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Transition metal oxides (TMOs) exhibit a broad spectrum of electronic, magnetic, and optical properties, making them intriguing materials for various technological applications. Soft x-ray absorption spectroscopy (XAS) is widely used to study TMOs, shedding light on their chemical state, electronic structure, orbital polarization, element-specific magnetism, and more. Different XAS acquisition modes feature different information depth regimes in the sample. Here, we employ two XAS acquisition modes, having surface-sensitive versus bulk probing depths, on the prototypical TMO SrVO3. We illustrate and elucidate a strong apparent discrepancy between the different modes, emphasizing the impact of the near-surface region on the interpretation of XAS data. These findings highlight the importance of the acquisition mode selection in XAS analysis. Moreover, the results highlight the role of the near-surface region not only in the characterization of TMOs, but also in the design of future nanoscale oxide electronics.

Controlling magnetic anisotropy (MA) is important in a variety of applications including magnetic memories, spintronic sensors, and skyrmion-based data distribution. The perovskite manganite family provides a fertile playground for complex, intricate, and potentially useful structure-magnetism relations. Here we report on the MA that emerges in 10% Ru substituted $La_{0.7}Sr_{0.3}MnO_{3}$ (Ru-LSMO) films for which strong perpendicular magnetization and anisotropic in-plane magnetization are found. These moderately compressively strained films possess a rich microstructure, consisting of coherently strained phase which evolves into a one dimensional (1D) periodically-modulated structure above a critical thickness. We illustrate how 10% Ru substitution plays a crucial role behind the observed MA, and how the structural distortion and 1D periodic structural modulation produce the anisotropic in-plane magnetization. We highlight the practical significance of the observed MA, which could pave the way towards the realization of cutting-edge oxide-based room temperature spintronic memory devices., Comment: 3 manuscript figures and 8 supplement figures

In Mott materials strong electron correlation yields a spectrum of complex electronic structures. Recent synthesis advancements open realistic opportunities for harnessing Mott physics to design transformative devices. However, a major bottleneck in realizing such devices remains the lack of control over the electron correlation strength. This stems from the complexity of the electronic structure, which often veils the basic mechanisms underlying the correlation strength. Here, we present control of the correlation strength by tuning the degree of orbital overlap using picometer-scale lattice engineering. We illustrate how bandwidth control and concurrent symmetry breaking can govern the electronic structure of a correlated $SrVO_3$ model system. We show how tensile and compressive biaxial strain oppositely affect the $SrVO_3$ in-plane and out-of-plane orbital occupancy, resulting in the partial alleviation of the orbital degeneracy. We derive and explain the spectral weight redistribution under strain and illustrate how high tensile strain drives the system towards a Mott insulating state. Implementation of such concepts will drive correlated electron phenomena closer towards new solid state devices and circuits. These findings therefore pave the way for understanding and controlling electron correlation in a broad range of functional materials, driving this powerful resource for novel electronics closer towards practical realization.

Resistive switching devices herald a transformative technology for memory and computation, offering considerable advantages in performance and energy efficiency. Here we employ a simple and scalable material system of conductive oxide interfaces and leverage their unique properties for a new type of resistive switching device. For the first time, we demonstrate an $\rm Al_2O_3-TiO_2$ based valence-change resistive switching device, where the conductive oxide interface serves both as the back electrode and as a reservoir of defects for switching. The amorphous-polycrystalline $\rm Al_2O_3-TiO_2$ conductive interface is obtained following the technological path of simplifying the fabrication of the two-dimensional electron gases (2DEGs), making them more scalable for practical mass integration. We combine physical analysis of the device chemistry and microstructure with comprehensive electrical analysis of its switching behavior and performance. We pinpoint the origin of the resistive switching to the conductive oxide interface, which serves as the bottom electrode and as a reservoir of oxygen vacancies. The latter plays a key role in valence-change resistive switching devices. The new device, based on scalable and complementary metal-oxide-semiconductor (CMOS) technology-compatible fabrication processes, opens new design spaces towards increased tunability and simplification of the device selection challenge., Comment: Supplementary Material included at the end

Due to the strong lattice-property relationships which exist in complex oxide epitaxial layers, their electronic and magnetic properties can be modulated by structural distortions induced at the atomic scale. The modification and control can be affected at coherent heterointerfaces by epitaxial strain imposed by the substrate or by structural modifications to accommodate the film-substrate symmetry mismatch. Often these act in conjunction with a strong dependence on the layer thickness, especially for ultrathin layers. Moreover, as a result of these effects, the temperature dependence of the structure may deviate largely from that of the bulk. The temperature-dependent structure of 3 to 44 unit cell thick ferromagnetic SrRuO$_3$ films grown on Nb-doped SrTiO$_3$ substrates are investigated using a combination of high-resolution synchrotron X-ray diffraction and high-resolution electron microscopy. This aims to shed light on the intriguing magnetic and magnetotransport properties of epitaxial SRO layers, subjected to extensive investigations lately. The oxygen octahedral tilts and rotations are found to be strongly dependent on the temperature, the film thickness, and the distance away from the film-substrate interface. As a striking manifestation of the coupling between magnetic order and lattice structure, the Invar effect is observed below the ferromagnetic transition temperature in epitaxial layers as thin as 8 unit cells, similar to bulk ferromagnetic SrRuO$_3$.

