Two-dimensional electron gas with six-fold symmetry at the (111) surface of KTaO3

Light-induced giant enhancement of nonreciprocal transport at KTaO3-based interfaces

Nonlinear transport is a unique functionality of noncentrosymmetric systems, which reflects profound physics, such as spin-orbit interaction, superconductivity and band geometry. However, it remains highly challenging to enhance the nonreciprocal transport for promising rectification devices. Here, we observe a light-induced giant enhancement of nonreciprocal transport at the superconducting and epitaxial CaZrO3/KTaO3 (111) interfaces. The nonreciprocal transport coefficient undergoes a giant increase with three orders of magnitude up to 105 A-1 T-1. Furthermore, a strong Rashba spin-orbit coupling effective field of 14.7 T is achieved with abundant high-mobility photocarriers under ultraviolet illumination, which accounts for the giant enhancement of nonreciprocal transport coefficient. Our first-principles calculations further disclose the stronger Rashba spin-orbit coupling strength and the longer relaxation time in the photocarrier excitation process, bridging the light-property quantitative relationship. Our work provides an alternative pathway to boost nonreciprocal transport in noncentrosymmetric systems and facilitates the promising applications in opto-rectification devices and spin-orbitronic devices.

Cycling-Induced Capacity Increase of Bulk and Artificially Layered LiTaO3 Anodes in Lithium-Ion Batteries

Tantalum-based oxide electrodes have recently drawn much attention as promising anode materials owing to their hybrid Li+ storage mechanism. However, the utilization of LiTaO3 electrode materials that can deliver a high theoretical capacity of 568 mAh g-1 has been neglected. Herein, we prepare a layered LiTaO3 electrode formed artificially by restacking LiTaO3 nanosheets using a facile synthesis method and investigate the Li+ storage performance of this electrode compared with its bulk counterpart. The designed artificially layered anode reaches specific capacities of 474, 290, and 201 mAh g-1, respectively, at 56 (>500 cycles), 280 (>1000 cycles), and 1120 mAg-1 (>2000 cycles) current densities. We also determine that the Li+ storage capacity of the layered LiTaO3 demonstrates a cycling-induced capacity increase after a certain number of cycles. Adopting various characterization techniques on LiTaO3 electrodes before and after electrochemical cycling, we attribute the origin of the cycling-induced improvement of the Li+ storage capacity in these electrodes to the amorphization of the electrode after cycling, formation of metallic tantalum during the partially irreversible conversion mechanism, lower activation overpotential of electrodes due to the formation of Li-rich species by the lithium insertion mechanism, and finally the intrinsic piezoelectric behavior of LiTaO3 that can regulate Li+ diffusion kinetics.

Designing spin and orbital sources of Berry curvature at oxide interfaces

Quantum materials can display physical phenomena rooted in the geometry of electronic wavefunctions. The corresponding geometric tensor is characterized by an emergent field known as the Berry curvature (BC). Large BCs typically arise when electronic states with different spin, orbital or sublattice quantum numbers hybridize at finite crystal momentum. In all the materials known to date, the BC is triggered by the hybridization of a single type of quantum number. Here we report the discovery of the first material system having both spin- and orbital-sourced BC: LaAlO₃/SrTiO₃ interfaces grown along the [111] direction. We independently detect these two sources and probe the BC associated to the spin quantum number through the measurements of an anomalous planar Hall effect. The observation of a nonlinear Hall effect with time-reversal symmetry signals large orbital-mediated BC dipoles. The coexistence of different forms of BC enables the combination of spintronic and optoelectronic functionalities in a single material.

Superfluid stiffness of a KTaO3-based two-dimensional electron gas

After almost twenty years of intense work on the celebrated LaAlO₃/SrTiO₃system, the recent discovery of a superconducting two-dimensional electron gas (2-DEG) in (111)-oriented KTaO₃-based heterostructures injects new momentum to the field of oxides interface. However, while both interfaces share common properties, experiments also suggest important differences between the two systems. Here, we report gate tunable superconductivity in 2-DEGs generated at the surface of a (111)-oriented KTaO₃ crystal by the simple sputtering of a thin Al layer. We extract the superfluid stiffness of the 2-DEGs and show that its temperature dependence is consistent with a node-less superconducting order parameter having a gap value larger than expected within a simple BCS weak-coupling limit model. The superconducting transition follows the Berezinskii-Kosterlitz-Thouless scenario, which was not reported on SrTiO₃-based interfaces. Our finding offers innovative perspectives for fundamental science but also for device applications in a variety of fields such as spin-orbitronics and topological electronics.

