Two-dimensional electron gases (2DEGs) at the surfaces of KTaO3 have become an exciting platform for exploring strong spin-orbit coupling, Rashba physics, and low-carrier-density superconductivity. Yet, a large fraction of reported KTaO3-based 2DEGs has been realized through chemically complex overlayers that both generate carriers and can obscure the native electronic structure, making spectroscopic access to the underlying 2DEG challenging. Here, we demonstrate a simple and direct method to generate a superconducting 2DEG on KTaO3(111) using Mg-induced surface reduction in molecular-beam epitaxy. Mg has an extremely low sticking coefficient at elevated temperatures, enabling the formation of an ultrathin (<1-2 monolayer) MgO layer that is transparent to soft X-ray photoemission spectroscopy (XPS) and angle-resolved photoemission spectroscopy (ARPES). This allows direct measurement of the surface chemistry and low-energy electronic structure of the pristine reduced surface without the need for a several-nanometer-thick capping layer. XPS shows clear reduction of Ta5+ to lower oxidation states, while ARPES reveals a parabolic Ta 5d conduction band with a similar to 150 meV bandwidth and additional subband features arising from quantum confinement. Transport measurements confirm a superconducting transition below 0.7 K. Together, these results demonstrate a chemically straightforward and controllable pathway for fabricating spectroscopically accessible superconducting 2DEGs on KTaO3(111) and provide a powerful new platform for investigating the mechanisms underlying orientation-dependent superconductivity in KTaO3-based oxide interfaces.

The sensitivity of low-dimensional superconductors to fluctuations gives rise to emergent behaviors beyond the conventional Bardeen-Cooper-Schrieffer framework. Anisotropy is one such manifestation often linked to spatially modulated electronic responses and unconventional pairing mechanisms. Pronounced in-plane anisotropy recently reported at KTaO3-based oxide interfaces has been interpreted as indicative of a stripelike superconducting texture, yet its microscopic origin and formation pathway remain unresolved. Here, we show that disorder in MgO/KTaO3(111) heterostructures broadens the superconducting transition and reveals transport signatures suggesting a percolative evolution from localized superconducting coherence to a stripelike texture. The stripe width extracted from vortex-dynamical responses is comparable to the spin precession length, suggesting a self-organized modulation influenced by spin-orbit coupling and reduced lattice symmetry. These results highlight disorder as a tuning parameter for superconductivity in two-dimensional quantum materials.

The perovskite ABO3 structure serves as the foundation for diverse functional and quantum materials, yet its applications are hindered by challenges in control of film stoichiometry and the precise construction of interfaces, particularly compared to conventional semiconductors. While a layer-by-layer growth mode is frequently cited, we demonstrate that many transition-metal perovskite oxides self-assemble via an energetically favorable layer-inversion mechanism. This phenomenon can be strategically exploited to fine-tune stoichiometry and surface termination at any point during growth. Layer inversion produces consistent behavior in electron diffraction rocking curves and diffracted-beam intensity oscillations during alternating A- and B-site shuttered growth across various polar and nonpolar surfaces. We introduce a model that accurately interprets these oscillations, enabling an entirely in situ method for precise relative and absolute calibration of multielemental A- and B-site fluxes at the percent level. This approach is successfully applied to the growth of a single-phase high-entropy oxide film.

To explore how anion substitution modifies the existing magnetism in strongly correlated oxides, we investigate local electronic states and magnetic ordering in nickel oxide (NiO) induced by substituting oxygen (O) with nitrogen (N). Each N introduces an additional N 2p hole and modifies the magnetic moment of a neighboring nickel (Ni) cation site, as the exchange interaction between this hole and the Ni eg electrons exceeds the Ni-O-Ni superexchange interaction. This leads to the formation of Ni-N-Ni centers consisting of five spins, without perturbing the antiferromagnetic NiO lattice. These centers are studied using density functional theory and confirmed through high-resolution spectroscopy on N-substituted NiO thin films grown by molecular beam epitaxy. This type of magnetic design may enable future advances in quantum technologies based on strongly correlated materials, such as quantum sensors and spin-based qubits.

