Epitaxial thin-film heterostructures of the strongly spin-orbit coupled Mott-insulator Sr2IrO4 (SIO) and the cuprate high temperature superconductor YBa2Cu3O7 - delta (YBCO) are grown with pulsed laser deposition (PLD). A high crystalline quality is confirmed with X ray diffraction. The magnetic order of single SIO layers is studied with dc magnetization and low-energy muon spin rotation measurements and resembles that of the bulk material with a canted antiferromagnetic order. The electronic normal state and superconducting properties of YBCO (10, 12, or 14 nm)-SIO (20 nm) and inversely stacked SIO (20nm)-YBCO (10, 12, or 14 nm) bilayers are studied with dc resistivity measurements and found to be strongly dependent on the sequence of the layer stacking. The YBCO-SIO bilayers with d(YBCO) = 14nm, 12 nm, and 10 nm are all metallic and superconducting with an onset temperature around 85K and zero resistivity below 65K. To the contrary, for the inversely stacked SIO-YBCO bilayers a metallic and superconducting response occurs only at d(YBCO) = 14 nm, whereas and 10 nm are electronic insulators. This highlights that a long-ranged localization and/or depletion of the YBCO charge carriers occurs at the SIO-YBCO interface that is very anomalous and remains to be understood.

The family of infinite-layer nickelates promises important insights into the mechanism of unconventional superconductivity. Since superconductivity has so far only been observed in epitaxial thin films, heteroepitaxy with the substrate or a capping layer possibly plays an important role. Here, we use soft x-ray spectroscopy to investigate superlattices as a potential approach for a targeted material design of high-temperature superconductors. We observe modulations in valence state and oxygen coordination in topotactically reduced artificial superlattices with repeating interfaces between nickelate layers and layers of materials commonly used as substrates and capping layers. Our results show that depending on the interlayer material metallic conductivity akin to the parent infinite-layer compounds is achieved. Depth-resolved electronic structure measured by resonant x-ray reflectivity reveals a reconstructed ligand field and valence state at the interface, which is confined to one or two unit cells. The central layers are predominantly monovalent nickel, but linear dichroism analysis reveals considerable disorder in the oxygen removal sites. We observe a quantitative correlation of this disorder with the interlayer material that is important for future modeling and design strategies.

Many electronic and electrochemical devices rely on the exchange of light elements such as hydrogen and oxygen with the environment. Understanding and tailoring the device functionality requires accurate information about the concentration and chemical bonding of such species inside a solid, which is particularly difficult if several species are exchanged. In LaNiO3 thin films in situ transport experiments reveal a re-entrant metal-insulator transition upon hydrogen exposure. The origin of this unusual behavior can be understood by combining information about the stoichiometry and chemical bonding of hydrogen and oxygen as determined by neutron reflectometry and x-ray absorption spectroscopy, respectively. In addition to the metallic parent phase, an insulating phase with composition LaNiO2.65 and a re-entrant metallic phase with composition LaNiO2.15(OH)0.5 are identified. They can be inter-converted by redox reactions in different external environments. The methodology employed offers new insights into the mechanisms underlying the influence of hydrogen in functional devices.

Complex oxides are well known to develop oxygen ordering patterns with well defined periodicities, but their interplay with electronic correlations remains largely unexplored. Here, we report resonant and non-resonant x-ray diffraction data indicating a four-unit-cell periodic superstructure related to oxygen vacancy ordering in La_2-xCa_xCuO_4 films with doping levels from the underdoped (x = 0.15) to the extremely overdoped (x = 0.50) regime. Whereas the lattice superstructure is temperature independent up to 300 K, a strongly temperature dependent electronic charge density wave (CDW) is observed in the underdoped and slightly overdoped regimes (x≤0.20). The periodicity, in-plane and out-of-plane correlation lengths of the CDW are locked in by the lattice superstructure. Our results highlight the necessity to consider lattice and electronic energetics on equal footing in the high-temperature oxygen-disordered phase to explain oxygen ordering phenomena in complex oxides.

