Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session S09: Complex oxide heterostructures - FerroelectricsFocus
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Sponsoring Units: DMP Chair: Marta Gibert, Univ of Geneva Room: LACC 301A |
Thursday, March 8, 2018 11:15AM - 11:51AM |
S09.00001: Resonant transport assisted by a charged domain wall in a ferroelectric tunnel junction Invited Speaker: Jacobo Santamaria Interfacially induced electronic states at oxide interfaces can be used to tailor novel functionalities. In this talk we discuss the possibility to exploit the unavoidable defects at interfaces to tailor novel electronic states. In particular, oxygen vacancies, difficult to detect and in many cases also to avoid in oxides heterostructures, may have drastic effects on materials properties. Under the strong electric fields developing in nanostructures at moderate voltages, oxygen vacancies may accumulate at interfaces and drastically alter their physical properties through their associated strain and doping fields. Here we will describe the controlled and manipulation of the oxygen vacancy profile in a ferroelectric tunnel barrier, showing that oxygen vacancies may stabilize unexpected domain structures. We describe the nucleation of a charged domain wall in a ferroelectric nanometer thick tunnel barrier, and how it controls transport by a resonant tunneling mechanism assisted by the highly confined screening electronic states [1]. This result provides a major step forward towards the concept “The Wall is the Device”, to exploit the electronic properties of domain walls for ferroelectric tunnel barriers with new functionalities. |
Thursday, March 8, 2018 11:51AM - 12:03PM |
S09.00002: Prediction of complex ferroelectric textures through second-principles simulations Pablo Garcia-Fernandez, Jorge Iniguez, Javier Junquera The combination of a ferroelectric, like PbTiO3, and a dielectric, such as SrTiO3, in a layered superlattice allows observing intrincate polarization patterns like vortex arrays1 or bubble domains2 that are sensitive to fields and strain3 and display exotic properties like negative capacitance.4 Understanding these systems requires complementing experiment with realistic simulations. This task is difficult due to the nanometric length scale of the characteristic dipole orders, the need for atomic resolution owing to strong coupling between various distortions of the underlying perovskite lattice and the small energy differences between the many stable configurations. Here, we tackle this problem using SCALE-UP5 an implementation of our second-principles DFT method6 that allows us studying the effect of periodicity/strain/external fields/temperature. Our predictions, fully consistent with experiment, show the presence of chirality and non-trivial topological structures. |
Thursday, March 8, 2018 12:03PM - 12:15PM |
S09.00003: Resonant Tunneling through a Ferroelectric Domain Wall Ming Li, Ling-Ling Tao, Evgeny Tsymbal Electron transport across a ferroelectric barrier has attracted interest due to the fundamental physics and potential applications. A prototypal example is the ferroelectric tunnel junction (FTJ), made of two metallic electrodes separated by a thin ferroelectric film. It's been shown experimentally that a head-to-head (HTH) ferroelectric domain structure can be formed in a La1-xSrxMnO3(LSMO)/BaTiO3(BTO)/La1-xSrxMnO3 FTJ with the domain wall parallel to the plane. Using first-principles density-functional calculations we explore electron transport in the LSMO/BTO/LSMO FTJ. We find that the assumed La1-xSrxO/TiO2 termination at both interfaces stabilizes two ferroelectric domains with polarization pointing at each other. The HTH domain wall in the middle of the BTO layer produces a V-shaped electrostatic potential profile, which pushes the conduction band minimum about 0.17 eV below the Fermi energy and forms a two-dimensional electron gas (2DEG) around the domain wall. Our quantum-transport calculations predict an enhanced transmission at special transverse wave vectors k, indicating resonant tunneling through the 2DEG. The microscopic physics of the resonant tunneling is understood from the analysis of real-space scattering states and the local k-dependent density of states. |
Thursday, March 8, 2018 12:15PM - 12:27PM |
