Fluorinated High-Voltage Electrolytes To Stabilize Nickel-Rich Lithium Batteries

As state-of-the-art (SOA) lithium-ion (Li-ion) batteries approach their specific energy limit (∼250 Wh kg-1), layer-structured, nickel-rich (Ni-rich) lithium transition metal oxide-based cathode materials, e.g., LiNi0.8Mn0.1Co0.1O2 (NMC811), have attracted great interest owing to their practical high specific capacities (>200 mAhg-1). Coupled with their high average discharge voltages (∼4 V vs Li/Li+), Ni-rich cathode-based lithium batteries possess a great potential to achieve much higher specific energies (>350 Wh kg-1 at the cell level) than the SOA Li-ion counterparts. In addition, Ni-rich oxides are low-cost battery cathode materials due to their low cobalt contents. However, Ni-rich cathode-based lithium batteries suffer quick capacity degradations upon cycling, particularly at high upper cutoff voltages (e.g., ≥4.5 V vs Li/Li+), due to crystal structure changes of the active cathode materials and parasitic side reactions at the electrolyte/electrode interfaces. In this study, a fluorinated-solvent-based, high-voltage stable electrolyte (HVE), i.e., 1 M Li bis(trifluoromethanesulfonyl)imide (LiTFSI) in fluoroethylene carbonate (FEC), bis(2,2,2-trifluoroethyl) carbonate (FDEC), and methyl (2,2,2-trifluoroethyl) carbonate (FEMC) with Li difluoro(oxalate)borate (LiDFOB) additive, was formulated and evaluated in Li/NMC811 battery cells. To the best of our knowledge, this class of electrolyte has not been investigated for Ni-rich cathode-based lithium batteries. Li/NMC811 cells with HVE exhibited a superior long-term cycle performance stability, maintaining ∼80% capacity after ∼500 cycles at a high cutoff voltage of 4.5 V (vs Li/Li+) than a baseline carbonate-solvent-based electrolyte (BE). The superior cycle stability of the Li/NMC811 cells is attributed to the inherently high-voltage stability of HVE, supported by the physical and electrochemical analyses. This conclusion is supported by our density functional theory (DFT) modeling where HVE shows a less tendency of deprotonation/oxidation than BE, leading to the observed cycle stability. The findings in this study are important to help tackle the technical challenges facing Ni-rich cathode-based lithium batteries to realize their high energy density potentials with a long cycle life.

Schottky Barrier Control of Self-Polarization for a Colossal Ferroelectric Resistive Switching

Controlling the domain evolution is critical both for optimizing ferroelectric properties and for designing functional electronic devices. Here we report an approach of using the Schottky barrier formed at the metal/ferroelectric interface to tailor the self-polarization states of a model ferroelectric thin film heterostructure system SrRuO₃/(Bi,Sm)FeO₃. Upon complementary investigations of the piezoresponse force microscopy, electric transport measurements, X-ray photoelectron/absorption spectra, and theoretical studies, we demonstrate that Sm doping changes the concentration and spatial distribution of oxygen vacancies with the tunable host Fermi level which modulates the SrRuO₃/(Bi,Sm)FeO₃ Schottky barrier and the depolarization field, leading to the evolution of the system from a single domain of downward polarization to polydomain states. Accompanied by such modulation on self-polarization, we further tailor the symmetry of the resistive switching behaviors and achieve a colossal on/off ratio of ∼1.1 × 10⁶ in the corresponding SrRuO₃/BiFeO₃/Pt ferroelectric diodes (FDs). In addition, the present FD also exhibits a fast operation speed of ∼30 ns with a potential for sub-nanosecond and an ultralow writing current density of ∼132 A/cm². Our studies provide a way for engineering self-polarization and reveal its strong link to the device performance, facilitating FDs as a competitive memristor candidate used for neuromorphic computing.

Prominent Size Effects without a Depolarization Field Observed in Ultrathin Ferroelectric Oxide Membranes

The increasing miniaturization of electronics requires a better understanding of material properties at the nanoscale. Many studies have shown that there is a ferroelectric size limit in oxides, below which the ferroelectricity will be strongly suppressed due to the depolarization field, and whether such a limit still exists in the absence of the depolarization field remains unclear. Here, by applying uniaxial strain, we obtain pure in-plane polarized ferroelectricity in ultrathin SrTiO_{3} membranes, providing a clean system with high tunability to explore ferroelectric size effects especially the thickness-dependent ferroelectric instability with no depolarization field. Surprisingly, the domain size, ferroelectric transition temperature, and critical strain for room-temperature ferroelectricity all exhibit significant thickness dependence. These results indicate that the stability of ferroelectricity is suppressed (enhanced) by increasing the surface or bulk ratio (strain), which can be explained by considering the thickness-dependent dipole-dipole interactions within the transverse Ising model. Our study provides new insights into ferroelectric size effects and sheds light on the applications of ferroelectric thin films in nanoelectronics.