Ultrahigh Carrier Mobilities in Ferroelectric Domain Wall Corbino Cones at Room Temperature

Recently, electrically conducting heterointerfaces between dissimilar band insulators (such as lanthanum aluminate and strontium titanate) have attracted considerable research interest. Charge transport and fundamental aspects of conduction have been thoroughly explored. Perhaps surprisingly, similar studies on conceptually much simpler conducting homointerfaces, such as domain walls, are not nearly so well developed. Addressing this disparity, magnetoresistance is herein reported in approximately conical 180° charged domain walls, in partially switched ferroelectric thin-film single-crystal lithium niobate. This system is ideal for such measurements: first, the conductivity difference between domains and domain walls is unusually large (a factor of 10¹³ ) and hence currents driven through the thin film, between planar top and bottom electrodes, are overwhelmingly channeled along the walls; second, when electrical contact is made to the top and bottom of the domain walls and a magnetic field is applied along their cone axes, then the test geometry mirrors that of a Corbino disk: a textbook arrangement for geometric magnetoresistance measurement. Data imply carriers with extremely high room-temperature Hall mobilities of up to ≈3700 cm² V^-1 s^-1 . This is an unparalleled value for oxide interfaces (and for bulk oxides) comparable to mobilities in other systems seen at cryogenic, rather than at room, temperature.

Tunable Spin and Orbital Edelstein Effect at (111) LaAlO3/SrTiO3 Interface

Converting charge current into spin current is one of the main mechanisms exploited in spintronics. One prominent example is the Edelstein effect, namely, the generation of a magnetization in response to an external electric field, which can be realized in systems with lack of inversion symmetry. If a system has electrons with an orbital angular momentum character, an orbital magnetization can be generated by the applied electric field, giving rise to the so-called orbital Edelstein effect. Oxide heterostructures are the ideal platform for these effects due to the strong spin–orbit coupling and the lack of inversion symmetries. Beyond a gate-tunable spin Edelstein effect, we predict an orbital Edelstein effect an order of magnitude larger then the spin one at the (111) LaAlO3/SrTiO3 interface for very low and high fillings. We model the material as a bilayer of t2g orbitals using a tight-binding approach, whereas transport properties are obtained in the Boltzmann approach. We give an effective model at low filling, which explains the non-trivial behaviour of the Edelstein response, showing that the hybridization between the electronic bands crucially impacts the Edelstein susceptibility.

Evidence of a 2D Electron Gas in a Single-Unit-Cell of Anatase TiO2 (001)

The formation and the evolution of electronic metallic states localized at the surface, commonly termed 2D electron gas (2DEG), represents a peculiar phenomenon occurring at the surface and interface of many transition metal oxides (TMO). Among TMO, titanium dioxide (TiO₂ ), particularly in its anatase polymorph, stands as a prototypical system for the development of novel applications related to renewable energy, devices and sensors, where understanding the carrier dynamics is of utmost importance. In this study, angle-resolved photo-electron spectroscopy (ARPES) and X-ray absorption spectroscopy (XAS) are used, supported by density functional theory (DFT), to follow the formation and the evolution of the 2DEG in TiO₂ thin films. Unlike other TMO systems, it is revealed that, once the anatase fingerprint is present, the 2DEG in TiO₂ is robust and stable down to a single-unit-cell, and that the electron filling of the 2DEG increases with thickness and eventually saturates. These results prove that no critical thickness triggers the occurrence of the 2DEG in anatase TiO₂ and give insight in formation mechanism of electronic states at the surface of TMO.

