Real-space imaging of periodic nanotextures in thin films via phasing of diffraction data.

New properties and exotic quantum phenomena can form due to periodic nanotextures, including Moire patterns, ferroic domains, and topologically protected magnetization and polarization textures. Despite the availability of powerful tools to characterize the atomic crystal structure, the visualization of nanoscale strain-modulated structural motifs remains challenging. Here, we develop nondestructive real-space imaging of periodic lattice distortions in thin epitaxial films and report an emergent periodic nanotexture in a Mott insulator. Specifically, we combine iterative phase retrieval with unsupervised machine learning to invert the diffuse scattering pattern from conventional X-ray reciprocal-space maps into real-space images of crystalline displacements. Our imaging in PbTiO3/SrTiO3 superlattices exhibiting checkerboard strain modulation substantiates published phase-field model calculations. Furthermore, the imaging of biaxially strained Mott insulator Ca2RuO4 reveals a strain-induced nanotexture comprised of nanometer-thin metallic-structure wires separated by nanometer-thin Mott-insulating-structure walls, as confirmed by cryogenic scanning transmission electron microscopy (cryo-STEM). The nanotexture in Ca2RuO4 film is induced by the metal-to-insulator transition and has not been reported in bulk crystals. We expect the phasing of diffuse X-ray scattering from thin crystalline films in combination with cryo-STEM to open a powerful avenue for discovering, visualizing, and quantifying the periodic strain-modulated structures in quantum materials.

Vanishing nematic order beyond the pseudogap phase in overdoped cuprate superconductors.

During the last decade, translational and rotational symmetry-breaking phases-density wave order and electronic nematicity-have been established as generic and distinct features of many correlated electron systems, including pnictide and cuprate superconductors. However, in cuprates, the relationship between these electronic symmetry-breaking phases and the enigmatic pseudogap phase remains unclear. Here, we employ resonant X-ray scattering in a cuprate high-temperature superconductor [Formula: see text] (Nd-LSCO) to navigate the cuprate phase diagram, probing the relationship between electronic nematicity of the Cu 3d orbitals, charge order, and the pseudogap phase as a function of doping. We find evidence for a considerable decrease in electronic nematicity beyond the pseudogap phase, either by raising the temperature through the pseudogap onset temperature T* or increasing doping through the pseudogap critical point, p*. These results establish a clear link between electronic nematicity, the pseudogap, and its associated quantum criticality in overdoped cuprates. Our findings anticipate that electronic nematicity may play a larger role in understanding the cuprate phase diagram than previously recognized, possibly having a crucial role in the phenomenology of the pseudogap phase.

Electronic nematicity in Sr2RuO4.

We have measured the angle-resolved transverse resistivity (ARTR), a sensitive indicator of electronic anisotropy, in high-quality thin films of the unconventional superconductor Sr2RuO4 grown on various substrates. The ARTR signal, heralding the electronic nematicity or a large nematic susceptibility, is present and substantial already at room temperature and grows by an order of magnitude upon cooling down to 4 K. In Sr2RuO4 films deposited on tetragonal substrates the highest-conductivity direction does not coincide with any crystallographic axis. In films deposited on orthorhombic substrates it tends to align with the shorter axis; however, the magnitude of the anisotropy stays the same despite the large lattice distortion. These are strong indications of actual or incipient electronic nematicity in Sr2RuO4.

Interfacial Electron-Phonon Coupling Constants Extracted from Intrinsic Replica Bands in Monolayer FeSe/SrTiO_{3}.

The observation of replica bands by angle-resolved photoemission spectroscopy has ignited interest in the study of electron-phonon coupling at low carrier densities, particularly in monolayer FeSe/SrTiO_{3}, where the appearance of replica bands has motivated theoretical work suggesting that the interfacial coupling of electrons in the FeSe layer to optical phonons in the SrTiO_{3} substrate might contribute to the enhanced superconducting pairing temperature. Alternatively, it has also been recently proposed that such replica bands might instead originate from extrinsic final state losses associated with the photoemission process. Here, we perform a quantitative examination of replica bands in monolayer FeSe/SrTiO_{3}, where we are able to conclusively demonstrate that the replica bands are indeed signatures of intrinsic electron-boson coupling, and not associated with final state effects. A detailed analysis of the energy splittings and relative peak intensities between the higher-order replicas, as well as other self-energy effects, allows us to determine that the interfacial electron-phonon coupling in the system corresponds to a value of λ=0.19±0.02, providing valuable insights into the enhancement of superconductivity in monolayer FeSe/SrTiO_{3}. The methodology employed here can also serve as a new and general approach for making more rigorous and quantitative comparisons to theoretical calculations of electron-phonon interactions and coupling constants.

