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Antiferroelectric materials have seen a resurgence of interest because of proposed applications in a number of energy-efficient technologies. Unfortunately, relatively few families of antiferroelectric materials have been identified, precluding many proposed applications. Here, we propose a design strategy for the construction of antiferroelectric materials using interfacial electrostatic engineering. We begin with a ferroelectric material with one of the highest known bulk polarizations, BiFeO3. By confining thin layers of BiFeO3 in a dielectric matrix, we show that a metastable antiferroelectric structure can be induced. Application of an electric field reversibly switches between this new phase and a ferroelectric state. The use of electrostatic confinement provides an untapped pathway for the design of engineered antiferroelectric materials with large and potentially coupled responses., Science Advances, 8 (5), ISSN:2375-2548

Hexagonal ABO3 oxides ( A, B = cation) are a class of rich materials for realizing novel quantum phenomena. Their hexagonal symmetry, oxygen trigonal bipyramid coordination, and quasi-two dimensional layering give rise to properties distinct from those of the cubic ABO3 perovskites. As bulk materials, most of the focus in this class of materials has been on the rare-earth manganites, RMnO3 ( R = rare earth); these materials display coupled ferroelectricity and antiferromagnetic order. In this review, we focus on the thin-film manifestations of the hexagonal ABO3 oxides. We cover the stability of the hexagonal oxides and substrates which can be used to template the hexagonal structure. We show how the thin-film geometry not only allows for further tuning of the bulk-stable manganites but also allows for the realization of metastable hexagonal oxides such as the RFeO3 that combine ferroelectricity with weak ferromagnetic order. The thin-film geometry is a promising platform to stabilize additional metastable hexagonal oxides to search for predicted high-temperature superconductivity and topological phases in this class of materials.

Since the discovery of high-temperature superconductivity in the copper oxide materials, there have been sustained efforts to both understand the origins of this phase and discover new cuprate-like superconducting materials. One prime materials platform has been the rare-earth nickelates and indeed superconductivity was recently discovered in the doped compound Nd$_{0.8}$Sr$_{0.2}$NiO$_2$. Undoped NdNiO$_2$ belongs to a series of layered square-planar nickelates with chemical formula Nd$_{n+1}$Ni$_n$O$_{2n+2}$ and is known as the 'infinite-layer' ($n = \infty$) nickelate. Here, we report the synthesis of the quintuple-layer ($n = 5$) member of this series, Nd$_6$Ni$_5$O$_{12}$, in which optimal cuprate-like electron filling ($d^{8.8}$) is achieved without chemical doping. We observe a superconducting transition beginning at $\sim$13 K. Electronic structure calculations, in tandem with magnetoresistive and spectroscopic measurements, suggest that Nd$_6$Ni$_5$O$_{12}$ interpolates between cuprate-like and infinite-layer nickelate-like behavior. In engineering a distinct superconducting nickelate, we identify the square-planar nickelates as a new family of superconductors which can be tuned via both doping and dimensionality., Comment: 21 pages, 4 figures

We construct ferroelectric ${{(\mathrm{LuFeO}}_{3})}_{m}/{(\mathrm{LuFe}}_{2}{\mathrm{O}}_{4})$ superlattices with varying index $m$ to study the effect of confinement on topological defects. We observe a thickness-dependent transition from neutral to charged domain walls and the emergence of fractional vortices. In thin ${\mathrm{LuFeO}}_{3}$ layers, the volume fraction of domain walls grows, lowering the symmetry from $P{6}_{3}cm$ to $P3c1$ before reaching the nonpolar $P{6}_{3}/mmc$ state, analogous to the group-subgroup sequence observed at the high-temperature ferroelectric to paraelectric transition. Our study shows how dimensional confinement stabilizes textures beyond those in bulk ferroelectric systems.

Interface materials offer a means to achieve electrical control of ferrimagnetism at room temperature as was recently demonstrated in (LuFeO3)m/(LuFe2O4)1 superlattices. A challenge to understanding the inner workings of these complex magnetoelectric multiferroics is the multitude of distinct Fe centres and their associated environments. This is because macroscopic techniques characterize average responses rather than the role of individual iron centres. Here, we combine optical absorption, magnetic circular dichroism and first-principles calculations to uncover the origin of high-temperature magnetism in these superlattices and the charge-ordering pattern in the m = 3 member. In a significant conceptual advance, interface spectra establish how Lu-layer distortion selectively enhances the Fe2+ → Fe3+ charge-transfer contribution in the spin-up channel, strengthens the exchange interactions and increases the Curie temperature. Comparison of predicted and measured spectra also identifies a non-polar charge ordering arrangement in the LuFe2O4 layer. This site-specific spectroscopic approach opens the door to understanding engineered materials with multiple metal centres and strong entanglement., Understanding the inner workings of complex magnetoelectric multiferroics remains a challenge, as macroscopic techniques characterize average responses rather than the role of individual iron centers. Here, the authors reveal the origin of high-temperature magnetism in multiferroic superlattices.

