Darrell G. Schlom

F16. 00001: Superconducting Sr 2 RuO 4 thin films with record transition temperature up to 2 K by molecular-beam epitaxy

There is significant technological interest in developing ever faster switching between different electronic and magnetic states of matter. Manipulating properties at terahertz rates requires accessing the intrinsic timescales of both electrons and associated phonons, which is possible with short-pulse photoexcitation. However, in many Mott insulators, the electronic transition is accompanied by the nucleation and growth of percolating domains of the changed lattice structure, leading to empirical timescales dominated by slowly coarsening dynamics. Here we use time-resolved X-ray diffraction and reflectivity measurements to show that the photoinduced insulator-to-metal transition in an epitaxially strained Mott insulating thin film occurs without observable domain formation and coarsening effects, allowing the study of the intrinsic electronic and lattice dynamics. Above a fluence threshold, the initial electronic excitation …

S08. 00004: MBE synthesis and in situ ARPES of epitaxial thin films of the heavy fermion superconductor CeCoIn5

The coupling between ferroelectric and ferroelastic order parameters in transition metal oxide systems offer unique opportunities to employ strain as a tuning parameter of polar order at the nanoscale limit with large scale technological implications. KNbO 3 is an archetypal ferroelectric with a rich structural phase diagram in the bulk and provides the possibility of stabilising non-trivial phases through strain-engineering which are otherwise thermodynamically inaccessible. The ability to grow KNbO 3 thin films by sub-oxide molecular beam epitaxy allows the application of large strains with a high degree of control in epitaxial thin films. The new stabilised structure is probed through temperature dependent second harmonic generation, synchrotron-based x-ray reciprocal space mapping and transmission electron microscopy which reveals a giant enhancement in ferroelectric Tc, polarisation and nonlinear optical …

We report the use of suboxide molecular-beam epitaxy (S-MBE) to grow α-(AlxGa1− x) 2O3 films on (110) sapphire substrates over the 0< x< 0.95 range of aluminum content. In S-MBE, 99.98% of the gallium-containing molecular beam arrives at the substrate in a preoxidized form as gallium suboxide (Ga2O). This bypasses the rate-limiting step of conventional MBE for the growth of gallium oxide (Ga2O3) from a gallium molecular beam and allows us to grow fully epitaxial α-(AlxGa1− x) 2O3 films at growth rates exceeding 1 μm/h and relatively low substrate temperature (Tsub= 605±15○ C). The ability to grow α-(AlxGa1− x) 2O3 over the nominally full composition range is confirmed by Vegard’s law applied to the x-ray diffraction data and by optical bandgap measurements with ultraviolet–visible spectroscopy. We show that S-MBE allows straightforward composition control and bandgap selection for α-(AlxGa1− x …

FIG. 1. Γ-diagrams of β− Ga2O3 (201) and ϵ− Ga2O3 (001) on AlN (0001).(a) Γ of β− Ga2O3 (201) as a function of ϕGa. For MOCATAXY, ϕIn= 0.5 nm− 2 s− 1 was additionally supplied, resulting in a growth rate increase ΔΓ of β− Ga2O3 (201) as indicated in the figure. The expansion of the O-rich regime by MOCATAXY is indicated by ΔϕO at the corresponding stoichiometric points.(b) Γ of β− Ga2O3 (201) as a function of ϕGa2O= 1/2ϕGa by S-MBE and MBE. 30 The growth of Ga2O3 by S-MBE takes place in the adsorption-controlled regime with maximized Γ, whereas the growth of Ga2O3 by MBE is kinetically forbidden at the same ϕGa2O= 1/2ϕGa, indicated by ΔΓ in the figure.(c) Γ of β− Ga2O3 (201) and ϵ− Ga2O3 (001) as a function of TG by MBE and MOCATAXY. At ϕO= 3.0 nm− 2 s− 1, the gray and gray-dashed areas in panel (c) refer to the regime where β− Ga2O3 and ϵ− Ga2O3 can be synthesized by …

Single-crystal substrate materials with crystal structures and lattice parameters matching a desired epitaxial film are an enabling technology for many critical materials. Such substrates are best grown by bulk techniques that benefit from the substrate material being congruently melting. The shortage of congruently melting perovskites in critical lattice parameter ranges has been addressed herein by a search for new congruently melting compositions. A solid solution of two perovskites can be congruently melting at a minimum temperature under specified thermodynamic conditions where the coefficient matrix has a zero determinant. This is called an indifferent point. A wide variety of perovskite solid solutions were investigated to identify compounds that have an indifferent melting minimum and a cubic crystal structure with favorable lattice constants in the range of 0.390–0.412 nm. Solid solution pairs that form such …

Advancements of high-power laser sources in the THz frequency range have opened up opportunities for coherent intense excitation of infrared (IR)-active phonons in crystalline materials. The nonlinear phononics mechanism utilizes anharmonic coupling between driven IR-active modes and other lattice modes to induce changes in crystal structure and functional properties on picosecond timescales. Recent experimental and theoretical works have noted the potential for phonon-induced strains on these short time scales [npj Quantum Mater. 5, 95 (2020), PRL 129, 167401 (2022)]. A natural way to measure the response of the lattice due to these THz pulses is by employing X-ray diffraction to observe changes to the position and intensities of Bragg peaks.We show, using theory and first-principle calculations, that these lattice anharmonicities are responsible for the expansion of the c-axis in a biaxially strained thin …

A hallmark of many unconventional superconductors is the presence of many-body interactions that give rise to broken-symmetry states intertwined with superconductivity. Recent resonant soft X-ray scattering experiments report commensurate 3a0 charge density wave order in infinite-layer nickelates, which has important implications regarding the universal interplay between charge order and superconductivity in both cuprates and nickelates. Here we present X-ray scattering and spectroscopy measurements on a series of NdNiO2+x samples, which reveal that the signatures of charge density wave order are absent in fully reduced, single-phase NdNiO2. The 3a0 superlattice peak instead originates from a partially reduced impurity phase where excess apical oxygens form ordered rows with three-unit-cell periodicity. The absence of any observable charge density wave order in NdNiO2 highlights a crucial …

Strain-engineering is a powerful means to tune the polar, structural, and electronic instabilities of ferroelectrics. KTaO 3 is an incipient ferroelectric, with a very large spin-orbit coupling, in which highly anisotropic superconductivity emerges near a polar instability in electron doped samples [1, 2]. Growth of high-quality epitaxial films provides an opportunity to use epitaxial strain to finely tune electronic and polar instabilities in KTaO 3. Using a molecular beam of the suboxides TaO 2 emanating from effusion cells containing Ta 2 O 5 combination with a molecular beam of potassium emanating from an indium-potassium intermetallic in an oxidant (~ 10% O 3+ 90% O 2) background pressure of 1x10–6 Torr, KTaO 3 films are grown under conditions of excess potassium in an absorption-controlled regime. Biaxial strains ranging from to are imposed on the commensurately strained KTaO 3 films by growing them upon …

We conduct local magnetic measurements on superconducting thin-film samples of SrRuO using scanning Superconducting Quantum Interference Device (SQUID) susceptometry. From the diamagnetic response, we extract the magnetic penetration depth, , which exhibits a quadratic temperature dependence at low temperatures. Although a quadratic dependence in high-purity bulk samples has been attributed to non-local electrodynamics, our analysis suggests that in our thin-film samples the presence of scattering is the origin of the quadratic dependence. While we observe micron-scale variations in the diamagnetic response and superconducting transition temperature, the form of the temperature dependence of is independent of position. Finally, we characterize flux trapping in superconducting rings lithographically fabricated from the thin films, paving the way to systematic device-based tests of the superconducting order parameter in SrRuO.

