to type I with increasing doping level, which reconciles the striking inconsistencies among the earlier studies. We further find that the strongly nonlinear dielectric response of the SrTiO3 substrates to the electric field is essential for the understanding of the band arrangement at the LaAlO3/SrTiO3 heterointerface.

Novel two-dimensional electron systems at the interfaces and surfaces of transition-metal oxides recently have attracted much attention as they display tunable, intriguing properties that can be exploited in future electronic devices. Here we show that a high-mobility quasi-two-dimensional electron system with strong spin-orbit coupling can be induced at the surface of a KTaO_3 (001) crystal by pulsed laser deposition of a disordered LaAlO_3 film. The momentum-resolved electronic structure of the buried electron system is mapped out by hard x-ray angle-resolved photoelectron spectroscopy. From a comparison to calculations it is found that the band structure deviates from that of electron-doped bulk KTaO_3 due to the confinement to the interface. Nevertheless, the Fermi surface appears to be clearly three-dimensional. From the k broadening of the Fermi surface and core-level depth profiling we estimate the extension of the electron system to be at least 1 nm but not much larger than 2 nm, respectively.

Abstract Thin films of transition metal oxides open up a gateway to nanoscale electronic devices beyond silicon characterized by novel electronic functionalities. While such films are commonly prepared in an oxygen atmosphere, they are typically considered to be ideally terminated with the stoichiometric composition. Using the prototypical correlated metal SrVO3 as an example, it is demonstrated that this idealized description overlooks an essential ingredient: oxygen adsorbing at the surface apical sites. The oxygen adatoms, which are present even if the films are kept in an ultrahigh vacuum environment and not explicitly exposed to air, are shown to severely affect the intrinsic electronic structure of a transition metal oxide film. Their presence leads to the formation of an electronically dead surface layer but also alters the band filling and the electron correlations in the thin films. These findings highlight that it is important to take into account surface apical oxygen or—mutatis mutandis—the specific oxygen configuration imposed by a capping layer to predict the behavior of ultrathin films of transition metal oxides near the single unit‐cell limit.

Depositing disordered Al on top of SrTiO3 is a cheap and easy way to create a two-dimensional electron system in the SrTiO3 surface layers. To facilitate future device applications, we passivate the heterostructure by a disordered LaAlO3 capping layer to study the electronic properties by complementary x-ray photoemission spectroscopy and transport measurements on the very same samples. We also tune the electronic interface properties by adjusting the oxygen pressure during film growth.

The heterostructure consisting of the Mott insulator LaVO$_3$ and the band insulator SrTiO$_3$ is considered a promising candidate for future photovoltaic applications. Not only does the (direct) excitation gap of LaVO$_3$ match well the solar spectrum, but its correlated nature and predicted built-in potential, owing to the non-polar/polar interface when integrated with SrTiO$_3$, also offer remarkable advantages over conventional solar cells. However, experimental data beyond the observation of a thickness-dependent metal-insulator transition is scarce and a profound, microscopic understanding of the electronic properties is still lacking. By means of soft and hard X-ray photoemission spectroscopy as well as resistivity and Hall effect measurements we study the electrical properties, band bending, and band alignment of LaVO$_3$/SrTiO$_3$ heterostructures. We find a critical LaVO$_3$ thickness of five unit cells, confinement of the conducting electrons to exclusively Ti 3$d$ states at the interface, and a potential gradient in the film. From these findings we conclude on electronic reconstruction as the driving mechanism for the formation of the metallic interface in LaVO$_3$/SrTiO$_3$.

BaBiO3 (BBO) is well known as the parent material for the high-T-c superconducting compounds Ba1-xKxBiO3 and BaPb1-xBixO3. In its pristine state, BBO is a charge-ordered (CO) insulator, resulting from a static breathing distortion of the BiO6 octahedra with alternating long and short bond lengths. Recently, it has been reported that the CO state is suppressed for BBO films grown on SrTiO3 (STO) below a thickness of approximately 4 nm, possibly resulting in a metallic phase. While we do confirm structural modifications in our BBO/Nb:STO samples in this thickness range by Raman spectroscopy and electron diffraction, in situ photoemission evidences that these changes are accompanied by a Bi deficit and that the films remain insulating. We hence conclude that, in line with previous findings for the BBO/STO interface, the thickness-controlled suppression of the CO state is not purely driven by the two-dimensional confinement but rather originates from modifications of the composition and structure inherent to the epitaxial growth of BBO on SrTiO3 (001).

Ferromagnetism at LaAlO3/SrTiO3 interfaces has so far been studied by various experimental methods to determine the magnetic origin of the domain structure. Here, we successfully observe granular ferromagnetic domains by laser-excited photoemission electron microscopy, which enables magnetic imaging with a high spatial resolution and a high sensitivity to the interfacial states. The LaAlO3 thickness dependence of ferromagnetic domain formation above 3 unit cells is consistent with the formation of a two-dimensional electron gas. Furthermore, the domain structure does not show temperature dependence between 15 and 300 K, but we observe a strong correlation with the density of oxygen vacancies. From our results, we infer that the localized carriers induced by oxygen vacancies act as magnetic moments, which are coupled by indirect exchange interaction through itinerant carriers of the two-dimensional electron gas at the LaAlO3/SrTiO3 interface.

