Antiferromagnetic half-skyrmions and bimerons at room temperature.

In the quest for post-CMOS (complementary metal-oxide-semiconductor) technologies, driven by the need for improved efficiency and performance, topologically protected ferromagnetic 'whirls' such as skyrmions1-8 and their anti-particles have shown great promise as solitonic information carriers in racetrack memory-in-logic or neuromorphic devices1,9-11. However, the presence of dipolar fields in ferromagnets, which restricts the formation of ultrasmall topological textures3,6,8,9,12, and the deleterious skyrmion Hall effect, when skyrmions are driven by spin torques9,10,12, have thus far inhibited their practical implementation. Antiferromagnetic analogues, which are predicted to demonstrate relativistic dynamics, fast deflection-free motion and size scaling, have recently become the subject of intense focus9,13-19, but they have yet to be experimentally demonstrated in natural antiferromagnetic systems. Here we realize a family of topological antiferromagnetic spin textures in α-Fe2O3-an Earth-abundant oxide insulator-capped with a platinum overlayer. By exploiting a first-order analogue of the Kibble-Zurek mechanism20,21, we stabilize exotic merons and antimerons (half-skyrmions)8 and their pairs (bimerons)16,22, which can be erased by magnetic fields and regenerated by temperature cycling. These structures have characteristic sizes of the order of 100 nanometres and can be chemically controlled via precise tuning of the exchange and anisotropy, with pathways through which further scaling may be achieved. Driven by current-based spin torques from the heavy-metal overlayer, some of these antiferromagnetic textures could emerge as prime candidates for low-energy antiferromagnetic spintronics at room temperature1,9-11,23.

Heterogeneous integration of single-crystalline complex-oxide membranes.

Complex-oxide materials exhibit a vast range of functional properties desirable for next-generation electronic, spintronic, magnetoelectric, neuromorphic, and energy conversion storage devices1-4. Their physical functionalities can be coupled by stacking layers of such materials to create heterostructures and can be further boosted by applying strain5-7. The predominant method for heterogeneous integration and application of strain has been through heteroepitaxy, which drastically limits the possible material combinations and the ability to integrate complex oxides with mature semiconductor technologies. Moreover, key physical properties of complex-oxide thin films, such as piezoelectricity and magnetostriction, are severely reduced by the substrate clamping effect. Here we demonstrate a universal mechanical exfoliation method of producing freestanding single-crystalline membranes made from a wide range of complex-oxide materials including perovskite, spinel and garnet crystal structures with varying crystallographic orientations. In addition, we create artificial heterostructures and hybridize their physical properties by directly stacking such freestanding membranes with different crystal structures and orientations, which is not possible using conventional methods. Our results establish a platform for stacking and coupling three-dimensional structures, akin to two-dimensional material-based heterostructures, for enhancing device functionalities8,9.

Superconductivity in undoped BaFe2As2 by tetrahedral geometry design.

Fe-based superconductors exhibit a diverse interplay between charge, orbital, and magnetic ordering. Variations in atomic geometry affect electron hopping between Fe atoms and the Fermi surface topology, influencing magnetic frustration and the pairing strength through changes of orbital overlap and occupancies. Here, we experimentally demonstrate a systematic approach to realize superconductivity without chemical doping in BaFe2As2, employing geometric design within an epitaxial heterostructure. We control both tetragonality and orthorhombicity in BaFe2As2 through superlattice engineering, which we experimentally find to induce superconductivity when the As-Fe-As bond angle approaches that in a regular tetrahedron. This approach to superlattice design could lead to insights into low-dimensional superconductivity in Fe-based superconductors.

Electron pairing without superconductivity.

Strontium titanate (SrTiO3) is the first and best known superconducting semiconductor. It exhibits an extremely low carrier density threshold for superconductivity, and possesses a phase diagram similar to that of high-temperature superconductors--two factors that suggest an unconventional pairing mechanism. Despite sustained interest for 50 years, direct experimental insight into the nature of electron pairing in SrTiO3 has remained elusive. Here we perform transport experiments with nanowire-based single-electron transistors at the interface between SrTiO3 and a thin layer of lanthanum aluminate, LaAlO3. Electrostatic gating reveals a series of two-electron conductance resonances-paired electron states--that bifurcate above a critical pairing field Bp of about 1-4 tesla, an order of magnitude larger than the superconducting critical magnetic field. For magnetic fields below Bp, these resonances are insensitive to the applied magnetic field; for fields in excess of Bp, the resonances exhibit a linear Zeeman-like energy splitting. Electron pairing is stable at temperatures as high as 900 millikelvin, well above the superconducting transition temperature (about 300 millikelvin). These experiments demonstrate the existence of a robust electronic phase in which electrons pair without forming a superconducting state. Key experimental signatures are captured by a model involving an attractive Hubbard interaction that describes real-space electron pairing as a precursor to superconductivity.