Raising the Curie temperature ${T}_{\mathrm{C}}$ of the highly spin-polarized semiconductor EuO by doping it with rare-earth elements is a strategy to make EuO more technologically relevant to spintronics. The increase of ${T}_{\mathrm{C}}$ with free carrier density $n$ and the surprisingly low dopant activation $p$, found in Gd-doped EuO thin films [Mairoser et al., Phys. Rev. Lett. 105, 257206 (2010)], raised the important question of whether ${T}_{\mathrm{C}}$ could be considerably enhanced by increasing $p$. Using a low-temperature growth method for depositing high-quality Lu-doped EuO films we attain high dopant activation ($p$) values of up to 67%, effectively more than doubling $p$ as compared to adsorption-controlled growth of Lu- and Gd-doped EuO. Relating $n, p$, and lattice compression of La- and Lu-doped EuO films grown at different temperatures to the ${T}_{\mathrm{C}}$ of these samples allows us to identify several different mechanisms influencing ${T}_{\mathrm{C}}$ and causing an experimental maximum in ${T}_{\mathrm{C}}$. In addition, scanning transmission electron microscopy in combination with electron energy loss spectroscopy measurements on La-doped EuO indicate that extensive dopant clustering is one, but not the sole reason for dopant deactivation in rare-earth doped EuO films.

Using epitaxial stabilization, we synthesized single-phase (001)-oriented thin films of DyFe2O4+x on (111) MgAl2O4 substrates by molecular-beam epitaxy. The metastable DyFe2O4 polymorph formed is isostructural to known trigonal ferrimagnetic RFe2O4 phases with space group R3̄m, where R = Ho to Lu. The epitaxial DyFe2O4 thin films have two in-plane orientation relationships: [100] DyFe2O4 || 211̄ MgAl2O4 plus a twin variant related by a 60° in-plane rotation. DyFe2O4 is not bulk stable and has never been synthesized before. Indeed, it has been predicted to be on the edge energetically of what may be possible to stabilize. The fact that the RFe2O4 phase is stable for all elements leading up to dysprosium (Ho–Lu) leads us to believe that DyFe2O4 could be a “remnant metastable phase,” one which, given the right thermodynamic conditions, could become the lowest free energy phase. We find that although we are able to get structurally very close to R3̄m DyFe2O4, the films are not stoichiometric as they have an increased c lattice parameter, indicative of extra oxygen as is sometimes seen in other RFe2O4 phases. The unintended surplus oxygen opens questions regarding what may be achievable using such tricks as epitaxial stabilization to access metastable phases and whether this indeed constitutes “remnant metastability.”

A novel approach to on-demand improvement of electronic properties in complex-oxide ferroelectrics is demonstrated whereby ion bombardment - commonly used in classic semiconductor materials - is applied to the PbTiO3 system. The result is deterministic reduction in leakage currents by 5 orders of magnitude, improved ferroelectric switching, and unprecedented insights into the nature of defects and intergap state evolution in these materials.