Resonant elastic X-ray scattering at the V K edge reveals systematic variations of the resonant forbidden (002) reflection in thin films of spinel LiV 2 O 4 deposited on MgAl 2 O 4 , SrTiO 3 , and MgO.

Ohmic or Schottky contacts in micro-and nano electronic devices are formed by metal-semiconductor bilayer systems, based on elemental metals or thermally more stable metallic compounds (germanides, silicides). The control of their electronic properties remains challenging as their structure formation is not yet fully understood. We have studied the phase and microstructure evolution during sputter deposition and postgrowth annealing of Pd/a-Ge bilayer systems with different Pd/Ge ratios (Pd:Ge, 2Pd:Ge, and 4Pd:Ge). The room -temperature deposition of up to 30 nm Pd was monitored by simultaneous, in situ synchrotron X-ray diffraction, X-ray reflectivity, and optical stress measurements. With this portfolio of complementary real-time methods, we could identify the microstructural origins of the resistivity evolution during contact formation: Real-time X-ray diffraction measurements indicate a coherent, epitaxial growth of Pd(111) on the individual crystallites of the initially forming, polycrystalline Pd2Ge[111] layer. The crystallization of the Pd2Ge interfacial layer causes a characteristic change in the real-time wafer curvature (tensile peak), and a significant drop of the resistivity after 1.5 nm Pd deposition. In addition, we could confirm the isostructural interface formation of Pd/a-Ge and Pd/a-Si. Subtle differences between both interfaces originate from the lattice mismatch at the interface between compound and metal. The solid-state reaction during subsequent annealing was studied by real-time X-ray diffraction and complementary UHV surface analysis. We could establish the link between phase and microstructure formation during deposition and annealing-induced solid-state reaction: The thermally induced reaction between Pd and a-Ge proceeds via diffusion-controlled growth of the Pd2Ge seed crystallites. The second-phase (PdGe) formation is nucleation-controlled and takes place only when a sufficient Ge reservoir exists. The real-time access to structure and electronic properties on the nanoscale opens new paths for the knowledge-based formation of ultrathin metal/semiconductor contacts.

Thin films comprised of 2D materials have attracted significant attention, as they can be assembled into novel functional materials. However, to further enhance their application scope, it is necessary to harvest the large property space of 2D materials by fine-tuning them on a molecular level, e.g., by intercalation. In order to fully exploit the potential of intercalated 2D materials and design their properties, it is vital to gain a fundamental understanding of the underlying intercalation mechanism. In this work, we present a method for the quantitative analysis of changes in the peak profile and position observed by in situ synchrotron measurements upon the intercalation of the guest molecule into the layered host. We do this by monitoring the intercalation of n-butylamine (3 M in ethanol) into a H3Sb3P2O14 thin film in real time. Our approach includes a state-of-the-art recursive supercell approach that accounts for peak broadening and shifting caused by randomly occurring intercalation, which enabled quantitative Rietveld refinements of the XRD patterns obtained during the interstratification process. This allowed us to reveal the transient formation of intermediates and the critical role of ethanol, which acts as a vehicle for amine intercalation into the layered host.

Heterostructuring provides different ways to manipulate the orbital degrees of freedom and to tailor orbital occupations in transition-metal oxides. However, the reliable prediction of these modifications remains a challenge. Here we present a detailed investigation of the relationship between the crystal and electronic structure in YVO3-LaAlO3 superlattices by combining ab initio theory, scanning transmission electron microscopy, and x-ray diffraction. Density functional theory simulations including an on-site Coulomb repulsion term accurately predict the crystal structure and, in conjunction with x-ray diffraction, provide an explanation for the lifting of degeneracy of the vanadium dxz and dyz orbitals that was recently observed in this system. In addition, we unravel the combined effects of electronic confinement and octahedral connectivity by disentangling their impact from that of epitaxial strain. Our results demonstrate that the specific orientation of the substrate and the thickness of the YVO3 slabs in the multilayer can be utilized to reliably engineer orbital polarization.

