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In this work, the high entropy oxide A2CuO4 Ruddlesden-Popper (La0.2Pr0.2Nd0.2Sm0.2Eu0.2)2CuO4 is explored by charge doping with Ce+4 and Sr+2 at concentrations known to induce superconductivity in the simple parent compounds, Nd2CuO4 and La2CuO4. Electron doped (La0.185Pr0.185Nd0.185Sm0.185Eu0.185Ce0.075)2CuO4 and hole doped (La0.18Pr0.18Nd0.18Sm0.18Eu0.18Sr0.1)2CuO4 are synthesized and shown to be single crystal, epitaxially strained, and highly uniform. Transport measurements demonstrate that all as-grown films are insulating regardless of doping. Annealing studies show that resistivity can be tuned by modifying oxygen stoichiometry and inducing metallicity but without superconductivity. These results in turn are connected to extended x-ray absorption fine structure (EXAFS) results indicating that the lack of superconductivity in the high entropy cuprates likely originates from a large distortion within the Cu-O plane ({\sigma}2>0.015 {\AA}2) due to A-site cation size variance, which drives localization of charge carriers. These findings describe new opportunities for controlling charge- and orbital-mediated functional responses in Ruddlesden-Popper crystal structures, driven by balancing of cation size and charge variances that may be exploited for functionally important behaviors such as superconductivity, antiferromagnetism, and metal-insulator transitions, while opening less understood phase spaces hosting doped Mott insulators, strange metals, quantum criticality, pseudogaps, and ordered charge density waves.

Controlling electronic band structure is an important step toward harnessing quantum materials for practical uses. This is exemplified in the spin-polarized bands near the Fermi level of a ferromagnet where small variation to the Berry curvature can be used to control the intrinsic anomalous Hall effect (AHE). Since these pathways are highly sensitive to crystal structure, iterative post-synthesis manipulation of the underlying lattice can act as a fundamental probe and provide new opportunities for functionalization. In this work, depth-dependent magnetic inhomogeneity in a $4d$ itinerant ferromagnet ${\mathrm{SrRuO}}_{3}$ film is controlled by applying low energy He ion irradiation. Combined magnetometry, polarized neutron reflectometry, and magnetotransport experiments demonstrate that the Berry phase and associated AHE can be continuously and iteratively modified post-synthesis. The findings of this work should be widely applicable to other quantum materials where crystal distortions and symmetry are tightly bound to band structure and resulting Berry phase effects.

The surfaces of rutile transition-metal oxides $({\mathrm{TMO}}_{2})$ are widely investigated for catalysis, photoelectrochemical solar cells, memristors, and supercapacitors, but their structures have remained controversial. Here we employ density functional theory to predict that a universal behavior of metallic ${\mathrm{TMO}}_{2}$ surfaces, i.e., the stoichiometric ${\mathrm{TMO}}_{2}$ surfaces, exhibit a contrast reversal in simulated scanning tunneling microscopy (STM) images at different scanning biases. The predictions are verified by experimental STM imaging of ${\mathrm{RuO}}_{2}(110)$ surfaces and this feature is shown to enable accurate determinations of the ${\mathrm{TMO}}_{2}(110)$ surface structures under various conditions. This work provides different insights into the electronic properties of ${\mathrm{TMO}}_{2}(110)$ surfaces and offers an effective method to directly map the surface structure and point defects using bias-dependent STM.

In classical morphotropic piezoelectric materials, rhombohedral and tetragonal phase variants can energetically compete to form a mixed phase regime with improved functional properties. While the discovery of morphotropic-like phases in multiferroic BiFeO3 films has broadened this definition, accessing these phase spaces is still typically accomplished through isovalent substitution or heteroepitaxial strain which do not allow for continuous modification of phase composition postsynthesis. Here, we show that it is possible to use low-energy helium implantation to tailor morphotropic phases of epitaxial BiFeO3 films postsynthesis in a continuous and iterative manner. Applying this strain doping approach to morphotropic films creates a new phase space based on internal and external lattice stress that can be seen as an analogue to temperature–composition phase diagrams of classical morphotropic ferroelectric systems.