A single-phase multiferroic material is constructed, in which ferroelectricity and strong magnetic ordering are coupled near room temperature, enabling direct electric-field control of magnetism. Materials that exhibit coupled ferroelectric and magnetic ordering are attractive candidates for use in future memory devices, but such materials are rare and typically exhibit their desirable properties only at low temperatures. Julia Mundy and colleagues now describe and successfully implement a strategy for building artificial layered materials in which ferroelectricity and magnetism are both present, and coupled near room temperature. Materials that exhibit simultaneous order in their electric and magnetic ground states hold promise for use in next-generation memory devices in which electric fields control magnetism1,2. Such materials are exceedingly rare, however, owing to competing requirements for displacive ferroelectricity and magnetism3. Despite the recent identification of several new multiferroic materials and magnetoelectric coupling mechanisms4,5,6,7,8,9,10,11,12,13,14,15, known single-phase multiferroics remain limited by antiferromagnetic or weak ferromagnetic alignments, by a lack of coupling between the order parameters, or by having properties that emerge only well below room temperature, precluding device applications2. Here we present a methodology for constructing single-phase multiferroic materials in which ferroelectricity and strong magnetic ordering are coupled near room temperature. Starting with hexagonal LuFeO3—the geometric ferroelectric with the greatest known planar rumpling16—we introduce individual monolayers of FeO during growth to construct formula-unit-thick syntactic layers of ferrimagnetic LuFe2O4 (refs 17, 18) within the LuFeO3 matrix, that is, (LuFeO3)m/(LuFe2O4)1 superlattices. The severe rumpling imposed by the neighbouring LuFeO3 drives the ferrimagnetic LuFe2O4 into a simultaneously ferroelectric state, while also reducing the LuFe2O4 spin frustration. This increases the magnetic transition temperature substantially—from 240 kelvin for LuFe2O4 (ref. 18) to 281 kelvin for (LuFeO3)9/(LuFe2O4)1. Moreover, the ferroelectric order couples to the ferrimagnetism, enabling direct electric-field control of magnetism at 200 kelvin. Our results demonstrate a design methodology for creating higher-temperature magnetoelectric multiferroics by exploiting a combination of geometric frustration, lattice distortions and epitaxial engineering.

Emergent phenomena at polar-nonpolar oxide interfaces have been studied intensely in pursuit of next-generation oxide electronics and spintronics. Here we report the disentanglement of critical thicknesses for electron reconstruction and the emergence of ferromagnetism in polar-mismatched LaMnO_{3}/SrTiO_{3} (001) heterostructures. Using a combination of element-specific x-ray absorption spectroscopy and dichroism, and first-principles calculations, interfacial electron accumulation, and ferromagnetism have been observed within the polar, antiferromagnetic insulator LaMnO_{3}. Our results show that the critical thickness for the onset of electron accumulation is as thin as 2 unit cells (UC), significantly thinner than the observed critical thickness for ferromagnetism of 5 UC. The absence of ferromagnetism below 5 UC is likely induced by electron overaccumulation. In turn, by controlling the doping of the LaMnO_{3}, we are able to neutralize the excessive electrons from the polar mismatch in ultrathin LaMnO_{3} films and thus enable ferromagnetism in films as thin as 3 UC, extending the limits of our ability to synthesize and tailor emergent phenomena at interfaces and demonstrating manipulation of the electronic and magnetic structures of materials at the shortest length scales.

We report cryogenic magnetic force microscopy (MFM) studies of a 200-nm-thick hexagonal ($h$) ${\mathrm{LuFeO}}_{3}$ film grown by molecular-beam epitaxy on a (111)-oriented yttria-stabilized cubic-zirconia substrate. Labyrinthlike domains $\ensuremath{\sim}1.8\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$ in size were observed after zero-field cooling below the N\'eel temperature, ${T}_{\mathrm{N}}\ensuremath{\approx}147$ K, where weak ferromagnetic order ($P{6}_{3}$cm) with a canted moment of ${M}_{\mathrm{S}}\ensuremath{\approx}0.02\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$ exists. At 6 K, MFM images of the magnetization reversal process reveal a typical domain behavior of a pinning-dominated hard magnet. The pinning strength is substantially reduced at elevated temperature. The temperature dependence of the domain contrast demonstrates that our MFM is able to detect the domain contrast of magnets with tiny magnetic moments ($\ensuremath{\sim}0.002\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$). An upper limit of the linear magnetoelectric coefficient of $h\ensuremath{-}{\mathrm{LuFeO}}_{3}$ (${\ensuremath{\alpha}}_{zz}l6\phantom{\rule{0.28em}{0ex}}\mathrm{ps}/\mathrm{m}$) is estimated by magnetoelectric force microscopy measurements.

