Tunable Rashba spin-orbit interaction at oxide interfaces

Spin-orbit interactions, time-reversal symmetry, and spin selection

Spin selective transport is usually associated with spin-orbit interactions. However, these interactions are invariant under time-reversal symmetry, and the Onsager relations and Bardarson's theorem imply that such interactions cannot yield spin selectivity for transport through a junction between two electronic reservoirs. Here, we review several ways to overcome this restriction, using a Zeeman magnetic field, the Aharonov-Bohm phase, time-dependent electric fields that generate time-dependent spin-orbit interactions, time-dependent transients, more than two terminals, leakage, and more than one level per ion on the junction. Our considerations focus on the transport of noninteracting electrons at low temperatures. A possible connection with the phenomenon of chiral-induced spin selectivity is pointed out in one of the systems considered.

Unearthing the emerging properties at buried oxide heterointerfaces: the γ-Al2O3/SrTiO3 heterostructure

The symmetry breaking that is formed when oxide layers are combined epitaxially to form heterostructures has led to the emergence of new functionalities beyond those observed in the individual parent materials. SrTiO3-based heterostructures have played a central role in expanding the range of functional properties arising at the heterointerface and elucidating their mechanistic origin. The heterostructure formed by the epitaxial combination of spinel γ-Al2O3 and perovskite SrTiO3 constitutes a striking example with features distinct from perovskite/perovskite counterparts such as the archetypical LaAlO3/SrTiO3 heterostructure. Here, non-isomorphic epitaxial growth of γ-Al2O3 on SrTiO3 can be achieved even at room temperature with the epitaxial union of the two distinct crystal structures resulting in modification of the functional properties by the broken cationic symmetry. The heterostructure features oxygen vacancy-mediated conductivity with dynamically adjustable electron mobilities as high as 140 000 cm2 V-1 s-1 at 2 K, strain-tunable magnetism and an unsaturated linear magnetoresistance exceeding 80 000% at 15 T and 2 K. Here, we review the structural, electronic and magnetic characteristics of the γ-Al2O3/SrTiO3 heterostructure with a particular emphasis on elucidating the underlying mechanistic origins of the various properties. We further show that γ-Al2O3/SrTiO3 may break new grounds for tuning the electronic and magnetic properties through dynamic defect engineering and polarity modifications, and also for band engineering, symmetry breaking and silicon integration.

Anomalous temperature dependence of spin-orbit coupling in Al2O3/SrTiO3and Al2O3/KTaO3heterostructures

Perovskite oxide-based heterostructures exhibit a range of exotic physical properties such as two-dimensional superconductivity, interface magnetism, tunable Kondo effect, and tunable spin-orbit coupling. Here, the magnetotransport properties of Al2O3/SrTiO3and Al2O3/KTaO3heterostructures are studied. Both Kondo effect and spin-orbit coupling-induced weak antilocalization (WAL) effect are observed at low temperatures. By analyzing the WAL curves, the spin relaxation time is extracted. Surprisingly, the extracted spin relaxation time unexpectedly decreases on increasing temperature in all samples. This indicates that the strength of the spin-orbit coupling is progressively enhanced on increasing temperature, conflicting with theoretical prediction. This anomalous temperature dependence is explained by the interplay between the Kondo effect and the D'yakonov-Perel spin relaxation mechanism.

Dirac-Like Fermions Anomalous Magneto-Transport in a Spin-Polarized Oxide 2D Electron System

In a 2D electron system (2DES) the breaking of the inversion, time-reversal and bulk crystal-field symmetries is interlaced with the effects of spin-orbit coupling (SOC) triggering exotic quantum phenomena. Here, epitaxial engineering is used to design and realize a 2DES characterized simultaneously by ferromagnetic order, large Rashba SOC and hexagonal band warping at the (111) interfaces between LaAlO3, EuTiO3, and SrTiO3 insulators. The 2DES displays anomalous quantum corrections to the magneto-conductance driven by the time-reversal-symmetry breaking occurring below the magnetic transition temperature. The results are explained by the emergence of a non-trivial Berry phase and competing weak anti-localization/weak localization back-scattering of Dirac-like fermions, mimicking the phenomenology of gapped topological insulators. These findings open perspectives for the engineering of novel spin-polarized functional 2DES holding promises in spin-orbitronics and topological electronics.

Enabling 2D Electron Gas with High Room-Temperature Electron Mobility Exceeding 100 cm2 Vs-1 at a Perovskite Oxide Interface

In perovskite oxide heterostructures, bulk functional properties coexist with emergent physical phenomena at epitaxial interfaces. Notably, charge transfer at the interface between two insulating oxide layers can lead to the formation of a 2D electron gas (2DEG) with possible applications in, e.g., high-electron-mobility transistors and ferroelectric field-effect transistors. So far, the realization of oxide 2DEGs is, however, largely limited to the interface between the single-crystal substrate and epitaxial film, preventing their deliberate placement inside a larger device architecture. Additionally, the substrate-limited quality of perovskite oxide interfaces hampers room-temperature (RT) 2DEG performance due to notoriously low electron mobility. In this work, the controlled creation of an interfacial 2DEG at the epitaxial interface between perovskite oxides BaSnO3 and LaInO3 is demonstrated with enhanced RT electron mobility values up to 119 cm2 Vs-1-the highest RT value reported so far for a perovskite oxide 2DEG. Using a combination of state-of-the-art deposition modes during oxide molecular beam epitaxy, this approach opens up another degree of freedom in optimization and in situ control of the interface between two epitaxial oxide layers away from the substrate interface. Thus this approach is expected to apply to the general class of perovskite oxide 2DEG systems and to enable their improved compatibility with novel device concepts and integration across materials platforms.

Ultrafast dynamics of two-dimensional electron gas at Al2O3/SrTiO3 interface studied by surface terahertz spectroscopy

Two-dimensional electron gas (2DEG) confined at the interface of SrTiO3-based heterostructure exhibits intriguing electronic and optoelectronic properties. In this work, we study the ultrafast dynamics of 2DEG at the Al2O3/SrTiO3 interface using surface-specific terahertz difference-frequency spectroscopy. Through proper polarization selection, we have resolved simultaneously the evolution of 2DEG confined in the quantum well and the surface potential after optical pump with different photon energies. The hot electrons excited from 2DEG with pump photon energy below the band gap relax to the ground state through two processes, a fast interfacial process associated with electron-electron and electron-phonon scattering on the order of tens of picoseconds and a slow surface-to-bulk transport on the order of hundreds of picoseconds. When the pump photon energy exceeds the bandgap, electrons are directly injected from the valence band to 2DEG at the interface and relax via electron-hole recombination in 3 ps. The recorded dynamic change of interfacial potential provides the key information to identify the drift and diffusion of photocarriers at the interface. Our results broaden the horizon of investigation on the comprehension of complex oxide interfaces and their photonics capabilities.