Chlorine evolution reaction electrocatalysis on RuO2(110) and IrO2(110) grown using molecular-beam epitaxy.

We report the electrocatalysis of the chlorine evolution reaction (CER) on well-defined RuO2(110) and IrO2(110) surfaces. RuO2 and IrO2 are known for their capabilities to catalyze the CER. Until now, the CER measurements have only been reported on well-defined RuO2 surfaces and only at high Cl- concentrations. We present the CER measurement and the role of Cl- at lower concentration on single-orientation RuO2(110) and IrO2(110) films. We find that RuO2(110) is two orders of magnitude more active than IrO2(110). Moreover, we observe the correlation between the CER activity and the Oad formation potential on RuO2 and IrO2, supporting the prior suggestion that the Oad is the active site for the CER. We further use the reaction order analysis to support the Volmer-Heyrovsky mechanism of the CER, which was previously suggested from the Tafel slope analysis. Our finding highlights the importance of the surface Oad species on oxides for the CER electrocatalysis and suggests the electrochemical formation of Clad on Oad (for example, Cl- + Oad ↔ OClad + e-) as the crucial step in the CER electrocatalysis.

Measurements of Oxygen Electroadsorption Energies and Oxygen Evolution Reaction on RuO2(110): A Discussion of the Sabatier Principle and Its Role in Electrocatalysis.

We report the hydroxide (OHad) and oxide (Oad) experimental electroadsorption free energies, their dependences on pH, and their correlations to the oxygen evolution reaction (OER) electrocatalysis on RuO2(110) surface. The Sabatier principle predicts that catalyst is most active when the intermediate stabilization is moderate, not too strong such that the bound intermediate disrupts the subsequent catalytic cycle, nor too weak such that the surface is ineffective. For decades, researchers have used this concept to rationalize the activity trend of many OER electrocatalysts including RuO2, which is among the state-of-the-art OER catalysts. In this article, we report an experimental assessment of the Sabatier principle by comparing the oxygen electroadsorption energy to the OER electrocatalysis for the first time on RuO2. We find that the OHad and Oad electroadsorption energies on RuO2(110) depend on pH and obey the scaling relation. However, we did not observe a direct correlation between the OHad and Oad electroadsorption energies and the OER activity in the comparative analysis that includes both RuO2(110) and IrO2(110). Our result raises a question of whether the Sabatier principle can describe highly active electrocatalysts, where the kinetic aspects may influence the electrocatalysis more strongly than the electroadsorption energy, which captures only the thermodynamics of the intermediates and not yet kinetics.

Engineering Carrier Effective Masses in Ultrathin Quantum Wells of IrO_{2}.

The carrier effective mass plays a crucial role in modern electronic, optical, and catalytic devices and is fundamentally related to key properties of solids such as the mobility and density of states. Here we demonstrate a method to deterministically engineer the effective mass using spatial confinement in metallic quantum wells of the transition metal oxide IrO_{2}. Using a combination of in situ angle-resolved photoemission spectroscopy measurements in conjunction with precise synthesis by oxide molecular-beam epitaxy, we show that the low-energy electronic subbands in ultrathin films of rutile IrO_{2} have their effective masses enhanced by up to a factor of 6 with respect to the bulk. The origin of this strikingly large mass enhancement is the confinement-induced quantization of the highly nonparabolic, three-dimensional electronic structure of IrO_{2} in the ultrathin limit. This mechanism lies in contrast to that observed in other transition metal oxides, in which mass enhancement tends to result from complex electron-electron interactions and is difficult to control. Our results demonstrate a general route towards the deterministic enhancement and engineering of carrier effective masses in spatially confined systems, based on an understanding of the three-dimensional bulk electronic structure.