Micrometer-tall vertically aligned single-crystalline CoFe2O4 nanobrush architectures with extraordinarily large aspect ratio have been achieved by the precise control of a kinetic and thermodynamic non-equilibrium pulsed laser epitaxy process. Direct observations by scanning transmission electron microscopy reveal that the nanobrush crystal is mostly defect-free by nature, and epitaxially connected to the substrate through a continuous 2D interface layer. In contrast, periodic dislocations and lattice defects such as anti-phase boundaries and twin boundaries are frequently observed in the 2D interface layer, suggesting that interface misfit strain relaxation under a non-equilibrium growth condition plays a critical role in the self-assembly of such artificial architectures. Magnetic property measurements have found that the nanobrushes exhibit a saturation magnetization value of 6.16 B/f.u., which is much higher than the bulk value. The discovery not only enables insights into an effective route for fabricating unconventional high-quality nanostructures, but also demonstrates a novel magnetic architecture with potential applications in nanomagnetic devices.

We report the synthesis of single-crystal iron germanium nanowires via chemical vapor deposition without the assistance of any catalysts. The assembly of single-crystal FeGe2 nanowires with tetragonal C16 crystal structure shows anisotropic magnetic behavior along the radial direction or the growth axial direction, with both antiferromagnetic and ferromagnetic orders. Single FeGe2 nanowire devices were fabricated using e-beam lithography. Electronic transport measurement in these devices show two resistivity anomalies near 250 K and 200 K which are likely signatures of the two spin density wave states in FeGe2.

In this study, high-performance multilayer WSe2 field-effect transistor (FET) devices with carrier type control are demonstrated via thickness modulation and a remote oxygen plasma surface treatment. Carrier type control in multilayer WSe2 FET devices with Cr/Au contacts is initially demonstrated by modulating the WSe2 thickness. The carrier type evolves with increasing WSe2 channel thickness, being p-type, ambipolar, and n-type at thicknesses 5 nm, respectively. The thickness-dependent carrier type is attributed to changes in the bandgap of WSe2 as a function of the thickness and the carrier band offsets relative to the metal contacts. Furthermore, we present a strong hole carrier doping effect via remote oxygen plasma treatment. It non-degenerately converts n-type characteristics into p-type and enhances field-effect hole mobility by three orders of magnitude. This work demonstrates progress towards the realization of high-performance multilayer WSe2 FETs with carrier type control, potentially extendable to other transition metal dichalcogenides, for future electronic and optoelectronic applications.

Discovering high-performance energy storage materials is indispensable for renewable energy, electric vehicle performance, and mobile computing. Owing to the open atomic framework and good room temperature conductivity, bronze-phase vanadium dioxide [VO2(B)] has been regarded as a highly promising electrode material for Li ion batteries. However, previous attempts were unsuccessful to show the desired cycling performance and capacity without chemical modification. Here, we show with epitaxial VO2(B) films that one can accomplish the theoretical limit for capacity with persistent charging–discharging cyclability owing to the high structural stability and unique open pathways for Li ion conduction. Atomic-scale characterization by scanning transmission electron microscopy and density functional theory calculations also reveal that the unique open pathways in VO2(B) provide the most stable sites for Li adsorption and diffusion. Thus, this work ultimately demonstrates that VO2(B) is a highly promising energy ...

The impact of single-axis lattice expansion on the optical response of $\mathrm{BiFe}{\mathrm{O}}_{3}$ films is examined. Low-energy He implantation is used to tailor morphotropic phases of $\mathrm{BiFe}{\mathrm{O}}_{3}$ films and study changes in their optical spectra with continuously increasing lattice expansion. He ion implantation of epitaxial rhombohedral (R)- and tetragonal (T)-like $\mathrm{BiFe}{\mathrm{O}}_{3}$ films induces uniaxial out-of-plane strain that, on R-like films, eventually leads to a complete R-T phase transition. This approach allows us to provide insights into the optical response of $\mathrm{BiFe}{\mathrm{O}}_{3}$ films. Strain doping of T-like films leads to a significant redshift of the optical absorption spectra that is theoretically explained by a lowering of Fe $3d\phantom{\rule{0.28em}{0ex}}{t}_{2g}$ states. R-like films, on the other hand, show a less-pronounced sensitivity to uniaxial strain and a blueshift of about 250 meV at the strain-induced R-T transition. The results demonstrate that strain doping allows a deeper examination of the optical properties of epitaxial phases that are otherwise impossible to access by standard epitaxy.