Devices with tunable magnetic noncollinearity are important components of superconducting electronics and spintronics, but they typically require epitaxial integration of several complex materials. The spin‐polarized neutron reflectometry measurements on La 1− x Sr x MnO 3 homojunction arrays with modulated Sr concentration reported herein have led to the discovery of magnetic fan structures with highly noncollinear alignment of Mn spins and an emergent periodicity twice as large as the array's unit cell. The neutron data show that these magnetic superstructures can be fully long‐range ordered, despite the gradual modulation of the doping level created by charge transfer and chemical intermixing. The degree of noncollinearity can be effectively adjusted by low magnetic fields. Notwithstanding their chemical and structural simplicity, oxide homojunctions thus show considerable promise as a platform for tunable complex magnetism and as a powerful design element of spintronic devices.

Magnonic devices operating at terahertz frequencies offer intriguing prospects for high-speed electronics with minimal energy dissipation However, guiding and manipulating terahertz magnons via external parameters present formidable challenges. Here we report the results of magnetic Raman scattering experiments on the antiferromagnetic spin-orbit Mott insulator Sr 2 IrO 4 under uniaxial stress. We find that the energies of zone-center magnons are extremely stress sensitive: lattice strain of 0.1% increases the magnon energy by 40%. The magnon response is symmetric with respect to the sign of the applied stress (tensile or compressive), but depends strongly on its direction in the IrO 2 planes. A theory based on coupling of the spin-orbit-entangled iridium magnetic moments to lattice distortions provides a quantitative explanation of the Raman data and a comprehensive framework for the description of magnon-lattice interactions in magnets with strong spin-orbit coupling. The possibility to efficiently manipulate the propagation of terahertz magnons via external stress opens up multifold design options for reconfigurable magnonic devices.

Oxide heterostructures provide unique opportunities to modify the properties of quantum materials through a targeted manipulation of spin, charge, and orbital states. Here, we use resonant x-ray reflectometry to probe the electronic structure of thin slabs of YVO3 embedded in a superlattice with LaAlO3. We extend the previously established methods of reflectometry analysis to a general form applicable to t(2g) electron systems and extract quantitative depth-dependent x-ray linear dichroism profiles. Our data reveal an artificial, layered orbital polarization, where the average occupation of xz and yz orbitals in the interface planes next to LaAlO3 is inverted compared to the central part of the YVO3 slab. This phase is stable down to 30 K and the bulklike orbital ordering transitions are absent. We identify the key mechanism for the electronic reconstruction to be a combination of epitaxial strain and spatial confinement by the LaAlO3 layers, in good agreement with predictions from ab initio theory.

We present the electronic and structural properties of monolayer ${\mathrm{WSe}}_{2}$ grown by pulsed-laser deposition on monolayer graphene (MLG) on SiC. The spin splitting in the ${\mathrm{WSe}}_{2}$ valence band at $\overline{\mathrm{K}}$ was ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{SO}}=0.469\ifmmode\pm\else\textpm\fi{}0.008$ eV, as determined by angle-resolved photoemission spectroscopy. Synchrotron-based grazing-incidence in-plane x-ray diffraction (XRD) revealed the in-plane lattice constant of monolayer ${\mathrm{WSe}}_{2}$ to be ${a}_{{\mathrm{WSe}}_{2}}=3.2757\ifmmode\pm\else\textpm\fi{}0.0008$ \AA{}. This indicates a lattice compression of $\ensuremath{-}0.19$% relative to bulk ${\mathrm{WSe}}_{2}$. By using the experimentally determined graphene lattice constant (${a}_{\mathrm{MLG}}=2.4575\ifmmode\pm\else\textpm\fi{}0.0007$ \AA{}), we found that a $3\ifmmode\times\else\texttimes\fi{}3$ unit cell of the slightly compressed ${\mathrm{WSe}}_{2}$ is perfectly commensurate with a $4\ifmmode\times\else\texttimes\fi{}4$ graphene lattice with a mismatch below 0.03%, which could explain why the monolayer ${\mathrm{WSe}}_{2}$ is compressed on MLG. From XRD and first-principles calculations, we conclude that the observed size of strain will affect ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{SO}}$ only on the order of a few meV. In addition, angle-resolved, ultraviolet, and x-ray photoelectron spectroscopies shed light on the band alignment between ${\mathrm{WSe}}_{2}$ and MLG/SiC and indicate electron transfer from graphene to the ${\mathrm{WSe}}_{2}$ monolayer. As further revealed by atomic force microscopy, the ${\mathrm{WSe}}_{2}$ island size depends on the number of carbon layers on top of the SiC substrate. This suggests that the epitaxy of ${\mathrm{WSe}}_{2}$ favors the weak van der Waals interactions with graphene, while it is perturbed by the influence of the SiC substrate and its carbon buffer layer.

