Suppression of the critical thickness threshold for conductivity at the LaAlO3/SrTiO3 interface

Surface triggered stabilization of metastable charge-ordered phase in SrTiO3

Charge ordering (CO), characterized by a periodic modulation of electron density and lattice distortion, has been a fundamental topic in condensed matter physics, serving as a potential platform for inducing novel functional properties. The charge-ordered phase is known to occur in a doped system with high d-electron occupancy, rather than low occupancy. Here, we report the realization of the charge-ordered phase in electron-doped (100) SrTiO3 epitaxial thin films that have the lowest d-electron occupancy i.e., d1-d0. Theoretical calculation predicts the presence of a metastable CO state in the bulk state of electron-doped SrTiO3. Atomic scale analysis reveals that (100) surface distortion favors electron-lattice coupling for the charge-ordered state, and triggering the stabilization of the CO phase from a correlated metal state. This stabilization extends up to six unit cells from the top surface to the interior. Our approach offers an insight into the means of stabilizing a new phase of matter, extending CO phase to the lowest electron occupancy and encompassing a wide range of 3d transition metal oxides.

Capping-layer-mediated lattice mismatch and redox reaction in SrTiO3-based bilayers

It is well known that the traditional two-dimensional electron system (2DES) hosted by the SrTiO₃substrate can exhibit diverse electronic states by modifying the capping layer in heterostructures. However, such capping layer engineering is less studied in the SrTiO₃-layer-carried 2DES (or bilayer 2DES), which is different from the traditional one on transport properties but more applicable to the thin-film devices. Here, several SrTiO₃bilayers are fabricated by growing various crystalline and amorphous oxide capping layers on the epitaxial SrTiO₃layers. For the crystalline bilayer 2DES, the monotonical reduction on the interfacial conductance, as well as carrier mobility, is recorded on increasing the lattice mismatch between the capping layers and epitaxial SrTiO₃layer. The mobility edge raised by the interfacial disorders is highlighted in the crystalline bilayer 2DES. On the other hand, when increasing the concentration of Al with high oxygen affinity in the capping layer, the amorphous bilayer 2DES becomes more conductive accompanied by the enhanced carrier mobility but almost constant carrier density. This observation cannot be explained by the simple redox-reaction model, and the interfacial charge screening and band bending need to be considered. Moreover, when the capping oxide layers have the same chemical composition but with different forms, the crystalline 2DES with a large lattice mismatch is more insulating than its amorphous counterpart, and vice versa. Our results shed some light on understanding the different dominant role in forming the bilayer 2DES using crystalline and amorphous oxide capping layer, which may be applicable in designing other functional oxide interfaces.

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.

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.

Physical insights into the Au growth on the surface of a LaAlO3/SrTiO3 heterointerface

Au growth on the LAO/STO substrate generates an optical peak in the wavelength region of 600–1200 nm due to the interaction of the Au s and dz2 orbitals with the O pz orbital of the LAO film.

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.

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.

Mapping spin-charge conversion to the band structure in a topological oxide two-dimensional electron gas

While spintronics has traditionally relied on ferromagnetic metals as spin generators and detectors, spin-orbitronics exploits the efficient spin-charge interconversion enabled by spin-orbit coupling in non-magnetic systems. Although the Rashba picture of split parabolic bands is often used to interpret such experiments, it fails to explain the largest conversion effects and their relationship with the electronic structure. Here, we demonstrate a very large spin-to-charge conversion effect in an interface-engineered, high-carrier-density SrTiO₃ two-dimensional electron gas and map its gate dependence on the band structure. We show that the conversion process is amplified by enhanced Rashba-like splitting due to orbital mixing and in the vicinity of avoided band crossings with topologically non-trivial order. Our results indicate that oxide two-dimensional electron gases are strong candidates for spin-based information readout in new memory and transistor designs. Our results also emphasize the promise of topology as a new ingredient to expand the scope of complex oxides for spintronics.

