Session A37a: Complex Oxide Interfaces and Heterostructures - LaAlO3/SrTiO3

Transport anisotropy controlled by oxygen vacancy concentration in (111) LaAlO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ heterostructures

The 2-dimensional electron gas that forms at the interface between the band insulators LaAlO3 and SrTiO3 (LAO/STO) has proven to be a rich playground for studying tunable, strongly correlated behavior, with the majority of previous studies focusing on the (001) orientation of the heterostructure. In this work we focus on the (111) orientation of the LAO/STO heterostructure, which has only recently been shown to be conducting. Previously we have shown that the (111) 2DEG exhibits anisotropy along two, orthogonal, crystal directions, and that the anisotropy is strongly tuned via an electrostatic back-gate potential.1 In this work, we show that the anisotropy in the electronic properties at the (111) LaAlO3 and SrTiO3 interface can be tuned by post-growth treatments, such as Oxygen annealing, Ar/H2 annealing, and UV exposure. Specifically, this treatment strongly affects the dependence of the device resistance, carrier concentration, and directional anisotropy on electrostatic back-gate voltage. We attribute these effects to changes in the oxygen vacancy concentration at the interface. 1) S. Davis, V. Chandrasekhar, Z. Huang, K. Han, Ariando, T. Venkatesan, Arxiv, 2015 1603.04538.   

Anisotropic magnetoresistance in LaAlO$_3$/SrTiO$_3$ (111) interface

The oxide interfaces can be tailored by controlling the crystal symmetry, which can be achieved by changing the crystallographic orientation of the interface. For example, the LaAlO$_3$/SrTiO$_3$ (111) interface has a hexagonal symmetry in contrast to fourfold symmetry of (100) and (110) interfaces. Here, we present the in-plane anisotropic magnetoresistance (AMR) measurements of LaAlO$_3$/SrTiO$_3$ (111) interface. Our AMR data indicates the presence of crystalline six-fold anisotropic component due to the symmetry of the interface. Moreover, we observe an additional uniaxial AMR contribution, which presumably originates from the domain boundaries present in SrTiO$_3$ substrate.   

Quantum transport through mesoscopic LaAlO3/SrTiO3 devices fabricated by a new epitaxial lift off process

Recent studies of mesoscopic devices based on the two-dimensional electron gas present at the LaAlO3/SrTiO3 interface have revealed rich quantum phenomena, such as superconducting quantum interference, electron interference, and universal conductance fluctuations. The edges of such devices are usually defined by an amorphous AlO3 (or LaAlO3) hard mask prior to the epitaxial LaAlO3 growth step. We present here a new high resolution ( under 100 nm) fabrication method, allowing us to create mesoscopic devices by ``lifting off'' the LaAlO3 layers from desired regions, leaving behind atomically flat surfaces. This method yields devices with clean boundaries between the conducting (4 or more unit cells of LaAlO3) and insulating (2 unit cells of LaAlO3) regions, free from any charged traps that may reside in amorphous materials. By introducing an additional lift off step we are able to create lateral junctions between the LaAlO3/SrTiO3 and other materials (specifically Metal/ LaAlO3/SrTiO3). We will present this method and our investigation of quantum transport through several devices, among them cavities exhibiting long electron coherence length.   

Carrier Density at LaAlO3/SrTiO3 Interfaces: Evidence of Electronic Reconstruction.

The origin of the 2D electron gas at the LaAlO3/SrTiO3 interface has been a controversial subject ever since its discovery. A serious inconsistency with the most accepted mechanism, an electronic reconstruction in response to a polar discontinuity at the interface, is that the carrier densities reported experimentally are invariably lower than the expected value except under conditions where reduction of SrTiO3 substrate is suspected. We have grown LaAlO3 films of different stoichiometry on TiO2-terminated SrTiO3 substrates using atomic layer-by-layer laser molecular beam epitaxy (ALL-Laser MBE), in which La2O3 and Al2O3 targets were sequentially ablated in 37 mTorr oxygen. The high oxygen pressure during growth prevents the possible oxygen reduction in SrTiO3, ensures that the LaAlO3 films are sufficiently oxygenated, and suppresses the La-Sr intermixing due to the bombardment effect. X-ray linear dichroism (XLD) and x-ray magnetic circular dichroism (XMCD) measurements show characteristics of oxygenated samples. In the electronic reconstruction picture, instead of the charge transfer of half of an electron in the case of a sufficiently thick stoichiometric LaAlO3, a LaAlO3 film thickness dependence is expected as well as a linear dependence on stoichiometry. Our experimental results on carrier densities in 10 nm-thick LaAl1$+$yO3(1$+$0.5y) films agree quantitatively with the theoretical expectations, lending a strong support for the electronic reconstruction mechanism.   