ABSTRACT β-Ga2O3 is actively touted as the next ultrawide bandgap material for power electronics. To fully utilize its high intrinsic critical electric field, development of high-quality robust large-barrier height junctions is essential. To this end, various high-work function metals, metal oxides, and hole-conducting oxides have been deposited on Ga2O3, primarily formed by sputter deposition. Unfortunately, reports to date indicate that measured barrier heights often deviate from the Schottky–Mott model as well as x-ray photoelectron spectroscopy (XPS) extractions of conduction band offsets, suggesting significant densities of electrically active defects at these junctions. We report Schottky diodes made from noble metal oxides, IrO2 and RuO2, deposited by ozone molecular beam epitaxy (ozone MBE) with barrier heights near 1.8 eV. These barriers show close agreement across extraction methods and robust to high …

We present an integrated procedure for the synthesis of infinite-layer nickelates using molecular-beam epitaxy with gas-phase reduction by atomic hydrogen. We first discuss challenges in the growth and characterization of perovskite NdNiO/SrTiO, arising from post growth crack formation in stoichiometric films. We then detail a procedure for fully reducing NdNiO films to the infinite-layer phase, NdNiO, using atomic hydrogen; the resulting films display excellent structural quality, smooth surfaces, and lower residual resistivities than films reduced by other methods. We utilize the in situ nature of this technique to investigate of the role that SrTiO capping layers play in the reduction process, illustrating their importance in preventing the formation of secondary phases at the exposed nickelate surface. A comparative bulk- and surface-sensitive study indicates formation of a polycrystalline crust on the film surface serves to limit the reduction process.

S57. 00004: Molecular-beam epitaxy synthesis and ARPES studies of SrMoO3 thin films with record low resistivities*

In bulk SrRuO, the strong sensitivity of the superconducting transition temperature to nonmagnetic impurities provides robust evidence for a superconducting order parameter that changes sign around the Fermi surface. In superconducting epitaxial thin-film SrRuO, the relationship between and the residual resistivity , which in bulk samples is taken to be a proxy for the low-temperature elastic scattering rate, is far less clear. Using high-energy electron irradiation to controllably introduce point disorder into bulk single-crystal and thin-film SrRuO, we show that is suppressed in both systems at nearly identical rates. This suggests that part of in films comes from defects that do not contribute to superconducting pairbreaking, and establishes a quantitative link between the superconductivity of bulk and thin-film samples.

The drive toward non‐von Neumann device architectures has led to an intense focus on insulator‐to‐metal (IMT) and the converse metal‐to‐insulator (MIT) transitions. Studies of electric field‐driven IMT in the prototypical VO2 thin‐film channel devices are largely focused on the electrical and elastic responses of the films, but the response of the corresponding TiO2 substrate is often overlooked, since it is nominally expected to be electrically passive and elastically rigid. Here, in‐operando spatiotemporal imaging of the coupled elastodynamics using X‐ray diffraction microscopy of a VO2 film channel device on TiO2 substrate reveals two new surprises. First, the film channel bulges during the IMT, the opposite of the expected shrinking in the film undergoing IMT. Second, a microns thick proximal layer in the substrate also coherently bulges accompanying the IMT in the film, which is completely unexpected. Phase …

Magnons, elementary excitations of the magnetic order, have been manifested as ultralow energy information carriers for future technological applications such as magneto-electric spin orbit coupled logic. Bismuth ferrite (BiFeO 3, BFO) is the only insulating antiferromagnet that exhibits multiple order parameters and has been demonstrated to be useful for electric field manipulation of the magnetic order [1], as well as control of magnon transport [2]. The coercive electric field in BFO, however, is large, limiting its practical usefulness. La-substitution (LBFO) has been proposed to mitigate this challenge [3], yet the effects of La-substitution on the local magnetic properties are not well understood. In this work, we experimentally demonstrate the antiferromagnetic non-local magnon spin transport in LBFO and detected via inverse spin Hall effect of Pt. While BFO exhibits well defined ferroelectric domains, LBFO has mixed …

Various embodiments disclosed herein provide for several methods of fabricating p-type semiconductors with industrially relevant hole mobilities that are back end of the line (BEOL) compatible. A first method of fabrication includes forming a buffer layer on a substrate, forming a palladium oxide layer over the buffer layer, annealing the palladium oxide layer, and then forming a cap layer over the palladium oxide layer, then cooling the stack, wherein each step is performed at a variety of predefined temperatures. Each of the substrate, buffer layer and cap layer can be magnesium oxide. A second method includes forming a palladium oxide layer over a titanium dioxide substrate, annealing the stack, and then cooling the stack, all performed at a different variety of predefined temperatures.

Recent work on searching for high mobility p-type oxides have led to the identification of several promising p-type oxide candidates. Among them, post-transition metal cations, including Sn2+, Pb2+, Sb3+, Tl1+, and Bi3+ are good materials space for high performance p-type oxides design due to their occupied lone-pair s orbitals hybridizing with the O-2p orbital at the valence band edge. However, few of them have been experimentally proven applicable until now, due to their weak phase stability such as SnO or limited p-type dopability like Ba2BiTaO6. In this work, we focus on Pb2+ chemistry which has not been previously investigated, to design phase stable and hole dopable p-type oxides. It is found that the atomic orbital energy level of Pb-6s in Pb2+-based oxides is deep and thus guarantees a stable Pb2+ valence stability as well as compound phase stability. While monoxide PbO has spontaneous formation …

Thermally excited magnons in an antiferromagnetic insulator can give rise to a non-local inverse spin Hall voltage in a detector wire in a planar device structure. Recently, the multiferroic bismuth ferrite (BFO) has been demonstrated to exhibit electric field controlled magnon transport, offering tantalizing opportunities for electrically controllable spintronic devices. The magnetic order of BFO, however, is complex, hosting a spin cycloid tied to the ferroelectric polarization, and the physics of spin transport in BFO remains a significant question. Here, we present a phenomenological model for magnon spin transport in polar antiferromagnets, inspired by the symmetries associated with the material and the device geometry. The model summarizes the physics of magnon spin transport in a single function η representing the extent to which a given magnon mode will contribute to the inverse spin Hall voltage in the detector …

Rare earth dopants are one of the most extensively studied optical emission centers for a broad range of applications such as laser optoelectronics, sensing, lighting, and quantum information technologies due to their narrow optical linewidth and exceptional coherence properties. Epitaxial doped oxide thin films can serve as a promising and controlled host to investigate rare-earth dopants suitable for scalable quantum memories, on-chip lasers and amplifiers. Here, we report high-quality epitaxial thin films of Tm-doped CaZrO grown by pulsed laser deposition for infrared optoelectronic and quantum memory applications. We perform extensive structural and chemical characterization to probe the crystallinity of the films and the doping behavior. Low temperature photoluminescence measurements show sharp radiative transitions in the short-wave infrared range of 1.75 - 2 \mu m.

T16. 00009: Oxygen ordered phases and resonant scattering measurements of the infinite-layer nickelates*

Here, we report that a source of Si impurities commonly observed on (010) β-Ga 2 O 3 is from exposure of the surface to air. Moreover, we find that a 15 min hydrofluoric acid (HF)(49%) treatment reduces the Si density by approximately 1 order of magnitude on (010) β-Ga 2 O 3 surfaces. This reduction in Si is critical for the elimination of the often observed parasitic conducting channel, which negatively affects transport properties and lateral transistor performance. After the HF treatment, the sample must be immediately put under vacuum, for the Si fully returns within 10 min of additional air exposure. Finally, we demonstrate that performing a 30 min HF (49%) treatment on the substrate before growth has no deleterious effect on the structure or on the epitaxy surface after subsequent Ga 2 O 3 growth.

Immediately following its first synthesis in the 1980's Ba 3 In 2 O 6 was, due to its structural similarity to La (2-x) Sr x CaCu 2 O 6 and La 2 CaCu 2 O (6+ Δ), hypothesized to be a high-T c superconductor. Ba 3 In 2 O 6 being highly hygroscopic, however, inhibited any characterization of its transport properties. In the following years the material was all but forgotten until recently machine learning predicted its T c to be 45.9 K. To answer the question whether Ba 3 In 2 O 6 is a high T c superconductor, Ba 3 In 2 O 6 films were grown by molecular-beam epitaxy and capped by amorphous SiO 2. The indium species were supplied by a newly developed suboxide source emanating a beam of In 2 O. In the course of successfully growing epitaxial Ba 3 In 2 O 6 films, the previously unknown member of the same Ruddlesden-Popper series, Ba 4 In 2 O 7, was also epitaxially grown. Despite the high quality of our films, which …

Frustrated by reproducibility in electrical measurements on ferroelectric films, Lane Martin, Jon-Paul Maria and Darrell Schlom discuss tactics to reliably synthesize ‘good’ ferroelectric samples, especially in the search for superior materials and device heterostructures.