Influence of Surface Adsorption on the Oxygen Evolution Reaction on IrO2(110).

A catalyst functions by stabilizing reaction intermediates, usually through surface adsorption. In the oxygen evolution reaction (OER), surface oxygen adsorption plays an indispensable role in the electrocatalysis. The relationship between the adsorption energetics and OER kinetics, however, has not yet been experimentally measured. Herein we report an experimental relationship between the adsorption of surface oxygen and the kinetics of the OER on IrO2(110) epitaxially grown on a TiO2(110) single crystal. The high quality of the IrO2 film grown using molecular-beam epitaxy affords the ability to extract the surface oxygen adsorption and its impact on the OER. By examining a series of electrolytes, we find that the adsorption energy changes linearly with pH, which we attribute to the electrified interfacial water. We support this hypothesis by showing that an electrolyte salt modification can lead to an adsorption energy shift. The dependence of the adsorption energy on pH has implications for the OER kinetics, but it is not the only factor; the dependence of the OER electrocatalysis on pH stipulates two OER mechanisms, one operating in acidic solution and another operating in alkaline solution. Our work points to the subtle adsorption-kinetics relationship in the OER and highlights the importance of the interfacial electrified interaction in electrocatalyst design.

Electron Doping of the Parent Cuprate La_{2}CuO_{4} without Cation Substitution.

In the cuprates, carrier doping of the Mott insulating parent state is necessary to realize superconductivity as well as a number of other exotic states involving charge or spin density waves. Cation substitution is the primary method for doping carriers into these compounds, and is the only known method for electron doping in these materials. Here, we report electron doping without cation substitution in epitaxially stabilized thin films of La_{2}CuO_{4} grown via molecular-beam epitaxy. We use angle-resolved photoemission spectroscopy to directly measure their electronic structure and conclusively determine that these compounds are electron doped with a carrier concentration of 0.09±0.02 e^{-}/Cu. We propose that intrinsic defects, most likely oxygen vacancies, are the sources of doped electrons in these materials. Our results suggest a new approach to electron doping in the cuprates, one which could lead to a more detailed experimental understanding of their properties.

Polycrystalline graphene with single crystalline electronic structure.

We report the scalable growth of aligned graphene and hexagonal boron nitride on commercial copper foils, where each film originates from multiple nucleations yet exhibits a single orientation. Thorough characterization of our graphene reveals uniform crystallographic and electronic structures on length scales ranging from nanometers to tens of centimeters. As we demonstrate with artificial twisted graphene bilayers, these inexpensive and versatile films are ideal building blocks for large-scale layered heterostructures with angle-tunable optoelectronic properties.

Spontaneous Supercrystal Formation During a Strain-Engineered Metal-Insulator Transition.

Mott metal-insulator transitions possess electronic, magnetic, and structural degrees of freedom promising next-generation energy-efficient electronics. A previously unknown, hierarchically ordered, and anisotropic supercrystal state is reported and its intrinsic formation characterized in-situ during a Mott transition in a Ca2RuO4 thin film. Machine learning-assisted X-ray nanodiffraction together with cryogenic electron microscopy reveal multi-scale periodic domain formation at and below the film transition temperature (TFilm ≈ 200-250 K) and a separate anisotropic spatial structure at and above TFilm. Local resistivity measurements imply an intrinsic coupling of the supercrystal orientation to the material's anisotropic conductivity. These findings add a new degree of complexity to the physical understanding of Mott transitions, opening opportunities for designing materials with tunable electronic properties.

Quantum many-body interactions in digital oxide superlattices.

Controlling the electronic properties of interfaces has enormous scientific and technological implications and has been recently extended from semiconductors to complex oxides that host emergent ground states not present in the parent materials. These oxide interfaces present a fundamentally new opportunity where, instead of conventional bandgap engineering, the electronic and magnetic properties can be optimized by engineering quantum many-body interactions. We use an integrated oxide molecular-beam epitaxy and angle-resolved photoemission spectroscopy system to synthesize and investigate the electronic structure of superlattices of the Mott insulator LaMnO(3) and the band insulator SrMnO(3). By digitally varying the separation between interfaces in (LaMnO(3))(2n)/(SrMnO(3))(n) superlattices with atomic-layer precision, we demonstrate that quantum many-body interactions are enhanced, driving the electronic states from a ferromagnetic polaronic metal to a pseudogapped insulating ground state. This work demonstrates how many-body interactions can be engineered at correlated oxide interfaces, an important prerequisite to exploiting such effects in novel electronics.