Effect of electrostatic confinement on the dome-shaped superconducting phase diagram at the LaAlO3/SrTiO3 interface

The two-dimensional electron gas (2DEG) at the LaAlO[Formula: see text]/SrTiO[Formula: see text] (LAO/STO) interface exhibits gate-tunable superconductivity with a dome-like shape of critical temperature as a function of electron concentration. This behavior has not been unambiguously explained yet. Here, we develop a microscopic model based on the Schrödinger-Poisson approach to determine the electronic structure of the LAO/STO 2DEG, which we then apply to study the principal characteristics of the superconducting phase within the real-space pairing mean-field approach. For the electron concentrations reported in the experiment, we successfully reproduce the dome-like shape of the superconducting gap. According to our analysis such behavior results from the interplay between the Fermi surface topology and the gap symmetry, with the dominant extended s-wave contribution. Similarly as in the experimental report, we observe a bifurcation effect in the superconducting gap dependence on the electron density when the 2DEG is electrostatically doped either with the top gate or the bottom gate. Our findings explains the dome-shaped phase diagram of the considered heterostucture with good agreement with the experimental data which, in turn, strongly suggest the appearance of the extended s-wave symmetry of the gap in 2DEG at the LAO/STO interface.

Single Layer Control of Nanoscale Metal-Insulator Transition at the LaAlO3/SrTiO3 Interface

Modern quantum device fabrication often requires precisely adding and removing materials in situ at nanoscales, which is challenging for high-quality correlated oxide devices. In this work, we present a novel nanofabrication method that remotely controls the interfacial metal-insulator transition at the LaAlO3/SrTiO3 interface by selectively removing an LaAlO3 overlayer using a diamond tip. Remarkably, we observe a large force window within which single atomic layer precision of control is achievable. Our results confirm the critical thickness and charge transfer mechanism through a layer-by-layer removal process at the interface. Additionally, high-quality nanodevices, including nanochannels and single electron transistors, are successfully fabricated using this method. This nonvolatile and high-precision nanofabrication method provides a promising oxide platform for quantum engineering by harnessing the rich electron correlations at the nanoscale.

Even-integer quantum Hall effect in an oxide caused by a hidden Rashba effect

In the presence of a high magnetic field, quantum Hall systems usually host both even- and odd-integer quantized states because of lifted band degeneracies. Selective control of these quantized states is challenging but essential to understand the exotic ground states and manipulate the spin textures. Here we demonstrate the quantum Hall effect in Bi2O2Se thin films. In magnetic fields as high as 50 T, we observe only even-integer quantum Hall states, but there is no sign of odd-integer states. However, when reducing the thickness of the epitaxial Bi2O2Se film to one unit cell, we observe both odd- and even-integer states in this Janus (asymmetric) film grown on SrTiO3. By means of a Rashba bilayer model based on the ab initio band structures of Bi2O2Se thin films, we can ascribe the only even-integer states in thicker films to the hidden Rasbha effect, where the local inversion-symmetry breaking in two sectors of the [Bi2O2]2+ layer yields opposite Rashba spin polarizations, which compensate with each other. In the one-unit-cell Bi2O2Se film grown on SrTiO3, the asymmetry introduced by the top surface and bottom interface induces a net polar field. The resulting global Rashba effect lifts the band degeneracies present in the symmetric case of thicker films.

Theory of spin and orbital Edelstein effects

In systems with broken spatial inversion symmetry, such as surfaces, interfaces, or bulk systems lacking an inversion center, the application of a charge current can generate finite spin and orbital densities associated with a nonequilibrium magnetization, which is known as spin and orbital Edelstein effect (SEE and OEE), respectively. Early reports on this current-induced magnetization focus on two-dimensional Rashba systems, in which an in-plane nonequilibrium spin density is generated perpendicular to the applied charge current. However, until today, a large variety of materials have been theoretically predicted and experimentally demonstrated to exhibit a sizeable Edelstein effect, which comprises contributions from the spin as well as the orbital degrees of freedom, and whose associated magnetization may be out of plane, nonorthogonal, and even parallel to the applied charge current, depending on the system's particular symmetries. In this review, we give an overview on the most commonly used theoretical approaches for the discussion and prediction of the SEE and OEE. Further, we introduce a selection of the most intensely discussed materials exhibiting a finite Edelstein effect, and give a brief summary of common experimental techniques.

Giant Tunability of Rashba Splitting at Cation-Exchanged Polar Oxide Interfaces by Selective Orbital Hybridization

The 2D electron gas (2DEG) at oxide interfaces exhibits extraordinary properties, such as 2D superconductivity and ferromagnetism, coupled to strongly correlated electrons in narrow d-bands. In particular, 2DEGs in KTaO3 (KTO) with 5d t2g orbitals exhibit larger atomic spin-orbit coupling and crystal-facet-dependent superconductivity absent for 3d 2DEGs in SrTiO3 (STO). Herein, by tracing the interfacial chemistry, weak anti-localization magneto-transport behavior, and electronic structures of (001), (110), and (111) KTO 2DEGs, unambiguously cation exchange across KTO interfaces is discovered. Therefore, the origin of the 2DEGs at KTO-based interfaces is dramatically different from the electronic reconstruction observed at STO interfaces. More importantly, as the interface polarization grows with the higher order planes in the KTO case, the Rashba spin splitting becomes maximal for the superconducting (111) interfaces approximately twice that of the (001) interface. The larger Rashba spin splitting couples strongly to the asymmetric chiral texture of the orbital angular moment, and results mainly from the enhanced inter-orbital hopping of the t2g bands and more localized wave functions. This finding has profound implications for the search for topological superconductors, as well as the realization of efficient spin-charge interconversion for low-power spin-orbitronics based on (110) and (111) KTO interfaces.