Motivated by the potential to create electronic and magnetic characteristics that are not accessible in bulk systems, the study of low-dimensional metallic delafossites with the general chemical formula ABO2 is gaining popularity. Understanding the surface behavior of the delafossite compounds is crucial due to their distinct layer structure, which gives rise to diverse surface states, electronic reconstructions, and surface reconstructions. We study epitaxial PtCoO 2, PdCoO 2, and PdCrO 2 films by combining molecular-beam epitaxy and angle-resolved photoemission spectroscopy. Through precise control of surface termination and treatment, we observed distinct surface reconstructions on the PtCoO 2 films, PdCoO 2 films, and PdCrO 2 films. Notably, these reconstructions have not been reported in prior studies of delafossites. Furthermore, our computational analysis reveals the relative stability of the BO 2 surface …

W05. 00010: Ultraviolet Raman spectroscopy characterization of epitaxial SrTiO 3 films grown with substrate temperature gradient*

Spin waves in magnetic materials are promising information carriers for future computing technologies due to their ultra-low energy dissipation and long coherence length. Antiferromagnets are strong candidate materials due, in part, to their stability to external fields and larger group velocities. Multiferroic aniferromagnets, such as BiFeO (BFO), have an additional degree of freedom stemming from magnetoelectric coupling, allowing for control of the magnetic structure, and thus spin waves, with electric field. Unfortunately, spin-wave propagation in BFO is not well understood due to the complexity of the magnetic structure. In this work, we explore long-range spin transport within an epitaxially engineered, electrically tunable, one-dimensional (1D) magnonic crystal. We discover a striking anisotropy in the spin transport parallel and perpendicular to the 1D crystal axis. Multiscale theory and simulation suggests that this preferential magnon conduction emerges from a combination of a population imbalance in its dispersion, as well as anisotropic structural scattering. This work provides a pathway to electrically-reconfigurable magnonic crystals in antiferromagnets.

Y62. 00013: Enhancement of Spin-Orbit Torque Generation from Antiferromagnetic Ordering in PdCrO 2*

A magnon is a collective excitation of the spin structure in a magnetic insulator and can transmit spin angular momentum with negligible dissipation. This quantum of a spin wave has always been manipulated through magnetic dipoles (that is, by breaking time-reversal symmetry). Here we report the experimental observation of chiral spin transport in multiferroic BiFeO3 and its control by reversing the ferroelectric polarization (that is, by breaking spatial inversion symmetry). The ferroelectrically controlled magnons show up to 18% modulation at room temperature. The spin torque that the magnons in BiFeO3 carry can be used to efficiently switch the magnetization of adjacent magnets, with a spin–torque efficiency comparable to the spin Hall effect in heavy metals. Utilizing such controllable magnon generation and transmission in BiFeO3, an all-oxide, energy-scalable logic is demonstrated composed of spin–orbit …

The magnetoelectric behavior of epitaxial Fe-Ga microstructures on top of (001)-oriented PMN-PT piezoelectric substrate is imaged with magnetic X-ray microscopy. Additionally, micron-scale strain distribution in PMN-PT is characterized by X-ray microdiffraction and examined with respect to the results of the Fe-Ga magnetoelectric switching. The magnetic reorientation is found to be strongly correlated with size, shape and crystallographic orientation of the microstructures. In the case of square-shaped strutures, size dictates the degree of influence of the strain distribution on both the initialization of the ground state and on the dynamics of the magnetic reorientation during application of voltage. On the other hand, the case of the elliptical microstructures demonstrates the importance of the orientation of the long axis with respect to the crystallographic directions of the PMN-PT, which leads to completely different …

The rare earth nickelate perovskites, RENiO 3 RE= La, Pr, Nd,..., exhibit a variety of intertwined phase transitions including a structural rearrangement, charge disproportionation, antiferromagnetic order, and a metal-to-insulator (MIT) transition. Epitaxial thin films have offered a fruitful platform to engineer these phases using doping, epitaxial strain, heterostructuring, and confinement. For example, it has been previously established that in ultrathin films of LaNiO 3 under compressive strain the metallic ground state is suppressed in favor of an insulating one as the film thickness is reduced. In this talk we illustrate an opposite effect where the MIT in highly tensily strained NdNiO 3/SrTiO 3 (100) films can be continuously suppressed by decreasing the film thickness. At a critical thickness we observe a complete suppression of the insulating ground state in favor of a metallic one. We discuss the investigation of this …

Aluminum plays a central role in the world of electronic oxide materials. Yet, aluminum sources are very difficult to handle during oxide molecular-beam epitaxy, the main reason for which is the high oxidization potential of aluminum. In this work, we present a thorough study of the behavior of aluminum sources during oxide thermal laser epitaxy. We identify two distinct operating regimes. At high laser-beam fluences, the source emanates reproducible fluxes independent of an applied oxygen pressure of< 10− 1 hPa. At lower beam fluences, the flux increases with increasing oxygen pressure (⁠< 10− 1 hPa) due to suboxide formation. We demonstrate reproducible rate control over a flux range of 5 orders of magnitude, which can be expanded further. These results demonstrate that thermal laser epitaxy does not present the challenges associated with the evaporation of aluminum during oxide molecular-beam epitaxy.

We demonstrate the use of a chip-scale, coplanar waveguide resonator in temperature-dependent Electron Spin Resonance (ESR) spectroscopy of defects in the ultra-wide bandgap semiconductor β-Ga 2 O 3. The study of point defects in β-Ga 2 O 3, many of which are spin active, is essential to establish it as a material platform for high-power electronics. Further, ESR spectroscopy has been a useful tool in the study of defects in semiconductors owing to its sensitivity to a low density of spin-active defects, its ability to readily differentiate between various defect charge states, and to discern asymmetries in the defects' environment by extracting the associated g-tensor. However, commercially available ESR spectrometers utilize bulky cavity resonators which are not sensitive to a low density of spin-defects in sub-micron epitaxial thin-films, at interfaces, or on surfaces due to the resonator's mode volume being much …

Despite significant achievements in characterizing the properties of Sr 2 RuO 4 over the last three decades, the precise nature of its electronic ground state is still unresolved. In this work, we provide a missing piece of the puzzle by uncovering evidence of electronic nematic order in the normal state of Sr 2 RuO 4, revealed by ultrafast time-resolved optical dichroism measurements of uniaxially strained thin films. This nematic order, whose domains are aligned by the strain, spontaneously breaks the fourfold rotational symmetry of the crystal. The temperature dependence of the dichroism resembles an Ising-like order parameter, and optical pumping induces a coherent oscillation of its amplitude mode. The existence of electronic nematic order in the normal state of Sr 2 RuO 4 may have consequences for the form and mechanism of superconductivity in this material.

Epitaxial untwinned SrRuO3 thin films were grown on (110)-oriented DyScO3 substrates by molecular-beam epitaxy. We report an exceptional sample with a residual resistivity ratio (RRR), ρ [300 K]/ρ [4 K] of 205 and a ferromagnetic Curie temperature, TC, of 168.3 K. We compare the properties of this sample to other SrRuO3 films grown on DyScO3 (110) with RRRs ranging from 8.8 to 205, and also compare it to the best reported bulk single crystal of SrRuO3. We determine that SrRuO3 thin films grown on DyScO3 (110) have an enhanced TC as long as the RRR of the thin film is above a minimum electrical quality threshold. This RRR threshold is about 20 for SrRuO3. Films with lower RRR exhibit TCs that are significantly depressed from the intrinsic strain-enhanced value.

We report a resonant inelastic x-ray scattering investigation of ultrathin epitaxial films of Ba 2 IrO 4, and compare their low-energy magnetic and spin-orbit excitations to those of their sister compound Sr 2 IrO 4. Due to the 180∘ Ir-O-Ir bond, the bandwidth of the magnon and spin orbiton is significantly larger in Ba 2 IrO 4, making it difficult to describe these two types of excitations as separate well-defined quasiparticles. Both types of excitations are found to be quite sensitive to the effect of epitaxial strain. In addition, we find that the d-level inversion observed in Sr 2 IrO 4 is absent in Ba 2 IrO 4, as predicted in recent theoretical studies. Our results illustrate that the magnetic properties of Ba 2 IrO 4 are substantially different from those of Sr 2 IrO 4, suggesting that these materials need to be examined more carefully with electron itinerancy taken into account.