In the growing field of spintronic devices incorporating antiferromagnetic materials, control of the domain configuration and Néel axis orientation is critical for technological implementations. Here we show by X-ray magnetic linear dichroism in photoelectron emission microscopy how antiferromagnetic properties of LaFeO3 (LFO) thin films can be tailored through epitaxial strain. LFO films were grown via molecular beam epitaxy with precise stoichiometric control, using substrates that span a range of strain states—from compressive to tensile—and crystal symmetries, including different crystallographic orientations. First, we show that epitaxial strain dictates the Néel axis orientation, shifting it from completely in-plane under compressive strain to completely out-of-plane under tensile strain, regardless of the substrate crystal symmetry. Second, we find that LFO films grown on cubic substrates exhibit a fourfold distribution of antiferromagnetic domains, but can be controlled by varying the substrate miscut, while those on orthorhombic substrates, regardless of strain state, form large-scale monodomains, a highly desirable feature for spintronic applications. Precise control over antiferromagnetic domain configurations and Néel axis orientation is essential for technological advancement of spintronic devices. Here, the authors use epitaxial strain to tailor the magnetic properties of LaFeO3 thin films, demonstrating a crystal engineering approach which may have much wider applicability.

The perovskite ABO3 structure forms the basis for a wide range of functional and quantum materials discussed in the literature, although applications so far have been limited in part because accurate control of film stoichiometry and the ability to precisely construct heterostructures continues to lag behind their conventional semiconductor counterparts. While layer-by-layer growth mode is often cited for perovskite films, most transition-metal perovskite oxides self-assemble via an energetically-favorable layer-inversion mechanism whose phenomenology we demonstrate can be exploited to precisely adjust the stoichiometry and surface termination at any point during growth. Layer inversion results in a universal behavior of the in situ electron diffraction rocking curves and diffracted-beam intensity oscillations during alternating A- and B-site shuttered growth for a wide range of polar and nonpolar surfaces. We present a model that permits unambiguous interpretation of the oscillations, providing for the first time a completely in situ method for precise relative and absolute calibration of multielemental A- and B-site fluxes at the percent level, and apply it to successfully grow a single-phase high-entropy oxide film.

Click to increase image sizeClick to decrease image size Additional informationFundingThis research was undertaken thanks in part to funding from the Max Planck-UBC-UTokyo Centre for Quantum Materials and the Canada First Research Excellence Fund, Quantum Materials and Future Technologies Program. The work at the University of British Columbia was also supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canada Foundation for Innovation (CFI), the British Columbia Knowledge Development Fund (BCKDF), and the Canada Research Chairs Program (A.D.). The work at the Canadian Light Source was supported by CFI, NSERC, the National Research Council (NRC), the Canadian Institutes of Health Research (CIHR), the Government of Saskatchewan, and the University of Saskatchewan.n

Interlayer excitons in solid‐state systems have emerged as candidates for realizing novel platforms ranging from excitonic transistors and optical qubits to exciton condensates. Interlayer excitons have been discovered in 2D transition metal dichalcogenides, with large exciton binding energies and the ability to form various van der Waals heterostructures. Here, an oxide system consisting of a single unit cell of Mg 2 TiO 4 on MgO (001) is proposed as a platform for hosting interlayer excitons. Using a combination of density functional theory (DFT) calculations, molecular beam epitaxy growth, and in situ crystal truncation rod measurements, it is shown that the Mg 2 TiO 4 ‐MgO interface can be precisely controlled to yield an internal electric field suitable for hosting interlayer excitons. The atoms in the polar Mg 2 TiO 4 layers are observed to be displaced to reduce polarity at the interface with the non‐polar MgO (001) surface. Such polarity‐driven atomic displacements strongly affect electrostatics of the film and the interface, resulting in localization of filled and empty band‐edge states in different layers of the Mg 2 TiO 4 film. The DFT calculations suggest that the electronic structure is favorable for localization of photoexcited electrons in the bottom layer and holes in the top layer, which may bind to form interlayer exciton states.

Experimental manipulation of superconductivity is of paramount importance, not only for practical applications but also for identifying the key factors involved in electron pairing. In this work, we have undertaken a meticulous study of the superconductivity in a series of titanium compounds with a rocksalt structure, synthesized as epitaxial films. We find that substituting nitrogen (N) for oxygen (O) in titanium monoxide (TiO) with the stoichiometry close to TiO$_{0.6}$N$_{0.4}$ leads to superconductivity with a transition temperature (T$_c$) of ~2.6 K, about five times higher than that of TiO at ~0.5 K and half as high as the T$_c$ of ~6 K in titanium nitride (TiN). However, Eliashberg theoretical calculations predict similar Tc in TiO, Ti oxynitride and TiN. The analysis of electron mean free path suggests the presence of significant disorder in TiO and a remarkable reduction in the impact of disorder in oxynitrides. Density functional theory (DFT) calculations reveal that disorder decreases the coherence of electronic states for non-zero momenta, which would degrade the influence of electron-phonon. Our findings demonstrate the disorder and superconductivity depend strongly on the N/O ratio, highlighting the critical role of disorder for superconductivity in this series of Ti(O,N) materials.