Ferromagnetism at LaAlO3/SrTiO3 interfaces has so far been studied by various experimental methods to determine the magnetic origin of the domain structure. Here, we successfully observe granular f...

The Mott transistor is a paradigm for a new class of electronic devices—often referred to by the term Mottronics—which are based on charge correlations between the electrons. Since correlation‐induced insulating phases of most oxide compounds are usually very robust, new methods have to be developed to push such materials right to the boundary to the metallic phase in order to enable the metal–insulator transition to be switched by electric gating. Here, it is demonstrated that thin films of the prototypical Mott insulator LaTiO 3 grown by pulsed laser deposition under oxygen atmosphere are readily tuned by excess oxygen doping across the line of the band‐filling controlled Mott transition in the electronic phase diagram. The detected insulator to metal transition is characterized by a strong change in resistivity of several orders of magnitude. The use of suitable substrates and capping layers to inhibit oxygen diffusion facilitates full control of the oxygen content and renders the films stable against exposure to ambient conditions. These achievements represent a significant advancement in control and tuning of the electronic properties of LaTiO 3+ x thin films making it a promising channel material in future Mottronic devices.

When ferromagnetic films become ultrathin, key properties such as the Curie temperature and the saturation magnetization are usually depressed. This effect is thoroughly investigated in magnetic oxides such as half-metallic manganites, but much less in ferrimagnetic insulating perovskites such as rare-earth titanates RTiO3 , despite their appeal to design correlated 2D electron gases. Here, the magnetic properties of epitaxial DyTiO3 thin films are reported. While films thicker than about 50 nm show a bulk-like response, at low thickness a surprising increase of the saturation magnetization is observed. This behavior is described using a classical model of a "dead layer" but assuming that this layer is actually "living," that is, it responds to the magnetic field with a strong paramagnetic susceptibility. Through depth-dependent X-ray absorption and photoemission spectroscopy, it is shown that the "living-dead layer" corresponds to surface regions where magnetic (S = 1/2) Ti3+ ions are replaced by nonmagnetic Ti4+ ions. Hysteresis cycles at the Dy M 5 and Ti L 3 edges indicate that the surface Ti4+ ions decouple the Dy3+ ions, thus unleashing their strong paramagnetic response. Finally, it is shown how capping the DyTiO3 film can help increase the Ti3+ content near the surface and thus recover a better ferrimagnetic behavior.

We report gate-tunable memristive switching in patterned LaAlO3/SrTiO3 interfaces at cryogenic temperatures. The application of voltages in the order of a few volts to the back gate of the device allows controlling and switching on and off the inherent memory functionality (memristance). For large and small gate voltages, a simple non-linear resistance characteristic is observed, while a pinched hysteresis loop and memristive switching occur in an intermediate voltage range. The memristance is further controlled by the density of oxygen vacancies, which is tuned by annealing the sample at 300 degrees C in a nitrogen atmosphere. Depending on the annealing time, the memristance at zero gate voltage can be switched on and off, leading to normally-on and normally-off memristors. The presented device offers reversible and irreversible control of memristive characteristics by gate voltages and annealing, respectively, which may allow compensating fabrication variabilities of memristors that complicate the realization of large memristor-based neural networks. Published by AIP Publishing.

The spinel/perovskite heterointerface gamma-Al2O3/SrTiO3 hosts a two-dimensional electron system (2DES) with electron mobilities exceeding those in its all-perovskite counterpart LaAlO3/SrTiO3 by more than an order of magnitude, despite the abundance of oxygen vacancies which act as electron donors as well as scattering sites. By means of resonant soft x-ray photoemission spectroscopy and ab initio calculations, we reveal the presence of a sharply localized type of oxygen vacancies at the very interface due to the local breaking of the perovskite symmetry. We explain the extraordinarily high mobilities by reduced scattering resulting from the preferential formation of interfacial oxygen vacancies and spatial separation of the resulting 2DES in deeper SrTiO3 layers. Our findings comply with transport studies and pave the way towards defect engineering at interfaces of oxides with different crystal structures.

More than a decade after the discovery of the two-dimensional electron system (2DES) at the interface between the band insulators LaAlO3 (LAO) and SrTiO3 (STO) its microscopic origin is still under debate. Several explanations have been proposed, the main contenders being electron doping by oxygen vacancies and electronic reconstruction, i.e., the redistribution of electrons to the interface to minimize the electrostatic energy in the polar LAO film. However, no experiment thus far could provide unambiguous information on the microscopic origin of the interfacial charge carriers. Here we utilize a novel experimental approach combining photoelectron spectroscopy (PES) with highly brilliant synchrotron radiation and apply it to a set of samples with varying key parameters that are thought to be crucial for the emergence of interfacial conductivity. Based on microscopic insight into the electronic structure, we obtain results tipping the scales in favor of polar discontinuity as a generic, robust driving force for the 2DES formation. Likewise, other functionalities such as magnetism or superconductivity might be switched in all-oxide devices by polarity-driven charge transfer.