Ultrafast Optically Induced Perturbation of Oxygen Octahedral Rotations in Multiferroic BiFeO3 Thin Films.

The functional properties of complex oxides, including magnetism and ferroelectricity, are closely linked to subtle structural distortions. Ultrafast optical excitations provide the means to manipulate structural features and ultimately to affect the functional properties of complex oxides with picosecond-scale precision. We report that the lattice expansion of multiferroic BiFeO3 following above-bandgap optical excitation leads to distortion of the oxygen octahedral rotation (OOR) pattern. The continuous coupling between OOR and strain was probed using time-resolved X-ray free-electron laser diffraction with femtosecond time resolution. Density functional theory calculations predict a relationship between the OOR and the elastic strain consistent with the experiment, demonstrating a route to employing this approach in a wider range of systems. Ultrafast control of the functional properties of BiFeO3 thin films is enabled by this approach because the OOR phenomena are related to ferroelectricity, and via the Fe-O-Fe bond angles, the superexchange interaction between Fe atoms.

Subpicosecond Optical Stress Generation in Multiferroic BiFeO3.

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 oxides. The fast component of the stress indicates that structural parameters and properties of ferroelectric thin film materials can be optically modulated with 3 dB bandwidths of at least 0.5 THz.

Stabilization of Sr3Al2O6 Growth Templates for Ex Situ Synthesis of Freestanding Crystalline Oxide Membranes.

A new synthetic approach has recently been developed for the fabrication of freestanding crystalline perovskite oxide nanomembranes, which involves the epitaxial growth of a water-soluble sacrificial layer. By utilizing an ultrathin capping layer of SrTiO3, here we show that this sacrificial layer, as grown by pulsed laser deposition, can be stabilized in air and therefore be used as transferrable templates for ex situ epitaxial growth using other techniques. We find that the stability of these templates depends on the thickness of the capping layer. On these templates, freestanding superconducting SrTiO3 membranes were synthesized ex situ using molecular beam epitaxy, enabled by the lower growth temperature which preserves the sacrificial layer. This study paves the way for the synthesis of an expanded selection of freestanding oxide membranes and heterostructures with a wide variety of ex situ growth techniques.

Observation of Strong Polarization Enhancement in Ferroelectric Tunnel Junctions.

Ferroelectric heterostructures, with capability of storing data at ultrahigh densities, could act as the platform for next-generation memories. The development of new device paradigms has been hampered by the long-standing notion of inevitable ferroelectricity suppression under reduced dimensions. Despite recent experimental observation of stable polarized states in ferroelectric ultrathin films, the out-of-plane polarization components in these films are strongly attenuated compared to thicker films, implying a degradation of device performance in electronic miniaturization processes. Here, in a model system of BiFeO3/La0.7Sr0.3MnO3, we report observation of a dramatic out-of-plane polarization enhancement that occurs with decreasing film thickness. Our electron microscopy analysis coupled with phase-field simulations reveals a polarization-enhancement mechanism that is dominated by the accumulation of oxygen vacancies at interfacial layers. The results shed light on the interplay between polarization and defects in nanoscale ferroelectrics and suggest a route to enhance functionality in oxide devices.

Defect-Induced Hedgehog Polarization States in Multiferroics.

Continuous developments in nanotechnology require new approaches to materials synthesis that can produce novel functional structures. Here, we show that nanoscale defects, such as nonstoichiometric nanoregions (NSNRs), can act as nano-building blocks for creating complex electrical polarization structures in the prototypical multiferroic BiFeO_{3}. An array of charged NSNRs are produced in BiFeO_{3} thin films by tuning the substrate temperature during film growth. Atomic-scale scanning transmission electron microscopy imaging reveals exotic polarization rotation patterns around these NSNRs. These polarization patterns resemble hedgehog or vortex topologies and can cause local changes in lattice symmetries leading to mixed-phase structures resembling the morphotropic phase boundary with high piezoelectricity. Phase-field simulations indicate that the observed polarization configurations are mainly induced by charged states at the NSNRs. Engineering defects thus may provide a new route for developing ferroelectric- or multiferroic-based nanodevices.

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.

Localized excited charge carriers generate ultrafast inhomogeneous strain in the multiferroic BiFeO3.

We apply ultrafast x-ray diffraction with femtosecond temporal resolution to monitor the lattice dynamics in a thin film of multiferroic BiFeO3 after above-band-gap photoexcitation. The sound-velocity limited evolution of the observed lattice strains indicates a quasi-instantaneous photoinduced stress which decays on a nanosecond time scale. This stress exhibits an inhomogeneous spatial profile evidenced by the broadening of the Bragg peak. These new data require substantial modification of existing models of photogenerated stresses in BiFeO3: the relevant excited charge carriers must remain localized to be consistent with the data.

Atomic scale structure changes induced by charged domain walls in ferroelectric materials.

