Giant conductivity switching of LaAlO3/SrTiO3 heterointerfaces governed by surface protonation

Charge Transfer and Orbital Reconstruction at an Organic-Oxide Interface

The two-dimensional electron system (2DES) located at the surface of strontium titanate (STO) and at several other STO-based interfaces has been an established platform for the study of novel physical phenomena since its discovery. Here we report how the interfacing of STO and tetracyanoquinodimethane (TCNQ) results in a charge transfer that depletes the number of free carriers at the STO surface, with a strong impact on its electronic structure. Our study paves the way for efficient tuning of the electronic properties, which promises novel applications in the framework of oxide/organic-based electronics.

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.

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.

Nanoscale Control of the Metal-Insulator Transition at LaAlO3/KTaO3 Interfaces

Recent reports of superconductivity at KTaO₃ (KTO) (110) and (111) interfaces have sparked intense interest due to the relatively high critical temperature as well as other properties that distinguish this system from the more extensively studied SrTiO₃ (STO)-based heterostructures. Here, we report the reconfigurable creation of conducting structures at intrinsically insulating LaAlO₃/KTO(110) and (111) interfaces. Devices are created using two distinct methods previously developed for STO-based heterostructures: (1) conductive atomic-force microscopy lithography and (2) ultralow-voltage electron-beam lithography. At low temperatures, KTO(110)-based devices show superconductivity that is tunable by an applied back gate. A one-dimensional nanowire device shows single-electron-transistor (SET) behavior. A KTO(111)-based device is metallic but does not become superconducting. These reconfigurable methods of creating nanoscale devices in KTO-based heterostructures offer new avenues for investigating mechanisms of superconductivity as well as development of quantum devices that incorporate strong spin-orbit interactions, superconducting behavior, and nanoscale dimensions.

Fast ionic transport in SrTiO3/LaAlO3 heterostructure

We constructed SrTiO3/LaAlO3 (STO/LAO) hetero-structural composites, which achieved a high ionic conductivity of 0.24 S cm−1 at 520 °C.

Electronically reconfigurable complex oxide heterostructure freestanding membranes

In recent years, lanthanum aluminate/strontium titanate (LAO/STO) heterointerfaces have been used to create a growing family of nanoelectronic devices based on nanoscale control of LAO/STO metal-to-insulator transition. The properties of these devices are wide-ranging, but they are restricted by nature of the underlying thick STO substrate. Here, single-crystal freestanding membranes based on LAO/STO heterostructures were fabricated, which can be directly integrated with other materials via van der Waals stacking. The key properties of LAO/STO are preserved when LAO/STO membranes are formed. Conductive atomic force microscope lithography is shown to successfully create reversible patterns of nanoscale conducting regions, which survive to millikelvin temperatures. The ability to form reconfigurable conducting nanostructures on LAO/STO membranes opens opportunities to integrate a variety of nanoelectronics with silicon-based architectures and flexible, magnetic, or superconducting materials.

Effect of Rare Earth Elements at Amorphous ReAlO3/SrTiO3 (Re = La, Pr, Nd, Sm, Gd, and Tm) Heterointerfaces

Although the amorphous two-dimensional electron gas (a-2DEG) of oxides provides new opportunities to explore nanoelectronic as well as quantum devices, the intrinsic effect of rare earth (Re = La, Pr, Nd, Sm, Gd, and Tm) elements at ReAlO₃/SrTiO₃ heterointerfaces is still largely unknown and needs to be addressed systematically. Herein, we first propose that the ionization potential of Re elements is a critical factor for the 2DEG fabricated by chemical spin coating. Furthermore, the photoresponsive properties of heterointerfaces are investigated comprehensively with the ionization potential ranging from 35.79 to 41.69 eV. The results show that the sheet resistances significantly increase with increasing the ionization potential, and a resistance upturn phenomenon is observed at TmAlO₃/SrTiO₃ heterointerfaces, which can be attributed to the weak localization effect theoretically. The most important observation is the dramatic transition from negative (-178.3%, Re = La) to positive (+89.9%, Re = Gd) photoresponse at ReAlO₃/SrTiO₃ heterointerfaces under the irradiation of 405 nm light at 50 K. More remarkably, a unique recovery behavior of transient-persistent photoconductivity coexistence at low temperatures is discovered at the TmAlO₃/SrTiO₃ heterointerface. This work reveals an effective approach to tune the transport and photoresponsive properties by changing Re elements and paves the way for the application of all-oxide devices.