Ferroelectric domain walls hold great promise as functional two-dimensional materials because of their unusual electronic properties. Particularly intriguing are the so-called charged walls where a polarity mismatch causes local, diverging electrostatic potentials requiring charge compensation and hence a change in the electronic structure. These walls can exhibit significantly enhanced conductivity and serve as a circuit path. The development of all-domain-wall devices, however, also requires walls with controllable output to emulate electronic nano-components such as diodes and transistors. Here we demonstrate electric-field control of the electronic transport at ferroelectric domain walls. We reversibly switch from resistive to conductive behaviour at charged walls in semiconducting ErMnO3 . We relate the transition to the formation—and eventual activation—of an inversion layer that acts as the channel for the charge transport. The findings provide new insight into the domain-wall physics in ferroelectrics and foreshadow the possibility to design elementary digital devices for all-domain-wall circuitry. © 2017. This is the authors' accepted and refereed manuscript to the article. The final authenticated version is available online at: https://www.nature.com/articles/nmat4878#abstract

Diverse topological defects arise in hexagonal manganites, such as ferroelectric vortices, as well as neutral and charged domain walls. The topological defects are intriguing because their low symmetry enables unusual couplings between structural, charge, and spin degrees of freedom, holding great potential for novel types of functional 2D and 1D systems. Despite the considerable advances in analyzing the different topological defects in hexagonal manganites, the understanding of their key intrinsic properties is still rather limited and disconnected. In particular, a rapidly increasing number of structural variants is reported without clarifying their relation, leading to a zoo of seemingly unrelated topological textures. Here, we combine picometer-precise scanning-transmission-electron microscopy with Landau theory modeling to clarify the inner structure of topological defects in Er1–xZrxMnO3. By performing a comprehensive parametrization of the inner atomic defect structure, we demonstrate that one primary length scale drives the morphology of both vortices and domain walls. Our findings lead to a unifying general picture of this type of structural topological defects. We further derive novel fundamental and universal properties, such as unusual bound-charge distributions and electrostatics at the ferroelectric vortex cores with emergent U(1) symmetry., Nano Letters, 17 (10), ISSN:1530-6984, ISSN:1530-6992

© 2017. This is the authors' accepted and refereed manuscript to the article. Locked until 4.2.2018 due to copyright restrictions. The final authenticated version is available online at: https://doi.org/10.1017/S1431927617008844

Certain materials contain both electric dipoles and magnetic moments. An experiment demonstrates that these properties can be coupled in previously unrecognized ways, leading to advanced functionality. Domain patterns inverted by a uniform magnetic field.

We have combined hard X-ray photoelectron spectroscopy with angular dependent O K-edge and V L-edge X-ray absorption spectroscopy to study the electronic structure of metallic and insulating end point phases in 4.1 nm thick (14 units cells along the c-axis of VO2) films on TiO2(001) substrates, each displaying an abrupt MIT centered at ~300 K with width20 K and a resistance change of ΔR/R10(3). The dimensions, quality of the films, and stoichiometry were confirmed by a combination of scanning transmission electron microscopy with electron energy loss spectroscopy, X-ray spectroscopy, and resistivity measurements. The measured end point phases agree with their bulk counterparts. This clearly shows that, apart from the strain induced change in transition temperature, the underlying mechanism of the MIT for technologically relevant dimensions must be the same as the bulk for this orientation.

Hyperspectral imaging (also known as spectrum imaging) requires software for extracting the signatures present in every spectrum. However, commercial software available for spectrum analysis remains expensive, complicated, and often not transparent regarding the internal workings and approximations made. For user facilities, educational institutes, and other settings where multiple users on a single tool can be expected, the limited availability of software becomes the bottleneck to data analysis, user training, and throughput. The Cornell Spectrum Imager (CSI) was developed as a universal data analysis tool to be freely distributed, to run on all computers, and to minimize training. This is accomplished by using one simple interface for imaging, cathodoluminescence, Raman, Electron Energy Loss Spectroscopic (EELS), and EDX data analysis. This article demonstrates the CSI plugins for ImageJ by guiding you through the basic workflow for processing EELS maps.

A promising electrocatalyst prototype of low Pt mole fraction, intermetallic nanoparticles of Cu3Pt, has been prepared using a simple impregnation-reduction method, followed by a post heat-treatment. Two dealloying methods (electrochemical and chemical) were implemented to control the atomic-level morphology and improve performance for the oxygen reduction reaction (ORR). The morphology and elemental composition of the dealloyed nanoparticles were characterized at angstrom resolution using an aberration-corrected scanning transmission electron microscope equipped with an electron energy loss spectrometer. We found that the electrochemical dealloying method led to the formation of a thin Pt skin of ca. 1 nm in thickness with an ordered Cu3Pt core structure, while chemical leaching gave rise to a “spongy” structure with no ordered structure being preserved. A three-dimensional tomographic reconstruction indicated that numerous voids were formed in the chemically dealloyed nanoparticles. Both dealloying meth...