Cross-Correlation between Strain, Ferroelectricity, and Ferromagnetism in Epitaxial Multiferroic CoFe2O4/BaTiO3 Heterostructures

Multiferroic biphase systems with robust ferromagnetic and ferroelectric response at room temperature would be ideally suitable for voltage-controlled nonvolatile memories. Understanding the role of strain and charges at interfaces is central for an accurate control of the ferroelectricity as well as of the ferromagnetism. In this paper, we probe the relationship between the strain and the ferromagnetic/ferroelectric properties in the layered CoFe₂O₄/BaTiO₃ (CFO/BTO) model system. For this purpose, ultrathin epitaxial bilayers, ranging from highly strained to fully relaxed, were grown by molecular beam epitaxy on Nb:SrTiO₃(001). The lattice characteristics, determined by X-ray diffraction, evidence a non-intuitive cross-correlation: the strain in the bottom BTO layer depends on the thickness of the top CFO layer and vice versa. Plastic deformation participates in the relaxation process through dislocations at both interfaces, revealed by electron microscopy. Importantly, the switching of the BTO ferroelectric polarization, probed by piezoresponse force microscopy, is found dependent on the CFO thickness: the larger is the latter, the easiest is the BTO switching. In the thinnest thickness regime, the tetragonality of BTO and CFO has a strong impact on the 3d electronic levels of the different cations, which were probed by X-ray linear dichroism. The quantitative determination of the nature and repartition of the magnetic ions in CFO, as well as of their magnetic moments, has been carried out by X-ray magnetic circular dichroism, with the support of multiplet calculations. While bulklike ferrimagnetism is found for 5-15 nm thick CFO layers with a magnetization resulting as expected from the Co²⁺ ions alone, important changes occur at the interface with BTO over a thickness of 2-3 nm because of the formation of Fe²⁺ and Co³⁺ ions. This oxidoreduction process at the interface has strong implications concerning the mechanisms of polarity compensation and coupling in multiferroic heterostructures.

A possible superconductor-like state at elevated temperatures near metal electrodes in an LaAlO3/SrTiO3 interface

We experimentally investigated the transport properties near metal electrodes installed on a conducting channel in a LaAlO3/SrTiO3 interface. The local region around the Ti and Al electrodes has a higher electrical conductance than that of other regions, where the upper limits of the temperature and magnetic field can be well defined. Beyond these limits, the conductance abruptly decreases, as in the case of a superconductor. The samples with the Ti- or Al-electrode have an upper-limit temperature of approximately 4 K, which is 10 times higher than the conventional superconducting critical temperature of LaAlO3/SrTiO3 interfaces and delta-doped SrTiO3. This phenomenon is explained by the mechanism of electron transfer between the metal electrodes and electronic d-orbitals in the LaAlO3/SrTiO3 interface. The transferred electrons trigger a phase transition to a superconductor-like state. Our results contribute to the deep understanding of the superconductivity in the LaAlO3/SrTiO3 interface and will be helpful for the development of high-temperature interface superconductors.

A Living-Dead Magnetic Layer at the Surface of Ferrimagnetic DyTiO3 Thin Films

When ferromagnetic films become ultrathin, key properties such as the Curie temperature and the saturation magnetization are usually depressed. This effect is thoroughly investigated in magnetic oxides such as half-metallic manganites, but much less in ferrimagnetic insulating perovskites such as rare-earth titanates RTiO3 , despite their appeal to design correlated 2D electron gases. Here, the magnetic properties of epitaxial DyTiO3 thin films are reported. While films thicker than about 50 nm show a bulk-like response, at low thickness a surprising increase of the saturation magnetization is observed. This behavior is described using a classical model of a "dead layer" but assuming that this layer is actually "living," that is, it responds to the magnetic field with a strong paramagnetic susceptibility. Through depth-dependent X-ray absorption and photoemission spectroscopy, it is shown that the "living-dead layer" corresponds to surface regions where magnetic (S = 1/2) Ti3+ ions are replaced by nonmagnetic Ti4+ ions. Hysteresis cycles at the Dy M 5 and Ti L 3 edges indicate that the surface Ti4+ ions decouple the Dy3+ ions, thus unleashing their strong paramagnetic response. Finally, it is shown how capping the DyTiO3 film can help increase the Ti3+ content near the surface and thus recover a better ferrimagnetic behavior.