In-plane charged domain walls with memristive behaviour in a ferroelectric film

Domain-wall nanoelectronics is considered to be a new paradigm for non-volatile memory and logic technologies in which domain walls, rather than domains, serve as an active element. Especially interesting are charged domain walls in ferroelectric structures, which have subnanometre thicknesses and exhibit non-trivial electronic and transport properties that are useful for various nanoelectronics applications^1-3. The ability to deterministically create and manipulate charged domain walls is essential to realize their functional properties in electronic devices. Here we report a strategy for the controllable creation and manipulation of in-plane charged domain walls in BiFeO₃ ferroelectric films a few nanometres thick. By using an in situ biasing technique within a scanning transmission electron microscope, an unconventional layer-by-layer switching mechanism is detected in which ferroelectric domain growth occurs in the direction parallel to an applied electric field. Based on atomically resolved electron energy-loss spectroscopy, in situ charge mapping by in-line electron holography and theoretical calculations, we show that oxygen vacancies accumulating at the charged domain walls are responsible for the domain-wall stability and motion. Voltage control of the in-plane domain-wall position within a BiFeO₃ film gives rise to multiple non-volatile resistance states, thus demonstrating the key functional property of being a memristor a few unit cells thick. These results promote a better understanding of ferroelectric switching behaviour and provide a new strategy for creating unit-cell-scale devices.

Ab initio study of alloying of MnBi to enhance the energy product

The DFT calculations show that filing empty interstitial sites of MnBi leads to stable alloys with Li, O, F, Sc, Y, Rh, Pd among which alloy with Li, O, Rh, Pd and Pt have larger magnetization and enhanced energy products (up to 20%) than bulk MnBi.

Epitaxial antiperovskite/perovskite heterostructures for materials design

Engineered heterostructures formed by complex oxide materials are a rich source of emergent phenomena and technological applications. In the quest for new functionality, a vastly unexplored avenue is interfacing oxide perovskites with materials having dissimilar crystallochemical properties. Here, we propose a unique class of heterointerfaces based on nitride antiperovskite and oxide perovskite materials as a previously unidentified direction for materials design. We demonstrate the fabrication of atomically sharp interfaces between nitride antiperovskite Mn₃GaN and oxide perovskites (La_0.3Sr_0.7)(Al_0.65Ta_0.35)O₃ and SrTiO₃. Using atomic-resolution imaging/spectroscopic techniques and first-principles calculations, we determine the atomic-scale structure, composition, and bonding at the interface. The epitaxial antiperovskite/perovskite heterointerface is mediated by a coherent interfacial monolayer that interpolates between the two antistructures. We anticipate our results to be an important step for the development of functional antiperovskite/perovskite heterostructures, combining their unique characteristics such as topological properties for ultralow-power applications.

Evaluating the Thermoelectric Properties of BaTiS3 by Density Functional Theory

BaTiS₃ is a semiconductor with a small bandgap of ∼0.5 eV and strong transport anisotropy caused primarily by structural anisotropy; it contains well-separated octahedral columns along the [0001] direction and low lattice thermal conductivity, appealing for thermoelectric applications. Here, we evaluate the prospect of BaTiS₃ as a thermoelectric material by using the linearized electron and phonon Boltzmann transport theory based on the first-principles density functional band structure calculations. We find sizable values of the key thermoelectric parameters, such as the maximum power factor PF = 928 μW K^-2 and the maximum figure of merit ZT = 0.48 for an electron-doped sample and PF = 74 μW K^-2 and ZT = 0.17 for a hole-doped sample at room temperature, and a small doping level of ±0.25e per unit cell. The increase in temperature yields an increase in both the power factor and the figure of merit, reaching large values of PF = 3078 μW K^-2 and ZT = 0.77 for the electron-doped sample and PF = 650 μW K^-2 and ZT = 0.62 for the hole-doped sample at 800 K. Our results elucidate the promise of BaTiS₃ as a material for the thermoelectric power generator.