Optically Controllable 2D Material/Complex Oxide Heterointerface

Heterostructures play a vital role in functional devices on the basis of the individual constituents. Non-conventional heterostructures formed by stacking 2D materials onto structurally distinct materials are of great interest in achieving novel phenomena that are both scientifically and technologically relevant. Here, a heterostructure based on a 2D (molybdenum ditelluride) MoTe2 and an amorphous strontium titanium oxide (a-STO) thin film is reported. The heterostructure functions as a high-performance photodetector, which exhibits anomalous negative photoresponse in the pristine device due to the scattering effect from the light-induced Oδ- ions. The photoresponsivity and the specific detectivity are found to be >104 AW-1 and >1013 Jones, respectively, which are significantly higher than those in standard MoTe2 devices. Moreover, through tuning the light programming time, the photodetection behavior of the MoTe2/a-STO heterostructure experiences a dynamic evolution from negative to positive. This is due to the optically controllable modulation of the interfacial states, which is further confirmed by the X-ray photoelectron spectroscopy and photoluminescence measurements. It is envisioned that the 2D material/a-STO heterostructure could be a potential platform for exploring new functional devices.

Engineered spin-orbit interactions in LaAlO3/SrTiO3-based 1D serpentine electron waveguides

The quest to understand, design, and synthesize new forms of quantum matter guides much of contemporary research in condensed matter physics. One-dimensional (1D) electronic systems form the basis for some of the most interesting and exotic phases of quantum matter. Here, we describe a family of quasi-1D nanostructures, based on LaAlO₃/SrTiO₃ electron waveguides, in which a sinusoidal transverse spatial modulation is imposed. These devices display unique dispersive features in the subband spectra, namely, a sizeable shift (∼7 T) in the spin-dependent subband minima, and fractional conductance plateaus. The first property can be understood as an engineered spin-orbit interaction associated with the periodic acceleration of electrons as they undulate through the nanowire (ballistically), while the second property signifies the presence of enhanced electron-electron scattering in this system. The ability to engineer these interactions in quantum wires contributes to the tool set of a 1D solid-state quantum simulation platform.

Visualizing Charge Transport and Nanoscale Electrochemistry by Hyperspectral Kelvin Probe Force Microscopy

Charge-transport and electrochemical processes are heavily influenced by the local microstructure. Kelvin probe force microscopy (KPFM) is a widely used technique to map electrochemical potentials at the nanometer scale; however, it offers little information on local charge dynamics. Here, we implement a hyperspectral KPFM approach for spatially mapping bias-dependent charge dynamics in timescales ranging from the sub-millisecond to the second regime. As a proof of principle, we investigate the role mobile surface charges play in a three-unit-cell LaAlO₃/SrTiO₃ oxide heterostructure. We explore machine learning approaches to assist with visualization, pattern recognition, and interpretation of the information-rich data sets. Linear unmixing methods reveal hidden bias-dependent interfacial processes, most likely water splitting, which are essentially unnoticed by functional fitting of the dynamic response alone. Hyperspectral KPFM will be beneficial for investigating nanoscale charge transport and local reactivity in systems involving a possible combination of electronic, ionic, and electrochemical phenomena.