Insulator-to-Metal Transition at Oxide Interfaces Induced by WO3 Overlayers

Interfaces between complex oxides constitute a unique playground for two-dimensional electron systems (2DESs), where superconductivity and magnetism can arise from combinations of bulk insulators. The 2DES at the LaAlO₃/SrTiO₃ interface is one of the most studied in this regard, and its origin is determined by the polar field in LaAlO₃ as well as by the presence of point defects, like oxygen vacancies and intermixed cations. These defects usually reside in the conduction channel and are responsible for a decrease of the electronic mobility. In this work, we use an amorphous WO₃ overlayer to obtain a high-mobility 2DES in WO₃/LaAlO₃/SrTiO₃ heterostructures. The studied system shows a sharp insulator-to-metal transition as a function of both LaAlO₃ and WO₃ layer thickness. Low-temperature magnetotransport reveals a strong magnetoresistance reaching 900% at 10 T and 1.5 K, the presence of multiple conduction channels with carrier mobility up to 80 000 cm² V^-1 s^-1, and quantum oscillations of conductance.

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.

Side Gate Tunable Josephson Junctions at the LaAlO3/SrTiO3 Interface

Novel physical phenomena arising at the interface of complex oxide heterostructures offer exciting opportunities for the development of future electronic devices. Using the prototypical LaAlO₃/SrTiO₃ interface as a model system, we employ a single-step lithographic process to realize gate-tunable Josephson junctions through a combination of lateral confinement and local side gating. The action of the side gates is found to be comparable to that of a local back gate, constituting a robust and efficient way to control the properties of the interface at the nanoscale. We demonstrate that the side gates enable reliable tuning of both the normal-state resistance and the critical (Josephson) current of the constrictions. The conductance and Josephson current show mesoscopic fluctuations as a function of the applied side gate voltage, and the analysis of their amplitude enables the extraction of the phase coherence and thermal lengths. Finally, we realize a superconducting quantum interference device in which the critical currents of each of the constriction-type Josephson junctions can be controlled independently via the side gates.

Highly efficient and tunable spin-to-charge conversion through Rashba coupling at oxide interfaces

The spin-orbit interaction couples the electrons' motion to their spin. As a result, a charge current running through a material with strong spin-orbit coupling generates a transverse spin current (spin Hall effect, SHE) and vice versa (inverse spin Hall effect, ISHE). The emergence of SHE and ISHE as charge-to-spin interconversion mechanisms offers a variety of novel spintronic functionalities and devices, some of which do not require any ferromagnetic material. However, the interconversion efficiency of SHE and ISHE (spin Hall angle) is a bulk property that rarely exceeds ten percent, and does not take advantage of interfacial and low-dimensional effects otherwise ubiquitous in spintronic hetero- and mesostructures. Here, we make use of an interface-driven spin-orbit coupling mechanism-the Rashba effect-in the oxide two-dimensional electron system (2DES) LaAlO₃/SrTiO₃ to achieve spin-to-charge conversion with unprecedented efficiency. Through spin pumping, we inject a spin current from a NiFe film into the oxide 2DES and detect the resulting charge current, which can be strongly modulated by a gate voltage. We discuss the amplitude of the effect and its gate dependence on the basis of the electronic structure of the 2DES and highlight the importance of a long scattering time to achieve efficient spin-to-charge interconversion.

Band engineering in a van der Waals heterostructure using a 2D polar material and a capping layer

Van der Waals (vdW) heterostructures are expected to play a key role in next-generation electronic and optoelectronic devices. In this study, the band alignment of a vdW heterostructure with 2D polar materials was studied using first-principles calculations. As a model case study, single-sided fluorographene (a 2D polar material) on insulating (h-BN) and metallic (graphite) substrates was investigated to understand the band alignment behavior of polar materials. Single-sided fluorographene was found to have a potential difference along the out-of-plane direction. This potential difference provided as built-in potential at the interface, which shift the band alignment between h-BN and graphite. The interface characteristics were highly dependent on the interface terminations because of this built-in potential. Interestingly, this band alignment can be modified with a capping layer of graphene or BN because the capping layer triggered electronic reconstruction near the interface. This is because the bonding nature is not covalent, but van der Waals, which made it possible to avoid Fermi-level pinning at the interface. The results of this study showed that diverse types of band alignment can be achieved using polar materials and an appropriate capping layer.