Characterization of the onset of a phase transition is often challenging due to the fluctuations of the correlation length scales of the order parameters. This is especially true for second-order structural-phase transition due to minute changes involved in the relevant lattice constants. A classic example is the cubic-to-tetragonal second-order phase transition in SrTiO(3) (STO), which is so subtle that it is still unresolved. Here, we demonstrate an approach to resolve this issue by epitaxially grown rhombohedral La(0.7)Sr(0.3)MnO(3) (LSMO) thin films on the cubic STO (100) substrate. The shear strain induced nanotwinning waves in the LSMO film are extremely sensitive to the cubic-to-tetragonal structural-phase transitions of the STO substrate. Upon cooling from room temperature, the development of the nanotwinning waves is spatially inhomogeneous. Untwinned, atomically flat domains, ranging in size from 100 to 300 nm, start to appear randomly in the twinned phase between 265 and 175 K. At ∼139 K, the untwinned, atomically flat domains start to grow rapidly into micrometer scale and finally become dominant at ∼108 K. These results indicate that the low-temperature tetragonal precursor phase of STO has already nucleated at 265 K, significantly higher than the critical temperature of STO (∼105 K). Our work paves a pathway to visualize the onset stages of structural-phase transitions that are too subtle to be observed using direct-imaging methods.

Thickness dependent strain relaxation effects are utilized to study the impact of crystal anisotropy on the optical band gap of epitaxial SnO2 films grown by pulsed laser deposition on (0001)-oriented sapphire substrates. An X-ray diffraction analysis reveals that all films are under tensile biaxial in-plane strain and that strain relaxation occurs with increasing thickness. Variable angle spectroscopic ellipsometry shows that the optical band gap of the SnO2 films continuously increases with increasing film thickness. This increase in the band gap is linearly related to the strain state of the films, which indicates that the main origin of the band gap change is strain relaxation. The experimental observation is in excellent agreement with results from density functional theory for biaxial in-plane strain. This work demonstrates that strain is an effective way to tune the band gap of SnO2 films and suggests that strain engineering is an appealing route to tailor the optical properties of oxide semiconductors.

Self-poling of ferroelectric films, i.e., a preferred, uniform direction of the ferroelectric polarization in as-grown samples is often observed yet poorly understood despite its importance for device applications. The multiferroic perovskite BiFeO3, which crystallizes in two distinct structural polymorphs depending on applied epitaxial strain, is well known to exhibit self-poling. This study investigates the effect of self-poling on the monoclinic domain configuration and the switching properties of the two polymorphs of BiFeO3 (R′ and T′) in thin films grown on LaAlO3 substrates with slightly different La0.3Sr0.7MnO3 buffer layers. This study shows that the polarization state formed during the growth acts as “imprint” on the polarization and that switching the polarization away from this self-poled direction can only be done at the expense of the sample's monoclinic domain configuration. The observed reduction of the monoclinic domain size is largely reversible; hence, the domain size is restored when the polarization is switched back to its original orientation. This is a direct consequence of the growth taking place in the polar phase (below Tc). Switching the polarization away from the preferred configuration, in which defects and domain patterns synergistically minimize the system's energy, leads to a domain state with smaller (and more highly strained and distorted) monoclinic domains.

Amorphous metal-oxide semiconductors offer the high carrier mobilities and excellent large-area uniformity required for high performance, transparent, flexible electronic devices; however, a critical bottleneck to their widespread implementation is the need to activate these materials at high temperatures which are not compatible with flexible polymer substrates. The highly controllable activation of amorphous indium gallium zinc oxide semiconductor channels using ionic liquid gating at room temperature is reported. Activation is controlled by electric field-induced oxygen migration across the ionic liquid-semiconductor interface. In addition to activation of unannealed devices, it is shown that threshold voltages of a transistor can be linearly tuned between the enhancement and depletion modes. Finally, the first ever example of transparent flexible thin film metal oxide transistor on a polyamide substrate created using this simple technique is demonstrated. This study demonstrates the potential of field-induced activation as a promising alternative to traditional postdeposition thermal annealing which opens the door to wide scale implementation into flexible electronic applications.

The optical band gap of the prototypical semiconducting oxide SnO2 is shown to be continuously controlled through single axis lattice expansion of nanometric films induced by low-energy helium implantation. While traditional epitaxy-induced strain results in Poisson driven multidirectional lattice changes shown to only allow discrete increases in bandgap, we find that a downward shift in the band gap can be linearly dictated as a function of out-of-plane lattice expansion. Our experimental observations closely match density functional theory that demonstrates that uniaxial strain provides a fundamentally different effect on the band structure than traditional epitaxy-induced multiaxes strain effects. Charge density calculations further support these findings and provide evidence that uniaxial strain can be used to drive orbital hybridization inaccessible with traditional strain engineering techniques.