Coexistence of Ferromagnetism and Superconductivity at KTaO3 Heterointerfaces

The coexistence of superconductivity and ferromagnetism is a long-standing issue in superconductivity due to the antagonistic nature of these two ordered states. Experimentally identifying and characterizing novel heterointerface superconductors that coexist with magnetism presents significant challenges. Here, we report the observation of two-dimensional long-range ferromagnetic order in a KTaO3 heterointerface superconductor, showing the coexistence of superconductivity and ferromagnetism. Remarkably, our direct current superconducting quantum interference device measurements reveal an in-plane magnetization hysteresis loop persisting above room temperature. Moreover, first-principles calculations and X-ray magnetic circular dichroism measurements provide decisive insights into the origin of the observed robust ferromagnetism, attributing it to oxygen vacancies that localize electrons in nearby Ta 5d states. Our findings suggest KTaO3 heterointerfaces as time-reversal symmetry breaking superconductors, injecting fresh momentum into the exploration of the intricate interplay between superconductivity and magnetism enhanced by the strong spin-orbit coupling inherent to the heavy Ta in 5d orbitals.

Spin Hall effect modulated by an electric field in asymmetric two-dimensional MoSiAs2Se

Spin current generation from charge current in nonmagnetic materials promises an energy-efficient scheme for manipulating magnetization in spintronic devices. In some asymmetric two-dimensional (2D) materials, the Rashba and valley effects coexist owing to strong spin-orbit coupling (SOC), which induces the spin Hall effect due to spin-momentum locking of both effects. Herein, we propose a new Janus structure MoSiAs2Se with both valley physics and the Rashba effect and reveal an effective way to modulate the properties of this structure. The results demonstrated that applying an external electric field is an effective means to modulating the electronic properties of MoSiAs2Se, leading to both type I-II phase transitions and semiconductor-metal phase transitions. Furthermore, the coexistence of the Rashba and valley effects in monolayer MoSiAs2Se contributes to the spin Hall effect (SHE). The magnitude and direction of spin Hall conductivity can also be manipulated with an out-of-plane electric field. Our results enrich the physics and materials of the Rashba and valley systems, opening new opportunities for the applications of 2D Janus materials in spintronic devices.

Emerging Two-Dimensional Conductivity at the Interface between Mott and Band Insulators

Intriguingly, conducting perovskite interfaces between ordinary band insulators are widely explored, whereas similar interfaces with Mott insulators are still not quite understood. Here, we address the (001), (110), and (111) interfaces between the LaTiO_{3} Mott, and large band gap KTaO_{3} insulators. Based on first-principles calculations, we reveal a mechanism of interfacial conductivity, which is distinct from a formerly studied one applicable to interfaces between polar wideband insulators. Here, the key factor causing conductivity is the matching of oxygen octahedra tilting in KTaO_{3} and LaTiO_{3} which, due to a small gap in the LaTiO_{3} results in its sensitivity to the crystal structure, yields metallization of its overlayer and following charge transfer from Ti to Ta. Our findings, also applicable to other Mott insulators interfaces, shed light on the emergence of conductivity observed in LaTiO_{3}/KTaO_{3} (110) where the "polar" arguments are not applicable and on the emergence of superconductivity in these structures.

Novel 2DEG System at the HfO2/STO Interface

Multifunctional integration in a single device has always been a hot research topic, especially for contradictory phenomena, one of which is the coexistence of ferroelectricity and metallicity. The complex oxide heterostructures, as symmetric breaking systems, provide a great possibility to incorporate different properties. Moreover, finding a series of oxide heterostructures to achieve this goal remains as a challenge. Here, taking the advantage of different physical phenomena, we use H2 plasma to pretreat the SrTiO3 (STO) substrate and then fabricate HfO2/STO heterostructures with it. The novel, well-repeatable metallic two-dimensional electron gas (2DEG) is directly obtained at the heterointerfaces without any further complex procedures, while the obvious ferroelectric-like behavior and Rashba spin-orbit coupling are also observed. The understanding of the mechanism, as well as the modified facile preparation procedure, would be meaningful for further development of ferroelectric metal in complex oxide heterostructures.

Extreme magnetoresistance at high-mobility oxide heterointerfaces with dynamic defect tunability

Magnetic field-induced changes in the electrical resistance of materials reveal insights into the fundamental properties governing their electronic and magnetic behavior. Various classes of magnetoresistance have been realized, including giant, colossal, and extraordinary magnetoresistance, each with distinct physical origins. In recent years, extreme magnetoresistance (XMR) has been observed in topological and non-topological materials displaying a non-saturating magnetoresistance reaching 103-108% in magnetic fields up to 60 T. XMR is often intimately linked to a gapless band structure with steep bands and charge compensation. Here, we show that a linear XMR of 80,000% at 15 T and 2 K emerges at the high-mobility interface between the large band-gap oxides γ-Al2O3 and SrTiO3. Despite the chemically and electronically very dissimilar environment, the temperature/field phase diagrams of γ-Al2O3/SrTiO3 bear a striking resemblance to XMR semimetals. By comparing magnetotransport, microscopic current imaging, and momentum-resolved band structures, we conclude that the XMR in γ-Al2O3/SrTiO3 is not strongly linked to the band structure, but arises from weak disorder enforcing a squeezed guiding center motion of electrons. We also present a dynamic XMR self-enhancement through an autonomous redistribution of quasi-mobile oxygen vacancies. Our findings shed new light on XMR and introduce tunability using dynamic defect engineering.

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.

Robust topological superconductivity in spin-orbit coupled systems at higher-order van Hove filling

Van Hove singularities in proximity to the Fermi level promote electronic interactions and generate diverse competing instabilities. It is also known that a nontrivial Berry phase derived from spin-orbit coupling can introduce an intriguing decoration into the interactions and thus alter correlated phenomena. However, it is unclear how and what type of new physics can emerge in a system featured by the interplay between van Hove singularities (VHSs) and the Berry phase. Here, based on a general Rashba model on the square lattice, we comprehensively explore such an interplay and its significant influence on the competing electronic instabilities by performing a parquet renormalization group analysis. Despite the existence of a variety of comparable fluctuations in the particle-particle and particle-hole channels associated with higher-order VHSs, we find that the chiral p±ip pairings emerge as two stable fixed trajectories within the generic interaction parameter space, namely the system becomes a robust topological superconductor. The chiral pairings stem from the hopping interaction induced by the nontrivial Berry phase. The possible experimental realization and implications are discussed. Our work sheds new light on the correlated states in quantum materials with strong spin-orbit coupling (SOC) and offers fresh insights into the exploration of topological superconductivity.