The high beam current and subangstrom resolution of aberration-corrected scanning transmission electron microscopes has enabled electron energy loss spectroscopy (EELS) mapping with atomic resolution. These spectral maps are often dose limited and spatially oversampled, leading to low counts/channel and are thus highly sensitive to errors in background estimation. However, by taking advantage of redundancy in the dataset map, one can improve background estimation and increase chemical sensitivity. We consider two such approaches—linear combination of power laws and local background averaging—that reduce background error and improve signal extraction. Principal component analysis (PCA) can also be used to analyze spectrum images, but the poor peak-to-background ratio in EELS can lead to serious artifacts if raw EELS data are PCA filtered. We identify common artifacts and discuss alternative approaches. These algorithms are implemented within the Cornell Spectrum Imager, an open source software package for spectroscopic analysis.

The thousandfold increase in data-collection speed enabled by aberration-corrected optics allows us to overcome an electron microscopy paradox - how to obtain atomic-resolution chemical structure in individual nanoparticles, yet record a statistically significant sample from an inhomogeneous population. This allowed us to map hundreds of Pt-Co nanoparticles to show atomic-scale elemental distributions across different stages of the catalyst aging in a proton-exchange-membrane fuel cell, and relate Pt-shell thickness to treatment, particle size, surface orientation, and ordering., 28 pages, 5 figures, accepted, nano letters

Strain and dimensional confinement can be used to tune magnetic and ferroelectric properties or enhance device performance. In epitaxial films, the different lattice spacing of an underlying substrate can be used to impose a biaxial strain. We have studied the effect of the substrate-induced biaxial strain on the electrical conductivity and magnetic properties of manganite compounds. Epitaxial (001) La0.7Sr0.3MnO3 thin films have been grown by reactive molecular-beam epitaxy (MBE) on single crystalline substrates, varying the substrate-induced biaxial strain from −2.3% to +3.2%. The strain caused the Curie temperature to decrease by up to 100 K in very good agreement with the predictions of Millis et al. [A. J. Millis, T. Darling, and A. Migliori, J. Appl. Phys. 83, 1588 (1998)]. The effect of dimensional confinement on manganites was also investigated in thin films by synthesizing a superlattice of two formula-unit-thick layers of CaMnO3 separated by CaO double layers, i.e., the n=2 Ruddlesden-Popper phase Ca3Mn2O7. Magnetization measurements on 30 nm thick (001)-oriented Ca3Mn2O7 thin films grown on (110) YAlO3 substrates by MBE reveal a Neél transition temperature of TN=115 K, similar to bulk Ca3Mn2O7, but 10 K lower than thick CaMnO3 films grown on this same substrate.

Epitaxy is widely employed to create highly oriented crystalline films. A less appreciated, but nonetheless powerful means of creating such films is via topotactic transformation, in which a chemical reaction transforms a single crystal of one phase into a single crystal of a different phase, which inherits its orientation from the original crystal. Topotactic reactions may be applied to epitactic films to substitute, add or remove ions to yield epitactic films of different phases. Here we exploit a topotactic reduction reaction to provide a non-ultra-high vacuum (UHV) means of growing highly oriented single crystalline thin films of the easily over-oxidized half-metallic semiconductor europium monoxide (EuO) with a perfection rivalling that of the best films of the same material grown by molecular-beam epitaxy or UHV pulsed-laser deposition. As the technique only requires high-vacuum deposition equipment, it has the potential to drastically improve the accessibility of high-quality single crystalline films of EuO as well as other difficult-to-synthesize compounds., In the absence of matching substrates, the growth of oxide thin films can be challenging. Here, the authors demonstrate the growth of EuO thin films via a topotactic reaction, where a chemical reaction transforms a single crystal of one phase into that of another.

The concept ‘the interface is the device' is embodied in a wide variety of interfacial electronic phenomena and associated applications in oxide materials, ranging from catalysts and clean energy systems to emerging multifunctional devices. Many device properties are defined by the band alignment, which is often influenced by interface dipoles. On the other hand, the ability to purposefully create and control interface dipoles is a relatively unexplored degree of freedom for perovskite oxides, which should be particularly effective for such ionic materials. Here we demonstrate tuning the band alignment in perovskite metal-semiconductor heterojunctions over a broad range of 1.7 eV. This is achieved by the insertion of positive or negative charges at the interface, and the resultant dipole formed by the induced screening charge. This approach can be broadly used in applications where decoupling the band alignment from the constituent work functions and electron affinities can enhance device functionality., Controlling the alignment of bands at oxide interfaces is crucial for developing them into useful devices. By inserting charges into the interface to generate dipoles, Yajima et al. show tuning of the band alignment between SrRuO3/Nb:SrTiO3 by 1.7 eV.