Gate-Tunable Optical Nonlinearities and Extinction in Graphene/LaAlO3/SrTiO3 Nanostructures.

We explore the ultrafast optical response of graphene subjected to intense (∼106 V/cm) local (∼10 nm) electric fields. Nanoscale gating of graphene is achieved using a voltage-biased, SrTiO3-based conductive nanowire junction "written" directly under the graphene and isolated from it by an insulating ultrathin (<2 nm) LaAlO3 barrier. Upon illumination with ultrafast visible-to-near-infrared (VIS-NIR) light pulses, the local field from the nanojunction creates a strong gate-tunable second-order nonlinearity in the graphene and produces a substantial difference-frequency (DFG) and sum-frequency generation (SFG) response detected by the nanojunction. Spectrally sharp, gate-tunable extinction features (>99.9%) are observed in the VIS-NIR and SFG spectral ranges, in parameter regimes that are positively correlated with the enhanced nonlinear response. The observed graphene-light interaction and nonlinear response are of fundamental interest and open the way for future exploitation in graphene-based optical devices such as phase shifters, modulators, and nanoscale THz sources.

Large and Reconfigurable Infrared Photothermoelectric Effect at Oxide Interfaces.

To maximize the photovoltaic efficiency, it is highly desirable to enable the electricity conversion from low energy photons and to extract the excessive energy from hot carriers. Here we report a large photovoltage generation at the LaAlO3/SrTiO3 interfaces from infrared photons with energies far below the oxide bandgaps. This effect is a result of the photoexcitation of hot carriers in metasurface electrical contacts and the subsequent thermoelectric charge separations by the interfacial two-dimensional electron gas (2DEG). Reaching a room-temperature responsivity of 4.4 V/W, such light-to-charge conversion can be spatially controlled and reconfigured through the patterning of 2DEG using conducting atomic force microscope. Compatible for broadband applications, our results demonstrate a new path toward efficient and programmable light sensing using oxide-based low-dimensional electron systems.

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 TiSr 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.

Quantized Ballistic Transport of Electrons and Electron Pairs in LaAlO3/SrTiO3 Nanowires.

SrTiO3-based heterointerfaces support quasi-two-dimensional (2D) electron systems that are analogous to III-V semiconductor heterostructures, but also possess superconducting, magnetic, spintronic, ferroelectric, and ferroelastic degrees of freedom. Despite these rich properties, the relatively low mobilities of 2D complex-oxide interfaces appear to preclude ballistic transport in 1D. Here we show that the 2D LaAlO3/SrTiO3 interface can support quantized ballistic transport of electrons and (nonsuperconducting) electron pairs within quasi-1D structures that are created using a well-established conductive atomic-force microscope (c-AFM) lithography technique. The nature of transport ranges from truly single-mode (1D) to three-dimensional (3D), depending on the applied magnetic field and gate voltage. Quantization of the lowest e2/ h plateau indicate a ballistic mean-free path lMF ∼ 20 µm, more than 2 orders of magnitude larger than for 2D LaAlO3/SrTiO3 heterostructures. Nonsuperconducting electron pairs are found to be stable in magnetic fields as high as B = 11 T and propagate ballistically with conductance quantized at 2 e2/ h. Theories of one-dimensional (1D) transport of interacting electron systems depend crucially on the sign of the electron-electron interaction, which may help explain the highly ballistic transport behavior. The 1D geometry yields new insights into the electronic structure of the LaAlO3/SrTiO3 system and offers a new platform for the study of strongly interacting 1D electronic systems.

Control of Epitaxial BaFe2As2 Atomic Configurations with Substrate Surface Terminations.

Atomic layer controlled growth of epitaxial thin films of unconventional superconductors opens the opportunity to discover novel high temperature superconductors. For instance, the interfacial atomic configurations may play an important role in superconducting behavior of monolayer FeSe on SrTiO3 and other Fe-based superconducting thin films. Here, we demonstrate a selective control of the atomic configurations in Co-doped BaFe2As2 epitaxial thin films and its strong influence on superconducting transition temperatures by manipulating surface termination of (001) SrTiO3 substrates. In a combination of first-principles calculations and high-resolution scanning transmission electron microscopy imaging, we show that Co-doped BaFe2As2 on TiO2-terminated SrTiO3 is a tetragonal structure with an atomically sharp interface and with an initial Ba layer. In contrast, Co-doped BaFe2As2 on SrO-terminated SrTiO3 has a monoclinic distortion and a BaFeO3- x initial layer. Furthermore, the superconducting transition temperature of Co-doped BaFe2As2 ultrathin films on TiO2-terminated SrTiO3 is significantly higher than that on SrO-terminated SrTiO3, which we attribute to shaper interfaces with no lattice distortions. This study allows the design of the interfacial atomic configurations and the effects of the interface on superconductivity in Fe-based superconductors.