The MBE growth of arbitrarily thick SrTiO$_{\mathrm{3}}$/LaAlO$_{\mathrm{3}}$ quantum well heterostructures

The dozens of novel electronic, magnetic, and optical phenomena discovered in oxide thin film systems in recent years suggest that these systems could eventually be used to fabricate devices that extend the functionality of traditional III-V semiconductor-based devices. Before such devices can become a reality, however, it is necessary to demonstrate the ability to fabricate high-quality oxide thin film heterostructures of arbitrary thickness. Here, we report on the structural quality of MBE-grown SrTiO$_{\mathrm{3}}$/LaAlO$_{\mathrm{3}}$ quantum well heterostructures at thicknesses approaching those that would be required for practical device fabrication. Through \textit{in situ} RHEED and \textit{ex situ} XRD and STEM measurements, we demonstrate the quality of our heterostructures does not depend on total sample thickness, indicating that it should be possible to fabricate practical devices from the STO/LAO system. Furthermore, we demonstrate an improvement in interfacial quality over previously-reported PLD-grown samples via a statistical analysis of structural variations at the STO/LAO interfaces. Notably, the well-established asymmetry between the $n$- and $p$-type STO/LAO interfaces is not present in our samples.   

Dual critical thickness for lattice and electronic instabilities in the LaAlO$_{3}$/SrTiO$_{3}$ interface.

When matter is strongly downscaled, entirely new properties can emerge that are not present in the parent bulk crystals. A paradigmatic example is the quantum well formed at the interface between SrTiO$_{3}$ (STO) and LaAlO$_{3}$ (LAO). Their charge mismatch leads to a built-in electric field inside the LAO film, which eventually induces the formation of a two-dimensional electron system at the boundary. In addition to this electronic reconstruction, here we show that the LAO/STO system also undergoes a ``phononic reconstruction'' strongly coupled to the inbuilt electric fields. More specifically, we have discovered an unexpected mechanism whereby nonpolar antiferrodistortive rotations of AlO$_{6}$ octahedra couple to the internal electrostatic fields. Two basic facts can explain such an unanticipated observation: (i) the extreme confinement of LAO vibration modes, which leads to a ``phonon reconstruction'' that has been so far overlooked; (ii) the emergence and subsequent decline of internal electric fields, resulting from the polar/nonpolar character of this interface. Such internal fields modulate the intensity of the antiferrodistortive modes, providing a new pathway to tailor functional properties that can be extrapolated to other oxide interfaces, beyond the case studied here.   

Depth dependence of charge density and orbital symmetry at the LaAlO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ interface from resonant x-ray reflectometry

The interface between the insulators LaAlO$_{\mathrm{3}}$ and SrTiO$_{\mathrm{3}}$ is known to accommodate a two-dimensional electron liquid (2DEL)---a high mobility electron system which exhibits superconductivity, magnetism, and correlations. While this flagship oxide heterostructure shows promise for electronics applications, questions remain regarding the origin and microscopic properties of the 2DEL, largely because the electronic structures of such buried interfaces are inherently difficult to probe. Here we utilize resonant x-ray reflectivity to perform a comprehensive examination of the 2DEL. We discover a high charge density near 0.5 electrons per interfacial unit cell for samples above the critical LaAlO$_{\mathrm{3\thinspace }}$thickness, a quantity predicted by theory but previously elusive in experiments, and extract the spatial profile of both the orbital and electronic reconstructions. We additionally study effects of post-annealing treatments in oxygen environments on the electronic and atomic structures near the interface. Our results provide insights into this often controversial interface, and show resonant x-ray reflectometry to be an ideal tool for the study of oxide heterostructure phenomena.   

Nanomechanical Probes of Sketched LaAlO$_{3}$/SrTiO$_{3}$ Single-Electron Transistors

Nanoscale devices that manipulate single electrons present an exciting platform for the observation of electronic and mechanical effects. By utilizing the locally tunable metal-insulator transition at the interface of LaAlO$_{3}$/SrTiO$_{3}$, we can create single-electron transistors using conducting atomic force microscope (c-AFM) lithography. The piezoelectric nature of LaAlO$_{3}$/SrTiO$_{3}$ gives way to an expected coupling between mechanical motion and electric charge within the device. We can test this effect by applying pressure to the device using an insulating AFM tip while measuring changes in electron density. A cryogenic AFM system is used to examine these effects, as many of the most interesting properties of these devices are only observed at low temperatures.   

Controlling the dual mechanisms of oxide interface doping

The formation of two dimensional electron gas (2DEG) at LaAlO3/SrTiO3 interfaces involves multiple electronic and structural causes. The interplay between them makes the investigation of individual mechanism very challenging. Here we demonstrate the nanoscale selective control of two interface doping pathways: charge transfers from surface adsorbed protons and oxygen vacancies created in LaAlO3 layers. The selective control is achieved by combining intensive electric field generated by conducting AFM probe which controls both the creation/migration of oxygen vacancies and the surface proton density, with plasma assisted surface hydroxylation and solvent based proton solvation that act mainly on surface adsorbates. Robust nanoscale reversible metal-insulator transition was achieved at the interfaces with the LaAlO3 layer thicker than the critic thickness. Different combinations of the experimental methods and doping mechanisms enable highly flexible tuning of the 2DEG's carrier density, mobility and sensitivity to ambient environments. The reversible and independent controls of surface states and vacancies add to the fundamental material research capabilities and can benefit future exploration of designed 2DEG nanoelectronics.   