We report on the magnetic structure and ordering of hexagonal ${\mathrm{LuFeO}}_{3}$ films of variable thickness grown by molecular-beam epitaxy on YSZ (111) and ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ (0001) substrates. These crystalline films exhibit long-range structural uniformity dominated by the polar $P{6}_{3}cm$ phase, which is responsible for the paraelectric to ferroelectric transition that occurs above 1000 K. Using bulk magnetometry and neutron diffraction, we find that the system orders into a ferromagnetically canted antiferromagnetic state via a single transition below 155 K regardless of film thickness, which is substantially lower than that previously reported in hexagonal ${\mathrm{LuFeO}}_{3}$ films. The symmetry of the magnetic structure in the ferroelectric state implies that this material is a strong candidate for linear magnetoelectric coupling and control of the ferromagnetic moment directly by an electric field.

Electronic changes at polar interfaces between transition metal oxides offer the tantalizing possibility to stabilize novel ground states yet can also cause unintended reconstructions in devices. The nature of these interfacial reconstructions should be qualitatively different for metallic and insulating films as the electrostatic boundary conditions and compensation mechanisms are distinct. Here we directly quantify with atomic-resolution the charge distribution for manganite-titanate interfaces traversing the metal-insulator transition. By measuring the concentration and valence of the cations, we find an intrinsic interfacial electronic reconstruction in the insulating films. The total charge observed for the insulating manganite films quantitatively agrees with that needed to cancel the polar catastrophe. As the manganite becomes metallic with increased hole doping, the total charge build-up and its spatial range drop substantially. Direct quantification of the intrinsic charge transfer and spatial width should lay the framework for devices harnessing these unique electronic phases.

We report the growth of ultrathin VO$_{2}$ films on rutile TiO$_{2}$ (001) substrates via reactive molecular-beam epitaxy. The films were formed by the cyclical deposition of amorphous vanadium and its subsequent oxidation and transformation to VO$_{2}$ via solid-phase epitaxy. Significant metal-insulator transitions were observed in films as thin as 2.3 nm, where a resistance change {\Delta}R/R of 25 was measured. Low angle annular dark field scanning transmission electron microscopy was used in conjunction with electron energy loss spectroscopy to study the film/substrate interface and revealed the vanadium to be tetravalent and the titanium interdiffusion to be limited to 1.6 nm., Comment: 25 pages, 6 figures

A new family of tunable microwave dielectrics with unparalleled performance at frequencies up to 125 GHz at room temperature has been created, using dimensionality to add and control a local ferroelectric instability in a system with exceptionally low dielectric loss. Tunable dielectric materials are valuable components for complex microwave circuitry, yet such materials tend to suffer losses when operated at microwave frequencies owing to intrinsic defects in their structures. Che-Hui Lee and colleagues have selected a family of dielectrics known to exhibit exceptionally low loss, and now show how these materials can be engineered to boost their tunability and attain levels of performance that rival all known tunable microwave dielectrics. The miniaturization and integration of frequency-agile microwave circuits—relevant to electronically tunable filters, antennas, resonators and phase shifters—with microelectronics offers tantalizing device possibilities, yet requires thin films whose dielectric constant at gigahertz frequencies can be tuned by applying a quasi-static electric field1. Appropriate systems such as BaxSr1−xTiO3 have a paraelectric–ferroelectric transition just below ambient temperature, providing high tunability1,2,3. Unfortunately, such films suffer significant losses arising from defects. Recognizing that progress is stymied by dielectric loss, we start with a system with exceptionally low loss—Srn+1TinO3n+1 phases4,5—in which (SrO)2 crystallographic shear6,7 planes provide an alternative to the formation of point defects for accommodating non-stoichiometry8,9. Here we report the experimental realization of a highly tunable ground state arising from the emergence of a local ferroelectric instability10 in biaxially strained Srn+1TinO3n+1 phases with n ≥ 3 at frequencies up to 125 GHz. In contrast to traditional methods of modifying ferroelectrics—doping1,2,3,11,12 or strain13,14,15,16—in this unique system an increase in the separation between the (SrO)2 planes, which can be achieved by changing n, bolsters the local ferroelectric instability. This new control parameter, n, can be exploited to achieve a figure of merit at room temperature that rivals all known tunable microwave dielectrics3.

