Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session K46: Complex Oxide Interfaces & Heterostructures -- Interfacial two dimensional electron gasFocus
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Sponsoring Units: DMP Chair: Hanghui Chen, New York University Room: BCEC 212 |
Wednesday, March 6, 2019 8:00AM - 8:36AM |
K46.00001: Two dimensional t2g electron gas at the oxide interface Invited Speaker: Alexander Demkov The discovered revolutionary class of polar oxide heterostructures holds tremendous promise for exploiting the physical properties of the novel 2DEG formed at the oxide/oxide interface. SrTiO3 is a widely used substrate for the growth of other functional oxide thin films. The reactivity of the substrate with respect to the film during deposition, particularly with regard to redox reactions, has typically been glossed over. We have recently demonstrated by depositing a variety of metals and measuring the in situ core level spectra of both the metal and SrTiO3 that, depending on the oxide formation energy and work function of the metal, there will be an interfacial layer of oxygen-deficient SrTiO3 at the interface with the top oxide film. I will focus on the integrated highly spin-split ferromagnetic semiconductor EuO onto perovskite SrTiO3 (001). A careful deposition of Eu metal by molecular beam epitaxy resulted in EuO growth via oxygen out-diffusion from SrTiO3. This in turn leaves behind a highly conductive interfacial layer through generation of oxygen vacancies. Below the Curie temperature of 70 K of EuO, this spin-polarized two-dimensional t2g electron gas at the EuO/SrTiO3 interface displays very large positive linear magnetoresistance. |
Wednesday, March 6, 2019 8:36AM - 8:48AM |
K46.00002: Engineering BaTiO3 Based Ferroelectric Materials with Reactive Molecular Dynamics Simulations Dundar Yilmaz, Dooman Akbarian, Panchapakesan Ganesh, Adri C Van Duin
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Wednesday, March 6, 2019 8:48AM - 9:00AM |
K46.00003: Engineered Helicity of One-Dimensional LaAlO3/SrTiO3 Nanowires Megan Briggeman, Jianan Li, Mengchen Huang, Anthony Tylan-Tyler, Hyungwoo Lee, Jungwoo Lee, Chang-Beom Eom, Patrick Irvin, Jeremy Levy Quantum transport in 1D geometries is fascinating in its own right, but it can also be regarded as a building-block for a variety of quantum devices. We have developed a flexible platform for creating 1D nanostructures at the LaAlO3/SrTiO3 interface using a conductive-AFM lithography technique [1]. Straight nanowire segments behave as electron waveguides with subband occupation that can be tuned with a gate and an external magnetic field [2]. We can periodically perturb this waveguide, with 10 nm periodicity, in two ways. “Kronig-Penney” type modulation results in periodic vertical displacement of the electron waveguide, and sinusoidal lateral displacement of the nanowire can also be achieved. Combining the two perturbations in quadrature yields a helical nanowire which exhibits striking oscillatory transmission as a function of both magnetic field and chemical potential. We discuss these results in terms of an engineered axial in-plane spin-orbit interaction within the spiral electron waveguide. |
Wednesday, March 6, 2019 9:00AM - 9:12AM |
K46.00004: Determination of Spin-Orbit Scattering Lifetime at the Interface of LaAlO3/SrTiO3 from the Superconducting Upper Critical Fields Wei-Li Lee, Akhilesh Singh, Tsung-Chi Wu, Ming-Yuan Song, Ming-Chin Chen, Chi-Sheng Li The intrinsic mechanism of the spin-orbit coupling at the LaAlO3 / SrTiO3 interface remains a debatable issue. Rashba-type spin-orbit coupling is an appealing candidate that has been demonstrated by several magnetotransport results. On the other hand, the atomic spinorbit coupling was also shown to play an important role, particularly when the Fermi level is close to the Liftshitz point. Unlike previous works, we focus on the measurements of the anisotropic and superconducting upper critical fields in gated LaAlO3 / SrTiO3 devices. By rigorous fittings of the Hc2-T curves using both the Werthamer-Helfand-Hohenberg theory |
Wednesday, March 6, 2019 9:12AM - 9:24AM |
K46.00005: Nanoscale Inhomogeneous Energy Landscape in LaAlO3/SrTiO3 Heterostructures Aditi Nethwewala, Megan Briggeman, Jianan Li, Yuhe Tang, Hyungwoo Lee, Jungwoo Lee, Chang-Beom Eom, Patrick Irvin, Jeremy Levy The energy landscape of LaAlO3/SrTiO3 (LAO/STO) heterostructures plays an important role in determining the transport phenomena. An inhomogeneous energy landscape can lead to the breakdown of 2D transport and the emergence of 1D transport. Thus, knowledge of the energy landscape of the system can play a significant role in understanding correlated electron systems. Here we report a minimally invasive probing technique to map the energy landscape of LAO/STO heterostructures. Using conductive atomic force microscope (c-AFM) lithography [1], we write nanoscale cross-shaped electron waveguides, or “nanocrosses”. Low temperature magnetotransport measurements reveal signs of spatial inhomogeneity. Anisotropic magnetoconductance and anomalous Hall resistance also indicate inhomogeneity. The nanocross geometry provides a powerful tool for exploring the energy landscape of the system and its implications under one frame. |
