Symmetry breaking at the (111) interfaces of SrTiO3 hosting a two-dimensional electron system (original) (raw)
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Physical Review Letters, 2014
Recent findings show the emergence of two-dimensional electron gases (2DEGs) at LaAlO 3 =SrTiO 3 interfaces along different orientations; yet details on band reconstructions have remained so far unknown. Via x-ray linear dichroism spectroscopy, we demonstrate that crystal symmetry imposes distinctive 2DEG orbital hierarchies on (001)-and (110)-oriented quantum wells, allowing selective occupancy of states of different symmetry. Such orientational tuning expands the possibilities for electronic engineering of 2DEGs and opens up enticing opportunities to understand the link between orbital symmetry and complex correlated states at LaAlO 3 =SrTiO 3 quantum wells.
Applied Physics Letters, 2011
In the artificially tailored heterostructures of certain materials, a polar discontinuity across the interface introduces a large energy cost. The total energy of such systems may be reduced either by electronic reconstruction leading to the interface phases or by simple atomic reconstruction by inter-site cation mixing. While most of the experiments and theoretical calculations assume an abrupt interface, in this work, we consider the La/Sr inter-site disorder across the interface of SrTiO3/LaTiO3 heterostructures and study its energetics and electronic structure properties. The calculations find that inter-site mixing of La/Sr atoms across the interface also reduces the total energy. However, the extent of such disorder is found to be dramatically minimized by allowing the interfacial atoms to relax fully and that for such systems, the changes in the electronic structure are negligible.
Physical Review B, 2013
A combined experimental and theoretical investigation of the electronic structure of the archetypal oxide heterointerface system LaAlO3 on SrTiO3 is presented. High-resolution, hard x-ray photoemission is used to uncover the occupation of Ti 3d states and the relative energetic alignment -and hence internal electric fields -within the LaAlO3 layer. Firstly, the Ti 2p core level spectra clearly show occupation of Ti 3d states already for two unit cells of LaAlO3. Secondly, the LaAlO3 core levels were seen to shift to lower binding energy as the LaAlO3 overlayer thickness, n, was increased -agreeing with the expectations from the canonical electron transfer model for the emergence of conductivity at the interface. However, not only is the energy offset of only ∼300meV between n = 2 (insulating interface) and n = 6 (metallic interface) an order of magnitude smaller than the simple expectation, but it is also clearly not the sum of a series of unit-cell by unit-cell shifts within the LaAlO3 block. Both of these facts argue against the simple charge-transfer picture involving a cumulative shift of the LaAlO3 valence bands above the SrTiO3 conduction bands, resulting in charge transfer only for n ≥ 4. We discuss effects which could frustrate this elegant and simple charge transfer model, concluding that although it cannot be ruled out, photodoping by the x-ray beam is unlikely to be the cause of the observed behavior. Turning to the theoretical data, our density functional simulations show that the presence of oxygen vacancies at the LaAlO3 surface at the 25% level reverses the direction of the internal field in the LaAlO3. Therefore, taking the experimental and theoretical results together, a consistent picture emerges for real-life samples in which nature does not wait until n = 4 and already for n = 2, mechanisms other than internal-electric-field-driven electron transfer from idealized LaAlO3 to near-interfacial states in the SrTiO3 substrate are active in heading off the incipient polarization catastrophe that drives the physics in these systems.