We have used atomic layer-by-layer molecular beam epitaxy to synthesize coherently lattice-matched thin films of the high-temperature superconductor DyBa2Cu3O7−x with minimal defect density. A systematic set of x-ray reciprocal-space maps reveals tetragonal and orthorhombic structures with different twinning patterns and elucidates their evolution with the thickness, the oxygenation state, and the epitaxial relationship with the substrate. We also show that films with more pronounced orthorhombicity exhibit lower normal-state resistivities and higher superconducting transition temperatures. These findings provide guidance for the synthesis of optimized superconducting heterostructures and devices.

X-ray diffraction is measured on individual bilayer and multilayer graphene single-crystals and combined with electrochemically induced lithium intercalation. In-plane Bragg peaks are observed by grazing incidence diffraction. Focusing the incident beam down to an area of about 10 μm × 10 μm, individual flakes are probed by specular X-ray reflectivity. By deploying a recursive Parratt algorithm to model the experimental data, we gain access to characteristic crystallographic parameters of the samples. Notably, it is possible to directly extract the bi/multilayer graphene c-axis lattice parameter. The latter is found to increase upon lithiation, which we control using an on-chip peripheral electrochemical cell layout. These experiments demonstrate the feasibility of in situ X-ray diffraction on individual, micron-sized single crystallites of few- and bilayer two-dimensional materials.

We establish strain engineering of ruthenium oxides as a method to controllably induce phase transitions between electronic ground states with vastly different electrical and magnetic properties. Specifically, we show that the epitaxial strain acting on Ca2RuO4 thin films on NdCaAlO4 (110), LaAlO3 (100), and LaSrAlO4 (001) substrates induces a transition from the Mott-insulating phase of bulk Ca2RuO4 into a metallic phase. Magnetometry and spin-polarized neutron reflectometry reveal a low-temperature, small-moment ferromagnetic state in Ca2RuO4 films on LaAlO3 (100) and LaSrAlO4(001). Thin-film structures may open up new ways to investigate and utilize the electronic response of ruthenates to lattice modification.

Atomically thin films of WSe2 from one monolayer up to 8 layers were deposited on an Al2O3 r-cut (11¯02) substrate using a hybrid-Pulsed Laser Deposition (PLD) system where a laser ablation of pure W is combined with a flux of Se. Specular X-ray reflectivities of films were analysed and were consistent with the expected thickness. Raman measurement and atomic force microscopy confirmed the formation of a WSe2 monolayer and its spatial homogeneity over the substrate. Grazing-incidence X-ray diffraction uncovered an in-plane texture in which WSe2 [101¯0] preferentially aligned with Al2O3 [112¯0]. These results present a potential to create 2D transition metal dichalcogenides by PLD, where the growth kinetics can be steered in contrast to common growth techniques like chemical vapor deposition and molecular beam epitaxy.

Transition metal oxides are promising candidates for thermoelectric applications, because they are stable at high temperature and because strong electronic correlations can generate large Seebeck coefficients, but their thermoelectric power factors are limited by the low electrical conductivity. We report transport measurements on Ca3Co4O9 films on various perovskite substrates and show that reversible incorporation of oxygen into SrTiO3 and LaAlO3 substrates activates a parallel conduction channel for p-type carriers, greatly enhancing the thermoelectric performance of the film-substrate system at temperatures above 450 °C. Thin-film structures that take advantage of both electronic correlations and the high oxygen mobility of transition metal oxides thus open up new perspectives for thermopower generation at high temperature.