Wednesday, March 6, 2019 9:24AM - 9:36AM |
K46.00006: Two-dimensional type-II Dirac fermions in a LaAlO3/LaNiO3/LaAlO3 quantum well Lingling Tao, Evgeny Y Tsymbal The type-II Dirac fermions that are characterized by a tilted Dirac cone and anisotropic magnetotransport properties have been recently proposed theoretically and confirmed experimentally. Here, we predict the emergence of two-dimensional (2D) type-II Dirac fermions in LaAlO3/LaNiO3/LaAlO3 quantum-well structures. Using first-principles calculations and model analyses, we show that the Dirac points are formed at the crossing between the dx2−y2 and dz2 bands protected by the mirror symmetry. The energy position of the Dirac points can be tuned to appear at the Fermi energy by changing the quantum-well width. For the quantum-well structure with a two-unit-cell-thick LaNiO3 layer, we predict the coexistence of the type-II Dirac point and the closed nodal line. The results are analyzed and interpreted using a tight-binding model and symmetry arguments. Our findings offer a practical way to realize 2D type-II Dirac fermions in oxide heterostructures. |
Wednesday, March 6, 2019 9:36AM - 9:48AM |
K46.00007: Density-functional prediction of a spin-orbital entangled two-dimensional electron gas at LaAlO3/SrIrO3 (001) interface Churna Bhandari, Sashi Sekhar Satpathy With the recent advances on the epitaxial growth techniques, epitaxy grown ultrathin films of SrIrO3 have brought in a considerable research interest as they exhibit thickness dependent metal-insulator transition and other interesting transport properties. SrIrO3 is a Mott insulator below three or four layers of SrIrO3 grown on the SrTiO3 substrate. Using density-functional methods, we study the (001) interface between the non-polar SrIrO3 and the polar LaAlO3 material, where the LaO layer is in contact with the IrO2 layer. We predict the formation of a spin-orbital entangled 2DEG at the interface due to the polar catastrophe, analogous to the n-type LaAlO3/Sr2IrO4 interface1.The predicted 2DEG is very well localized at a single layer of Ir near the interface occupying the Jeff =1/2 conduction bands (upper Hubbard band). The 2DEG leads to a ferromagnetic interface, which is different from the antiferromagnetic bulk. |
Wednesday, March 6, 2019 9:48AM - 10:00AM |
K46.00008: Silicon-Integrated Transition Metal Oxide Thin Film Quantum Structures J. Elliott Ortmann, Sunah Kwon, Agham Posadas, Moon Kim, Alexander Demkov While the SrTiO3/LaAlO3 (STO/LAO) system has garnered widespread attention since the discovery of a two-dimensional electron gas at the interface of these two band insulators, recent efforts have instead focused on its optical properties. The huge 2.4 eV conduction band offset between STO and LAO allows for charge confinement in STO quantum wells and the optical and electrical modulation of carriers between confined states. Such heterostructures could find use in a multitude of next-generation electrical, optical, and electro-optical devices. However, the technological relevance of such devices hinges on the ability to successfully integrate high-quality STO/LAO quantum structures with silicon. Here, we demonstrate the monolithic integration of high-quality STO/LAO multiple quantum wells on silicon (001) with molecular beam epitaxy. We present electron diffraction, X-ray diffraction and electron microscopy studies establishing the excellent uniformity and crystalline quality with which such heterostructures can be fabricated. Finally, we present examples of how silicon-integrated STO/LAO quantum structures could be used in device fabrication. |
Wednesday, March 6, 2019 10:00AM - 10:12AM |
K46.00009: Transport regimes of a split gate superconducting quantum point contact in the two-dimensional LaAlO3/SrTiO3 superfluid Holger Thierschmann, Emre Mulazimoglu, Manca Nicola, Srijit Goswami, Teun M Klapwijk, Andrea Caviglia Ever since the observation of the quantized resistance in a point contact in GaAs/AlGaAs heterostructures it has been a long standing goal to achieve similar experimental conditions also in superconductors. Being formed with split gate technology, these structures represent in an ideal manner equilibrium reservoirs which are connected only through a few electron mode channel. Here we demonstrate the formation of a superconducting quantum point contact (SQPC) with split gate technology in a two-dimensional superconductor, utilizing the unique gate tunability of the superfluid at the LaAlO3/SrTiO3 interface. When the constriction is tuned through metallic split gates we identify three regimes of transport: First, SQPC for which the supercurrent is carried only by a few quantum transport channels. Second, superconducting island strongly coupled to the equilibrium reservoirs. Third, charge island with a discrete spectrum weakly coupled to the reservoirs. |
Wednesday, March 6, 2019 10:12AM - 10:24AM |