Formation of the coherent heavy fermion liquid at the hidden order transition in URu2Si2.

We present high-resolution angle-resolved photoemission spectra of the heavy-fermion superconductor URu2Si2. Detailed measurements as a function of both photon energy and temperature allow us to disentangle a variety of spectral features, revealing the evolution of the low-energy electronic structure across the "hidden order" transition. Above the transition, our measurements reveal the existence of weakly dispersive states that exhibit a large scattering rate and do not appear to shift from above to below the Fermi level, as previously reported. Upon entering the hidden order phase, these states rapidly hybridize with light conduction band states and transform into a coherent heavy fermion liquid, coincident with a dramatic drop in the scattering rate. This evolution is in stark contrast with the gradual crossover expected in Kondo lattice systems, which we attribute to the coupling of the heavy fermion states to the hidden order parameter.

Nodeless superconducting phase arising from a strong (π, π) antiferromagnetic phase in the infinite-layer electron-doped Sr(1-x)La(x)CuO2 compound.

The asymmetry between electron and hole doping remains one of the central issues in high-temperature cuprate superconductivity, but our understanding of the electron-doped cuprates has been hampered by apparent discrepancies between the only two known families: Re(2-x)Ce(x)CuO4 and A(1-x)La(x)CuO2. Here we report in situ angle-resolved photoemission spectroscopy measurements of epitaxially stabilized Sr(1-x)La(x)CuO2 thin films synthesized by oxide molecular-beam epitaxy. Our results reveal a strong coupling between electrons and (π, π) antiferromagnetism that induces a Fermi surface reconstruction which pushes the nodal states below the Fermi level. This removes the hole pocket near (π/2, π/2), realizing nodeless superconductivity without requiring a change in the symmetry of the order parameter and providing a universal understanding of all electron-doped cuprates.

Comparing Thickness and Doping-Induced Effects on the Normal States of Infinite-Layer Electron-Doped Cuprates: Is There Anything to Learn?

We grew Sr1-xLaxCuO2 thin films and SrCuO2/Sr0.9La0.1CuO2/SrCuO2 trilayers by reflection high-energy diffraction-calibrated layer-by-layer molecular beam epitaxy, to study their electrical transport properties as a function of the doping and thickness of the central Sr0.9La0.1CuO2 layer. For the trilayer samples, as already observed in underdoped SLCO films, the electrical resistivity versus temperature curves as a function of the central layer thickness show, for thicknesses thinner than 20 unit cells, sudden upturns in the low temperature range with the possibility for identifying, in the normal state, the T* and a T** temperatures, respectively, separating high-temperature linear behavior and low-temperature quadratic dependence. By plotting the T* and T** values as a function of TConset for both the thin films and the trilayers, the data fall on the same curves. This result suggests that, for the investigated trilayers, the superconducting critical temperature is the important parameter able to describe the normal state properties and that, in the limit of very thin central layers, such properties are mainly influenced by the modification of the energy band structure and not by interface-related disorder.

Interfacial charge transfer and persistent metallicity of ultrathin SrIrO3/SrRuO3 heterostructures.

Interface quantum materials have yielded a plethora of previously unknown phenomena, including unconventional superconductivity, topological phases, and possible Majorana fermions. Typically, such states are detected at the interface between two insulating constituents by electrical transport, but whether either material is conducting, transport techniques become insensitive to interfacial properties. To overcome these limitations, we use angle-resolved photoemission spectroscopy and molecular beam epitaxy to reveal the electronic structure, charge transfer, doping profile, and carrier effective masses in a layer-by-layer fashion for the interface between the Dirac nodal-line semimetal SrIrO3 and the correlated metallic Weyl ferromagnet SrRuO3. We find that electrons are transferred from the SrIrO3 to SrRuO3, with an estimated screening length of λ = 3.2 ± 0.1 Å. In addition, we find that metallicity is preserved even down to a single SrIrO3 layer, where the dimensionality-driven metal-insulator transition typically observed in SrIrO3 is avoided because of strong hybridization of the Ir and Ru t2g states.