Hybridization-controlled charge transfer and induced magnetism at correlated oxide interfaces

At interfaces between conventional materials, band bending and alignment are classically controlled by differences in electrochemical potential. Applying this concept to oxides in which interfaces can be polar and cations may adopt a mixed valence has led to the discovery of novel two-dimensional states between simple band insulators such as LaAlO3 and SrTiO3. However, many oxides have a more complex electronic structure, with charge, orbital and/or spin orders arising from strong Coulomb interactions between transition metal and oxygen ions. Such electronic correlations offer a rich playground to engineer functional interfaces but their compatibility with the classical band alignment picture remains an open question. Here we show that beyond differences in electron affinities and polar effects, a key parameter determining charge transfer at correlated oxide interfaces is the energy required to alter the covalence of the metal-oxygen bond. Using the perovskite nickelate (RNiO3) family as a template, we probe charge reconstruction at interfaces with gadolinium titanate GdTiO3. X-ray absorption spectroscopy shows that the charge transfer is thwarted by hybridization effects tuned by the rare-earth (R) size. Charge transfer results in an induced ferromagnetic-like state in the nickelate, exemplifying the potential of correlated interfaces to design novel phases. Further, our work clarifies strategies to engineer two-dimensional systems through the control of both doping and covalence.

Tailoring LaAlO3/SrTiO3 Interface Metallicity by Oxygen Surface Adsorbates

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

Universal Fabrication of 2D Electron Systems in Functional Oxides

2D electron systems (2DESs) in functional oxides are promising for applications, but their fabrication and use, essentially limited to SrTiO3 -based heterostructures, are hampered by the need for growing complex oxide overlayers thicker than 2 nm using evolved techniques. It is demonstrated that thermal deposition of a monolayer of an elementary reducing agent suffices to create 2DESs in numerous oxides.

Polarization effects on the interfacial conductivity in LaAlO3/SrTiO3heterostructures: a first-principles study

First-principles calculations predict the normalized electron carrier density (μ/μ₀), the mobility (m*/m₀), and the conductivity (σ/σ₀) in LaAlO₃/SrTiO₃as a function of uniaxial strain.

Mitochondrial targeted β-lapachone induces mitochondrial dysfunction and catastrophic vacuolization in cancer cells

Mitochondria play important roles in tumor cell physiology and survival by providing energy and metabolites for proliferation and metastasis. As part of their oncogenic status, cancer cells frequently produce increased levels of mitochondrial-generated reactive oxygen species (ROS). However, extensive stimulation of ROS generation in mitochondria has been shown to be able to induce cancer cell death, and is one of the major mechanisms of action of many anticancer agents. We hypothesized that enhancing mitochondrial ROS generation through direct targeting of a ROS generator into mitochondria will exhibit tumor cell selectivity, as well as high efficacy in inducing cancer cell death. We thus synthesized a mitochondrial targeted version of β-lapachone (XJB-Lapachone) based on our XJB mitochondrial targeting platform. We found that the mitochondrial targeted β-lapachone is more efficient in inducing apoptosis compared to unconjugated β-lapachone, and the tumor cell selectivity is maintained. XJB-Lapachone also induced extensive cellular vacuolization and autophagy at a concentration not observed with unconjugated β-lapachone. Through characterization of mitochondrial function we revealed that XJB-Lapachone is indeed more capable of stimulating ROS generation in mitochondria, which led to a dramatic mitochondrial uncoupling and autophagic degradation of mitochondria. Taken together, we have demonstrated that targeting β-lapachone accomplishes higher efficacy through inducing ROS generation directly in mitochondria, resulting in extensive mitochondrial and cellular damage. XJB-Lapachone will thus help to establish a novel platform for the design of next generation mitochondrial targeted ROS generators for cancer therapy.