Quantum fluctuations in low-dimensional systems and near quantum phase transitions have significant influences on material properties. Yet, it is difficult to experimentally gauge the strength and importance of quantum fluctuations. Here we provide a resonant inelastic x-ray scattering study of magnon excitations in Mott insulating cuprates. From the thin film of SrCuO, single- and bi-magnon dispersions are derived. Using an effective Heisenberg Hamiltonian generated from the Hubbard model, we show that the single magnon dispersion is only described satisfactorily when including significant renormalization stemming from quantum fluctuations. Comparative results on LaCuO indicate that quantum fluctuations are much stronger in SrCuO suggesting closer proximity to a magnetic quantum critical point. Monte Carlo calculations suggest an exotic incommensurate magnetic order as the ground state that competes with the antiferromagnetic N\'eel order. Our results indicate that SrCuO -- due to strong quantum fluctuations -- is a unique starting point for the exploration of novel magnetic ground states.

Renewable energy sources such as solar and wind are critical to combatting global warming. Nevertheless, their intermittent energy generation requires the development of large-scale grid energy storage, in contrast to the ondemand generation of coal-based power plants. Sodium-ion batteries offer a promising potential technology, yet because sodium ions are larger than lithium ions, sodium-ion intercalation results in more drastic structural rearrangements. An improved understanding of structural dynamics and ionic diffusion pathways is crucial to developing more durable sodium-ion batteries. Here, we synthesize epitaxial NaxCoO2 by using molecular-beam epitaxy and topotactic transformation. In the synthesized epitaxial films, the CoO2 layers are canted with respect to the film surface, allowing electrochemical extraction of sodium ions, which we confirm via ex situ X-ray diffraction. We anticipate the epitaxial …

We report a synthetic route to achieve high electron mobility at room temperature in epitaxial La: BaSnO 3/SrZrO 3 heterostructures prepared on several oxide substrates. Room-temperature mobilities of 157, 145, and 143 cm 2 V− 1 s− 1 are achieved for heterostructures grown on DyScO 3 (110), MgO (001), and TbScO 3 (110) crystalline substrates, respectively. This is realized by first employing pulsed laser deposition to grow at very high temperature the SrZrO 3 buffer layer to reduce dislocation density in the active layer, then followed by the epitaxial growth of an overlaying La: BaSnO 3 active layer by molecular-beam epitaxy. Structural properties of these heterostructures are investigated, and the extracted upper limit of threading dislocations is well below 1.0× 10 10 cm− 2 for buffered films on DyScO 3, MgO, and TbScO 3 substrates. The present results provide a promising route toward achieving high mobility in …

Coherent X-ray diffraction imaging (CXDI) is a lensless imaging technique by directly inverting the oversampled diffraction pattern into real-space images. Because of its non-destructive nature and ability to image sub-picometer atomic-lattice displacements with nanometer resolutions, CXDI has been applied to visualize biological cells, strain in nanocrystals, and recently operando changes in energy materials. Yet, it is limited to objects spatially confined in all three dimensions. Here, we will demonstrate the extension of CXDI to mapping periodic lattice distortions in an extended object. By combining the conventional CXDI iterative computation and an unsupervised machine learning clustering algorithm, we imaged the periodic nanotextures in epitaxially strained Ca 2 RuO 4 thin films grown on LaAlO 3 substrate via the inversion of its diffraction pattern. The result reveals the formation of striped periodic …

Oxide heterostructures exhibit a vast variety of unique physical properties. Examples are unconventional superconductivity in layered nickelates and topological polar order in (PbTiO)/(SrTiO) superlattices. Although it is clear that variations in oxygen content are crucial for the electronic correlation phenomena in oxides, it remains a major challenge to quantify their impact. Here, we measure the chemical composition in multiferroic (LuFeO)/(LuFeO) superlattices, revealing a one-to-one correlation between the distribution of oxygen vacancies and the electric and magnetic properties. Using atom probe tomography, we observe oxygen vacancies arranging in a layered three-dimensional structure with a local density on the order of 10 cm, congruent with the formula-unit-thick ferrimagnetic LuFeO layers. The vacancy order is promoted by the locally reduced formation energy and plays a key role in stabilizing the ferroelectric domains and ferrimagnetism in the LuFeO and LuFeO layers, respectively. The results demonstrate the importance of oxygen vacancies for the room-temperature multiferroicity in this system and establish an approach for quantifying the oxygen defects with atomic-scale precision in 3D, giving new opportunities for deterministic defect-enabled property control in oxide heterostructures.

Renewable energy sources such as solar and wind are critical to combatting global warming. Nevertheless, their intermittent energy generation requires the development of large-scale grid energy storage, in contrast to the on-demand generation of coal-based power plants. Sodium-ion batteries offer a promising potential technology, yet because sodium ions are larger than lithium ions, sodium-ion intercalation results in more drastic structural rearrangements. An improved understanding of structural dynamics and ionic diffusion pathways is crucial to developing more durable sodium-ion batteries. Here, we synthesize epitaxial NaxCoO2 by using molecular-beam epitaxy and topotactic transformation. In the synthesized epitaxial films, the CoO2 layers are canted with respect to the film surface, allowing electrochemical extraction of sodium ions, which we confirm via ex situ X-ray diffraction. We anticipate the epitaxial …

Ruthenates are an intensively investigated class of materials because of their wide variety of ground states which include superconductivity in Sr 2 RuO 4, metamagnetism in Sr 3 Ru 2 O 7, ferromagnetism in SrRuO 3 and antiferromagnetism in Ca 2 RuO 4, etc.. Epitaxial strain has always been a key knob to tune these ground states and realize new functionalities in the Ruddlesden-Popper ruthenates when grown in thin-film form. Recently, RuO 2, the parent binary oxide of these ruthenates has attracted attention because of the discoveries of antiferromagnetism [4, 5], and superconductivity in this material. Here, we employ molecular-beam epitaxy to grow thin films of RuO 2 on isostructural (100)-oriented TiO 2 substrates and realize superconductivity at about 1 K. We show that the superconducting transition temperature (T c) and the nature of the superconducting dip changes as a function of the thickness of the …

Exotic properties associated with the emerging class of quantum materials are often coupled to picometer-scale lattice modulations in the host material. Although scanning transmission electron microscopy (STEM) is one of the foremost techniques for such atomic-scale characterization, conventional STEM imaging modalities are prone to artifacts arising from channeling effects, crystal mistilts and surface relaxations. Annular bright field (ABF) and differential phase contrast (DPC) techniques have welldocumented errors arising from variations in the diffraction condition [1, 2]. High-angle annular dark field (HAADF) imaging, although often more tolerant to diffraction artifacts, fails to capture the rich information encoded in the atomic positions of lighter atoms.With recent improvements in direct electron detectors [3, 4], electron ptychography has enabled reconstructions of the electrostatic scattering potential [5] and …

The concept of remote epitaxy involves a two-dimensional van der Waals layer covering the substrate surface, which still enable adatoms to follow the atomic motif of the underlying substrate. The mode of growth must be carefully defined as defects, e.g., pinholes, in two-dimensional materials can allow direct epitaxy from the substrate, which, in combination with lateral epitaxial overgrowth, could also form an epilayer. Here, we show several unique cases that can only be observed for remote epitaxy, distinguishable from other two-dimensional material-based epitaxy mechanisms. We first grow BaTiO3 on patterned graphene to establish a condition for minimizing epitaxial lateral overgrowth. By observing entire nanometer-scale nuclei grown aligned to the substrate on pinhole-free graphene confirmed by high-resolution scanning transmission electron microscopy, we visually confirm that remote epitaxy is operative …

We report a new synthetic route to achieve high electron mobility at room temperature in epitaxial La: BaSnO /SrZrO heterostructures. Room-temperature mobilities of 157, 145, and 143 cm V s are achieved for heterostructures grown on DyScO (110), MgO (001), and TbScO (110) crystalline substrates, respectively. This is realized by first employing pulsed laser deposition to grow at very high temperature the SrZrO buffer layer to reduce dislocation density in the active layer, then followed by the epitaxial growth of an overlaying La: BaSnO active layer by molecular beam epitaxy. Structural properties of these heterostructures are investigated, and the extracted upper limit of threading dislocations is well below cm for buffered films on DyScO , MgO, and TbScO substrates. The present results provide a promising route towards achieving high mobility in buffered La: BaSnO films …

Mott metal-insulator transitions possess electronic, magnetic, and structural degrees of freedom promising next generation energy-efficient electronics. We report a previously unknown, hierarchically ordered state during a Mott transition and demonstrate correlated switching of functional electronic properties. We elucidate in-situ formation of an intrinsic supercrystal in a Ca2RuO4 thin film. Machine learning-assisted X-ray nanodiffraction together with 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. Our findings add an additional degree of complexity to the physical understanding of Mott transitions, opening opportunities for designing materials with tunable electronic properties.