Manipulation of Spin-Orbit Torque in Tungsten Oxide/Manganite Heterostructure by Ionic Liquid Gating and Orbit Engineering

Spin-orbit coupling (SOC) is the interaction between electron's spin and orbital motion, which could realize a charge-to-spin current conversion and enable an innovative method to switch the magnetization by spin-orbit torque (SOT). Varied techniques have been developed to manipulate and improve the SOT, but the role of the orbit degree of freedom, which should have a crucial bearing on the SOC and SOT, is still confusing. Here, we find that the charge-to-spin current conversion and SOT in W3O8-δ/(La, Sr)MnO3 could be produced or eliminated by ionic liquid gating. Through tuning the preferential occupancy of Mn/W-d electrons from the in-plane (dx2-y2) to out-of-plane (d3z2-r2) orbit, the SOT damping-like field efficiency is nearly doubled due to the enhanced spin Hall effect and interfacial Rashba-Edelstein effect. These findings not only offer intriguing opportunities to control the SOT for high-efficient spintronic devices but also could be a fundamental step toward spin-orbitronics in the future.

The tunable anisotropic Rashba spin-orbit coupling effect in Pb-adsorbed Janus monolayer WSeTe

The spin-splitting properties of Pb-adsorbed monolayer Janus WSeTe are investigated based on first-principles calculations. The adsorbed system shows large Rashba splitting (the Rashba parameter is up to 0.75 eV Å), and we find that different adsorption layers (Te/Se adsorption layers) exhibit different significant features under spin-orbit coupling. Zeeman splitting and Rashba splitting co-exist at the high symmetry Γ point of the Te adsorption layer, while the Se adsorption layer exhibits anisotropic Rashba spin-orbit coupling. It was determined using k·p perturbation theory that Pb atom adsorption reduces the initial symmetry of the 2H-WSeTe monolayer and induces a strong spin-orbit coupling effect, so as to induce the anisotropic Rashba effect. Furthermore, the tunability of Rashba splitting was demonstrated by varying the adsorption concentration, adjusting the adsorption distance, and applying biaxial strain. This predicted adsorption system has potential value in spintronic devices.

Quantum Confinement and Electronic Structure at the Surface of van der Waals Ferroelectric α-In2Se3

Two-dimensional (2D) ferroelectric (FE) materials are promising compounds for next-generation nonvolatile memories due to their low energy consumption and high endurance. Among them, α-In2Se3 has drawn particular attention due to its in- and out-of-plane ferroelectricity, whose robustness has been demonstrated down to the monolayer limit. This is a relatively uncommon behavior since most bulk FE materials lose their ferroelectric character at the 2D limit due to the depolarization field. Using angle resolved photoemission spectroscopy (ARPES), we unveil another unusual 2D phenomenon appearing in 2H α-In2Se3 single crystals, the occurrence of a highly metallic two-dimensional electron gas (2DEG) at the surface of vacuum-cleaved crystals. This 2DEG exhibits two confined states, which correspond to an electron density of approximately 1013 electrons/cm2, also confirmed by thermoelectric measurements. Combination of ARPES and density functional theory (DFT) calculations reveals a direct band gap of energy equal to 1.3 ± 0.1 eV, with the bottom of the conduction band localized at the center of the Brillouin zone, just below the Fermi level. Such strong n-type doping further supports the quantum confinement of electrons and the formation of the 2DEG.

Strong Rashba parameter of two-dimensional electron gas at CaZrO3/SrTiO3 heterointerface

We synthesized a CaZrO3/SrTiO3 oxide heterostructure, which can serve as an alternative to LaAlO3/SrTiO3, and confirmed the generation of 2-dimensional electron gas (2-DEG) at the heterointerface. We analyzed the electrical-transport properties of the 2-DEG to elucidate its intrinsic characteristics. Based on the magnetic field dependence of resistance at 2 K, which exhibited Weak Anti-localization (WAL) behaviors, the fitted Rashba parameter values were found to be about 12-15 × 10-12 eV*m. These values are stronger than the previous reported Rashba parameters obtained from the 2-DEGs in other heterostructure systems and several layered 2D materials. The observed strong spin-orbit coupling (SOC) is attributed to the strong internal electric field generated by the lattice mismatch between the CaZrO3 layer and SrTiO3 substrate. This pioneering strong SOC of the 2-DEG at the CaZrO3/SrTiO3 heterointerface may play a pivotal role in the developing future metal oxide-based quantum nanoelectronics devices.

Ultra-thin magnetic film with giant phonon-drag for heat to spin current conversion

A tightly confined 2D electron gas with good carrier mobility and large spin-polarization is an essential ingredient for the implementation of spin-caloritronic conversion device technology. Here we give evidence that the SrTiO3/EuTiO3/LaAlO3 heterostructure is a prototype material for this purpose. The presence of Eu induces strong spin-polarization in the 2D electron gas spontaneously formed at the interface and ferromagnetic order at low temperature. Furthermore, tight 2D confinement and spin-polarization can be highly enhanced upon charge depletion, in turn generating huge thermopower associated with the phonon-drag mechanism. Most importantly, the remarkable difference in the population of the two spin channels results in the giant spin-polarized Seebeck effect and in turn, giant spin voltages of mV K-1 order at the two ends of an applied thermal gradient. Our results represent a strong assessment to the capabilities of this interface for low-temperature spin-caloritronic applications.

Reversible Photomodulation of Two-Dimensional Electron Gas in LaAlO3/SrTiO3 Heterostructures

Long-lived photoinduced conductance changes in LaAlO₃/SrTiO₃ (LAO/STO) heterostructures enable their use in optoelectronic memory applications. However, it remains challenging to quench the persistent photoconductivity (PPC) instantly and reproducibly, which limits the reversible optoelectronic switching. Herein, we demonstrate a reversible photomodulation of two-dimensional electron gas (2DEG) in LAO/STO heterostructures with high reproducibility. By irradiating UV pulses, the 2DEG at the LAO/STO interface is gradually transformed to the PPC state. Notably, the PPC can be completely removed by water treatment when two key requirements are met: (1) the moderate oxygen deficiency in STO and (2) the minimal band edge fluctuation at the interface. Through our X-ray photoelectron spectroscopy and electrical noise analysis, we reveal that the reproducible change in the conductivity of 2DEG is directly attributed to the surface-driven electron relaxation in the STO. Our results provide a stepping-stone toward developing optically tunable memristive devices based on oxide 2DEG systems.