With nearly complete spin polarization, the ferromagnetic semiconductor europium monoxide could enable next-generation spintronic devices by providing efficient ohmic spin injection into silicon. Spin injection is greatly affected by the quality of the interface between the injector and silicon. Here, we use atomic-resolution scanning transmission electron microscopy in conjunction with electron energy loss spectroscopy to directly image and chemically characterize a series of EuO|Si and EuO|YAlO3 interfaces fabricated using different growth conditions. We identify the presence of europium silicides and regions of disorder at the EuO|Si interfaces, imperfections that could significantly reduce spin injection efficiencies via spin-flip scattering.

Controlling the electronic properties of interfaces has enormous scientific and technological implications and has been recently extended from semiconductors to complex oxides which 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 utilize 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 LaMnO3 and the band insulator SrMnO3. By digitally varying the separation between interfaces in (LaMnO3)2n/(SrMnO3)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., Comment: 14 pages, 13 figures. For published article, see http://www.nature.com/nmat/journal/v11/n10/full/nmat3405.html

We present an electron tomography method that allows for the identification of hundreds of electrocatalyst nanoparticles with one-to-one correspondence before and after electrochemical aging. This method allows us to track, in three-dimensions, the trajectories and morphologies of each Pt-Co nanocatalyst on a fuel cell carbon support. In conjunction with the use of atomic-scale electron energy loss spectroscopic imaging, our experiment enables the correlation of performance degradation of the catalyst with changes in particle/interparticle morphologies, particle-support interactions, and the near-surface chemical composition. We found that aging of the catalysts under normal fuel cell operating conditions (potential scans from +0.6 to +1.0 V for 30,000 cycles) gives rise to coarsening of the nanoparticles, mainly through coalescence, which in turn leads to the loss of performance. The observed coalescence events were found to be the result of nanoparticle migration on the carbon support during potential cycling. This method provides detailed insights into how nanocatalyst degradation occurs in proton exchange membrane fuel cells (PEMFCs) and suggests that minimization of particle movement can potentially slow down the coarsening of the particles and the corresponding performance degradation.

We report the growth of (001)-oriented VO2 films as thin as 1.5 nm with abrupt and reproduciblemetal-insulator transitions (MIT) without a capping layer. Limitations to the growth of thinnerfilms with sharp MITs are discussed, including the Volmer-Weber type growth mode due to thehigh energy of the (001) VO2 surface. Another key limitation is interdiffusion with the (001) TiO2substrate, which we quantify using low angle annular dark field scanning transmission electronmicroscopy in conjunction with electron energy loss spectroscopy. We find that controlling islandcoalescence on the (001) surface and minimization of cation interdiffusion by using a low growthtemperature followed by a brief anneal at higher temperature are crucial for realizing ultrathin VO2films with abrupt MIT behavior. VC 2015 AIP Publishing LLC.

We demonstrate the ability to tune the thermal conductivity of homoepitaxial SrTiO3 films depositedby reactive molecular-beam epitaxy by varying growth temperature, oxidation environment, and cationstoichiometry. Both point defects and planar defects decrease the longitudinal thermal conductivity(k33), with the greatest decrease in films of the same composition observed for films containingplanar defects oriented perpendicular to the direction of heat flow. The longitudinal thermal conductivitycan be modified by as much as 80%—from 11.5W m1K1 for stoichiometric homoepitaxialSrTiO3 to 2W m1K1 for strontium-rich homoepitaxial Sr1þdTiOx films—by incorporating (SrO)2Ruddlesden-Popper planar defects.VC 2015 AIP Publishing LLC.

We measured the optical properties of epitaxial thin films of the metastable hexagonal polymorph of LuFeO3 by absorption spectroscopy, magnetic circular dichroism, and photoconductivity. Comparison with complementary electronic structure calculations reveals a 1.1 eV direct gap involving hybridized Fe 3dz2+O 2pz→Fe d excitations at the Γ and A points, with a higher energy direct gap at 2.0 eV. Both charge gaps nicely overlap the solar spectrum.