Controlling the Magnetic Properties of LaMnO3 /SrTiO3 Heterostructures by Stoichiometry and Electronic Reconstruction: Atomic-Scale Evidence

Interface-driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking are of importance for fundamental physics and device applications. How interfaces affect the interplay between charge, spin, orbital, and lattice degrees of freedom is the key to boosting device performance. In LaMnO₃ /SrTiO₃ (LMO/STO) polar-nonpolar heterostructures, electronic reconstruction leads to an antiferromagnetic to ferromagnetic transition, making them viable for spin filter applications. The interfacial electronic structure plays a critical role in the understanding of the microscopic origins of the observed magnetic phase transition, from antiferromagnetic at 5 unit cells (ucs) of LMO or below to ferromagnetic at 6 ucs or above, yet such a study is missing. Here, an atomic scale understanding of LMO/STO ambipolar ferromagnetism is offered by quantifying the interface charge distribution and performing first-principles density functional theory (DFT) calculations across this abrupt magnetic transition. It is found that the electronic reconstruction is confined within the first 3 ucs of LMO from the interface, and more importantly, it is robust against oxygen nonstoichiometry. When restoring stoichiometry, an enhanced ferromagnetic insulating state in LMO films with a thickness as thin as 2 nm (5 uc) is achieved, making LMO readily applicable as barriers in spin filters.

Spin Filtering in CrI3 Tunnel Junctions

The recently discovered magnetism of two-dimensional (2D) van der Waals crystals has attracted a lot of attention. Among these materials is CrI₃, a magnetic semiconductor, exhibiting transitions between ferromagnetic and antiferromagnetic orderings under the influence of an applied magnetic field. Here, using first-principles methods based on density functional theory, we explore spin-dependent transport in tunnel junctions formed of face-centered cubic Cu(111) electrodes and a CrI₃ tunnel barrier. We find about 100% spin polarization of the tunneling current for a ferromagnetically ordered four-monolayer CrI₃ and a tunneling magnetoresistance of about 3000% associated with a change of magnetic ordering in CrI₃. This behavior is understood in terms of the spin and wave-vector-dependent evanescent states in CrI₃, which control the tunneling conductance. We find a sizable charge transfer from Cu to CrI₃, which adds new features to the mechanism of spin filtering in CrI₃-based tunnel junctions. Our results elucidate the mechanisms of spin filtering in CrI₃ tunnel junctions and provide important insights for the design of magnetoresistive devices based on 2D magnetic crystals.

Publisher Correction: Anisotropic polarization-induced conductance at a ferroelectric-insulator interface

In the version of this Letter originally published, the right-hand arrow in Fig. 3b was incorrectly labelled; see correction note for details. Also, ref. 29 was incorrectly included in the reference list; it has now been removed.

Anisotropic polarization-induced conductance at a ferroelectric-insulator interface

Coupling between different degrees of freedom, that is, charge, spin, orbital and lattice, is responsible for emergent phenomena in complex oxide heterostrutures^1,2. One example is the formation of a two-dimensional electron gas (2DEG) at the polar/non-polar LaAlO₃/SrTiO₃ (LAO/STO)^3-7 interface. This is caused by the polar discontinuity and counteracts the electrostatic potential build-up across the LAO film³. The ferroelectric polarization at a ferroelectric/insulator interface can also give rise to a polar discontinuity^8-10. Depending on the polarization orientation, either electrons or holes are transferred to the interface, to form either a 2DEG or two-dimensional hole gas (2DHG)^11-13. While recent first-principles modelling predicts the formation of 2DEGs at the ferroelectric/insulator interfaces^9,10,12-14, experimental evidence of a ferroelectrically induced interfacial 2DEG remains elusive. Here, we report the emergence of strongly anisotropic polarization-induced conductivity at a ferroelectric/insulator interface, which shows a strong dependence on the polarization orientation. By probing the local conductance and ferroelectric polarization over a cross-section of a BiFeO₃-TbScO₃ (BFO/TSO) (001) heterostructure, we demonstrate that this interface is conducting along the 109° domain stripes in BFO, whereas it is insulating in the direction perpendicular to these domain stripes. Electron energy-loss spectroscopy and theoretical modelling suggest that the anisotropy of the interfacial conduction is caused by an alternating polarization associated with the ferroelectric domains, producing either electron or hole doping of the BFO/TSO interface.