Strong interlayer interactions in bilayer and trilayer moiré superlattices.

Moiré superlattices constructed from transition metal dichalcogenides have demonstrated a series of emergent phenomena, including moiré excitons, flat bands, and correlated insulating states. All of these phenomena depend crucially on the presence of strong moiré potentials, yet the properties of these moiré potentials, and the mechanisms by which they can be generated, remain largely open questions. Here, we use angle-resolved photoemission spectroscopy with submicron spatial resolution to investigate an aligned WS2/WSe2 moiré superlattice and graphene/WS2/WSe2 trilayer heterostructure. Our experiments reveal that the hybridization between moiré bands in WS2/WSe2 exhibits an unusually large momentum dependence, with the splitting between moiré bands at the Γ point more than an order of magnitude larger than that at K point. In addition, we discover that the same WS2/WSe2 superlattice can imprint an unexpectedly large moiré potential on a third, separate layer of graphene (g/WS2/WSe2), suggesting new avenues for engineering two-dimensional moiré superlattices.

Amorphization mechanism of SrIrO3 electrocatalyst: How oxygen redox initiates ionic diffusion and structural reorganization.

The use of renewable electricity to prepare materials and fuels from abundant molecules offers a tantalizing opportunity to address concerns over energy and materials sustainability. The oxygen evolution reaction (OER) is integral to nearly all material and fuel electrosyntheses. However, very little is known about the structural evolution of the OER electrocatalyst, especially the amorphous layer that forms from the crystalline structure. Here, we investigate the interfacial transformation of the SrIrO3 OER electrocatalyst. The SrIrO3 amorphization is initiated by the lattice oxygen redox, a step that allows Sr2+ to diffuse and O2- to reorganize the SrIrO3 structure. This activation turns SrIrO3 into a highly disordered Ir octahedral network with Ir square-planar motif. The final Sr y IrO x exhibits a greater degree of disorder than IrO x made from other processing methods. Our results demonstrate that the structural reorganization facilitated by coupled ionic diffusions is essential to the disordered structure of the SrIrO3 electrocatalyst.

Effects of Anisotropic Strain on Spin-Orbit Torque Produced by the Dirac Nodal Line Semimetal IrO2.

We report spin-torque ferromagnetic resonance studies of the efficiency of the damping-like (ξDL) spin-orbit torque exerted on an adjacent ferromagnet film by current flowing in epitaxial (001) and (110) IrO2 thin films. IrO2 possesses Dirac nodal lines (DNLs) in the band structure that are gapped by spin-orbit coupling, which could enable a very high spin Hall conductivity, σSH. We find that the (001) films do exhibit exceptionally high ξDL ranging from 0.45 at 293 K to 0.65 at 30 K, which sets the lower bounds of σSH to be 1.9 × 105 and 3.75 × 105 Ω-1 m-1, respectively, 10 times higher and of opposite sign than the theoretical prediction. Furthermore, ξDL and σSH are substantially reduced in anisotropically strained (110) films. We suggest that this high sensitivity to anisotropic strain is because of changes in contributions to σSH near the DNLs.

Realization of Epitaxial Thin Films of the Topological Crystalline Insulator Sr3 SnO.

Topological materials are derived from the interplay between symmetry and topology. Advances in topological band theories have led to the prediction that the antiperovskite oxide Sr3 SnO is a topological crystalline insulator, a new electronic phase of matter where the conductivity in its (001) crystallographic planes is protected by crystallographic point group symmetries. Realization of this material, however, is challenging. Guided by thermodynamic calculations, a deposition approach is designed and implemented to achieve the adsorption-controlled growth of epitaxial Sr3 SnO single-crystal films by molecular-beam epitaxy (MBE). In situ transport and angle-resolved photoemission spectroscopy measurements reveal the metallic and electronic structure of the as-grown samples. Compared with conventional MBE, the used synthesis route results in superior sample quality and is readily adapted to other topological systems with antiperovskite structures. The successful realization of thin films of Sr3 SnO opens opportunities to manipulate topological states by tuning symmetries via strain engineering and heterostructuring.