Field-effect control of superconductivity and Rashba spin-orbit coupling in top-gated LaAlO3/SrTiO3 devices

The recent development in the fabrication of artificial oxide heterostructures opens new avenues in the field of quantum materials by enabling the manipulation of the charge, spin and orbital degrees of freedom. In this context, the discovery of two-dimensional electron gases (2-DEGs) at LaAlO₃/SrTiO₃ interfaces, which exhibit both superconductivity and strong Rashba spin-orbit coupling (SOC), represents a major breakthrough. Here, we report on the realisation of a field-effect LaAlO₃/SrTiO₃ device, whose physical properties, including superconductivity and SOC, can be tuned over a wide range by a top-gate voltage. We derive a phase diagram, which emphasises a field-effect-induced superconductor-to-insulator quantum phase transition. Magneto-transport measurements show that the Rashba coupling constant increases linearly with the interfacial electric field. Our results pave the way for the realisation of mesoscopic devices, where these two properties can be manipulated on a local scale by means of top-gates.

Modulated two-dimensional charge-carrier density in LaTiO3-layer-doped LaAlO3/SrTiO3 heterostructure

The highly mobile two-dimensional electron gas (2DEG) formed at the polar/nonpolar LaAlO3/SrTiO3 (LAO/STO) heterostructure (HS) is a matter of great interest because of its potential applications in nanoscale solid-state devices. To realize practical implementation of the 2DEG in device design, desired physical properties such as tuned charge carrier density and mobility are necessary. In this regard, polar perovskite-based transition metal oxides can act as doping layers at the interface and are expected to tune the electronic properties of 2DEG of STO-based HS systems dramatically. Herein, we investigated the doping effects of LaTiO3(LTO) layers on the electronic properties of 2DEG at n-type (LaO)(+1)/(TiO2)(0) interface in the LAO/STO HS using spin-polarized density functional theory calculations. Our results indicate an enhancement of orbital occupation near the Fermi energy, which increases with respect to the number of LTO unit cells, resulting in a higher charge carrier density of 2DEG than that of undoped system. The enhanced charge carrier density is attributed to an extra electron introduced by the Ti 3d(1) orbitals from the LTO dopant unit cells. This conclusion is consistent with the recent experimental findings (Appl. Phys. Lett. 2013, 102, 091601). Detailed charge density and partial density of states analysis suggests that the 2DEG in the LTO-doped HS systems primarily comes from partially occupied dyz and dxz orbitals.

First-principles characterization of the critical thickness for forming metallic states in strained LaAlO₃/SrTiO₃(001) heterostructure

The emerging two-dimensional electron gas (2DEG) at the interface between polar LaAlO3 (LAO) and nonpolar SrTiO3 (STO) provides potential applications in low-dimensional nanoelectronic devices because of its exceptional electron transport property. To form 2DEG in the LAO/STO heterostructure (HS), a minimum thickness of approximately 4 unit cells of LAO is necessary. Herein, we modeled the n-type (TiO2)(0)/(LaO)(+1) HS by depositing (LAO)n (n = 4, 5, and 6) thin films on the STO substrate and explored strain effects on the critical thickness for forming 2DEG in the LAO/STO HS-based slab systems using first-principles electronic structure calculations. A vacuum layer was added along the [001] direction on the LAO film to resemble the actual epitaxial growth process of the materials. An insulator-to-metal transition is predicted in unstrained (LAO)n/STO systems when n ≥ 5. Our calculations indicate that O 2px/py states give rise to the surface conductivity, while Ti 3dxy states are responsible for the interfacial conductivity. For the tensilely strained HS system, an increased film thickness of LAO (n ≥ 6) is required to form the 2DEG, and a remarkable shift of O 2p orbitals toward higher energy in LAO layers is found, which is caused by the strain-induced change of the electrostatic potential. For the compressively strained HS system, the critical thickness of LAO film for forming 2DEG is between 5 and 6 unit cells of LAO. In addition, our calculations suggest that the interfacial charge carrier density and magnetic moment can be optimized when a moderate tensile strain is applied on the STO substrate in the ab-plane.