Charged domain walls (CDWs) are of significant scientific and technological importance as they have been shown to play a critical role in controlling the switching mechanism and electric, photoelectric, and piezoelectric properties of ferroelectric materials. The atomic scale structure and properties of CDWs, which are critical for understanding the emergent properties, have, however, been rarely explored. In this work, using a spherical-aberration-corrected transmission electron microscope with subangstrom resolution, we have found that the polarization bound charge of the CDW in rhombohedral-like BiFeO3 thin films not only induces the formation of a tetragonal-like crystal structure at the CDW but also stabilizes unexpected nanosized domains with new polarization states and unconventional domain walls. These findings provide new insights on the effects of bound charge on ferroelectric domain structures and are critical for understanding the electrical switching in ferroelectric thin films as well as in memory devices.

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.

Electronic origin of ultrafast photoinduced strain in BiFeO3.

Above-band-gap optical excitation produces interdependent structural and electronic responses in a multiferroic BiFeO(3) thin film. Time-resolved synchrotron x-ray diffraction shows that photoexcitation can induce a large out-of-plane strain, with magnitudes on the order of half of one percent following pulsed-laser excitation. The strain relaxes with the same nanosecond time dependence as the interband relaxation of excited charge carriers. The magnitude of the strain and its temporal correlation with excited carriers indicate that an electronic mechanism, rather than thermal effects, is responsible for the lattice expansion. The observed strain is consistent with a piezoelectric distortion resulting from partial screening of the depolarization field by charge carriers, an effect linked to the electronic transport of excited carriers. The nonthermal generation of strain via optical pulses promises to extend the manipulation of ferroelectricity in oxide multiferroics to subnanosecond time scales.

Spectroscopic detection of hopping induced mixed valence for Ti and Sc in GdSc1-xTixO3 for x > 0.165.

Only two of the first row transition metals have elemental oxides that are either ferro- or ferri-magnetic. These are CrO2 and Fe3O4. The electron spin alignment that promotes the ferro(i)magnetism is associated with a double exchange mechanism that requires mixed valence as well as metallic conductivity. This paper describes a novel way to realize these two necessary, but not sufficient conditions for double exchange magnetism. These are mixed valence and a hopping conductivity that promotes at least intra-plane electron spin alignment in a complex oxide perovskite host, A(B,C)O3. A is an ordinary metal, or a rare earth atom, B is a d0 transition metal, and C is a d(n) transition metal in which n > or = 1, as for example in GdSc1-xTi(x)O3. This article combines X-ray absorption spectroscopy, multiplet theory, charge transfer multiplet theory and degeneracy removal by Jahn-Teller effect mechanisms to demonstrate mixed valence for both Sc and Ti above a percolation threshold, x > 0.16, in which hopping transport gives rise to a metal to insulator transition.

Spontaneous vortex nanodomain arrays at ferroelectric heterointerfaces.

The polarization of the ferroelectric BiFeO(3) sub-jected to different electrical boundary conditions by heterointerfaces is imaged with atomic resolution using a spherical aberration-corrected transmission electron microscope. Unusual triangular-shaped nanodomains are seen, and their role in providing polarization closure is understood through phase-field simulations. Heterointerfaces are key to the performance of ferroelectric devices, and this first observation of spontaneous vortex nanodomain arrays at ferroelectric heterointerfaces reveals properties unlike the surrounding film including mixed Ising-Néel domain walls, which will affect switching behavior, and a drastic increase of in-plane polarization. The importance of magnetization closure has long been appreciated in multidomain ferromagnetic systems; imaging this analogous effect with atomic resolution at ferroelectric heterointerfaces provides the ability to see device-relevant interface issues. Extension of this technique to visualize domain dynamics is envisioned.

Self-regeneration of Pd-LaFeO3 catalysts: new insight from atomic-resolution electron microscopy.

Aberration-corrected transmission electron microscopy was used to study atomic-scale processes in Pd-LaFeO(3) catalysts. Clear evidence for diffusion of Pd into LaFeO(3) and out of LaFe(0.95)Pd(0.05)O(3-δ) under high-temperature oxidizing and reducing conditions, respectively, was found, but the extent to which these processes occurred was quite limited. These observations cast doubt that such phenomena play a significant role in a postulated mechanism of self-regeneration of this system as an automotive exhaust-gas catalyst.

Optical properties of (SrMnO3)n/(LaMnO3)2n superlattices: an insulator-to-metal transition observed in the absence of disorder.

We measure the optical conductivity, σ1(ω), of (SrMnO3)n/(LaMnO3)2n superlattices (SL) for n = 1, 3, 5, and 8 and 10 < T < 400 K. Data show a T-dependent insulator to metal transition (IMT) for n ≤ 3, driven by the softening of a polaronic mid-infrared band. At n = 5 that softening is incomplete, while at the largest-period n = 8 compound the MIR band is independent of T and the SL remains insulating. One can thus first observe the IMT in a Manganite system in the absence of the disorder due to chemical doping. Unsuccessful reconstruction of the SL optical properties from those of the original bulk materials suggests that (SrMnO3)n/(LaMnO3)2n heterostructures give rise to a novel electronic state.

Optical transient grating pumped X-ray diffraction microscopy for studying mesoscale structural dynamics.