Tuning of the magnetotransport properties of a spin-polarized 2D electron system using visible light

We report on the effects of visible light on the low temperature electronic properties of the spin-polarized two dimensional electron system (2DES) formed at the interfaces between LaAlO[Formula: see text], EuTiO[Formula: see text] and (001) SrTiO[Formula: see text]. A strong, persistent modulation of both longitudinal and transverse conductivity was obtained using light emitting diodes (LEDs) with emissions at different wavelengths in the visible spectrum range. In particular, Hall effect data show that visible light induces a non-volatile electron filling of bands with mainly 3d[Formula: see text] character, and at the same time an enhancement of the anomalous Hall effect associated to the magnetic properties of the system. Accordingly, a suppression of the weak-anti localization corrections to the magneto-conductance is found, which correlates with an enhancement of the spin-polarization and of the ferromagnetic character of 2DES. The results establish the LED-induced photo-doping as a viable route for the control of the ground state properties of artificial spin-polarized oxide 2DES.

Light-Induced Giant Rashba Spin-Orbit Coupling at Superconducting KTaO3 (110) Heterointerfaces

The 2D electron system (2DES) at the KTaO₃ surface or heterointerface with 5d orbitals hosts extraordinary physical properties, including a stronger Rashba spin-orbit coupling (RSOC), higher superconducting transition temperature, and potential of topological superconductivity. Herein, a huge enhancement of RSOC under light illumination achieved at a superconducting amorphous-Hf_0.5 Zr_0.5 O₂ /KTaO₃ (110) heterointerfaces is reported. The superconducting transition is observed with T_c = 0.62 K and the temperature-dependent upper critical field reveals the interaction between spin-orbit scattering and superconductivity. A strong RSOC with B_so = 1.9 T is revealed by weak antilocalization in the normal state, which undergoes sevenfold enhancement under light illumination. Furthermore, RSOC strength develops a dome-shaped dependence of carrier density with the maximum of B_so = 12.6 T achieved near the Lifshitz transition point n_c ≈ 4.1 × 10¹³ cm^-2 . The highly tunable giant RSOC at KTaO₃ (110)-based superconducting interfaces show great potential for spintronics.

Evidence of linear and cubic Rashba effect in non-magnetic heterostructure

TheLaAlO3/KTaO3system serves as a prototype to study the electronic properties that emerge as a result of spin-orbit coupling (SOC). In this article, we have used first-principles calculations to systematically study two types of defect-free (0 0 1) interfaces, which are termed as Type-I and Type-II. While the Type-I heterostructure produces a two dimensional (2D) electron gas, the Type-II heterostructure hosts an oxygen-rich 2D hole gas at the interface. Furthermore, in the presence of intrinsic SOC, we have found evidence of both cubic and linear Rashba interactions in the conduction bands of the Type-I heterostructure. On the contrary, there is spin-splitting of both the valence and the conduction bands in the Type-II interface, which are found to be only linear Rashba type. Interestingly, the Type-II interface also harbors a potential photocurrent transition path, making it an excellent platform to study the circularly polarized photogalvanic effect.

Atomic Force Manipulation of Single Magnetic Nanoparticles for Spin-Based Electronics

Magnetic nanoparticles (MNPs) are instrumental for fabrication of tailored nanomagnetic structures, especially where top-down lithographic patterning is not feasible. Here, we demonstrate precise and controllable manipulation of individual magnetite MNPs using the tip of an atomic force microscope. We verify our approach by placing a single MNP with a diameter of 50 nm on top of a 100 nm Hall bar fabricated in a quasi-two-dimensional electron gas (q2DEG) at the oxide interface between LaAlO₃ and SrTiO₃ (LAO/STO). A hysteresis loop due to the magnetic hysteresis properties of the magnetite MNPs was observed in the Hall resistance. Further, the effective coercivity of the Hall resistance hysteresis loop could be changed upon field cooling at different angles of the cooling field with respect to the measuring field. The effect is associated with the alignment of the MNP magnetic moment along the easy axis closest to the external field direction across the Verwey transition in magnetite. Our results can facilitate experimental realization of magnetic proximity devices using single MNPs and two-dimensional materials for spin-based nanoelectronics.

Experimental Evidence of t_{2g} Electron-Gas Rashba Interaction Induced by Asymmetric Orbital Hybridization

We report the control of Rashba spin-orbit interaction by tuning asymmetric hybridization between Ti orbitals at the LaAlO_{3}/SrTiO_{3} interface. This asymmetric orbital hybridization is modulated by introducing a LaFeO_{3} layer between LaAlO_{3} and SrTiO_{3}, which alters the Ti-O lattice polarization and traps interfacial charge carriers, resulting in a large Rashba spin-orbit effect at the interface in the absence of an external bias. This observation is verified through high-resolution electron microscopy, magnetotransport and first-principles calculations. Our results open hitherto unexplored avenues of controlling Rashba interaction to design next-generation spin orbitronics.

Direct visualization of Rashba-split bands and spin/orbital-charge interconversion at KTaO3 interfaces

Rashba interfaces have emerged as promising platforms for spin-charge interconversion through the direct and inverse Edelstein effects. Notably, oxide-based two-dimensional electron gases display a large and gate-tunable conversion efficiency, as determined by transport measurements. However, a direct visualization of the Rashba-split bands in oxide two-dimensional electron gases is lacking, which hampers an advanced understanding of their rich spin-orbit physics. Here, we investigate KTaO₃ two-dimensional electron gases and evidence their Rashba-split bands using angle resolved photoemission spectroscopy. Fitting the bands with a tight-binding Hamiltonian, we extract the effective Rashba coefficient and bring insight into the complex multiorbital nature of the band structure. Our calculations reveal unconventional spin and orbital textures, showing compensation effects from quasi-degenerate band pairs which strongly depend on in-plane anisotropy. We compute the band-resolved spin and orbital Edelstein effects, and predict interconversion efficiencies exceeding those of other oxide two-dimensional electron gases. Finally, we suggest design rules for Rashba systems to optimize spin-charge interconversion performance.

Visualization of the Rashbasplit bands in oxide two-dimensional electron gases is lacking, which hampers understanding of their rich spin-orbit physics. Here, the authors investigate KTaO₃ two dimensional electron gases and their Rashba-split bands.

The Giant Spin-to-Charge Conversion of the Layered Rashba Material BiTeI

Rashba spin-orbit coupling (SOC) could facilitate an efficient interconversion between spin and charge currents. Among various systems, BiTeI holds one of the largest Rashba-type spin splittings. Unlike other Rashba systems (e.g., Bi/Ag and Bi₂Se₃), an experimental investigation of the spin-to-charge interconversion in BiTeI remains to be explored. Through performing an angle-resolved photoemission spectroscopy (ARPES) measurement, such a large Rashba-type spin splitting with a Rashba parameter α_R = 3.68 eV Å is directly identified. By studying the spin pumping effect in the BiTeI/NiFe bilayer, we reveal a very large inverse Rashba-Edelstein length λ_IREE ≈ 1.92 nm of BiTeI at room temperature. Furthermore, the λ_IREE monotonously increases to 5.00 nm at 60 K, indicating an enhanced Rashba SOC at low temperature. These results suggest that BiTeI films with the giant Rashba SOC are promising for achieving efficient spin-to-charge interconversion, which could be implemented for building low-power-consumption spin-orbitronic devices.