Photoinduced Persistent Electron Accumulation and Depletion in LaAlO_{3}/SrTiO_{3} Quantum Wells

Persistent photoconductance is a phenomenon found in many semiconductors, by which light induces long-lived excitations in electronic states. Commonly, persistent photoexcitation leads to an increase of carriers (accumulation), though occasionally it can be negative (depletion). Here, we present the quantum well at the LaAlO_{3}/SrTiO_{3} interface, where in addition to photoinduced accumulation, a secondary photoexcitation enables carrier depletion. The balance between both processes is wavelength dependent, and allows tunable accumulation or depletion in an asymmetric manner, depending on the relative arrival time of photons of different frequencies. We use Green's function formalism to describe this unconventional photoexcitation, which paves the way to an optical implementation of neurobiologically inspired spike-timing-dependent plasticity.

Pascal conductance series in ballistic one-dimensional LaAlO3/SrTiO3 channels

One-dimensional electronic systems can support exotic collective phases because of the enhanced role of electron correlations. We describe the experimental observation of a series of quantized conductance steps within strongly interacting electron waveguides formed at the lanthanum aluminate-strontium titanate (LaAlO₃/SrTiO₃) interface. The waveguide conductance follows a characteristic sequence within Pascal's triangle: (1, 3, 6, 10, 15, …) ⋅ e ² /h, where e is the electron charge and h is the Planck constant. This behavior is consistent with the existence of a family of degenerate quantum liquids formed from bound states of n = 2, 3, 4, … electrons. Our experimental setup could provide a setting for solid-state analogs of a wide range of composite fermionic phases.

Unravelling oxygen-vacancy-induced electron transfer at SrTiO3-based heterointerfaces by transport measurement during growth

Numerous studies have shown that oxygen vacancies play an important role on the formation of two-dimensional electron gas (2DEG) at SrTiO₃-based heterointerfaces. Previously, it is widely believed that the main mechanism is that the oxygen vacancies in SrTiO₃ directly contribute electrons to the 2DEG. Here, we performed transport measurements during the creation of 2DEG for depositing amorphous LaAlO₃ on SrTiO₃ substrates and related heterostructures. Our result suggests that, unlike the previous viewpoint, in this kind of 2DEG the determinant mechanism is the electron transfer from the oxygen vacancies in the film grown on SrTiO₃, rather than the oxygen vacancies in SrTiO₃ themselves. This effect is so striking that an amorphous film of less than 10% monolayer coverage on SrTiO₃, or equivalently 0.04 nm, can already generate a highly conducting 2DEG. The present result may have a general implication and provide a possible way to understand the long-standing debate on the origin of 2DEG at SrTiO₃-based heterointerfaces.

Oxygen vacancies and hydrogen doping in LaAlO3/SrTiO3 heterostructures: electronic properties and impact on surface and interface reconstruction

We investigate the effect of oxygen vacancies and hydrogen dopants at the surface and inside slabs of [Formula: see text], [Formula: see text], and [Formula: see text]/[Formula: see text] heterostructures on the electronic properties by means of electronic structure calculations as based on density functional theory. Depending on the concentration, the presence of these defects in a [Formula: see text] slab can suppress the surface conductivity. In contrast, in insulating [Formula: see text] slabs already very small concentrations of oxygen vacancies or hydrogen dopant atoms induce a finite occupation of the conduction band. Surface defects in insulating [Formula: see text]/[Formula: see text] heterostructure slabs with three [Formula: see text] overlayers lead to the emergence of interface conductivity. Calculated defect formation energies reveal strong preference of hydrogen dopant atoms for surface sites for all structures and concentrations considered. Strong decrease of the defect formation energy of hydrogen adatoms with increasing thickness of the [Formula: see text] overlayer and crossover from positive to negative values, taken together with the metallic conductivity induced by hydrogen adatoms, seamlessly explains the semiconductor-metal transition observed for these heterostructures as a function of the overlayer thickness. Moreover, we show that the potential drop and concomitant shift of (layer resolved) band edges is suppressed for the metallic configuration. Finally, magnetism with stable local moments, which form atomically thin magnetic layers at the interface, is generated by oxygen vacancies either at the surface or the interface, or by hydrogen atoms buried at the interface. In particular, oxygen vacancies in the [Formula: see text] interface layer cause drastic downshift of the 3d e _g states of the Ti atoms neighboring the vacancies, giving rise to strongly localized magnetic moments, which add to the two-dimensional background magnetization.