As the parent compound of a promising solid electrolyte material Li3xLa2/3−xTiO3, the perovskite La2/3TiO3 has potential for advancing research on Li-intercalated ionic conductors. Epitaxial La2/3TiO3 films have been grown by molecular beam epitaxy using a growth process consisting of deposition and annealing cycles, with in situ monitoring by electron diffraction. X-ray absorption spectroscopy confirms the tetravalent state of Ti in La2/3TiO3, and the as-grown films are insulating. X-ray diffraction reveals the presence of half-order peaks, indicating a doubling of the pseudocubic perovskite unit cell due to the ordering of La vacancies in alternating A-site layers. These results demonstrate that single-phase, vacancy-ordered epitaxial films of La2/3TiO3 can be stabilized with excellent crystalline and electronic properties over wafer-sized areas, making possible Li-ion intercalation studies in films with well-defined domain boundary properties. Such boundaries are known to profoundly influence Li-ion conduction within the material. Understanding the effects of domain boundaries on Li-ion conduction could lead to improvements in solid-state battery technology and pave the way for the development of more efficient and safer energy storage devices.

Quantum phenomena such as superconductivity usually emerge in strongly correlated oxide systems as the result of the interplay of spin, charge, and orbital degrees of freedom. Adding ferromagnetism to two‐dimensional electron gases (2DEGs) at oxide interfaces is intriguing due to their scientifically exotic and technically valuable properties, which may lead to new fundamental understanding and multifunctional applications. Here, ferromagnetic 2DEGs at the interface of polar antiferromagnetic LaTiO 3 and nonpolar antiferromagnetic EuTiO 3 are generated. The magnetotransport properties of these 2DEGs depend on the thickness of LaTiO 3 that determines the carrier concentration, with all showing robust ferromagnetism up to 5.5 K. This magnetism is intrinsic to the strongly‐correlated 2D electron system and is highly sensitive and tunable based on the sample configuration. Thus, a prototype oxide system with magnetic functionality for spintronics and ferromagnetic semiconductors has been developed.

Abstract Two-dimensional electron gas (2DEG) systems generated at oxide interfaces that exhibit novel physics phenomena have opened up a new era for oxide-based electronics, photonics, and spintronics. The recent discovery of superconductivity plus the strong spin-orbital coupling naturally existing in the 2DEGs of KTaO3 (KTO) makes KTO an exciting platform for the interplay of the electronic and spin degree of freedom to create new physical properties. By directly placing KTO’s 2DEGs next to another strongly-correlated oxide with nontrivial topological nodes, we reveal direct evidence of topological states in the electronic transport properties of the KTO’s 2DEGs, due to the electronic reconstruction caused by the proximity effect. This adds potential for new functionality in KTO heterostructures.

Interface kinetics plays a crucial role in modulating the resistive switching mechanism for memristor devices with a Schottky junction. This study introduces H atoms by catalytic doping and examines the interfacial electrical transfer characteristics of the Pd/Nb‐doped SrTiO 3 (Nb‐STO). The I–V measurements show that H + doping at the Pd/Nb‐STO interface reduces the barrier height by 300 mV compared to the sample before H + doping. This reduction in barrier height is further correlated with the decrease in built‐in potential by 300 mV and depletion layer thickness from C–V measurements. The underlying reason for such a drastic change in resistive switching characteristics is the reduction of interface layer thickness. The work highlights the easy use of Pd metal to introduce H atoms to oxide materials and provides insight into their effects on switching mechanisms.

The choice and quality of single-crystal substrates for epitaxial thin film growth drastically influence the properties of the synthesized films. Among them, high-quality substrates with a polar surface are particularly difficult to prepare due to their inherent structural instability against electrostatic forces of the polar discontinuity. As a commonly used insulating substrate, the (111) cut of MgO shows alternating planes of Mg2+ and O2− that form a hexagonal unit cell at the surface, which is useful for thin films with similar crystal symmetries. We present a method to prepare such a surface by first introducing (3×3)R30° reconstructed MgO (111) surface via face-to-face annealing in an O2 environment. We then dissolve the top layers in acid to achieve an unreconstructed and also passivated surface. The structure, stoichiometry, and stability of the resulting surfaces are characterized. This work provides an easy and reliable way to prepare a MgO (111) polar surface ready for subsequent epitaxial growth.