Electrically and optically erasable non-volatile two-dimensional electron gas memory

The high-mobility two-dimensional electron gas (2DEG) generated at the interface between two wide-band insulators, LaAlO₃ (LAO) and SrTiO₃ (STO), is an extensively researched topic. In this study, we have successfully realized reversible switching between metallic and insulating states of the 2DEG system via the application of optical illumination and positive pulse voltage induced by the introduction of oxygen vacancies as reservoirs for electrons. The positive pulse voltage irreversibly drives the electron to the defect energy level formed by the oxygen vacancies, which leads to the formation of the insulating state. Subsequently, the metallic state can be achieved via optical illumination, which excites the trapped electron back to the 2DEG potential well. The ON/OFF state is observed to be robust with a ratio exceeding 10⁶; therefore, the interface can be used as an electrically and optically erasable non-volatile 2DEG memory.

Giant tunable Rashba spin splitting in two-dimensional polar perovskites TlSnX3 (X = Cl, Br, I)

The electronic structures and Rashba effect of two-dimensional polar tetragonal perovskites TlSnX₃ (X = Cl, Br, I) are investigated by first-principles density functional theory, and intrinsic Rashba effects are found around the Γ point. In particular, TlSnI₃ has the largest Rashba constant of 1.072 eV Å^-1. Additionally, TlSnBr₃ and TlSnI₃ respond strongly to the applied electric field, and the electric field responsivity of TlSnI₃ can reach 0.790 e Ų. Therefore, due to the large Rashba constants and strong electric field responses, these 2D polar perovskites are promising semiconductor materials with short channel lengths. The nano-scale short spin coherence length can keep the spin coherence of spin FETs, which is superior to the traditional 3D micron spin FETs, and will show a broad application prospect in the Rashba semiconductor field.

Electric field and strain engineering tuning Rashba spin splitting in quasi-one-dimensional organic-inorganic hybrid perovskites (MV)AI3Cl2 (MV = methylviologen, A = Bi, Sb)

We systematically study the Rashba spin texture of lead-free quasi-one-dimensional organic-inorganic hybrid perovskites (OIHP), (MV)AI₃Cl₂ (MV = methylviologen, A = Bi, Sb) with first-principles calculations. The k_x-k_y plane Rashba spin splitting was found to depend on the composition of Bi (Sb) and I atoms at band edges. Importantly, increasing ferroelectric polarization and the stretch along the z-direction can effectively enhance the amplitude of the Rashba spin splitting. This work provides an avenue for electric field and strain-controlled spin splitting and highlights the potential of quasi-one-dimensional OIHP for further applications in spin field effect transistors and photovoltaic cells.

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.

Manipulation of 2DEG at double-doped high-entropy heterointerfaces

Chemical doping is a dominating method for manipulating oxide two-dimensional electron gas (2DEG). However, enhancing the doping level while maintaining the metallic conduction remains a challenge, which limits detailed knowledge of 2DEG manipulation. Herein, we propose a concept of high-entropy heterointerface, which consists of a complex oxide (containing at least 5 elements) at either or both sides of the interface. By doubly doping Sr and Mn elements in the Nd and Al sites of NdAlO₃, we grow Nd_1-xSr_xAl_1-xMn_xO₃ (NSAMO) films onto SrTiO₃ (STO) substrates to fabricate NSAMO/STO high-entropy heterointerfaces with different thicknesses (2-30 nm) and a wide range of doping ratios x (0.14-0.56). The 2DEG conducting behavior is maintained until x = 0.42, which is higher compared with similar studies. The varying x results in the coexistence of rich properties like a weak anti-localization (0.14-0.42), abnormal Hall effect (0.28 & 0.42), Lifshitz transition (0.42) and stable structure. These results confirm the potential of this strategy to tailor 2DEG in all-oxide interfaces.

A Two-Dimensional Superconducting Electron Gas in Freestanding LaAlO3/SrTiO3 Micromembranes

Freestanding oxide membranes constitute an intriguing material platform for new functionalities and allow integration of oxide electronics with technologically important platforms such as silicon. Sambri et al. recently reported a method to fabricate freestanding LaAlO₃/SrTiO₃ (LAO/STO) membranes by spalling of strained heterostructures. Here, we first develop a scheme for the high-yield fabrication of membrane devices on silicon. Second, we show that the membranes exhibit metallic conductivity and a superconducting phase below ∼200 mK. Using anisotropic magnetotransport we extract the superconducting phase coherence length ξ ≈ 36-80 nm and establish an upper bound on the thickness of the superconducting electron gas d ≈ 17-33 nm, thus confirming its two-dimensional character. Finally, we show that the critical current can be modulated using a silicon-based backgate. The ability to form superconducting nanostructures of LAO/STO membranes, with electronic properties similar to those of the bulk counterpart, opens opportunities for integrating oxide nanoelectronics with silicon-based architectures.

Quantum Phase Transition in the Spin Transport Properties of Ferromagnetic Metal-Insulator-Metal Hybrid Materials

Perpendicular magnetic tunnel junctions provide a technologically important design platform for studying metal-insulator-metal heterostructure materials. Accurate characterization of the sensitivity of their electronic structure to proximity coupling effects based on first-principles calculations is key in the fundamental understanding of their emergent collective properties at macroscopic scales. Here, we use an effective field theory that combines ab initio calculations of the electronic structure within density functional theory with the plane waves calculation of the spin polarised conductance to gain insights into the proximity effect induced magnetoelectric couplings that arise in the transport of spin angular momentum when a monolayer tunnel barrier material is integrated into the magnetic tunnel junction. We find that the spin density of states exhibits a discontinuous change from half-metallic to the metallic character in the presence of monolayer hexagonal boron nitride when the applied electric field reaches a critical amplitude, and this signals a first order transition in the transport phase. This unravels an electric-field induced quantum phase transition in the presence of a monolayer hexagonal boron nitride tunnel barrier quite unlike molybdenum disulphide. The role of the applied electric field in the observed phase transition is understood in terms of the induced spin-flip transition and the charge transfer at the constituent interfaces. The results of this study show that the choice of the tunnel barrier layer material plays a nontrivial role in determining the magnetoelectric couplings during spin tunnelling under external field bias.