Finite range ferromagnetism and antiferromagnetism in two-dimensional (2D) systems within an isotropic Heisenberg model at non-zero temperature were originally proposed to be impossible. However, recent theoretical studies using an Ising model have shown that 2D magnetic crystals can exhibit magnetism. Experimental verification of existing 2D magnetic crystals in this system has remained exploratory. In this work we exfoliated CrSiTe3, a bulk ferromagnetic semiconductor, to mono- and few-layer 2D crystals onto a Si/SiO2 substrate. Raman spectra indicate good stability and high quality of the exfoliated flakes, consistent with the computed phonon spectra of 2D CrSiTe3, giving strong evidence for the existence of 2D CrSiTe3 crystals. When the thickness of the CrSiTe3 crystals is reduced to a few layers, we observed a clear change in resistivity at 80–120 K, consistent with theoretical calculations of the Curie temperature (Tc) of ∼80 K for the magnetic ordering of 2D CrSiTe3 crystals. The ferromagnetic mono- and few-layer 2D CrSiTe3 indicated here should enable numerous applications in nano-spintronics.

High-quality epitaxial growth of inter-metallic MnPt films on oxides is achieved, with potential for multiferroic heterostructure applications. Antisite-stabilized spin-flipping induces ferromagnetism in MnPt films, although it is robustly antiferromagnetic in bulk. Moreover, highly ordered antiferromagnetic MnPt films exhibit superiorly large exchange coupling with a ferromagnetic layer.

We report a giant, ~22%, electroresistance modulation for a metallic alloy above room temperature. It is achieved by a small electric field of 2 kV/cm via piezoelectric strain-mediated magnetoelectric coupling and the resulting magnetic phase transition in epitaxial FeRh/BaTiO3 heterostructures. This work presents detailed experimental evidence for an isothermal magnetic phase transition driven by tetragonality modulation in FeRh thin films, which is in contrast to the large volume expansion in the conventional temperature-driven magnetic phase transition in FeRh. Moreover, all the experimental results in this work illustrate FeRh as a mixed-phase model system well similar to phase-separated colossal magnetoresistance systems with phase instability therein., 6 pages, 3 figures

Highly strained BiFeO3 films transition into a true tetragonal state at 430 °C but remain polar to much higher temperatures (∼800 °C). Piezoelectric switching is only possible up to 300 °C, i.e., at temperatures for which strain stabilizes the stripe-like coexistence of multiple polymorphs.

We present a combined theoretical and experimental study of alloys of the high performance piezoelectric PZT (${\mathrm{PbZr}}_{0.5}{\mathrm{Ti}}_{0.5}{\mathrm{O}}_{3}$) with BZnT (${\mathrm{BiZn}}_{0.5}{\mathrm{Ti}}_{0.5}{\mathrm{O}}_{3}$) and BZnZr (${\mathrm{BiZn}}_{0.5}{\mathrm{Zr}}_{0.5}{\mathrm{O}}_{3}$), focusing on atomic displacements, ferroelectric polarization, and elastic stability. From theory we find that the 75-25 PZT-BZnT alloy has substantially larger cation displacements, and hence ferroelectric polarization than the PZT base material, on the tetragonal side of the phase diagram. We also find a possible morphotrophic phase boundary in this system by comparing displacement patterns and optimized $c/a$ ratios. Elastic stability calculations find the structures to be essentially stable. Experiments indicate the feasibility of sample synthesis within this alloy system, although measurements do not find significant polarization, probably due to a large coercive field.

The realization of a controllable metamagnetic transition from AFM to FM ordering would open the door to a plethora of new spintronics based devices that, rather than reorienting spins in a ferromagnet, harness direct control of a materials intrinsic magnetic ordering. In this study FeRh films with drastically reduced transition temperatures and a large magneto-thermal hysteresis were produced for magnetocaloric and spintronics applications. Remarkably, giant controllable magnetization changes (measured to be as high has ~25%) are realized by manipulating the strain transfer from the external lattice when subjected to two structural phase transitions of BaTiO3 (001) single crystal substrate. These magnetization changes are the largest seen to date to be controllably induced in the FeRh system. Using polarized neutron reflectometry we reveal how just a slight in plane surface strain change at ~290C results in a massive magnetic transformation in the bottom half of the film clearly demonstrating a strong lattice-spin coupling in FeRh. By means of these substrate induced strain changes we show a way to reproducibly explore the effects of temperature and strain on the relative stabilities of the FM and AFM phases in multi-domain metamagnetic systems. This study also demonstrates for the first time the depth dependent nature of a controllable magnetic order using strain in an artificial multiferroic heterostructure.