Over 100-THz bandwidth selective difference frequency generation at LaAlO3/SrTiO3 nanojunctions

The ability to combine continuously tunable narrow-band terahertz (THz) generation that can access both the far-infrared and mid-infrared regimes with nanometer-scale spatial resolution is highly promising for identifying underlying light-matter interactions and realizing selective control of rotational or vibrational resonances in nanoparticles or molecules. Here, we report selective difference frequency generation with over 100 THz bandwidth via femtosecond optical pulse shaping. The THz emission is generated at nanoscale junctions at the interface of LaAlO₃/SrTiO₃ (LAO/STO) that is defined by conductive atomic force microscope lithography, with the potential to perform THz spectroscopy on individual nanoparticles or molecules. Numerical simulation of the time-domain signal facilitates the identification of components that contribute to the THz generation. This ultra-wide-bandwidth tunable nanoscale coherent THz source transforms the LAO/STO interface into a promising platform for integrated lab-on-chip optoelectronic devices with various functionalities.

Highly Stable Persistent Photoconductivity with Suspended Graphene Nanoribbons

Graphene nanoribbon (GNR), also known as 1-dimensional graphene, with a non-zero band gap has a huge potential for various electrical and optoelectrical applications because of its high transparency, flexibility, controllable band gap, and unique edge states. Recent advances in the synthesis of GNR enable us to show the possibility of GNRs as future high performance electrical devices. However, the applicability of GNRs to optoelectrical devices is unclear. Here we report that suspended GNR devices can show persistent photoconductivity (PPC) with long decay time (over 72 h) and adequate environmental stability. Repeated non-volatile memory operation is also demonstrated with an integrated PPC device using GNRs. This very stable PPC device can be applied to a wide variety of fields such as ultra-low-power non-volatile memory, nanoscale imaging, and biological sensors. Our results have opened the door to advance the study of GNRs in novel directions such as optoelectrical applications.

Control of the Polarization of Ferroelectric Capacitors by the Concurrent Action of Light and Adsorbates

Ferroelectric perovskites hold promise of enhanced photovoltaic efficiency and photocatalytic activity. Consequently, the photoresponse of oxide ferroelectric thin films is an active field of research. In electrode/ferroelectric/electrode devices, internal charge in the ferroelectric, free charge in the electrodes, and buried adsorbates at interfaces combine to screen the ferroelectric polarization and to stabilize the polar state. Under illumination, photoinduced carriers and photodissociated adsorbates may disrupt the screening equilibrium, modifying the switchable polarization and altering its expected benefits. Here, we explore the photoresponse of BaTiO₃ thin films in a capacitor geometry, focusing on the effects of visible illumination on the remanent polarization. By combining ferroelectric and X-ray photoelectron spectroscopy, we discover that photoreaction of charge-screening H₂O-derived adsorbates at the buried metal-ferroelectric Pt/BaTiO₃ interface plays an unexpected pivotal role, enabling a substantial modulation (up to 75%) of the switchable remanent polarization by light. These findings illustrate that the synergy between photochemistry and photovoltaic activity at the surface of a ferroelectric material can be exploited to tune photoferroelectric activity.

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 l_MF ∼ 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.