Circular Photogalvanic Effect in Oxide Two-Dimensional Electron Gases

Two-dimensional electron gases (2DEGs) at the LaAlO_{3}/SrTiO_{3} interface have attracted wide interest, and some exotic phenomena are observed, including 2D superconductivity, 2D magnetism, and diverse effects associated with Rashba spin-orbit coupling. Despite the intensive investigations, however, there are still hidden aspects that remain unexplored. For the first time, here we report on the circular photogalvanic effect (CPGE) for the oxide 2DEG. Spin polarized electrons are selectively excited by circular polarized light from the in-gap states of SrTiO_{3} to 2DEG and are converted into electric current via the mechanism of spin-momentum locking arising from Rashba spin-orbit coupling. Moreover, the CPGE can be effectively modified by the density and distribution of oxygen vacancies. This Letter presents an effective approach to generate and manipulate the spin polarized current, paving the way toward oxide spintronics.

Robust Electronic Structure of Manganite-Buffered Oxide Interfaces with Extreme Mobility Enhancement

The electronic structure as well as the mechanism underlying the high-mobility two-dimensional electron gases (2DEGs) at complex oxide interfaces remain elusive. Herein, using soft X-ray angle-resolved photoemission spectroscopy (ARPES), we present the band dispersion of metallic states at buffered LaAlO₃/SrTiO₃ (LAO/STO) heterointerfaces where a single-unit-cell LaMnO₃ (LMO) spacer not only enhances the electron mobility but also renders the electronic structure robust toward X-ray radiation. By tracing the evolution of band dispersion, orbital occupation, and electron-phonon interaction of the interfacial 2DEG, we find unambiguous evidence that the insertion of the LMO buffer strongly suppresses both the formation of oxygen vacancies as well as the electron-phonon interaction on the STO side. The latter effect makes the buffered sample different from any other STO-based interfaces and may explain the maximum mobility enhancement achieved at buffered oxide interfaces.

Hysteretic temperature dependence of resistance controlled by gate voltage in LaAlO3/SrTiO3 heterointerface electron system

For two-dimensional electron gas device applications, it is important to understand how electrical-transport properties are controlled by gate voltage. Here, we report gate voltage-controllable hysteresis in the resistance–temperature characteristics of two-dimensional electron gas at LaAlO₃/SrTiO₃ heterointerface. Electron channels made of the LaAlO₃/SrTiO₃ heterointerface showed hysteretic resistance–temperature behavior: the measured resistance was significantly higher during upward temperature sweeps in thermal cycling tests. Such hysteretic behavior was observed only after application of positive back-gate voltages below 50 K in the thermal cycle, and the magnitude of hysteresis increased with the applied back-gate voltage. To explain this gate-controlled resistance hysteresis, we propose a mechanism based on electron trapping at impurity sites, in conjunction with the strong temperature-dependent dielectric constant of the SrTiO₃ substrate. Our model explains well the observed gate-controlled hysteresis of the resistance–temperature characteristics, and the mechanism should be also applicable to other SrTiO₃-based oxide systems, paving the way to applications of oxide heterostructures to electronic devices.

Topological Hall effect in SrRuO3thin films and heterostructures

Transition metal oxides hold a wide spectrum of fascinating properties endowed by the strong electron correlations. In 4dand 5doxides, exotic phases can be realized with the involvement of strong spin-orbit coupling (SOC), such as unconventional magnetism and topological superconductivity. Recently, topological Hall effects (THEs) and magnetic skyrmions have been uncovered in SrRuO₃thin films and heterostructures, where the presence of SOC and inversion symmetry breaking at the interface are believed to play a key role. Realization of magnetic skyrmions in oxides not only offers a platform to study topological physics with correlated electrons, but also opens up new possibilities for magnetic oxides using in the low-power spintronic devices. In this review, we discuss recent observations of THE and skyrmions in the SRO film interfaced with various materials, with a focus on the electric tuning of THE. We conclude with a discussion on the directions of future research in this field.

Fully Optical Modulation of the Two-Dimensional Electron Gas at the γ-Al2O3/SrTiO3 Interface

Two-dimensional electron gas (2DEG) formed at the heterointerface between two oxide insulators hosts plenty of emergent phenomena and provides new opportunities for electronics and photoelectronics. However, despite being long sought after, on-demand properties controlled through a fully optical illumination remain far from being explored. Herein, a giant tunability of the 2DEG at the interface of γ-Al₂O₃/SrTiO₃ through a fully optical gating is discovered. Specifically, photon-generated carriers lead to a delicate tunability of the carrier density and the underlying electronic structure, which is accompanied by the remarkable Lifshitz transition. Moreover, the 2DEG can be optically tuned to possess a maximum Rashba spin-orbit coupling, particularly at the crossing region of the sub-bands with different symmetries. First-principles calculations essentially well explain the optical modulation of γ-Al₂O₃/SrTiO₃. Our fully optical gating opens a new pathway for manipulating emergent properties of the 2DEGs and is promising for on-demand photoelectric devices.

Oxide Two-Dimensional Electron Gas with High Mobility at Room-Temperature

The prospect of 2-dimensional electron gases (2DEGs) possessing high mobility at room temperature in wide-bandgap perovskite stannates is enticing for oxide electronics, particularly to realize transparent and high-electron mobility transistors. Nonetheless only a small number of studies to date report 2DEGs in BaSnO3 -based heterostructures. Here, 2DEG formation at the LaScO3 /BaSnO3 (LSO/BSO) interface with a room-temperature mobility of 60 cm2  V-1  s-1 at a carrier concentration of 1.7 × 1013  cm-2 is reported. This is an order of magnitude higher mobility at room temperature than achieved in SrTiO3 -based 2DEGs. This is achieved by combining a thick BSO buffer layer with an ex situ high-temperature treatment, which not only reduces the dislocation density but also produces a SnO2 -terminated atomically flat surface, followed by the growth of an overlying BSO/LSO interface. Using weak beam dark-field transmission electron microscopy imaging and in-line electron holography technique, a reduction of the threading dislocation density is revealed, and direct evidence for the spatial confinement of a 2DEG at the BSO/LSO interface is provided. This work opens a new pathway to explore the exciting physics of stannate-based 2DEGs at application-relevant temperatures for oxide nanoelectronics.