Complex oxide films are highly anisotropic in the way they conduct electricity, which is due to phase separation. However, the origin of this metal–insulator phase coexistence has been unclear. Transport measurements now show that strain, rather than chemical inhomogeneity, is mainly responsible. The presence of electronic phase separation in complex materials has been linked to many types of exotic behaviour, such as colossal magnetoresistance, the metal–insulator transition and high-temperature superconductivity1,2,3,4; however, the mechanisms that drive the formation of coexisting electronic phases are still debated5,6,7,8. Here we report transport measurements that show a preferential orientation of electronic phase domains driven by anisotropic long-range elastic coupling between a complex oxide film and substrate. We induce anisotropic electronic-domain formation along one axis of a pseudocubic perovskite single-crystal thin-film manganite by epitaxially locking it to an orthorhombic substrate. Simultaneous temperature-dependent resistivity measurements along the two perpendicular in-plane axes show substantial differences in the metal–insulator transition temperature and extraordinarily high anisotropic resistivities, which indicate that percolative conduction channels open more readily along one axis. These findings suggest that the origin of phase coexistence is much more strongly influenced by strain than by local chemical inhomogeneity.

Strong Coulomb repulsion and spin–orbit coupling are known to give rise to exotic physical phenomena in transition metal oxides. Initial attempts to investigate systems, where both of these fundamental interactions are comparably strong, such as 3d and 5d complex oxide superlattices, have revealed properties that only slightly differ from the bulk ones of the constituent materials. Here we observe that the interfacial coupling between the 3d antiferromagnetic insulator SrMnO3 and the 5d paramagnetic metal SrIrO3 is enormously strong, yielding an anomalous Hall response as the result of charge transfer driven interfacial ferromagnetism. These findings show that low dimensional spin–orbit entangled 3d–5d interfaces provide an avenue to uncover technologically relevant physical phenomena unattainable in bulk materials., Whilst superlattices containing thin films of 5d transition metal oxides are expected to yield strong interfacial coupling, only weak effects have been observed. Here, the authors report strong coupling between 3d SrMnO3 and 5d SrIrO3 due to the interplay of strong Coulomb and spin orbit interactions.

The evolution of the electronic properties of electron-doped (Sr1-xLax)2IrO4 is experimentally explored as the doping limit of La is approached. As electrons are introduced, the electronic ground state transitions from a spin-orbit Mott phase into an electronically phase separated state, where long-range magnetic order vanishes beyond x = 0:02 and charge transport remains percolative up to the limit of La substitution (x =0:06). In particular, the electronic ground state remains inhomogeneous even beyond the collapse of the parent state's long-range antiferromagnetic order, while persistent short-range magnetism survives up to the highest La-substitution levels. Furthermore, as electrons are doped into Sr2IrO4, we observe the appearance of a low temperature magnetic glass-like state intermediate to the complete suppression of antiferromagnetic order. Universalities and di erences in the electron-doped phase diagrams of single layer and bilayer Ruddlesden-Popper strontium iridates are discussed.

We report on the use of helium ion implantation to independently control the out-of-plane lattice constant in epitaxial La(0.7)Sr(0.3)MnO(3) thin films without changing the in-plane lattice constants. The process is reversible by a vacuum anneal. Resistance and magnetization measurements show that even a small increase in the out-of-plane lattice constant of less than 1% can shift the metal-insulator transition and Curie temperatures by more than 100 °C. Unlike conventional epitaxy-based strain tuning methods which are constrained not only by the Poisson effect but by the limited set of available substrates, the present study shows that strain can be independently and continuously controlled along a single axis. This permits novel control over orbital populations through Jahn-Teller effects, as shown by Monte Carlo simulations on a double-exchange model. The ability to reversibly control a single lattice parameter substantially broadens the phase space for experimental exploration of predictive models and leads to new possibilities for control over materials' functional properties.

The electronic phase diagram of the weak spin-orbit Mott insulator (Sr(1-x)Lax)3Ir2O7 is determined via an exhaustive experimental study. Upon doping electrons via La substitution, an immediate collapse in resistivity occurs along with a narrow regime of nanoscale phase separation comprised of antiferromagnetic, insulating regions and paramagnetic, metallic puddles persisting until x~0.04. Continued electron doping results in an abrupt, first-order phase boundary where the Neel state is suppressed and a homogenous, correlated, metallic state appears with an enhanced spin susceptibility and local moments. As the metallic state is stabilized, a weak structural distortion develops and suggests a competing instability with the parent spin-orbit Mott state., 5 pages, 4 figures

The epitaxially stabilized metallic α-FeSi2 thin films on Si(001) were grown using pulsed laser deposition. While the bulk material of α-FeSi2 is a high temperature metastable phase and nonmagnetic, the thin film is stabilized at room temperature and shows unusual electronic transport and magnetic properties due to strain modification. The transport renders two different conducting states with a strong crossover at 50 K accompanied by an onset of ferromagnetism as well as a substantial magnetocaloric effect and magnetoresistance. These experimental results are discussed in terms of the unusual electronic structure of α-FeSi2 obtained within density functional calculations and Boltzmann transport calculations with and without strain. Our findings provide an example of a tailored material with interesting physics properties for practical applications.