Extreme Reconfigurable Nanoelectronics at the CaZrO3 /SrTiO3 Interface

Complex oxide heterostructures have fascinating emergent properties that originate from the properties of the bulk constituents as well as from dimensional confinement. The conductive behavior of the polar/nonpolar LaAlO3 /SrTiO3 interface can be reversibly switched using conductive atomic force microscopy (c-AFM) lithography, enabling a wide range of devices and physics to be explored. Here, extreme nanoscale control over the CaZrO3 /SrTiO3 (CZO/STO) interface, which is formed from two materials that are both nonpolar, is reported. Nanowires with measured widths as narrow as 1.2 nm are realized at the CZO/STO interface at room temperature by c-AFM lithography. These ultrathin nanostructures have spatial dimensions at room temperature that are comparable to single-walled carbon nanotubes, and hold great promise for alternative oxide-based nanoelectronics, as well as offer new opportunities to investigate the electronic structure of the complex oxide interfaces. The cryogenic properties of devices constructed from quasi-1D channels, tunnel barriers, and planar gates exhibit gate-tunable superconductivity, quantum oscillations, electron pairing outside of the superconducting regime, and quasi-ballistic transport. This newly demonstrated ability to control the metal-insulator transition at nonpolar oxide interface greatly expands the class of materials whose behavior can be patterned and reconfigured at extreme nanoscale dimensions.

Large Enhancement of 2D Electron Gases Mobility Induced by Interfacial Localized Electron Screening Effect

The interactions between delocalized and localized charges play important roles in correlated electron systems. Here, using a combination of transport measurements, spectroscopic ellipsometry (SE), and X-ray absorption spectroscopy (XAS) supported by theoretical calculations, we reveal the important role of interfacial localized charges and their screening effects in determining the mobility of (La_0.3 Sr_0.7 )(Al_0.65 Ta_0.35 )O₃ /SrTiO₃ (LSAT/SrTiO₃ ) interfaces. When the LSAT layer thickness reaches the critical value of 5 uc, the insulating interface abruptly becomes conducting, accompanied by the appearance of a new midgap state. This midgap state emerges at ≈1 eV below the Ti t_2g band and shows a strong character of Ti 3d_xy - O 2p hybridization. Increasing the LSAT layer from 5 to 18 uc, the number of localized charges increases, resulting in an enhanced screening effect and higher mobile electron mobility. This observation contradicts the traditional semiconductor interface where the localized charges always suppress the carrier mobility. These results demonstrate a new strategy to probe localized charges and mobile electrons in correlated electronic systems and highlight the important role of screening effects from localized charges in improving the mobile electron mobility at complex oxide interfaces.

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.

Physics of SrTiO3-based heterostructures and nanostructures: a review

This review provides a summary of the rich physics expressed within SrTiO3-based heterostructures and nanostructures. The intended audience is researchers who are working in the field of oxides, but also those with different backgrounds (e.g., semiconductor nanostructures). After reviewing the relevant properties of SrTiO3 itself, we will then discuss the basics of SrTiO3-based heterostructures, how they can be grown, and how devices are typically fabricated. Next, we will cover the physics of these heterostructures, including their phase diagram and coupling between the various degrees of freedom. Finally, we will review the rich landscape of quantum transport phenomena, as well as the devices that elicit them.

Tuning Up or Down the Critical Thickness in LaAlO3 /SrTiO3 through In Situ Deposition of Metal Overlayers

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO₃ and SrTiO₃ has attracted much attention from the oxide electronics community. One of its hallmark features is the existence of a critical LaAlO₃ thickness of 4 unit-cells (uc) for interfacial conductivity to emerge. In this paper, the chemical, electronic, and transport properties of LaAlO₃ /SrTiO₃ samples capped with different metals grown in a system combining pulsed laser deposition, sputtering, and in situ X-ray photoemission spectroscopy are investigated. The results show that for metals with low work function a q2DES forms at 1-2 uc of LaAlO₃ and is accompanied by a partial oxidation of the metal, a phenomenon that affects the q2DES properties and triggers the formation of defects. In contrast, for noble metals, the critical thickness is increased above 4 uc. The results are discussed in terms of a hybrid mechanism that incorporates electrostatic and chemical effects.