KTaO3 -The New Kid on the Spintronics Block

Long after the heady days of high-temperature superconductivity, the oxides came back into the limelight in 2004 with the discovery of the 2D electron gas (2DEG) in SrTiO₃ (STO) and several heterostructures based on it. Not only do these materials exhibit interesting physics, but they have also opened up new vistas in oxide electronics and spintronics. However, much of the attention has recently shifted to KTaO₃ (KTO), a material with all the "good" properties of STO (simple cubic structure, high mobility, etc.) but with the additional advantage of a much larger spin-orbit coupling. In this state-of-the-art review of the fascinating world of KTO, it is attempted to cover the remarkable progress made, particularly in the last five years. Certain unsolved issues are also indicated, while suggesting future research directions as well as potential applications. The range of physical phenomena associated with the 2DEG trapped at the interfaces of KTO-based heterostructures include spin polarization, superconductivity, quantum oscillations in the magnetoresistance, spin-polarized electron transport, persistent photocurrent, Rashba effect, topological Hall effect, and inverse Edelstein Effect. It is aimed to discuss, on a single platform, the various fabrication techniques, the exciting physical properties and future application possibilities of this family of materials.

Predicting the structural, electronic and magnetic properties of few atomic-layer polar perovskite

Density functional theory (DFT) calculations are performed to predict the structural, electronic and magnetic properties of electrically neutral or charged few-atomic-layer (AL) oxides based on polar perovskite KTaO3. Their properties vary greatly with the number of ALs (nAL) and the stoichiometric ratio. In the few-AL limit (nAL ≤ 14), the even AL (EL) systems with the chemical formula (KTaO3)n are semiconductors, while the odd AL (OL) systems with the formula Kn+1TanO3n+1 or KnTan+1O3n+2 are half-metal except for the unique KTa2O5 case which is a semiconductor due to the large Peierls distortions. After reaching a certain critical thickness (nAL > 14), the EL systems show ferromagnetic surface states, while ferromagnetism disappears in the OL systems. These predictions from fundamental complexity of polar perovskite when approaching the two-dimensional (2D) limit may be helpful for interpreting experimental observations later.

Electronic Structure of a Graphene-like Artificial Crystal of NdNiO3

Artificial complex-oxide heterostructures containing ultrathin buried layers grown along the pseudocubic [111] direction have been predicted to host a plethora of exotic quantum states arising from the graphene-like lattice geometry and the interplay between strong electronic correlations and band topology. To date, however, electronic-structural investigations of such atomic layers remain an immense challenge due to the shortcomings of conventional surface-sensitive probes with typical information depths of a few angstroms. Here, we use a combination of bulk-sensitive soft X-ray angle-resolved photoelectron spectroscopy (SX-ARPES), hard X-ray photoelectron spectroscopy (HAXPES), and state-of-the-art first-principles calculations to demonstrate a direct and robust method for extracting momentum-resolved and angle-integrated valence-band electronic structure of an ultrathin buckled graphene-like layer of NdNiO₃ confined between two 4-unit cell-thick layers of insulating LaAlO₃. The momentum-resolved dispersion of the buried Ni d states near the Fermi level obtained via SX-ARPES is in excellent agreement with the first-principles calculations and establishes the realization of an antiferro-orbital order in this artificial lattice. The HAXPES measurements reveal the presence of a valence-band bandgap of 265 meV. Our findings open a promising avenue for designing and investigating quantum states of matter with exotic order and topology in a few buried layers.

Tunable magnetic and half-metallic properties of the two-dimensional electron gas in LaAlO3/SrTiO3(111) heterostructures

Half-metallic materials have gained a lot of attention because of their unique properties and applications in spintronic devices. Despite the fact that these materials have been studied by several research groups there are very limited studies on their heterostructure (HS) systems. In the current study we have investigated the electronic and magnetic properties of (LaAlO3)6.5/(SrTiO3)2.5(111) HS using density functional theory (DFT) calculations. We demonstrate that the system exhibits a 100% spin-polarized two-dimensional electron gas (2DEG) which is extremely confined to the Ti 3d orbitals of the SrTiO3 layers. In particular, this system can keep its half-metallic properties under different in-plane strains from -3 to 2%. This property proves that this material has relatively stable half-metallic properties. In addition, the conducting and magnetic ground states of the system can also be tailored by changing in-plane strain and interfacial cation intermixing of La and Sr (Sr ⇔ La intermixing). By increasing the in-plane lattice parameters, this system has the ability to evolve from a nonmagnetic to a ferromagnetic metal and then to a half-metal and by further increasing the in-plane lattice parameter it becomes a ferromagnetic insulator. Sr ⇔ La intermixing can destroy the original half-metallic properties and the system exhibits an AFM Mott-type insulator phase. Our results demonstrate that the system has high potential for application in the field of spintronics, and opens the prospect of using LaAlO3/SrTiO3(111) HSs to explore quantum phase transitions.