A novel method has been proposed and tested to optimize the photorefractive multiple quantum wells device for biomedical imaging. The device acts as a dynamic holographic beam combiner in a holographic optical coherence imaging. By choosing an appropriate duty cycle of the external electric field, the Joule heat limit and saturation of exciton are decreased. We examine several characteristics that govern the use of the device.

In this work, we demonstrate that the optical bandgap of WO3 films can be continuously controlled through uniaxial strain induced by low-energy helium implantation. The insertion of He into epitaxially grown and coherently strained WO3 films can be used to induce single axis out-of-plane lattice expansion of up to 2%. Ellipsometric spectroscopy reveals that the optical bandgap is reduced by about 0.18 eV per percent expansion of the out-of-plane unit cell length. Density functional theory calculations show that this response is a direct result of changes in orbital degeneracy driven by changes in the octahedral rotations and tilts.

We investigate the effects of strain on antiferromagntic (AFM) single crystal thin films of La$_{1-x}$Sr$_{x}$MnO$_{3}$ (x = 0.6). Nominally unstrained samples have strong magnetoresistance with anisotropic magnetoresistances (AMR) of up to 8%. Compressive strain suppresses magnetoresistance but generates AMR values of up to 63%. Tensile strain presents the only case of a metal-insulator transition and demonstrates a previously unreported AMR behavior. In all three cases, we find evidence of magnetic ordering and no indication of a global ferromagnetic phase transition. These behaviors are attributed to epitaxy induced changes in orbital occupation driving different magnetic ordering types. Our findings suggest that different AFM ordering types have a profound impact on the AMR magnitude and character., Comment: http://dx.doi.org/10.1063/1.4892420

The efficacy of strained layer threading dislocation filter structures in single crystal epitaxial layers is evaluated using numerical modeling for (001) face-centred cubic materials, such as GaAs or Si(1-x)Ge(x), and (0001) hexagonal materials such as GaN. We find that threading dislocation densities decay exponentially as a function of the strain relieved, irrespective of the fraction of threading dislocations that are mobile. Reactions between threading dislocations tend to produce a population that is a balanced mixture of mobile and sessile in (001) cubic materials. In contrast, mobile threading dislocations tend to be lost very rapidly in (0001) GaN, often with little or no reduction in the immobile dislocation density. The capture radius for threading dislocation interactions is estimated to be approx. 40nm using cross section transmission electron microscopy of dislocation filtering structures in GaAs monolithically grown on Si. We find that the minimum threading dislocation density that can be obtained in any given structure is likely to be limited by kinetic effects to approx. 1.0e+04 to 1.0e+05 per square cm., Comment: 18 pages, 9 figures

Using combined theoretical and experimental approaches, we studied the structural and electronic origin of the magnetic structure in hexagonal LuFeO$_3$. Besides showing the strong exchange coupling that is consistent with the high magnetic ordering temperature, the previously observed spin reorientation transition is explained by the theoretically calculated magnetic phase diagram. The structural origin of this spin reorientation that is responsible for the appearance of spontaneous magnetization, is identified by theory and verified by x-ray diffraction and absorption experiments., 5 pages, 2 tables and 4 figures, Please contact us for the supplementary material. Accepted in Phys. Rev. B, in production

Physical and electrochemical phenomena at the surfaces of transition metal oxides and their coupling to local functionality remains one of the enigmas of condensed matter physics. Understanding the emergent physical phenomena at surfaces requires the capability to probe the local composition, map order parameter fields and establish their coupling to electronic properties. Here we demonstrate that measuring the sub-30-pm displacements of atoms from high-symmetry positions in the atomically resolved scanning tunnelling microscopy allows the physical order parameter fields to be visualized in real space on the single-atom level. Here, this local crystallographic analysis is applied to the in-situ-grown manganite surfaces. In particular, using direct bond-angle mapping we report direct observation of structural domains on manganite surfaces, and trace their origin to surface-chemistry-induced stabilization of ordered Jahn-Teller displacements. Density functional calculations provide insight into the intriguing interplay between the various degrees of freedom now resolved on the atomic level.