The discovery of greatly enhanced superconductivity at the interface between FeSe and SrTiO 3 has attracted enormous interest due to the potential of enhancing superconductivity through interfacial interactions. In our previous work, we confirmed the presence of intrinsic interfacial coupling of electrons in the FeSe layer to optical phonons in the adjacent SrTiO 3 substrate, but a clear causative connection between this phonon coupling and the apparent enhanced T c enhancement remains unestablished. In order to better understand the true contribution of interface phonon coupling on the resultant high-T c state, we consider a highly analogous system: alkali surface-doped FeSe films. This approach produces a controllably electron doped superconducting layer constrained to the film-vacuum interface, analogous to the FeSe/STO interface but lacking any phonon coupling effect. Here, using a simultaneous …

Molecular-beam epitaxy enables ultrathin functional materials to be combined in heterostructures to create emergent phenomena at the interface. Magnetic skyrmions are an example of an exciting phase found in such heterostructures. SrRuO3 and SrRuO3-based heterostructures have been at the center of the debate on whether a hump-like feature appearing in Hall resistivities is sufficient evidence to prove the presence of skyrmions in a material. To address the ambiguity, we synthesize a model heterostructure with engineered Berry curvature that combines, in parallel, a positive anomalous Hall effect (AHE) channel (a Sr0. 6Ca0. 4RuO3 layer) with a negative AHE channel (a SrRuO3 layer). We demonstrate that the two opposite AHE channels can be combined to artificially reproduce a “hump-like” feature, which closely resembles the hump-like feature typically attributed to the topological Hall effect and the …

RuO2 is a well-known electrocatalyst for the oxygen evolution reaction (OER), a critical electrochemical reaction in the water electrolyzer. The behavior of RuO2 during the OER is, however, a subject of debate, in particular, the corrosion mechanism. In this contribution, we report an electrochemical AFM (EC-AFM) study on well-defined RuO2 films grown using molecular beam epitaxy (MBE). We track the morphology of step edges prior, during, and after the OER in acid and alkaline electrolytes to gain insights into the corrosion behavior. Our EC-AFM experiment shows the potential range of RuO2 corrosion and hints at the role of strain and defects in the RuO2 stability. The heterogeneity of corrosion will be discussed in the context of the catalyst longevity along with possible mitigation strategies.

This patent document provides implementations and examples of circuits and devices based on low-energy consumption semiconductor structures exhibiting multi-valued states. In one aspect, a semiconductor device is configured to comprise: a multi-layer structure forming a magnetoelectric or multiferroic system to include a ferromagnetic, magnetostrictive layer that exhibits a biaxial magnetic anisotropy and an underlying metal structure exhibits a spin Hall effect to provide a conversion between electrical energy and magnetic energy with more than two distinctive magnetic states.

Metallic interfacial quantum materials are most readily detected at the interface between two insulating constituents by probes such as electrical transport. However, when either material is conducting, transport techniques become insensitive to interfacial properties. To overcome these limitations, we employ 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 ferromagnet SrRuO3. Through comparisons with first-principles theory calculations, we determine that electrons are transferred from the SrIrO3 to SrRuO3, with an estimated screening length of 3.2+/-0.1 Å. In addition, we find that metallicity is preserved even down to a single SrIrO 3 layer, where the dimensionality-driven metal …

Potassium niobate (KNbO3) is a lead-free perovskite ferroelectric structure, while potassium tantalate (KTaO3) is a quantum paraelectric material with a cubic structure. However, due to its high polarizability, a small chemical substitution or stress can drive KTaO3 to a ferroelectric state. K (Ta, Nb) O3 (KTN) is an unlimited solid solution of these compounds where Nb atoms replace Ta atoms. Depending on the Nb concentration, KTN can undergo a ferroelectric phase transition from the temperatures above room temperature to lower temperatures. However, the nature of this transition is debated and there is evidence for both a soft mode freezing (displacive transition) and an order-disorder transition1, 2. Electron microscopy can solve this puzzle by providing direct evidence of local and subtle lattice distortions and their correlation length with picometer resolution. This study investigates the nature of ferroelectric …

Recent research work on searching for high mobility p-type oxides has identified substantial promising p-type oxide candidates. However, very few of them have been experimentally proven with high p-type conductivity, due to their limited p-type dopability caused by the deep valence band edge (VBE). In this work, we report amorphous phase Ta2SnO6 (a-Ta2SnO6) possessing unusually shallow VBE that permits high p-type doping without hole-killing oxygen vacancy defects spontaneous formation, contrasting to crystalline Ta2SnO6 (c-Ta2SnO6). The shallow VBE in a-Ta2SnO6 also allows a low Schottky barrier height with contact metals and unique a−/c- Ta2SnO6 heterostructure device. The shallow VBE in a-Ta2SnO6 is due to the local structure disorder that circumvents the strong electrostatic Coulombic interaction between positively charged Ta5+ and Sn-5 s lone-pair electrons which accounts for the deep …

We present time-domain THz spectroscopy of thin films of the heavy-fermion superconductor CeCoIn. The complex optical conductivity is analyzed through a Drude model and extended Drude model analysis. Below the 40 K Kondo coherence temperature, a narrow Drude-like peak forms, as the result of the orbital - conduction electron hybridization and the formation of the heavy-fermion state. Via an extended Drude model analysis, we measure the frequency-dependent scattering rate () and effective mass (). This scattering rate shows a linear dependence on temperature, which matches the dependence of the resistivity as expected. Nonetheless, the width of the low-frequency Drude peak (characterized by a {\it renormalized} quasiparticle scattering rate () does show a dependence giving evidence for a hidden Fermi state.

Competition between ground states at phase boundaries can lead to significant changes in properties under stimuli, particularly when these ground states have different crystal symmetries. A key challenge is to stabilize and control the coexistence of symmetry-distinct phases. Using BiFeO3 layers confined between layers of dielectric TbScO3 as a model system, we stabilize the mixed-phase coexistence of centrosymmetric and non-centrosymmetric BiFeO3 phases at room temperature with antipolar, insulating and polar semiconducting behaviour, respectively. Application of orthogonal in-plane electric (polar) fields results in reversible non-volatile interconversion between the two phases, hence removing and introducing centrosymmetry. Counterintuitively, we find that an electric field ‘erases’ polarization, resulting from the anisotropy in octahedral tilts introduced by the interweaving TbScO3 layers. Consequently …

Magnetoelectric spin-orbit logic (MESO) devices comprise a magnetoelectric switch capacitor coupled to a spin-orbit coupling structure. The logic state of the MESO device is represented by the magnetization orientation of the ferromagnet of the magnetoelectric switch capacitor and the spin-orbit coupling structure converts the magnetization orientation of the ferromagnet to an output current. MESO devices in which all or at least some of the constituent layers of the device are perovskite materials can provide advantages such as improved control over the manufacturing of MESO devices and high quality interfaces between MESO layers due to the lattice matching of perovskite materials.

The simple, rutile oxide, RuO 2, is the parent material of the perovskite ruthenates and a source of current intrigue following the surprising discoveries of antiferromagnetism [1, 2], and strain-induced superconductivity in this material. The details of the magnetic structure and its relationship to the novel superconducting phase, however, remain open questions. Recent magnetic resonant scattering results from the highly-strained, superconducting films deviate from those in bulk RuO 2, and simulations of resonant diffraction from several antiferromagnetic motifs have led some researchers to question the evidence for collinear antiferromagnetism in RuO 2. These critics instead suggest a``chiral signature,''measured with circularly-polarized, resonant x-rays, as an unambiguous indicator of long-range magnetic order. Here we present the results of such an experiment, exploiting both circular and linear polarization …

Plasma discharge lamps are widely utilized in the practice of angle-resolved photoemission spectroscopy (ARPES) experiments as narrow-linewidth ultraviolet photon sources. However, many emission lines such as Ar-I, Ne-I, and Ne-II have closely spaced doublet emission lines, which result in superimposed replica on the measured ARPES spectra. Here, we present a simple method for subtracting the contribution of these doublet emission lines from photoemission spectra. Benchmarking against ARPES spectra of well-characterized 2D materials, we demonstrate that this algorithm manages to subtract the doublet signal and reproduce the key features of the monochromated He-I α spectra in a physically sound manner that reliably reproduces quantifiable dispersion relations and quasiparticle lifetimes.