Publisher Correction: Direct imaging of the electron liquid at oxide interfaces

In the version of this Letter originally published, in two instances in Fig. 1 the layers in the cross-sectional view of the (001) interface were incorrectly labelled: in Fig. 1b SrO⁺ should have read SrO⁰; in Fig. 1c LaO⁺, AlO₂^-, LaO⁺, TiO₂⁰, SrO⁺, TiO₂⁰ should have read LaO₃^3-, Al³⁺, LaO₃^3-, Ti⁴⁺, SrO₃^4-, Ti⁴⁺. In Fig. 3c the upper-right equation read -σ_s = -e/2a² but should have read -σ_s = e/2a² and in Fig. 3f the lower-right equation read -σ_s = -e/2√3a² but should have read σ_s = -e/2√3a². These errors have now been corrected in the online version of the Letter.

Ambipolar ferromagnetism by electrostatic doping of a manganite

Complex-oxide materials exhibit physical properties that involve the interplay of charge and spin degrees of freedom. However, an ambipolar oxide that is able to exhibit both electron-doped and hole-doped ferromagnetism in the same material has proved elusive. Here we report ambipolar ferromagnetism in LaMnO₃, with electron-hole asymmetry of the ferromagnetic order. Starting from an undoped atomically thin LaMnO₃ film, we electrostatically dope the material with electrons or holes according to the polarity of a voltage applied across an ionic liquid gate. Magnetotransport characterization reveals that an increase of either electron-doping or hole-doping induced ferromagnetic order in this antiferromagnetic compound, and leads to an insulator-to-metal transition with colossal magnetoresistance showing electron-hole asymmetry. These findings are supported by density functional theory calculations, showing that strengthening of the inter-plane ferromagnetic exchange interaction is the origin of the ambipolar ferromagnetism. The result raises the prospect of exploiting ambipolar magnetic functionality in strongly correlated electron systems.

Direct imaging of the electron liquid at oxide interfaces

The breaking of symmetry across an oxide heterostructure causes the electronic orbitals to be reconstructed at the interface into energy states that are different from their bulk counterparts ¹ . The detailed nature of the orbital reconstruction critically affects the spatial confinement and the physical properties of the electrons occupying the interfacial orbitals^2-4. Using an example of two-dimensional electron liquids forming at LaAlO₃/SrTiO₃ interfaces^5,6 with different crystal symmetry, we show that the selective orbital occupation and spatial quantum confinement of electrons can be resolved with subnanometre resolution using inline electron holography. For the standard (001) interface, the charge density map obtained by inline electron holography shows that the two-dimensional electron liquid is confined to the interface with narrow spatial extension (~1.0 ± 0.3 nm in the half width). On the other hand, the two-dimensional electron liquid formed at the (111) interface shows a much broader spatial extension (~3.3 ± 0.3 nm) with the maximum density located ~2.4 nm away from the interface, in excellent agreement with density functional theory calculations.

Direct observation of a two-dimensional hole gas at oxide interfaces

The discovery of a two-dimensional electron gas (2DEG) at the LaAlO₃/SrTiO₃ interface ¹ has resulted in the observation of many properties^2-5 not present in conventional semiconductor heterostructures, and so become a focal point for device applications^6-8. Its counterpart, the two-dimensional hole gas (2DHG), is expected to complement the 2DEG. However, although the 2DEG has been widely observed ⁹ , the 2DHG has proved elusive. Herein we demonstrate a highly mobile 2DHG in epitaxially grown SrTiO₃/LaAlO₃/SrTiO₃ heterostructures. Using electrical transport measurements and in-line electron holography, we provide direct evidence of a 2DHG that coexists with a 2DEG at complementary heterointerfaces in the same structure. First-principles calculations, coherent Bragg rod analysis and depth-resolved cathodoluminescence spectroscopy consistently support our finding that to eliminate ionic point defects is key to realizing a 2DHG. The coexistence of a 2DEG and a 2DHG in a single oxide heterostructure provides a platform for the exciting physics of confined electron-hole systems and for developing applications.