Leakage currents through insulators received continuous attention for decades, owing to their importance for a wide range of technologies, and interest in their fundamental mechanisms. This work investigates the leakage currents through atomic layer deposited (ALD) $Al_2O_3$, grown on $SrTiO_3$. This combination is not only a key building block of oxide electronics, but also a clean system for studying the leakage mechanisms without interfacial layers that form on most of the conventional bottom electrodes. We show how tiny differences in the deposition process can have a dramatic effect on the leakage behavior. Detailed analysis of the leakage behavior rules out Fowler-Nordheim tunneling (FNT) and thermionic emission, and leaves the trap-related mechanisms of trap-assisted tunneling (TAT) and Poole-Frenkel as the likely mechanisms. After annealing the sample in air, the currents are reduced, which is ascribed to transition from trap-based mechanism to FNT, due to the elimination of the traps. The dramatic role of the assumptions regarding the flat-band voltage used for analysis is critically discussed, and the sensitivity of the extracted parameters on this magnitude is quantitatively described. We show that field effect devices based on structures similar to those described here, should be able to modulate $>10^{13} cm^{-2}$ electrons. These results provide general guidelines for reducing and analyzing leakage currents in insulators, and highlight some of the possible approaches and pitfalls in their analysis., Comment: 15 pages, 4 figures

The abrupt metal insulator transition in $VO_2$ is attracting considerable interest from both fundamental and applicative angles. We report on DC I-V characteristics measured on $VO_2$ single crystals in the two-probe configuration at several ambient temperatures below the insulator-metal transition. The insulator-mixed-metal-insulator transition is induced by Joule heating above ambient temperature in the range of negative differential resistivity (NDR). In this range the stability of V(I) is governed by the load resistance $R_L$. Steady state I(V) is obtained for $R_L> |dV/dI|_{max}$ in the NDR regime. For $R_L< |dV/dI|_{max}$ there is switching between initial and final steady states associated with peaks in the Joule power, that are higher the lower $R_L$ is. The peaks caused by steep switching are superfluous and damaging the samples. On the other hand, the large $R_L$ needed for steady state is the main power consumer in the circuit at high currents. The present work is motivated by the need to avoid damaging switching in the NDR regime while reducing the power consumption in the circuit. It is shown here that large resistance modulation can be obtained under steady state conditions with reduced power consumption by increasing the ambient temperature of the device above room temperature., Comment: 11 pages, 5 figures (main text) + 7 pages, 6 figures (supplementary material)

Inspired by nature, we investigate the short-range order effect on the physical properties of amorphous materials. Amorphous Al2O3 thin films exhibit a higher proportion of their 4-coordinated Al sites close to the surface, causing variations in the average short-range order of the film. Below some thickness, the density of these films changes with size. In this work, we address the short-range order effect, through the thickness, on the electronic and optical properties of atomic layer deposited (ALD) Al2O3 thin films. Both the refractive index and the permittivity were found to vary with size. The refractive index increased with thickness, and for thick films (~50 nm) it was comparable to that of bulk amorphous Al2O3. The permittivity increased with thickness as well, but did not attain those of the bulk material. We discuss how these effects correlate with the density and short-range order. These results shed light on the size effects in thin amorphous oxides, and may guide the design of electronic and optical components and devices.

2D electron gases (2DEGs) formed at oxide interfaces provide a rich testbed for fundamental physics and device applications. While the discussion of the physical origins of this phenomenon continues, the recent discovery of oxide 2DEGs at non-epitaxial interfaces between amorphous and crystalline oxides provides useful insight onto this debate. Furthermore, using amorphous oxides offers a low-cost route towards realizing 2DEGs for device applications. In this work, the band offsets of a simple model system of amorphous-crystalline oxide interface are investigated. The model system consists of amorphous $Al_2O_3$ grown on single-crystalline (001) $SrTiO_3$. X-ray photoelectron spectroscopy is employed to study the chemical states, band gap and band offsets at the interface. The density of ionic defects near the interface is found to be below the detection limit, and the interface is found to be insulating. Analysis of the relative band structure yields significant interfacial barriers, exceeding 1.05 eV for holes and 2.0 eV for electrons. The barrier for holes is considerably larger than what is known for related materials systems, outlining the promise of using amorphous $Al_2O_3$ as an effective and simple insulator, an important building block for oxide-based field effect devices., Comment: 14 pages, 6 figures (including Supplementary Material). Accepted to JAP

Two dimensional electron gases (2DEGs) formed at the interfaces of oxide heterostructures draw considerable interest owing to their unique physics and potential applications. Growing such heterostructures on conventional semiconductors has the potential to integrate their functionality with semiconductor device technology. We demonstrate 2DEGs on a conventional semiconductor by growing $GdTiO_3-SrTiO_3$ on silicon. Structural analysis confirms the epitaxial growth of heterostructures with abrupt interfaces and a high degree of crystallinity. Transport measurements show the conduction to be an interface effect, with $\sim 9\times 10^{13} \; cm^{-2}$ electrons per interface. Good agreement is demonstrated between the electronic behavior of structures grown on Si and on an oxide substrate, validating the robustness of this approach to bridge between lab-scale samples to a scalable, technologically relevant materials system., Comment: 10 pages, 4 figures