Separating out the contributions of different scattering channels in strongly interacting metals is crucial in identifying the mechanisms that govern their properties. While momentum or current relaxation rates can be readily probed via \textit{dc} resistivity or optical/THz spectroscopy, distinguishing different kinds of inelastic scattering can be more challenging. Using nonlinear THz 2D coherent spectroscopy, we measure the rates of energy relaxation after THz excitation in the strongly interacting Fermi liquid, SrRuO. Energy relaxation is a bound on the total scattering and specifically a measure of contributions to the electron self-energy that arise from {\it inelastic} coupling to a bath. We observe two distinct energy relaxation channels: a fast process that we interpret as energy loss to the phonon system and a much slower relaxation that we interpret as arising from a non-equilibrium phonon effects and subsequent heat loss through diffusion. Interestingly, even the faster energy relaxation rate is at least an order of magnitude slower than the overall momentum relaxation rate, consistent with strong electron interactions and the dominance of energy-conserving umklapp or interband electron-electron scattering in momentum relaxation. The slowest energy relaxation rate decays on a sub-GHz scale, consistent with the relaxation dynamics of non-equilibrium phonons. Our observations reveal the versatility of nonlinear THz spectroscopy to measure the energy relaxation dynamics in correlated metals. Our work also highlights the need for improved theoretical understanding of such processes in interacting metals.

A ferroelectric device includes a substrate, a first electrode on the substrate, and a hexagonal ferroelectric material on the first electrode. The first electrode comprises a single crystal epitaxial material. By using a single crystal epitaxial material for an electrode to a hexagonal ferroelectric material, a high-quality material interface may be provided between these layers, thereby improving the performance of the ferroelectric device by allowing for a reduced coercive field.

Rare-earth nickelates exhibit a variety of intriguing phenomena, including a metal-insulator transition, long-range magnetic order, and ferroelectricity. Using a combination of reflection momentum-resolved electron energy loss spectroscopy (EELS), angle-resolved photoemission spectroscopy, and oxide molecular beam epitaxy, we study the collective excitations of NdNiO 3 thin films. We observe a multitude of collective excitations, including phonons, plasmons, and dd excitations in the high-temperature metallic phase. As the insulating antiferromagnetic phase is reached, we observe a strong suppression of the plasmons and novel, dipole-active modes emerge in the EELS spectra. These modes are close in energy to magnon excitations and exhibit highly asymmetric line shapes. We discuss the possible origin of these modes, including the relationship between magnetic excitations and the electric dipole degrees …

TbScO 3 is a wide bandgap semiconductor with potential applications in charge trap memory devices and acts as an alternate gate dielectric in fully depleted transistors and also a substrate for epitaxial thin film growth. TbScO 3 has an orthorhombic crystal structure, which gives rise to optical anisotropy. Generalized ellipsometric spectra are measured for multiple in-plane rotations of (110) and (001) oriented TbScO 3 single crystals over a photon energy range of 0.7–8.5 eV to determine the complex dielectric function (ε= ε 1+ iε 2) spectra for electric fields oscillating along each axis. A direct bandgap is identified at 6.50 eV, and above gap critical point transitions are found at 6.99, 7.14, 7.16, 7.21, and 7.42 eV.

Alkali antimonide semiconductor photocathodes provide a promising platform for the generation of high-brightness electron beams, which are necessary for the development of cutting-edge probes, including x-ray free electron lasers and ultrafast electron diffraction. Nonetheless, to harness the intrinsic brightness limits in these compounds, extrinsic degrading factors, including surface roughness and contamination, must be overcome. By exploring the growth of CsxSb thin films monitored by in situ electron diffraction, the conditions to reproducibly synthesize atomically smooth films of CsSb on 3C–SiC (100) and graphene-coated TiO2 (110) substrates are identified, and detailed structural, morphological, and electronic characterization is presented. These films combine high quantum efficiency in the visible (up to 1.2% at 400 nm), an easily accessible photoemission threshold of 566 nm, low surface roughness …

Strain-engineering is a powerful means to tune the polar, structural, and electronic instabilities of incipient ferroelectrics. KTaO 3 is near a polar instability and shows anisotropic superconductivity in electron-doped samples. Here, we demonstrate growth of high-quality KTaO 3 thin films by molecular-beam epitaxy. Tantalum was provided by either a suboxide source emanating a TaO 2 flux from Ta 2 O 5 contained in a conventional effusion cell or an electron-beam-heated tantalum source. Excess potassium and a combination of ozone and oxygen (10% O 3+ 90% O 2) were simultaneously supplied with the TaO 2 (or tantalum) molecular beams to grow the KTaO 3 films. Laue fringes suggest that the films are smooth with an abrupt film/substrate interface. Cross-sectional scanning transmission electron microscopy does not show any extended defects and confirms that the films have an atomically abrupt interface with …

Technologies for a transistor with a ferroelectric gate dielectric are disclosed. In the illustrative embodiment, a transistor has a ferroelectric gate dielectric that is lattice matched to the channel of the transistor. In one embodiment, the ferroelectric polarization changes when voltage is applied and removed from a gate electrode, facilitating switching of the transistor at a lower applied voltage. In another embodiment, the ferroelectric polarization of a gate dielectric of a transistor changes when the voltage is past a positive threshold value or a negative threshold value. Such a transistor can be used as a one transistor memory cell.

Ferroelectric nanomaterials are of interest in catalysis, nonvolatile memory, and neuromorphic computing among other applications because of their switchable structure that can alter the electronic and interface properties of a single material. The investigation of the role of polarization on the surface structure and chemistry of ferroelectric nanomaterials is a longstanding challenge, as it ideally requires a combination of both nanoscale imaging and chemical spectroscopy. In this work, we study a model ferroelectric BaTiO3 thin film by synchrotron X-ray scanning tunneling microscopy (SX-STM), a unique method that integrates nanoscale surface imaging and chemically sensitive spectroscopy. We find that polarization switching from downward to upward in (001) single-crystalline BaTiO3 thin films increases the intensity of X-ray absorption across Ba M, Ti L, and O K edges. Chemical mapping of nanometer-sized …

The past decade has witnessed dramatic progress related to various aspects of emergent topological polar textures in oxide nanostructures displaying vortices, skyrmions, merons, hopfions, dipolar waves, or labyrinthine domains, among others. For a long time, these nontrivial structures (the electric counterparts of the exotic spin textures) were not expected due to the high energy cost associated with the dipolar anisotropy: the smooth and continuous evolution of the local polarization to produce topologically protected structures would result in a large elastic energy penalty. However, it was discovered that the delicate balance and intricate interplay between the electric, elastic, and gradient energies can be altered in low-dimensional forms of ferroelectric oxide nanostructures. These can be tuned to manipulate order parameters in ways once considered impossible. This review addresses the historical context that …

Emerging memories such as resistive random access memory (ReRAM) and spin-transfer torque magnetic random access memory (STT-MRAM) are candidates for embedded last-level persistent cache. The maximum attainable array efficiency relies on pairing the storage element with a compatible access device or selector. Here, we leverage the high-temperature spin-state driven insulator-to-metal (IMT) phase transition in a strongly correlated oxide, LaCoO3 (LCO), to demonstrate a two-terminal bi-directional selector suitable for cross-point embedded non-volatile memory. Vertical selectors are fabricated and characterized using epitaxial heterostructures of LCO thin films grown on La0.5Sr0.5CoO3 (LSCO) bottom electrodes. We demonstrate electrically triggered abrupt IMT switching in the selector devices across a range of chip operating temperatures (>85 °C) with switching speed less than 20 ns, on current density of …

Defect engineering in perovskite thin films has attracted extensive attention recently due to the films’ atomic-scale modification, allowing for remarkable flexibility to design novel nanostructures for next generation nanodevices. However, the defect-assisted three-dimensional nanostructures in thin film matrices usually has large misfit strain and thus causes unstable thin film structures. In contrast, defect-assisted one- or two-dimensional nanostructures embedded in thin films can sustain large misfit strains without relaxation, which make them suitable for defect engineering in perovskite thin films. Here, we reported the fabrication and characterization of edge-type misfit dislocation-assisted two-dimensional BiMnOx nanochannels embedded in SrTiO3/La0.7Sr0.3MnO3/TbScO3 perovskite thin films. The nanochannels are epitaxially grown from the surrounding films without noticeable misfit strain. Diode-like current …

Utilizing the powerful combination of molecular-beam epitaxy (MBE) and angle-resolved photoemission spectroscopy (ARPES) we produce and study the effect of different terminating layers on the electronic structure of the metallic delafossite PdCoO 2. Attempts to introduce unpaired electrons and synthesize new antiferromagnetic metals akin to the isostructural compound PdCrO 2 have been made by replacing cobalt with iron in PdCoO 2 films grown by MBE. Using ARPES, we observe similar bulk bands in these PdCoO 2 films with Pd-, CoO 2-, and FeO 2-termination. Nevertheless, Pd-and CoO 2-terminated films show a reduced intensity of surface states. Additionally, we are able to epitaxially stabilize PdFe x Co 1-x O 2 films which show an anomaly in the derivative of the electrical resistance with respect to temperature at 20 K, but do not display pronounced magnetic order.