Artificial quantum confinement in LaAlO3/SrTiO3 heterostructures
Physical Review Materials
Heterostructures of transition metal oxides (TMO) perovskites represent an ideal platform to explore exotic phenomena involving the complex interplay between the spin, charge, orbital and lattice degrees of freedom available in these compounds. At the interface between such materials, this interplay can lead to phenomena that are present in none of the original constituents such as the formation of the interfacial 2D electron system (2DES) discovered at the LAO 3 /STO 3 (LAO/STO) interface. In samples prepared by growing a LAO layer onto a STO substrate, the 2DES is confined in a band bending potential well, whose width is set by the interface charge density and the STO dielectric properties, and determines the electronic band structure. Growing LAO (2 nm) /STO (x nm)/LAO (2 nm) heterostructures on STO substrates allows us to control the extension of the confining potential of the top 2DES via the thickness of the STO layer. In such samples, we explore the dependence of the electronic structure on the width of the confining potential using soft X-ray ARPES combined with ab-initio calculations. The results indicate that varying the thickness of the STO film modifies the quantization of the 3d t 2g bands and, interestingly, redistributes the charge between the d xy and d xz /d yz bands. Advances in thin-film deposition techniques have led to the discovery of a variety of phenomena in artificial heterostructures ranging from the quantum Hall effect to topological
Origin of metallicity of LaTiO3/SrTiO3 heterostructures
Physical Review B, 2008
It is shown that LaTiO3 in superlattices with SrTiO3 is not a Mott insulator but a strongly correlated metal. The tetragonal lattice geometry imposed by the SrTiO3 substrate leads to an increase of the Ti 3d t2g band width and a reversal of the t2g crystal field relative to the orthorhombic bulk geometry. Using dynamical mean field theory based on finite-temperature multi-band exact diagonalization we show that, as a result of these effects, local Coulomb interactions are not strong enough to induce a Mott transition in tetragonal LaTiO3. The metalicity of these heterostructures is therefore not an interface property but stems from all LaTiO3 planes. 71.27.+a., There exists currently considerable interest in the design of nano-materials with electronic properties that differ qualitatively from those of the constituent components in their bulk form. A particularly intriguing example is the formation of a thin metallic layer at the interface between two insulators. For instance, SrTiO 3 is a band insulator with an empty d band, whereas LaTiO 3 , with one d electron per site, is regarded as a textbook Mott insulator because of strong local Coulomb interactions [1]. Nevertheless, the pioneering work by Ohtomo et al. shows that LaTiO 3 /SrTiO 3 superlattices are metallic, where the overall conductivity depends on the thickness of the LaTiO 3 interlayers and on the spacing between them. This phenomenon appears to follow from the fact that the Ti interface layer between adjacent La and Sr planes formally exhibits a 3d 0.5 valency, in contrast to the 3d 1 and 3d 0 configurations of bulk LaTiO 3 and SrTiO 3 , respectively. Other examples are LaAlO 3 /SrTiO 3 heterostructures where a conducting interface was observed although both constituents are wide band gap perovskite insulators . Very recently, various other perovskite superlattices have been investigated experimentally .
Anisotropic two-dimensional electron gas at the LaAlO3/SrTiO3 (110) interface
Nature Communications, 2013
The observation of a high-mobility two-dimensional electron gas between two insulating complex oxides, especially LaAlO 3 /SrTiO 3 , has enhanced the potential of oxides for electronics. The occurrence of this conductivity is believed to be driven by polarization discontinuity, leading to an electronic reconstruction. In this scenario, the crystal orientation has an important role and no conductivity would be expected, for example, for the interface between LaAlO 3 and (110)-oriented SrTiO 3 , which should not have a polarization discontinuity. Here we report the observation of unexpected conductivity at the LaAlO 3 / SrTiO 3 interface prepared on (110)-oriented SrTiO 3 , with a LaAlO 3 -layer thickness-dependent metal-insulator transition. Density functional theory calculation reveals that electronic reconstruction, and thus conductivity, is still possible at this (110) interface by considering the energetically favourable (110) interface structure, that is, buckled TiO 2 /LaO, in which the polarization discontinuity is still present. The conductivity was further found to be strongly anisotropic along the different crystallographic directions with potential for anisotropic superconductivity and magnetism, leading to possible new physics and applications.
ACS Applied Materials & Interfaces, 2011
Transition metal oxides exhibit a huge variety of intrinsic functionalities such as magnetism, superconductivity, thermoelectricity, ferroelectricity or multiferroic behavior. 1À26 Experimentally, there is also growing interest in the creation of "oxide heterostructures" consisting of alternating layers of different transitionÀmetal oxides. 2,3 OxideÀoxide interfaces often give rise to novel physical phenomena not exhibited in the bulk constituents, due to the rearrangement of charge, spins, orbitals, lattice and the resulting rebalancing of their mutual interactions. Thus interface can be exploited to modify electronic structure and to tailor novel properties and behaviors.
Intrinsic origin of two-dimensional electron gas at the (001) surface of SrTiO 3
Physical Review B, 2015
The predictions of the polar catastrophe scenario to explain the occurrence of a metallic interface in heterostructures of the solid solution(LaAlO3)x(SrTiO3)1−x (LASTO:x) grown on (001) SrTiO3 were investigated as a function of film thickness and x. The films are insulating for the thinnest layers, but above a critical thickness, tc, the interface exhibits a constant finite conductivity which depends in a predictable manner on x. It is shown that tc scales with the strength of the built-in electric field of the polar material, and is immediately understandable in terms of an electronic reconstruction at the nonpolar-polar interface. These results thus conclusively identify the polarcatastrophe model as the intrinsic origin of the doping at this polar oxide interface. arXiv:1112.3532v1 [cond-mat.mtrl-sci]