Organic spintronic devices have been appealing because of the long spin lifetime of the charge carriers in the organic materials and their low cost, flexibility and chemical diversity. In previous studies, the control of resistance of organic spin valves is generally achieved by the alignment of the magnetization directions of the two ferromagnetic electrodes, generating magnetoresistance. Here we employ a new knob to tune the resistance of organic spin valves by adding a thin ferroelectric interfacial layer between the ferromagnetic electrode and the organic spacer: the magnetoresistance of the spin valve depends strongly on the history of the bias voltage, which is correlated with the polarization of the ferroelectric layer; the magnetoresistance even changes sign when the electric polarization of the ferroelectric layer is reversed. These findings enable active control of resistance using both electric and magnetic fields, opening up possibility for multi-state organic spin valves., Organic materials potentially offer a low-cost, flexible and environment-friendly route to spintronics. Here, the authors demonstrate an organic spin-valve device in which an electric field can control both the magnitude and the sign of magnetoresistance.

When confining an electronically phase we separated manganite film to the scale of its coexisting self-organized metallic and these insulating domains allows resistor-capacitor circuit-like responses while providing both electroresistive and magnetoresistive switching functionality.

A growth diagram of Lu-Fe-O compounds on MgO (111) substrates using pulsed laser deposition is constructed based on extensive growth experiments. The LuFe${}_{2}$O${}_{4}$ phase can only be grown in a small range of temperature and O${}_{2}$ pressure conditions. An understanding of the growth mechanism of Lu-Fe-O compound films is offered in terms of the thermochemistry at the surface. Superparamagnetism is observed in the LuFe${}_{2}$O${}_{4}$ film and is explained in terms of the effect of the impurity hexagonal LuFeO${}_{3}$ (h-LuFeO${}_{3})$ phase and structural defects.

The crystal and magnetic structures of single-crystalline hexagonal LuFeO$_3$ films have been studied using x-ray, electron and neutron diffraction methods. The polar structure of these films are found to persist up to 1050 K; and the switchability of the polar behavior is observed at room temperature, indicating ferroelectricity. An antiferromagnetic order was shown to occur below 440 K, followed by a spin reorientation resulting in a weak ferromagnetic order below 130 K. This observation of coexisting multiple ferroic orders demonstrates that hexagonal LuFeO$_3$ films are room-temperature multiferroics.

By reducing an electronically phase-separated manganite (${\mathrm{La}}_{1\ensuremath{-}y}{\mathrm{Pr}}_{y}$)${}_{x}{\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{MnO}}_{3}$ single-crystal thin film to dimensions on the order of the inherent phase domains, it is possible to isolate and monitor the behavior of single domains at a first-order transition. At this critical point, it is possible to study the coexistence, formation, and annihilation processes of discrete electronic phase domains. With this technique, we make several observations on the mechanisms leading to the metal-insulator transition in manganites. We observe that domain formation is emergent and random, the transition process from the metallic phase to the insulating phase takes longer than the reverse process, electric field effects are more influential in driving a phase transition than current-induced electron heating, and single domain transition dynamics can be tuned through careful application of temperature and electric field.

A prominent feature of complex oxides is the coexistence of competing electronic phases. The separation of metallic and insulating phases is believed to be responsible for a variety of emergent transport phenomena, including quantum criticality in ruthenates and colossal magnetoresistance (CMR) in manganites. Interestingly, the phase boundaries between neighboring phases can often be displaced by small perturbations such as chemical doping, heating, stress, and electric or magnetic field, leading to intriguing metal–insulator transitions (MITs). The association of the emergent MIT with electronic inhomogeneities is particularly pronounced in low-dimensional materials which are uniquely suited to studying the MIT and phase evolutions in response to modification of the order parameters. Here, we present a few examples to illustrate the intimate interplay between emergent MIT and the competing electronic phases in functional metal oxide materials, including a percolative MIT near the critical temperature of the Mott transition in a Mn-doped bilayer ruthenate Sr3Ru2O7 crystal surface, and the abrupt conductance changes and reemergent MIT in manganite nanowires of La5/8 − x Pr x Ca3/8MnO3. This experimental research has benefited from new developments in the fabrication and characterization of low-dimensional oxide materials and nanostructures. A rare glimpse of the microscopic phase separation, the dynamic phase percolation, and the strain-tuned MIT has been provided. The results indicate the critical role of electron–lattice interactions in phase separation and suggest that the origin of phase coexistence is much more strongly influenced by strain than local chemical inhomogeneity, both for ruthenates and manganites.