Shubnikov-de Haas-like Quantum Oscillations in Artificial One-Dimensional LaAlO_{3}/SrTiO_{3} Electron Channels.

The widely reported magnetoresistance oscillations in LaAlO_{3}/SrTiO_{3} heterostructures have invariably been attributed to the Shubnikov-de Haas (SdH) effect, despite a pronounced inconsistency with low-field Hall resistance measurements. Here we report SdH-like resistance oscillations in quasi-1D electron waveguides created at the LaAlO_{3}/SrTiO_{3} interface by conductive atomic force microscopy lithography. These oscillations can be directly attributed to magnetic depopulation of magnetoelectric subbands. Our results suggest that the SdH oscillations in 2D SrTiO_{3}-based systems may originate from naturally forming quasi-1D channels.

One-Dimensional Nature of Superconductivity at the LaAlO_{3}/SrTiO_{3} Interface.

We examine superconductivity in LaAlO_{3}/SrTiO_{3} channels with widths that transition from the 1D to the 2D regime. The superconducting critical current is independent of the channel width and increases approximately linearly with the number of parallel channels. Signatures of electron pairing outside of the superconducting regime are also found to be independent of the channel width. Collectively, these results indicate that superconductivity exists at the boundary of these channels and is absent within the interior region of the channels. The intrinsic 1D nature of superconductivity at the LaAlO_{3}/SrTiO_{3} interface imposes strong physical constraints on possible electron pairing mechanisms.

Tailoring the Doping Mechanisms at Oxide Interfaces in Nanoscale.

Here, we demonstrate the nanoscale manipulations of two types of charge transfer to the LaAlO3/SrTiO3 interfaces: one from surface adsorbates and another from oxygen vacancies inside LaAlO3 films. This method can be used to produce multiple insulating and metallic interface states with distinct carrier properties that are highly stable in air. By reconfiguring the patterning and comparing interface structures formed from different doping sources, effects of extrinsic and intrinsic material characters on the transport properties can be distinguished. In particular, a multisubband to single-subband transition controlled by the structural phases in SrTiO3 was revealed. In addition, the transient behaviors of nanostructures also provided a unique opportunity to study the nanoscale diffusions of adsorbates and oxygen vacancies in oxide heterostructures. Knowledge of such dynamic processes is important for nanodevice implementations.

Electron-Lattice Coupling in Correlated Materials of Low Electron Occupancy.

In correlated materials including transition metal oxides, electronic properties and functionalities are modulated and enriched by couplings between the electron and lattice degrees of freedom. These couplings are controlled by external parameters such as chemical doping, pressure, magnetic and electric fields, and light irradiation. However, the electron-lattice coupling relies on orbital characters, i.e., symmetry and occupancy, of t2g and eg orbitals, so that a large electron-lattice coupling is limited to eg electron system, whereas t2g electron system exhibits an inherently weak coupling. Here, we design and demonstrate a strongly enhanced electron-lattice coupling in electron-doped SrTiO3, that is, the t2g electron system. In ultrathin films of electron-doped SrTiO3 [i.e., (La0.25Sr0.75)TiO3], we reveal the strong electron-lattice-orbital coupling, which is manifested by extremely increased tetragonality and the corresponding metal-to-insulator transition. Our findings open the way of an active tuning of the charge-lattice-orbital coupling to obtain new functionalities relevant to emerging nanoelectronic devices.

Sharpened VO2 Phase Transition via Controlled Release of Epitaxial Strain.

Phase transitions in correlated materials can be manipulated at the nanoscale to yield emergent functional properties, promising new paradigms for nanoelectronics and nanophotonics. Vanadium dioxide (VO2), an archetypal correlated material, exhibits a metal-insulator transition (MIT) above room temperature. At the thicknesses required for heterostructure applications, such as an optical modulator discussed here, the strain state of VO2 largely determines the MIT dynamics critical to the device performance. We develop an approach to control the MIT dynamics in epitaxial VO2 films by employing an intermediate template layer with large lattice mismatch to relieve the interfacial lattice constraints, contrary to conventional thin film epitaxy that favors lattice match between the substrate and the growing film. A combination of phase-field simulation, in situ real-time nanoscale imaging, and electrical measurements reveals robust undisturbed MIT dynamics even at preexisting structural domain boundaries and significantly sharpened MIT in the templated VO2 films. Utilizing the sharp MIT, we demonstrate a fast, electrically switchable optical waveguide. This study offers unconventional design principles for heteroepitaxial correlated materials, as well as novel insight into their nanoscale phase transitions.