A spin-orbit playground: surfaces and interfaces of transition metal oxides

Within the last twenty years, the status of the spin-orbit interaction has evolved from that of a simple atomic contribution to a key effect that modifies the electronic band structure of materials. It is regarded as one of the basic ingredients for spintronics, locking together charge and spin degrees of freedom and recently it is instrumental in promoting a new class of compounds, the topological insulators. In this review, we present the current status of the research on the spin-orbit coupling in transition metal oxides, discussing the case of two semiconducting compounds, [Formula: see text] and [Formula: see text], and the properties of surface and interfaces based on these. We conclude with the investigation of topological effects predicted to occur in different complex oxides.

High-Mobility Spin-Polarized Two-Dimensional Electron Gases at EuO/KTaO_{3} Interfaces

Two-dimensional electron gases (2DEGs) at oxide interfaces, which provide unique playgrounds for emergent phenomena, have attracted increasing attention in recent years. While most of the previous works focused on the 2DEGs at LaAlO_{3}/SrTiO_{3} interfaces, here we report on a new kind of 2DEGs formed between a magnetic insulator EuO and a high-k perovskite KTaO_{3}. The 2DEGs are not only highly conducting, with a maximal Hall mobility of 111.6  cm^{2}/V s at 2 K, but also well spin polarized, showing strongly hysteretic magnetoresistance up to 25 K and well-defined anomalous Hall effect up to 70 K. Moreover, unambiguous correspondences between the hysteretic behaviors of 2DEGs and the EuO layer are captured, suggesting the proximity effect of the latter on the former. This is confirmed by the results of density-functional theory calculations: Through interlayer exchange, EuO drives the neighboring TaO_{2} layer into a ferromagnetic state. The present work opens new avenues for the exploration for high performance spin-polarized 2DEGs at oxide interfaces.

Strain effects on oxygen vacancy energetics in KTaO3

Due to lattice mismatch between epitaxial films and substrates, in-plane strain fields are produced in the thin films, with accompanying structural distortions, and ion implantation can be used to controllably engineer the strain throughout the film. Because of the strain profile, local defect energetics are changed. In this study, the effects of in-plane strain fields on the formation and migration of oxygen vacancies in KTaO₃ are investigated using first-principles calculations. In particular, the doubly positive charged oxygen vacancy (V) is studied, which is considered to be the main charge state of the oxygen vacancy in KTaO₃. We find that the formation energies for oxygen vacancies are sensitive to in-plane strain and oxygen position. The local atomic configuration is identified, and strong relaxation of local defect structure is mainly responsible for the formation characteristics of these oxygen vacancies. Based on the computational results, formation-dependent site preferences for oxygen vacancies are expected to occur under epitaxial strain, which can result in orders of magnitude differences in equilibrium vacancy concentrations on different oxygen sites. In addition, all possible migration pathways, including intra- and inter-plane diffusions, are considered. In contrast to the strain-enhanced intra-plane diffusion, the diffusion in the direction normal to the strained plane is impeded under the epitaxial strain field. These anisotropic diffusion processes can further enhance site preferences.

Extraction of the Rashba spin-orbit coupling constant from scanning gate microscopy conductance maps for quantum point contacts

We study the possibility for the extraction of the Rashba spin-orbit coupling constant for a two-dimensional electron gas with the conductance microscopy technique. Due to the interplay between the effective magnetic field due to the Rashba spin-orbit coupling and the external magnetic field applied within the plane of confinement, the electron backscattering induced by a charged tip of an atomic force microscope located above the sample leads to the spin precession and spin mixing of the incident and reflected electron waves between the QPC and the tip-induced 2DEG depletion region. This mixing leads to a characteristic angle-dependent beating pattern visible in the conductance maps. We show that the structure of the Fermi level, bearing signatures of the spin-orbit coupling, can be extracted from the Fourier transform of the interference fringes in the conductance maps as a function of the magnetic field direction. We propose a simple analytical model which can be used to fit the experimental data in order to obtain the spin-orbit coupling constant.