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 …

Knowing oxide-forming ability is vital to gain desired or avoid deleterious oxides formation through tuning oxidizing environment and materials chemistry. Here, we have conducted a comprehensive thermodynamic analysis of 137 binary oxides using the presently predicted Ellingham diagrams. It is found that the active elements to form oxides easily are the f-block elements (lanthanides and actinides), elements in the groups II, III, and IV (alkaline earth, Sc, Y, Ti, Zr, and Hf), and Al and Li; while the noble elements with their oxides nonstable and easily reduced are coinage metals (Cu, Ag, and especially Au), Pt-group elements, and Hg and Se. Machine learning based sequential feature selection indicates that oxide-forming ability can be represented by electronic structures of pure elements, for example, their d- and s-valence electrons, Mendeleev numbers, and the groups, making the periodic table a useful tool to tailor oxide-forming ability. The other key elemental features to correlate oxide-forming ability are thermochemical properties such as melting points and standard entropy at 298 K of pure elements. It further shows that the present Ellingham diagrams enable qualitatively understanding and even predicting oxides formed in multicomponent materials, such as the Fe-20Cr-20Ni alloy (in wt.%) and the equimolar high entropy alloy of AlCoCrFeNi, which are in accordance with thermodynamic calculations using the CALPHAD approach and experimental observations in the literature.

A Sn2+ based oxide, ilmenite SnTiO3, is investigated as a potential p-type oxide. Due to Sn2+ chemistry, ilmenite SnTiO3 shows a dispersive valence band with a wide band gap of 2.4 eV and a high hole mobility of ∼60 cm2 V−1 s−1. Thermodynamic studies confirm its phase stability and synthesizability. Defect calculations demonstrate its high hole dopability. Further band alignment calculations show its shallow valence band edge suitable for metal contact. Amorphous phase SnTiO3 is revealed to be a high mobility hole-dopable oxide. The results suggest that ilmenite and amorphous SnTiO3 is an experimentally realizable p-type oxide promising for future oxide electronics.

The invention relates to a method for growing a bulk single crystal, wherein the method comprises the steps of

The heteroepitaxial growth and phase formation of Ga2O3 on Al-polar AlN (0001) templates by molecular-beam epitaxy (MBE) are studied. Three different MBE approaches are employed:(i) conventional MBE,(ii) suboxide MBE (S-MBE), and (iii) metal-oxide-catalyzed epitaxy (MOCATAXY). We grow phase-pure β-Ga2O3 (201) and phase-pure ϵ/κ-Ga2O3 (001) with smooth surfaces by S-MBE and MOCATAXY. Thin film analysis shows that the crystallographic and surface features of the β-Ga2O3 (201)/AlN (0001) and ϵ/κ-Ga2O3 (001)/AlN (0001) epilayers are of high crystalline quality. Growth and phase diagrams are developed to synthesize Ga2O3 on AlN by MBE and MOCATAXY and to provide guidance to grow Ga2O3 on several non-oxide surfaces, eg, AlN, GaN, and SiC, by MBE, S-MBE, and MOCATAXY.

Low resistance non-alloyed ohmic contacts are realized by a metal-first process on homoepitaxial, heavily n þ doped (010) b-Ga2O3. The resulting contacts have a contact resistance (Rc) as low as 0.23 X-mm on an as-grown sample and exhibit nearly linear ohmic behavior even without a post-metallization anneal. The metal-first process was applied to form non-alloyed contacts on n þ (010) b-Ga2O3 grown by metalorganic chemical vapor deposition (MOCVD) as well as suboxide molecular beam epitaxy. Identical contacts fabricated on similar MOCVD samples by conventional liftoff processing exhibit highly rectifying Schottky behavior. Re-processing using the metal-first process after removal of the poor contacts by conventional methods does not improve the contacts; however, addition of a Ga-flux polishing step followed by re-processing using a metal-first process again results in low resistance, nearly linear …

We report the use of suboxide molecular-beam epitaxy (S-MBE) to grow β-Ga2O3 at a growth rate of∼ 1 µm/h with control of the silicon doping concentration from 5× 1016 to 1019 cm− 3. In S-MBE, pre-oxidized gallium in the form of a molecular beam that is 99.98% Ga2O, ie, gallium suboxide, is supplied. Directly supplying Ga2O to the growth surface bypasses the rate-limiting first step of the two-step reaction mechanism involved in the growth of β-Ga2O3 by conventional MBE. As a result, a growth rate of∼ 1 µm/h is readily achieved at a relatively low growth temperature (Tsub≈ 525 C), resulting in films with high structural perfection and smooth surfaces (rms roughness of< 2 nm on∼ 1 µm thick films). Silicon-containing oxide sources (SiO and SiO2) producing an SiO suboxide molecular beam are used to dope the β-Ga2O3 layers. Temperature-dependent Hall effect measurements on a 1 µm thick film with a …

A dielectric thin film includes a stack structure of a perovskite material layer including at least two Group II elements and a rocksalt layer on the perovskite material layer and including at least two Group II elements. A first content ratio of the at least two Group II elements included in the perovskite material layer may be the same as a second content ratio of the at least two Group II elements included in the rocksalt layer.

Single-crystal delafossite PdCoO 2 is known to have an extremely low intrinsic impurity concentration of∼ 0.001%, demonstrating extraordinarily high conductivity with a mean free path of∼ 20 μm at low temperatures. However, when grown as thin films, the resistivity at room temperature increases by a factor of 3–80 times, depending on the film thickness. Using scanning transmission electron microscopy, we identify different classes of defects for the single crystal vs epitaxial thin film. The dominant defect for single-crystal PdCoO 2 is found to be ribbonlike defects. For the thin films, we identify different types of defects arising in epitaxial thin films mainly due to substrate termination that disrupt the lateral connectivity of the conducting planes. Our results are consistent with the high conductivity of single crystals and increased electrical resistivity of the thin films compared to that of single crystals, suggesting that …

During the explosion of research on two-dimensional (2D) van der Waals crystals over the past couple decades, synthesis has steadily advanced to include epitaxial films but always with the low-energy plane of the 2D material parallel to the surface of the substrate. Here, we report epitaxial synthesis of a 2D van der Waals crystal, SnO, in which the layers are aligned perpendicular to the surface. This accomplishment is particularly important for SnO, a top candidate p-type transparent conducting oxide, because the crystallographic direction with the highest hole mobility lies in the plane of the film where it could be utilized in a transistor. We find that the discontinuous nanowire morphology of the films prevents fabrication of useful devices, but nonetheless, this discovery represents a milestone in the fields of 2D materials synthesis and p-type oxides.