K46.00010: Measurement of Helical Edge States in Ultra-thin LaTiO3/SrTiO3 Heterostructures Michael Veit, Di Yi, Remi Arras, Rossitza Pentcheva, Yuri Suzuki Emergent metallic behavior at the interface of the Mott insulator LaTiO3 and the band insulator SrTiO3 has been explained in terms of charge redistribution at the interface combined with strain-induced electronic structure modification. We have previously studied ultra-thin (3 unit cell thick) films of LaTiO3 on SrTiO3 substrates and found evidence for unexpectedly large spin-orbit coupling from quantum oscillations with an associated Berry phase, weak anti-localization in the magnetoresistance, and anisotropic in-plane magnetoresistance. In order to tune this material, we have electrically gated these films both on the back side of the SrTiO3 substrate and on top of the LaTiO3 film with an electrolytic gate. Most interestingly, we found a maximum in the resistance that is quantized to the quantum of conductance (2e2/h). This is independent of the dimensions of the sample. Additionally, by studying samples with four and six contact geometries, we measure nonlocal transport similar. These features are present up to a surprisingly high temperature of 230K. We attribute these observations to helical edge states at the LaTiO3/SrTiO3 interface. |
Wednesday, March 6, 2019 10:24AM - 10:36AM |
K46.00011: Anisotropic magnetoresistance for (111) SrTiO3 surfaces within Boltzmann transport theory Nazim Boudjada, Ilia Khait, Arun Paramekanti 2D electron gases formed at (111) surfaces and interfaces of SrTiO3 are highly sensitive to application of in-plane magnetic fields. Motivated by recent magnetoresistance experiments which report different crystalline symmetry components appearing at different gate voltages, we derive a multi-orbital single-layer Hamiltonian which includes spin-orbit coupling, orbital Rashba and broken inversion symmetry at the surface. The anisotropic magnetoresistance is calculated by solving the Boltzmann equation. An explanation for the experimental results observed for SrTiO3 is devised. |
Wednesday, March 6, 2019 10:36AM - 10:48AM |
K46.00012: Adding depth resolution to resonant inelastic X-ray scattering by means of standing-wave excitation Cheng-Tai Kuo, Shih-Chieh Lin, Yingying Peng, Yu-Cheng Shao, Gabriella Maria De Luca, Daniele Di Castro, Ismael Graff, Nicholas B Brookes, Yi-De Chuang, Mark Huijben, Lucio Braicovich, Giacomo Ghiringhelli, Charles Fadley Resonant inelastic X-ray scattering (RIXS) is a photon-in/photon-out synchrotron-based spectroscopy that uniquely probes the charge transfer, dd, magnetic, phonon and other excitations in correlated oxides and other systems.RIXS is considered to be a probe of bulk properties, reaching depths of the order of 1000 Å. It is thus desirable to give RIXS more quantitative depth resolution, for example to investigate interfaces in oxide heterostructures, which are known to show emergent properties (e.g. interface-induced ferromagnetism at the La1.85Sr0.15CuO4/La0.67Sr0.33MnO3 (LSCO/LSMO) heterostructures, 2D electron gases at LaAlO3/SrTiO3 (LAO/STO) heterostructure) not present in the single constituents. Here, we demonstrate that, by using standing-wave (SW) excitation from multilayer heterostructures, interface-specific RIXS information can be obtained. We present the results of SW-RIXS measurements on LSCO/LSMO and LAO/STO superlattices. SW effects are clearly observed on the RIXS excitations for these systems and were found to have different depth distribution. SW-RIXS will open up a new spatial dimension to this already powerful technique. |
Wednesday, March 6, 2019 10:48AM - 11:00AM |
K46.00013: Chemically specific termination control of oxide interfaces via layer-by-layer mean inner potential engineering Haoying Sun, Zhangwen Mao, Tianwei Zhang, Lu Han, Tingting Zhang, Xiangbin Cai, Xuyun Guo, Yingfei Li, Yipeng Zang, Wei Guo, Jianhui Song, Dianxiang Ji, Chenyi Gu, Chao Tang, Zhengbin Gu, Ning Wang, Ye Zhu, Darrell G. Schlom, Yuefeng Nie, Xiaoqing Pan Creating oxide interfaces with precise chemical specificity at the atomic layer level is desired for the engineering of quantum phases and electronic applications, but highly challenging, owing partially to the lack of in situ tools to monitor the chemical composition and completeness of the surface layer during growth. Here we report the in situ observation of atomic layer-by-layer inner potential variations by analysing the Kikuchi lines during epitaxial growth of strontium titanate, providing a powerful real-time technique to monitor and control the chemical composition during growth. A model combining the effects of mean inner potential and step edge density (roughness) reveals the underlying mechanism of the complex and previously not well-understood reflection high-energy electron diffraction oscillations observed in the shuttered growth of oxide films. General rules are proposed to guide the synthesis of atomically and chemically sharp oxide interfaces, opening up vast opportunities for the exploration of intriguing quantum phenomena at oxide interfaces. |
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