Tunneling Hot Spots in Ferroelectric SrTiO3

Strontium titanate (SrTiO₃) is the "silicon" in the emerging field of oxide electronics. While bulk properties of this material have been studied for decades, new unexpected phenomena have recently been discovered at the nanoscale, when SrTiO₃ forms an ultrathin film or an atomically sharp interface with other materials. One of the striking discoveries is room-temperature ferroelectricity in strain-free ultrathin films of SrTiO₃ driven by the Ti_Sr antisite defects, which generate a local dipole moment polarizing the surrounding nanoregion. Here, we demonstrate that these polar defects are not only responsible for ferroelectricity, but also propel the appearance of highly conductive channels, "hot spots", in the ultrathin SrTiO₃ films. Using a combination of scanning probe microscopy experimental studies and theoretical modeling, we show that the hot spots emerge due to resonant tunneling through localized electronic states created by the polar defects and that the tunneling conductance of the hot spots is controlled by ferroelectric polarization. Our finding of the polarization-controlled defect-assisted tunneling reveals a new mechanism of resistive switching in oxide heterostructures and may have technological implications for ferroelectric tunnel junctions. It is also shown that the conductivity of the hot spots can be modulated by mechanical stress, opening a possibility for development of conceptually new electronic devices with mechanically tunable resistive states.

Effects of pressure and strain on spin polarization of IrMnSb

A high degree of spin polarization in electron transport is one of the most sought-after properties of a material which can be used in spintronics-an emerging technology utilizing a spin degree of freedom in electronic devices. An ideal candidate to exhibit highly spin-polarized current would be a room temperature half-metal, a material which behaves as an insulator for one spin channel and as a conductor for the other spin channel. In this paper, we explore a semi-Heusler compound, IrMnSb, which has been reported to exhibit pressure induced half-metallic transition. We confirm that the bulk IrMnSb is a spin-polarized metal, with dominant contribution to electronic states at the Fermi energy from majority-spin electrons. Application of a uniform pressure shifts the Fermi level into the minority-spin energy gap, thus demonstrating pressure induced half-metallic transition. This behavior is explained by the reduction of the exchange splitting of the spin bands consistent with the Stoner model for itinerant magnetism. We find that the half-metallic transition is suppressed when instead of uniform pressure the bulk IrMnSb is exposed to biaxial strain. This suppression of half-metallicity is driven by the epitaxial strain induced tetragonal distortion, which lifts the degeneracy of the Mn 3d t _2g and e _g orbitals and reduces the minority-spin band gap under compressive strain, thus preventing half-metallic transition. Our calculations also indicate that in thin film geometry, surface states emerge in the minority-spin band gap, which has detrimental for practical applications impact on the spin polarization of IrMnSb.

Polarization-Mediated Modulation of Electronic and Transport Properties of Hybrid MoS2-BaTiO3-SrRuO3 Tunnel Junctions

Hybrid structures composed of ferroelectric thin films and functional two-dimensional (2D) materials may exhibit unique characteristics and reveal new phenomena due to the cross-interface coupling between their intrinsic properties. In this report, we demonstrate a symbiotic interplay between spontaneous polarization of the ultrathin BaTiO₃ ferroelectric film and conductivity of the adjacent molybdenum disulfide (MoS₂) layer, a 2D narrow-bandgap semiconductor. Polarization-induced modulation of the electronic properties of MoS₂ results in a giant tunneling electroresistance effect in the hybrid MoS₂-BaTiO₃-SrRuO₃ ferroelectric tunnel junctions (FTJs) with an OFF-to-ON resistance ratio as high as 10⁴, a 50-fold increase in comparison with the same type of FTJs with metal electrodes. The effect stems from the reversible accumulation-depletion of the majority carriers in the MoS₂ electrode in response to ferroelectric switching, which alters the barrier at the MoS₂-BaTiO₃ interface. Continuous tunability of resistive states realized via stable sequential domain structures in BaTiO₃ adds memristive functionality to the hybrid FTJs. The use of narrow band 2D semiconductors in conjunction with ferroelectric films provides a novel pathway for development of the electronic devices with enhanced performance.

On the structural origin of the single-ion magnetic anisotropy in LuFeO3

The electronic structure for the conduction bands of both hexagonal and orthorhombic LuFeO3 thin films have been measured using x-ray absorption spectroscopy at oxygen K (O K) edge. Dramatic differences in both the spectral features and the linear dichroism are observed. These differences in the spectra can be explained using the differences in crystal field splitting of the metal (Fe and Lu) electronic states and the differences in O 2p-Fe 3d and O 2p-Lu 5d hybridizations. While the oxidation states have not changed, the spectra are sensitive to the changes in the local environments of the Fe(3+) and Lu(3+) sites in the hexagonal and orthorhombic structures. Using the crystal-field splitting and the hybridizations that are extracted from the measured electronic structures and the structural distortion information, we derived the occupancies of the spin minority states in Fe(3+), which are non-zero and uneven. The single ion anisotropy on Fe(3+) sites is found to originate from these uneven occupancies of the spin minority states via spin-orbit coupling in LuFeO3.