From Low-Field Sondheimer Oscillations to High-Field Very Large and Linear Magnetoresistance in a SrTiO3-Based Two-Dimensional Electron Gas

Quantum materials harbor a cornucopia of exotic transport phenomena challenging our understanding of condensed matter. Among these, a giant, nonsaturating linear magnetoresistance (MR) has been reported in various systems, from Weyl semimetals to topological insulators. Its origin is often ascribed to unusual band structure effects, but it may also be caused by extrinsic sample disorder. Here, we report a very large linear MR in a SrTiO₃ two-dimensional electron gas and, by combining transport measurements with electron spectromicroscopy, show that it is caused by nanoscale inhomogeneities that are self-organized during sample growth. Our data also reveal semiclassical Sondheimer oscillations arising from interferences between helicoidal electron trajectories, from which we determine the 2DEG thickness. Our results bring insight into the origin of linear MR in quantum materials, expand the range of functionalities of oxide 2DEGs, and suggest exciting routes to explore the interaction of linear MR with features like Rashba spin-orbit coupling.

Resonant tunneling driven metal-insulator transition in double quantum-well structures of strongly correlated oxide

The metal-insulator transition (MIT), a fascinating phenomenon occurring in some strongly correlated materials, is of central interest in modern condensed-matter physics. Controlling the MIT by external stimuli is a key technological goal for applications in future electronic devices. However, the standard control by means of the field effect, which works extremely well for semiconductor transistors, faces severe difficulties when applied to the MIT. Hence, a radically different approach is needed. Here, we report an MIT induced by resonant tunneling (RT) in double quantum well (QW) structures of strongly correlated oxides. In our structures, two layers of the strongly correlated conductive oxide SrVO₃ (SVO) sandwich a barrier layer of the band insulator SrTiO₃. The top QW is a marginal Mott-insulating SVO layer, while the bottom QW is a metallic SVO layer. Angle-resolved photoemission spectroscopy experiments reveal that the top QW layer becomes metallized when the thickness of the tunneling barrier layer is reduced. An analysis based on band structure calculations indicates that RT between the quantized states of the double QW induces the MIT. Our work opens avenues for realizing the Mott-transistor based on the wave-function engineering of strongly correlated electrons.

Prediction of giant and ideal Rashba-type splitting in ordered alloy monolayers grown on a polar surface

A large and ideal Rashba-type spin-orbit splitting is desired for the applications of materials in spintronic devices and the detection of Majorana fermions in solids. Here, we propose an approach to achieve giant and ideal spin-orbit splittings through a combination of ordered surface alloying and interface engineering, that is, growing alloy monolayers on an insulating polar surface. We illustrate this unique strategy by means of first-principle calculations of buckled hexagonal monolayers of SbBi and PbBi supported on Al₂O₃(0001). Both systems display ideal Rashba-type states with giant spin-orbit splittings, characterized with energy offsets over 600 meV and momentum offsets over 0.3 Å⁻¹, respectively. Our study thus points to an effective way of tuning spin-orbit splitting in low-dimensional materials to draw immediate experimental interest.

Hot Electron Tunneling in Pt/LaAlO3/SrTiO3 Heterostructures for Enhanced Photodetection

LaAlO₃/SrTiO₃ (LAO/STO) heterostructures, in which a highly mobile two-dimensional electron gas (2DEG) is formed, have great potential for optoelectronic applications. However, the inherently high density of the 2DEG hinders the observation of photo-excitation effects in oxide heterostructures. Herein, a strong photoresponse of the 2DEG in a Pt/LAO/STO heterostructure is achieved by adopting a vertical tunneling configuration. The tunneling of the 2DEG through an ultrathin LAO layer is significantly enhanced by UV light irradiation, showing a maximum photoresponsivity of ∼1.11 × 10⁷%. The strong and reversible photoresponse is attributed to the thermionic emission of photoexcited hot electrons from the oxygen-deficient STO. Notably, the oxygen vacancy defects play a critical role in enhancing the tunneling photocurrent. Our systematic study on the hysteresis behavior and the light power dependency of the tunneling current consistently support the fact that the photoexcited hot electrons from the oxygen vacancies strongly contribute to the tunneling conduction under the UV light. This work offers valuable insights into a novel photodetection mechanism based on the 2DEG as well as into developing ultrathin optoelectronic devices based on the oxide heterostructures.

Spin-Charge Interconversion in KTaO3 2D Electron Gases

Oxide interfaces exhibit a broad range of physical effects stemming from broken inversion symmetry. In particular, they can display non-reciprocal phenomena when time reversal symmetry is also broken, e.g., by the application of a magnetic field. Examples include the direct and inverse Edelstein effects (DEE, IEE) that allow the interconversion between spin currents and charge currents. The DEE and IEE have been investigated in interfaces based on the perovskite SrTiO₃ (STO), albeit in separate studies focusing on one or the other. The demonstration of these effects remains mostly elusive in other oxide interface systems despite their blossoming in the last decade. Here, the observation of both the DEE and IEE in a new interfacial two-dimensional electron gas (2DEG) based on the perovskite oxide KTaO₃ is reported. 2DEGs are generated by the simple deposition of Al metal onto KTaO₃ single crystals, characterized by angle-resolved photoemission spectroscopy and magnetotransport, and shown to display the DEE through unidirectional magnetoresistance and the IEE by spin-pumping experiments. Their spin-charge interconversion efficiency is then compared with that of STO-based interfaces, related to the 2DEG electronic structure, and perspectives are given for the implementation of KTaO₃ 2DEGs into spin-orbitronic devices is compared.

Extraordinary phase coherence length in epitaxial halide perovskites

Inorganic halide perovskites have emerged as a promising platform in a wide range of applications from solar energy harvesting to computing and light emission. The recent advent of epitaxial thin film growth of halide perovskites has made it possible to investigate low-dimensional quantum electronic devices based on this class of materials. This study leverages advances in vapor-phase epitaxy of halide perovskites to perform low-temperature magnetotransport measurements on single-domain cesium tin iodide (CsSnI₃) epitaxial thin films. The low-field magnetoresistance carries signatures of coherent quantum interference effects and spin-orbit coupling. These weak anti-localization measurements reveal a micron-scale low-temperature phase coherence length for charge carriers in this system. The results indicate that epitaxial halide perovskite heterostructures are a promising platform for investigating long coherent quantum electronic effects and potential applications in spintronics and spin-orbitronics.

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Epitaxial halide perovskites with extraordinary quantum phase coherence

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Quantum transport properties with weak antilocalization observed in tetragonal CsSnI₃

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Demonstration of quasi-2d charge carrier behavior with of spin-orbit coupling

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Epitaxial halide perovskites emerging materials for quantum electronic applications