Tunable Magneto-electric Subbands in Oxide Electron Waveguides

Strontium titanate-based complex-oxide interfaces hold great promise for exploring new correlated electron physics and applications in quantum technologies. Previous reports show electron mobility can be greatly enhanced in 1D \footnote {Nano. Lett. \textbf{13}, 364 (2013), Phys. Rev. Lett.\textbf{17}, 096801 (2016)}, while the 2D interface can contain 1D channels due to the presence of ferroelastic domains. In addition, carrier density measurements at the 2D interface by Shubnikov-de Haas (SdH) oscillations and Hall effect reveal a large discrepancy. Here we fabricate quasi-1D electron waveguides at the LaAlO$_3$/SrTiO$_3$ (LAO/STO) interface to locally probe the interface. The conductance of the waveguides is fully quantized, and the corresponding magneto-electric subbands can be depopulated by increasing the magnetic field. The 2D carrier densities (10$^{12}$ cm$^{-2}$) extracted from magnetic depopulation are consistent with measurements by SdH oscillations at the 2D interface. Our results show that magneto-electric subbands of quasi-1D electron waveguides can reproduce known SdH signatures without discrepancies in electron density, and suggest that 2D SdH measurements may also arise from quasi-1D channels.   

Pascal Liquid Phase in Electronic Waveguides

Clean one-dimensional electron transport has been observed in very few material systems. The development of exceptionally clean electron waveguides formed at the interface between complex oxides LaAlO$_3$ and SrTiO$_3$ enables low-dimensional transport to be explored with newfound flexibility. This material system not only supports ballistic 1D transport\footnote{M. Tomczyk \textit{et al.}, PRL \textbf{117}, 096801 (2016)}, but possesses a rich phase diagram and strong attractive electron-electron interactions\footnote{G. Cheng \textit{et al.}, Nature \textbf{521}, 196 (2015)} which are not present in other solid-state systems. Here we report an unusual phenomenon in which quantized conductance increases by steps that themselves increase sequentially in multiples of $e^2/h$. The overall conductance exhibits a Pascal-like sequence: 1, 3, 6, 10... $e^2/h$, which we ascribe to ballistic transport of 1, 2, 3, 4 ... “bunches” of electrons. We will discuss how subband degeneracies can occur in non-interacting models that have carefully tuned parameters. Strong attractive interactions are required, however, for these subbands to “lock” together. This “Pascal liquid” phase provides a striking example of the consequences of strong attractive interactions in low-dimensional environments.   

$LaAlO_3/SrTiO_3$--A Tale of Two Magnetisms

Ten years since the first report of magnetism by Brinkman et al.\footnote{A. Brinkman \textit{et al.}, Nat. Mater. \textbf{6}, 493 (2007)}, a unified picture of magnetism at the two-dimensional electron system (2DES) between $LaAlO_3/SrTiO_3$ is still lacking. The understanding is further hindered by the complex interplay of magnetism and many other aspects of this system: multi-band superconductivity, quantum paraelectricity, multiferroicity, to name but a few. We argue that the reported magnetic signatures in this system can come from two principal origins\footnote{Y. Pai \textit{et al.}, arXiv:1610.00789)}: (1) a ferromagnetic long-range order resulting from local magnetic moments mediated by itinerant electrons, and (2) metamagnetic phenomena associated with electron pairing without superconductivity \footnote{G. Cheng \textit{et al.}, Nature \textbf{521}, 196 (2015)} . Finally, we discuss possible experimental tests of this framework.   

Probing 1D superlattices at the $LaAlO_3/SrTiO_3$ interface

Complex oxides and other quantum systems exhibit behavior that is currently too complex to be understood using analytic or computational methods. One approach is to use a configurable quantum system whose Hamiltonian can be mapped onto the system of interest. This approach, known as quantum simulation, requires a rich physical system whose quanta and interactions can be controlled precisely, at the level of single electrons and other degrees of freedom. Here we describe steps toward developing a quantum simulation platform, using the complex oxide heterostructure $LaAlO_3/SrTiO_3$, by creating quantum systems with features comparable to the mean spacing between electrons\footnote{C. Cen \textit{et al.}, Nat. Mater. \textbf{7}, 298 (2008)}. This interface has strong, sign changing, gate-tunable electron-electron interactions \footnote{G. Cheng \textit{et al.}, arXiv:1602.06029 (2016)} that can strongly influence the quantum ground state. We explore the magnetotransport properties of 1D superlattices, where periodic modulation produces reproducible dispersive features not seen in control structures. The results of these experiments can be compared with effective 1D model Hamiltonians to bridge experiment and theory and enable quantum simulation of more complex systems.