Epitaxial CrN thin films with high thermoelectric figure of merit

A large enhancement of the thermoelectric figure of merit is reported in single-crystalline films of CrN. The mechanism of the reduction of the lattice thermal conductivity in cubic CrN is similar to the resonant bonding in IV-VI compounds. Therefore, useful ideas from classic thermo-electrics can be applied to tune functionalities in transition metal nitrides and oxides.

Mechanical Tuning of LaAlO3/SrTiO3 Interface Conductivity

In recent years, complex-oxide heterostructures and their interfaces have become the focus of significant research activity, primarily driven by the discovery of emerging states and functionalities that open up opportunities for the development of new oxide-based nanoelectronic devices. The highly conductive state at the interface between insulators LaAlO3 and SrTiO3 is a prime example of such emergent functionality, with potential application in high electron density transistors. In this report, we demonstrate a new paradigm for voltage-free tuning of LaAlO3/SrTiO3 (LAO/STO) interface conductivity, which involves the mechanical gating of interface conductance through stress exerted by the tip of a scanning probe microscope. The mechanical control of channel conductivity and the long retention time of the induced resistance states enable transistor functionality with zero gate voltage.

The stability and surface termination of hexagonal LuFeO3

The surface termination and the nominal valence states for hexagonal LuFeO3 thin films grown on Al2O3(0 0 0 1) substrates were characterized by angle resolved x-ray photoemission spectroscopy. The Lu 4f, Fe 2p and O 1s core level spectra indicate that both the surface termination and the nominal valence depend on surface preparation, but the stable surface terminates in a Fe-O layer. This is consistent with the results of density functional calculations which predict that the Fe-O termination of LuFeO3(0 0 0 1) surface is energetically favorable and stable over a broad range of temperatures and oxygen partial pressures when it is reconstructed to eliminate surface polarity.

Room-temperature ferroelectricity in hexagonal TbMnO3 thin films

Piezoresponse force microscopy imaging in conjunction with first-principles calculations provide strong evidence for room-temperature ferroelectricity in epitaxially stabilized hexagonal TbMnO3 thin films, which in the bulk form are with orthorhombic structure. The obtained results demonstrate that new phases and functional properties of complex oxide materials can be strain-engineered using epitaxial growth.

Ferroelectric control of magnetocrystalline anisotropy at cobalt/poly(vinylidene fluoride) interfaces

Electric field control of magnetization is one of the promising avenues for achieving high-density energy-efficient magnetic data storage. Ferroelectric materials can be especially useful for that purpose as a source of very large switchable electric fields when interfaced with a ferromagnet. Organic ferroelectrics, such as poly(vinylidene fluoride) (PVDF), have an additional advantage of being weakly bonded to the ferromagnet, thus minimizing undesirable effects such as interface chemical modification and/or strain coupling. In this work we use first-principles density functional calculations of Co/PVDF heterostructures to demonstrate the effect of ferroelectric polarization of PVDF on the interface magnetocrystalline anisotropy that controls the magnetization orientation. We show that switching of the polarization direction alters the magnetocrystalline anisotropy energy of the adjacent Co layer by about 50%, driven by the modification of the screening charge induced by ferroelectric polarization. The effect is reduced with Co oxidation at the interface due to quenching the interface magnetization. Our results provide a new insight into the mechanism of the magnetoelectric coupling at organic ferroelectric/ferromagnet interfaces and suggest ways to achieve the desired functionality in practice.

Switchable induced polarization in LaAlO3/SrTiO3 heterostructures

Demonstration of a tunable conductivity of the LaAlO(3)/SrTiO(3) interfaces drew significant attention to the development of oxide electronic structures where electronic confinement can be reduced to the nanometer range. While the mechanisms for the conductivity modulation are quite different and include metal-insulator phase transition and surface charge writing, generally it is implied that this effect is a result of electrical modification of the LaAlO(3) surface (either due to electrochemical dissociation of surface adsorbates or free charge deposition) leading to the change in the two-dimensional electron gas (2DEG) density at the LaAlO(3)/SrTiO(3) (LAO/STO) interface. In this paper, using piezoresponse force microscopy we demonstrate a switchable electromechanical response of the LAO overlayer, which we attribute to the motion of oxygen vacancies through the LAO layer thickness. These electrically induced reversible changes in bulk stoichiometry of the LAO layer are a signature of a possible additional mechanism for nanoscale oxide 2DEG control on LAO/STO interfaces.