The development of high-performance p-type oxides with wide band gap and high hole mobility is critical for the application of oxide semiconductors in back-end-of-line (BEOL) complementary metal-oxide-semiconductor (CMOS) devices. SnO has been intensively studied as a high-mobility p-type oxide due to its low effective hole mass resulting from the hybridized O-2p/Sn-5s orbital character at the valence band edge. However, SnO has a very small band gap (∼0.7 eV) for practical p-type oxide devices. In this work, we report an engineering method to enhance the band gap and hole mobility in SnO. It is found that both the band gap and the hole mobility of a layer-structured SnO increase with the interlayer stacking spacing change. By exploiting this unique electronic structure feature, we propose expanding the interlayer spacing by interlayer intercalation to engineer the band gap and p-type mobility in SnO …

We outline a method to synthesize (ATiO 3) n AO Ruddlesden–Popper phases with high-n, where the A-site is a mixture of barium and strontium, by molecular-beam epitaxy. The precision and consistency of the method described is demonstrated by the growth of an unprecedented (SrTiO 3) 50 SrO epitaxial film. We proceed to investigate barium incorporation into the Ruddlesden–Popper structure, which is limited to a few percent in bulk, and we find that the amount of barium that can be incorporated depends on both the substrate temperature and the strain state of the film. At the optimal growth temperature, we demonstrate that as much as 33% barium can homogeneously populate the A-site when films are grown on SrTiO 3 (001) substrates, whereas up to 60% barium can be accommodated in films grown on TbScO 3 (110) substrates, which we attribute to the difference in strain. This detailed synthetic study of …

In its ground state, RuO 2 was long thought to be an ordinary metallic paramagnet. Recent neutron and x-ray diffraction revealed that bulk RuO 2 is an antiferromagnet with T N above 300 K. Furthermore, epitaxial strain induces superconductivity in thin films of RuO 2 below 2 K. Here, we present a resonant elastic x-ray scattering study at the Ru L 2 edge of the strained RuO 2 films exhibiting the strain-induced superconductivity. We observe an azimuthal modulation of the 100 Bragg peak consistent with bulk. Most notably, in the strained films displaying superconductivity, we observe a∼ 1 eV shift of the Ru e g orbitals to a higher energy. The energy shift is smaller in thicker, relaxed films and films with a different strain direction. Our results provide further evidence of the utility of epitaxial strain as a tuning parameter in complex oxides.

Multidimensional data generated from modern aberration-corrected electron microscopes offers many new opportunities to explore the rich structural information in materials and devices. With the access to both real and diffraction space, four-dimensional scanning transmission electron microscopy (4DSTEM) has been demonstrated a wide impact [1]. Electron ptychography is such an example. It uses 4DSTEM datasets to iteratively reconstruct the electrostatic potential of the sample and greatly benefits from the advancements of direct electron detectors [2]. It works well for samples with weak scattering [3] or thicknesses below a few nanometers [4]. For thick bulk samples, however, conventional ptychography has a degraded performance due to strong multiple scattering. In fact, multiple scattering affects all conventional imaging techniques, usually detrimentally, such as a reduction of the interpretable resolution [5 …

Functional properties of transition-metal oxides strongly depend on crystallographic defects; crystallographic lattice deviations can affect ionic diffusion and adsorbate binding energies. Scanning x-ray nanodiffraction enables imaging of local structural distortions across an extended spatial region of thin samples. Yet, localized lattice distortions remain challenging to detect and localize using nanodiffraction, due to their weak diffuse scattering. Here, we apply an unsupervised machine learning clustering algorithm to isolate the low-intensity diffuse scattering in as-grown and alkaline-treated thin epitaxially strained SrIrO 3 films. We pinpoint the defect locations, find additional strain variation in the morphology of electrochemically cycled SrIrO 3, and interpret the defect type by analyzing the diffraction profile through clustering. Our findings demonstrate the use of a machine learning clustering algorithm for identifying …

Barium hexaferrite (BaFe12O19 or BaM) is a commercially-important magnetic material with a ferrimagnetic order below~ 720 K. It is also studied as a quantum paraelectric candidate, and hence we are exploring its strain engineering as a possible pathway to creating a room-temperature magnetoelectric-coupled multiferroic. The projected crystal structure of BaM along the [100] direction is shown in Figure 1 (a) with the unit cell marked with a dashed white line. In its native centrosymmetric version, BaM belongs to the P63/mmc space group, and first-principles calculations have shown an unstable phonon mode associated with a displacement of the trigonal bipyramid (TBP) Fe 3+ ion off the mirror plane on which it lies [1]. This off-mirror plane (OMP) displacement and the TBP coordination of the Fe atom with adjacent O atoms is marked with a brown box in Figure 1 (a) and shown in detail in Figure 1 (b). Further …

Molecular-beam epitaxy (MBE) and more particularly suboxide MBE (S-MBE) and related structures are disclosed. S-MBE is disclosed that includes the use of a molecular beam of a suboxide that may be subsequently oxidized in a single step reaction to form an oxide film. By way of example, for a gallium oxide (Ga 2 O 3) film, a molecular beam including a suboxide of gallium (Ga 2 O) may be provided. S-MBE may be performed in adsorption-controlled regimes where there is an excess of source material containing species in order to promote high growth rates for oxide films with improved crystallinity. Source mixtures for providing molecular beams of suboxides are disclosed that include mixtures of a particular element and an oxide of the element in ratios that promote such adsorption-controlled growth regimes. Related structures include oxide films having increased thickness with reduced crystal defects …

The Ruddlesden–Popper (An+1BnO3n+1) compounds are highly tunable materials whose functional properties can be dramatically impacted by their structural phase n. The negligible differences in formation energies for different n can produce local structural variations arising from small stoichiometric deviations. Here, we present a Python analysis platform to detect, measure, and quantify the presence of different n-phases based on atomic-resolution scanning transmission electron microscopy (STEM) images. We employ image phase analysis to identify horizontal Ruddlesden–Popper faults within the lattice images and quantify the local structure. Our semiautomated technique considers effects of finite projection thickness, limited fields of view, and lateral sampling rates. This method retains real-space distribution of layer variations allowing for spatial mapping of local n-phases to enable quantification of …

We use resonant inelastic x-ray scattering to probe the propagation of plasmons in the electron-doped cuprate superconductor Sr 0.9 La 0.1 CuO 2. We detect a plasmon gap of∼ 120 meV at the two-dimensional Brillouin zone center, indicating that low-energy plasmons in Sr 0.9 La 0.1 CuO 2 are not strictly acoustic. The plasmon dispersion, including the gap, is accurately captured by layered t− J− V model calculations. A similar analysis performed on recent resonant inelastic x-ray scattering data from other cuprates suggests that the plasmon gap is generic and its size is related to the magnitude of the interlayer hopping t z. Our work signifies the three dimensionality of the charge dynamics in layered cuprates and provides a new method to determine t z.

Symmetry plays a central role in determining the polarization of spin currents induced by electric fields. It also influences how these spin currents generate spin-transfer torques in magnetic devices. Here we show that an out-of-plane damping-like torque can be generated in ruthenium dioxide (RuO2)/permalloy devices when the Néel vector of the collinear antiferromagnet RuO2 is canted relative to the sample plane. By measuring characteristic changes in all three components of the electric-field-induced torque vector as a function of the angle of the electric field relative to the crystal axes, we find that the RuO2 generates a spin current with a well-defined tilted spin orientation that is approximately parallel to the Néel vector. A maximum out-of-plane damping-like spin torque efficiency per unit electric field of 7 ± 1 × 103 Ω−1 m−1 is measured at room temperature. The observed angular dependence indicates …

We demonstrate the epitaxial growth of the first two members, and the n=∞ member of the homologous Ruddlesden–Popper series of Ba n+ 1 In n O 2.5 n+ 1 of which the n= 1 member was previously unknown. The films were grown by suboxide molecular-beam epitaxy where the indium is provided by a molecular beam of indium-suboxide [In 2 O (g)]. To facilitate ex situ characterization of the highly hygroscopic barium indate films, a capping layer of amorphous SiO 2 was deposited prior to air exposure. The structural quality of the films was assessed by x-ray diffraction, reflective high-energy electron diffraction, and scanning transmission electron microscopy.

Optical excitation leads to ultrafast stress generation in the prototypical multiferroic BiFeO3. The time scales of stress generation are set by the dynamics of the population of excited electronic states and the coupling of the electronic configuration to the structure. X-ray free-electron laser diffraction reveals high-wavevector subpicosecond-time scale stress generation following ultraviolet excitation of a BiFeO3 thin film. Stress generation includes a fast component with a 1/e rise time with an upper limit of 300 fs and longer-rise time components extending to 1.5 ps. The contributions of the fast and delayed components vary as a function of optical fluence, with a reduced a fast-component contribution at high fluence. The results provide insight into stress-generation mechanisms linked to the population of excited electrons and point to new directions in the application of nanoscale multiferroics and related ferroic complex …