A fundamental understanding of materials' structural dynamics, with fine spatial and temporal control, underpins future developments in electronic and quantum materials. Here, we introduce an optical transient grating pump and focused X-ray diffraction probe technique (TGXD) to examine the structural evolution of materials excited by modulated light with a precisely controlled spatial profile. This method adds spatial resolution and direct structural sensitivity to the established utility of a sinusoidal transient-grating excitation. We demonstrate TGXD using two thin-film samples: epitaxial BiFeO3, which exhibits a photoinduced strain (structural grating) with an amplitude proportional to the optical fluence, and FeRh, which undergoes a magnetostructural phase transformation. In BiFeO3, structural relaxation is location independent, and the strain persists on the order of microseconds, consistent with the optical excitation of long-lived charge carriers. The strain profile of the structural grating in FeRh, in comparison, deviates from the sinusoidal excitation and exhibits both higher-order spatial frequencies and a location-dependent relaxation. The focused X-ray probe provides spatial resolution within the engineered optical excitation profile, resolving the spatiotemporal flow of heat through FeRh locally heated above the phase transition temperature. TGXD successfully characterizes mesoscopic energy transport in functional materials without relying on a specific transport model.

Bursting at the seams: Rippled monolayer bismuth on NbSe2.

Bismuth, one of the heaviest semimetals in nature, ignited the interest of the materials physics community for its potential impact on topological quantum material systems that use its strong spin-orbit coupling and unique orbital hybridization. In particular, recent theoretical predictions of unique topological and superconducting properties of thin bismuth films and interfaces prompted intense research on the growth of submonolayers to a few monolayers of bismuth on different substrates. Similar to bulk rhombohedral bismuth, the initial growth of bismuth films on most substrates results in buckled bilayers that grow in either the (111) or (110) directions, with a lattice constant close to that of bulk Bi. By contrast, we show a new growth pattern for bismuth monolayers on NbSe2. We find that the initial growth of Bi can form a strongly bonded commensurate layer, resulting in a compressively strained two-dimensional (2D) triangular lattice. We also observed unique pattern of 1D ripples and domain walls is observed. The single layer of bismuth also introduces strong marks on the electronic properties at the surface.

Tailoring manganese oxide with atomic precision to increase surface site availability for oxygen reduction catalysis.

Controlling the structure of catalysts at the atomic level provides an opportunity to establish detailed understanding of the catalytic form-to-function and realize new, non-equilibrium catalytic structures. Here, advanced thin-film deposition is used to control the atomic structure of La2/3Sr1/3MnO3, a well-known catalyst for the oxygen reduction reaction. The surface and sub-surface is customized, whereas the overall composition and d-electron configuration of the oxide is kept constant. Although the addition of SrMnO3 benefits the oxygen reduction reaction via electronic structure and conductivity improvements, SrMnO3 can react with ambient air to reduce the surface site availability. Placing SrMnO3 in the sub-surface underneath a LaMnO3 overlayer allows the catalyst to maintain the surface site availability while benefiting from improved electronic effects. The results show the promise of advanced thin-film deposition for realizing atomically precise catalysts, in which the surface and sub-surface structure and stoichiometry are tailored for functionality, over controlling only bulk compositions.

Giant Resistive Switching via Control of Ferroelectric Charged Domain Walls.

Controlled switching of resistivity in ferroelectric thin films is demonstrated by writing and erasing stable, nanoscale, strongly charged domain walls using an in situ transmission electron microscopy technique. The resistance can be read nondestructively and presents the largest off/on ratio (≈10(5) ) ever reported in room-temperature ferroelectric devices, opening new avenues for engineering ferroelectric thin-film devices.

Epitaxial integration of a nanoscale BiFeO3 phase boundary with silicon.

The successful integration of the strain-driven nanoscale phase boundary of BiFeO3 onto a silicon substrate is demonstrated with extraordinary ferroelectricity and ferromagnetism. The detailed strain history is delineated through a reciprocal space mapping technique. We have found that a distorted monoclinic phase forms prior to a tetragonal-like phase, a phenomenon which may correlates with the thermal strain induced during the growth process.

Capturing ultrafast photoinduced local structural distortions of BiFeO3.

The interaction of light with materials is an intensively studied research forefront, in which the coupling of radiation energy to selective degrees of freedom offers contact-free tuning of functionalities on ultrafast time scales. Capturing the fundamental processes and understanding the mechanism of photoinduced structural rearrangement are essential to applications such as photo-active actuators and efficient photovoltaic devices. Using ultrafast x-ray absorption spectroscopy aided by density functional theory calculations, we reveal the local structural arrangement around the transition metal atom in a unit cell of the photoferroelectric archetype BiFeO3 film. The out-of-plane elongation of the unit cell is accompanied by the in-plane shrinkage with minimal change of interaxial lattice angles upon photoexcitation. This anisotropic elastic deformation of the unit cell is driven by localized electric field as a result of photoinduced charge separation, in contrast to a global lattice constant increase and lattice angle variations as a result of heating. The finding of a photoinduced elastic unit cell deformation elucidates a microscopic picture of photocarrier-mediated non-equilibrium processes in polar materials.