Angle-resolved photoemission spectroscopy studies of metallic surface and interface states of oxide insulators

Over the last decade, conducting states embedded in insulating transition metal oxides (TMOs) have served as gateways to discovering and probing surprising phenomena that can emerge in complex oxides, while also opening opportunities for engineering advanced devices. These states are commonly realized at thin film interfaces, such as the well-known case of LaAlO3 (LAO) grown on SrTiO3 (STO). In recent years, the use of angle-resolved photoemission spectroscopy (ARPES) to investigate the k-space electronic structure of such materials led to the discovery that metallic states can also be formed on the bare surfaces of certain TMOs. In this topical review, we report on recent studies of low-dimensional metallic states confined at insulating oxide surfaces and interfaces as seen from the perspective of ARPES, which provides a direct view of the occupied band structure. While offering a fairly broad survey of progress in the field, we draw particular attention to STO, whose surface is so far the best-studied, and whose electronic structure is probably of the most immediate interest, given the ubiquitous use of STO substrates as the basis for conducting oxide interfaces. The ARPES studies provide crucial insights into the electronic band structure, orbital character, dimensionality/confinement, spin structure, and collective excitations in STO surfaces and related oxide surface/interface systems. The obtained knowledge increases our understanding of these complex materials and gives new perspectives on how to manipulate their properties.

Atomically Precise Lateral Modulation of a Two-Dimensional Electron Liquid in Anatase TiO2 Thin Films

Engineering the electronic band structure of two-dimensional electron liquids (2DELs) confined at the surface or interface of transition metal oxides is key to unlocking their full potential. Here we describe a new approach to tailoring the electronic structure of an oxide surface 2DEL demonstrating the lateral modulation of electronic states with atomic scale precision on an unprecedented length scale comparable to the Fermi wavelength. To this end, we use pulsed laser deposition to grow anatase TiO₂ films terminated by a (1 × 4) in-plane surface reconstruction. Employing photostimulated chemical surface doping we induce 2DELs with tunable carrier densities that are confined within a few TiO₂ layers below the surface. Subsequent in situ angle-resolved photoemission experiments demonstrate that the (1 × 4) surface reconstruction provides a periodic lateral perturbation of the electron liquid. This causes strong backfolding of the electronic bands, opening of unidirectional gaps and a saddle point singularity in the density of states near the chemical potential.

Two-Dimensional Iron Tungstate: A Ternary Oxide Layer With Honeycomb Geometry

The exceptional physical properties of graphene have sparked tremendous interests toward two-dimensional (2D) materials with honeycomb structure. We report here the successful fabrication of 2D iron tungstate (FeWO _x ) layers with honeycomb geometry on a Pt(111) surface, using the solid-state reaction of (WO₃)₃ clusters with a FeO(111) monolayer on Pt(111). The formation process and the atomic structure of two commensurate FeWO _x phases, with (2 × 2) and (6 × 6) periodicities, have been characterized experimentally by combination of scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD) and understood theoretically by density functional theory (DFT) modeling. The thermodynamically most stable (2 × 2) phase has a formal FeWO₃ stoichiometry and corresponds to a buckled Fe²⁺/W⁴⁺ layer arranged in a honeycomb lattice, terminated by oxygen atoms in Fe-W bridging positions. This 2D FeWO₃ layer has a novel structure and stoichiometry and has no analogues to known bulk iron tungstate phases. It is theoretically predicted to exhibit a ferromagnetic electronic ground state with a Curie temperature of 95 K, as opposed to the antiferromagnetic behavior of bulk FeWO₄ materials.

Control of a two-dimensional electron gas on SrTiO₃(111) by atomic oxygen

We report on the formation of a two-dimensional electron gas (2DEG) at the bare surface of (111) oriented SrTiO3. Angle resolved photoemission experiments reveal highly itinerant carriers with a sixfold symmetric Fermi surface and strongly anisotropic effective masses. The electronic structure of the 2DEG is in good agreement with self-consistent tight-binding supercell calculations that incorporate a confinement potential due to surface band bending. We further demonstrate that alternate exposure of the surface to ultraviolet light and atomic oxygen allows tuning of the carrier density and the complete suppression of the 2DEG.