Magnetism at the Interface of Magnetic Oxide and Nonmagnetic Semiconductor Quantum Dots

Engineering interfaces specifically in quantum dot (QD) heterostructures provide several prospects for developing multifunctional building block materials. Precise control over internal structure by chemical synthesis offers a combination of different properties in QDs and allows us to study their fundamental properties, depending on their structure. Herein, we studied the interface of magnetic/nonmagnetic Fe₃O₄/CdS QD heterostructures. In this work, we demonstrate the decrease in the size of the magnetic core due to annealing at high temperature by the decrease in saturation magnetization and blocking temperature. Furthermore, surprisingly, in a prominently optically active and magnetically inactive material such as CdS, we observe the presence of substantial exchange bias in spite of the nonmagnetic nature of CdS QDs. The presence of exchange bias was proven by the increase in magnetic anisotropy as well as the presence of exchange bias field (H_E) during the field-cooled magnetic measurements. This exchange coupling was eventually traced to the in situ formation of a thin antiferromagnetic FeS layer at the interface. This is verified by the study of Fe local structure using X-ray absorption fine structure spectroscopy, demonstrating the importance of interface engineering in QDs.

First-Principles Prediction of Two-Dimensional Electron Gas Driven by Polarization Discontinuity in Nonpolar/Nonpolar AHfO3/SrTiO3 (A = Ca, Sr, and Ba) Heterostructures

By using first-principles electronic structure calculations, we explored the possibility of producing two-dimensional electron gas (2DEG) in nonpolar/nonpolar AHfO₃/SrTiO₃ (A = Ca, Sr, and Ba) heterostructures. Two types of nonpolar/nonpolar interfaces, (AO)⁰/(TiO₂)⁰ and (HfO₂)⁰/(SrO)⁰, each with AO and HfO₂ surface terminations, are modeled, respectively. The polarization domain and resulting interfacial electronic property are found to be more sensitive to the surface termination of the film rather than the interface model. As film thickness increases, an insulator-to-metal transition is found in all the heterostructures with HfO₂ surface termination: for (AO)⁰/(TiO₂)⁰ interfaces, predicted critical film thickness for an insulator-to-metal transition is about 7, 6, and 3 unit cells for CaHfO₃/SrTiO₃, SrHfO₃/SrTiO₃, and BaHfO₃/SrTiO₃, respectively; for (HfO₂)⁰/(SrO)⁰ interfaces, the critical film thickness is about 7.5, 5.5, and 4.5 unit cells, respectively. In contrast, for the heterostructures with AO surface termination, CaHfO₃/SrTiO₃ exhibits a much larger critical film thickness about 11-12 unit cells for an insulator-to-metal transition; while SrHfO₃/SrTiO₃ and BaHfO₃/SrTiO₃ do not show any polarization behavior even film thickness increases up to 20 unit cells. The strain-induced polarization behavior was well-elucidated from energy versus polarization profile. This work is expected to stimulate further experimental investigation to the interfacial conductivity in the nonpolar/nonpolar AHfO₃/SrTiO₃ HS.

High-Throughput Design of Two-Dimensional Electron Gas Systems Based on Polar/Nonpolar Perovskite Oxide Heterostructures

The two-dimensional electron gas (2DEG) formed at the interface between two insulating oxides such as LaAlO₃ and SrTiO₃ (STO) is of fundamental and practical interest because of its novel interfacial conductivity and its promising applications in next-generation nanoelectronic devices. Here we show that a group of combinatorial descriptors that characterize the polar character, lattice mismatch, band gap, and the band alignment between the perovskite-oxide-based band insulators and the STO substrate, can be introduced to realize a high-throughput (HT) design of SrTiO₃-based 2DEG systems from perovskite oxide quantum database. Equipped with these combinatorial descriptors, we have carried out a HT screening of all the polar perovskite compounds, uncovering 42 compounds of potential interests. Of these, Al-, Ga-, Sc-, and Ta-based compounds can form a 2DEG with STO, while In-based compounds exhibit a strain-induced strong polarization when deposited on STO substrate. In particular, the Ta-based compounds can form 2DEG with potentially high electron mobility at (TaO₂)⁺/(SrO)⁰ interface. Our approach, by defining materials descriptors solely based on the bulk materials properties, and by relying on the perovskite-oriented quantum materials repository, opens new avenues for the discovery of perovskite-oxide-based functional interface materials in a HT fashion.

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.