Two-dimensional electron gas systems (2DEGs) generated at the oxide interfaces that exhibit rich physics phenomena opened up an era for oxide-based electronics, photonics, and spintronics. The recent discovery of superconductivity plus the strong spin-orbital coupling naturally existing in the 2DEGs of KTaO3 (KTO) made KTO an exciting platform for the interplay of the electronic and spin degrees of freedom to create exotic physical properties. By directly placing KTO's 2DEGs next to another strongly-correlated oxide with nontrivial topological nodes, we reveal the anomalous effects which were induced by the topological states in the electronic transport properties of the KTO's 2DGEs, due to the electronic reconstruction caused by the proximity effect. This adds an additional prospect to the functions of KTO heterostructures.

Titanium monoxide (TiO), an important member of the rock salt 3d transition-metal monoxides, has not been studied in the stoichiometric single-crystal form. It has been challenging to prepare stoichiometric TiO due to the highly reactive Ti2+ We adapt a closely lattice-matched MgO(001) substrate and report the successful growth of single-crystalline TiO(001) film using molecular beam epitaxy. This enables a first-time study of stoichiometric TiO thin films, showing that TiO is metal but in proximity to Mott insulating state. We observe a transition to the superconducting phase below 0.5 K close to that of Ti metal. Density functional theory (DFT) and a DFT-based tight-binding model demonstrate the extreme importance of direct Ti-Ti bonding in TiO, suggesting that similar superconductivity exists in TiO and Ti metal. Our work introduces the new concept that TiO behaves more similar to its metal counterpart, distinguishing it from other 3d transition-metal monoxides.

Spin degree of freedom generally plays an important role in unconventional superconductivity. In many of the iron-based compounds, superconductivity is found in close proximity to long-range antiferromagnetic order, whereas monolayer FeSe grown on SrTiO 3 , with enhanced superconductivity, exhibits no magnetic or nematic ordering. Here we grow monolayer and multilayer FeSe on antiferromagnetic EuTiO 3 (001) layers, in an effort to introduce a spin polarization in proximity to the superconductivity of FeSe. By X-ray magnetic dichroism, we observe an antiferromagnet–ferromagnet switching on Eu and Ti sites in EuTiO 3 driven by the applied magnetic field, with no concomitant spin polarization on the Fe site of FeSe. Transport measurements show enhanced superconductivity of monolayer FeSe on EuTiO 3 with a transition temperature of ~30 K. The band structure revealed by photoemission spectroscopy is analogous to that of FeSe/SrTiO 3 . Our work creates a platform for the interplay of spin and unconventional superconductivity in the two-dimensional limit.

${\mathrm{EuTiO}}_{3}$, a band insulator, and ${\mathrm{LaTiO}}_{3}$, a Mott insulator, are both antiferromagnetic with transition temperatures \ensuremath{\sim}5.5 and \ensuremath{\sim}160 K, respectively. Here, we report the synthesis of ${\mathrm{Eu}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{TiO}}_{3}$ thin films with $x=0$ to 1 by oxide molecular beam epitaxy. The films in the full range have high crystalline quality and show no phase segregation, allowing us to carry out transport measurements to study their electrical and magnetic properties. From $x=0.03$ to 0.95, ${\mathrm{Eu}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{TiO}}_{3}$ films show conduction by electrons as charge carriers, with differences in carrier densities and mobilities, contrary to the insulating nature of pure ${\mathrm{EuTiO}}_{3}$ and ${\mathrm{LaTiO}}_{3}$. Following a rich phase diagram, the magnetic ground states of the films vary with increasing La-doping level, changing ${\mathrm{Eu}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{TiO}}_{3}$ from an antiferromagnetic insulator to an antiferromagnetic metal, a ferromagnetic metal, a paramagnetic metal, and back to an antiferromagnetic insulator. These emergent properties reflect the evolutions of the band structure, mainly at the Ti ${t}_{2\mathrm{g}}$ bands near the Fermi level, when ${\mathrm{Eu}}^{2+}$ are gradually replaced by ${\mathrm{La}}^{3+}$. This work sheds light on this method for designing the electrical and magnetic properties in strongly correlated oxides and completes the phase diagram of the titanate ${\mathrm{Eu}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{TiO}}_{3}$.