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.

Tailoring LaAlO3/SrTiO3 Interface Metallicity by Oxygen Surface Adsorbates.

We report an oxygen surface adsorbates induced metal-insulator transition at the LaAlO3/SrTiO3 interfaces. The observed effects were attributed to the terminations of surface Al sites and the resultant electron-accepting surface states. By controlling the local oxygen adsorptions, we successfully demonstrated the nondestructive patterning of the interface two-dimensional electron gas (2DEG). The obtained 2DEG structures are stable in air and also robust against general solvent treatments. This study provides new insights into the metal-insulator transition mechanism at the complex oxide interfaces and also a highly efficient technique for tailoring the interface properties.

Imprint Control of BaTiO3 Thin Films via Chemically Induced Surface Polarization Pinning.

Surface-adsorbed polar molecules can significantly alter the ferroelectric properties of oxide thin films. Thus, fundamental understanding and controlling the effect of surface adsorbates are crucial for the implementation of ferroelectric thin film devices, such as ferroelectric tunnel junctions. Herein, we report an imprint control of BaTiO3 (BTO) thin films by chemically induced surface polarization pinning in the top few atomic layers of the water-exposed BTO films. Our studies based on synchrotron X-ray scattering and coherent Bragg rod analysis demonstrate that the chemically induced surface polarization is not switchable but reduces the polarization imprint and improves the bistability of ferroelectric phase in BTO tunnel junctions. We conclude that the chemical treatment of ferroelectric thin films with polar molecules may serve as a simple yet powerful strategy to enhance functional properties of ferroelectric tunnel junctions for their practical applications.

Nanodomain Engineering in Ferroelectric Capacitors with Graphene Electrodes.

Polarization switching in ferroelectric capacitors is typically realized by application of an electrical bias to the capacitor electrodes and occurs via a complex process of domain structure reorganization. As the domain evolution in real devices is governed by the distribution of the nucleation centers, obtaining a domain structure of a desired configuration by electrical pulsing is challenging, if not impossible. Recent discovery of polarization reversal via the flexoelectric effect has opened a possibility for deterministic control of polarization in ferroelectric capacitors. In this paper, we demonstrate mechanical writing of arbitrary-shaped nanoscale domains in thin-film ferroelectric capacitors with graphene electrodes facilitated by a strain gradient induced by a tip of an atomic force microscope (AFM). A phase-field modeling prediction of a strong effect of graphene thickness on the threshold load required to initiate mechanical switching has been confirmed experimentally. Deliberate voltage-free domain writing represents a viable approach for development of functional devices based on domain topology and electronic properties of the domains and domain walls.

Micrometer-Scale Ballistic Transport of Electron Pairs in LaAlO_{3}/SrTiO_{3} Nanowires.

High-mobility complex-oxide heterostructures and nanostructures offer new opportunities for extending the paradigm of quantum transport beyond the realm of traditional III-V or carbon-based materials. Recent quantum transport investigations with LaAlO_{3}/SrTiO_{3}-based quantum dots reveal the existence of a strongly correlated phase in which electrons form spin-singlet pairs without becoming superconducting. Here, we report evidence for the micrometer-scale ballistic transport of electron pairs in quasi-1D LaAlO_{3}/SrTiO_{3} nanowire cavities. In the paired phase, Fabry-Perot-like quantum interference is observed, in sync with conductance oscillations observed in the superconducting regime (at a zero magnetic field). Above a critical magnetic field B_{p}, the electron pairs unbind and the conductance oscillations shift with the magnetic field. These experimental observations extend the regime of ballistic electronic transport to strongly correlated phases.

Broadband terahertz generation and detection at 10 nm scale.

Terahertz (0.1-30 THz) radiation reveals a wealth of information that is relevant for material, biological, and medical sciences with applications that span chemical sensing, high-speed electronics, and coherent control of semiconductor quantum bits. To date, there have been no methods capable of controlling terahertz (THz) radiation at molecular scales. Here we report both generation and detection of broadband terahertz field from 10 nm scale oxide nanojunctions. Frequency components of ultrafast optical radiation are mixed at these nanojunctions, producing broadband THz emission. These same devices detect THz electric fields with comparable spatial resolution. This unprecedented control, on a scale of 4 orders of magnitude smaller than the diffraction limit, creates a pathway toward THz-bandwidth spectroscopy and control of individual nanoparticles and molecules.