In strongly correlated electronic systems, such as manganites, the global transport behavior depends sensitively on the spin ordering, whose alteration often requires a large external magnetic field. Here we show that the spin ordering in manganites can be easily controlled by exchange field across the interface between a ferromagnet and a manganite. By depositing isolated ferromagnetic nanodots on a manganite thin film, we find that it is possible to overcome dimensionality and strain effects to raise the metal-insulator transition (MIT) temperature by over 200 K and increase the magnetoresistance by 5000%. The MIT temperature can also be tuned by controlling the density of the magnetic nanodots which indicates that the formation process of electronic phase separation can be controlled by the presence of magnetic nanodots.

Using in situ magneto-optical Kerr effect measurements and phenomenological modeling, we study the tunability in both the magnetization anisotropy and magnetic coupling of Fe nanodots on a curved Cu(111) substrate with varying vicinity. We observe that, as the terrace width $w$ decreases, the magnetization anisotropy increases monotonically, faster when $w$ is smaller than the nanodot size $d$. In contrast, the magnetic coupling strength also increases until $w\ensuremath{\sim}d$, after which it decreases steeply. These striking observations can be rationalized by invoking the counterintuitive dimensionality variation of the surface electrons mediating the interdot coupling: the electrons are confined to be one dimensional (1D) when $w\ensuremath{\ge}d$, but become quasi-2D when $wld$ due to enhanced electron spillover across the steps bridged by the nanodots.

The dynamics of first-order electronic phase transitions in complex transition metal oxides are not well understood but are crucial in understanding the emergent phenomena of electronic phase separation. We show that a manganite system reduced to the scale of its inherent electronic charge-ordered insulating and ferromagnetic metal phase domains allows for the direct observation of single electronic phase domain fluctuations within a critical regime of temperature and magnetic field at the metal-insulator transition.

The two hottest areas of research in condensed matter physics are complexity and nanoscale physics. Interestingly, these two areas have little overlap as most of the nanophysics research work is conducted using ?simple? materials of metals or semiconductors instead of complex materials such as transition metal oxides. However, due to the strong electronic correlation, it is exactly the transition metal oxides that will most likely lead to observations of striking new phenomena under spatial confinement. We will use perovskite manganites as model systems to demonstrate how spatial confinement can dramatically affect their transport and magnetic properties. The emerging magnetic and transport behavior is likely associated with the electronic phase separation under confined geometry in the manganites. Some of the new properties such as ultrasharp jumps of magnetoresistance and reentrant metal-insulator transition may have significant impact on fabricating oxides-based novel devices.

The metal-insulator transition is characterized as a single peak in the temperature-dependent resistivity measurements; exceptions to this have never been seen in any single crystal material system. We show that by reducing a single crystal manganite thin film to a wire with a width comparable to the mesoscopic phase-separated domains inherent in the material, a second and robust metal-insulator transition peak appears in the resistivity versus temperature measurement. This new observation suggests that spatial confinement is a promising route for the discovery of emergent physical phenomena in complex oxides.

Optical lithography is used to fabricate LPCMO wires starting from a single La5=80:3Pr0:3Ca3=8MnO3 (LPCMO) film epitaxially grown on a LaAlO3100 substrate. As the width of the wires is decreased, the resistivity of the LPCMO wires exhibits giant and ultrasharp steps upon varying temperature and magnetic field in the vicinity of the metal-insulator transition. The origin of the ultrasharp transitions is attributed to the effect of spatial confinement on the percolative transport in manganites. The strong spin-charge-lattice interaction in transition metal oxides (TMOs) often leads to a striking phenomenon called electronic phase separation (PS), which is identified with the coexistence of a range of exotic electronic and magnetic phases despite the single crystalline structure [1]. The role of the PS in the related physical properties such as high-T c superconducting and colossal magnetoresistance (CMR) is a hotly debated issue in the field of condensed matter physics. Spatial confinement is a very useful route to gain deeper insight into the nature of PS. In particular, when the spatial dimension of the TMOs is reduced to the characteristic PS length scale, one would expect that changes in the transport properties of the TMOs could be quite dramatic. A model system for this study is La5=8xPrxCa3=8MnO3 (LPCMO), a perovskite manganite that is famously known for its large-scale PS. Transmission electron microscopy has revealed submicrometer-scaled ferromagnetic (FM) and charge ordered (CO) domains [2]. The coexistence of FM=CO phases in LPCMO reflects the competition between the intrinsic properties of its two starting materials, i.e., FM La5=8Ca3=8MnO3 (TC 275 K) and CO antiferro