S09.00004: Size of ferroelectric field effects in oxide heterostructures Javier Villegas, Laura Begon-Lours, Victor Rouco, Anke Sander, Juan Trastoy, Karim Bouzehouane, Vincent Garcia, S. Fusil, Manuel Bibes, Agnes Barthelemy, Qiao Qiao, Manuel Roldan, Maria Varela, Jacobo Santamaria Ferroelectric field-effects allow the manipulation of the physical properties of strongly correlated oxides. This is achieved in heterostructures that combine these materials with ferroelectrics. Similarly, as in a transistor, a strong accumulation of charge is induced in the correlated oxide channel to screen the electric field from the ferroelectric gate, which results in a significant doping of the correlated material. Here we investigate the extrinsic factors that may limit the size of ferroelectric field-effects, which include among other the nanoscale structural and ferroelectric properties near the interface. The investigation is done via a combination of transport measurements (Hall effect and TC measurements), piezoresponse force microscopy (PFM), high-resolution scanning transmission electron microscopy (HR-STEM) and electron energy loss spectroscopy (EELS) in BiFeO3 (ferroelectric)/YBa2Cu3O7 (superconductor) heterostructures. We find that incomplete ferroelectric switching, probably associated to the interface atomic terminations, and changes in the carrier mobility that accompany the carrier concentration modulation, conspire to reduce the size of the field effects. |
Thursday, March 8, 2018 12:27PM - 12:39PM |
S09.00005: Phase Coexistence in Ultrathin Films of Multiferroic BiFeO3 Dan Ferenc Segedin, Julia Mundy, Liv Dedon, Ramamoorthy Ramesh An electrical polar discontinuity presents an opportunity to stabilize an emergent phase at an interface. Here, we grow thin films and superlattices of dielectric DyScO3 and ferroelectric BiFeO3, i.e. (BiFeO3)m/(DyScO3)n for various m on (110)-oriented DyScO3 single crystal substrates using pulsed laser deposition. The ferroelectric polarization in contact with the dielectric layer leads to a large electric field build-up in the absence of a compensating charge. We report the results of Scanning Transmission Electron Microscopy (STEM), X-Ray Diffraction (XRD) and Atomic-Force Microscopy (AFM) measurements, demonstrating that our films exhibit excellent structural quality. Atomic-scale images of the structure using STEM measurements shows that, for layer thicknesses of 4 nm (10 unit cells), BiFeO<span style="font-size:10.8333px">3</span> adopts an orthorhombic, anti-ferroelectric ground state. This work thus provides the first direct observation of this ground state in un-doped BiFeO3, demonstrating the ability of interfacial field to dramatically change the structural and electric ground state of an ultrathin film. |
Thursday, March 8, 2018 12:39PM - 12:51PM |
S09.00006: Deterministic and Robust Room–temperature Exchange Coupling in Monodomain Multiferroic BiFeO3 Heterostructures Bruce Davidson, Wittawat Saenrang, Francesco Maccherozzi, Jacob P. Podkaminer Podkaminer, Julian Irwin, Roger Johnson, John Freeland, Jorge Íñiguez, Jonathan Schad, Kellan Reierson, Joshua Frederick, Carlos Vaz, Ludovic Howald, Tae Heon Kim, Sangwoo Ryu, Michel Van Veenendaal, Paolo Radaelli, Sarnjeet Dhesi, Mark Rzchowski, Chang-Beom Eom Exploiting multiferroic BiFeO3 (BFO) thin films in spintronic devices requires deterministic and robust control of both the internal magnetoelectric coupling in BFO as well as exchange coupling of its antiferromagnetic (AF) order to a ferromagnetic overlayer. Previous reports of exchange coupling in films have utilized approaches based on multistep ferroelectric (FE) switching with multiple FE domains. Because domain walls can be responsible for fatigue, can contain localized charges either intrinsically or via defects, and present issues for device reproducibility and scaling, an alternative approach using a monodomain magnetoelectric state with single–step switching is desirable for practical devices. Here we demonstrate RT, deterministic and robust exchange coupling between monodomain BFO films and a Co overlayer that is “intrinsic” (without domain walls). Direct coupling between AF order in BFO and Co magnetization leads to ~90° in–plane Co moment rotation upon single-step FE switching, reproducible for hundreds of cycles[1]. This has important consequences for useful BFO-based magnetoelectric devices. |
Thursday, March 8, 2018 12:51PM - 1:03PM |
S09.00007: Extreme Reconfigurable Nanoelectronics at the CaZrO3/SrTiO3 Interface Lu Chen, Jianan Li, Yuhe Tang, Yunzhong Chen, Nini Pryds, Patrick Irvin, Jeremy Levy The metal-insulator transition at the interface between polar LaAlO3 (LAO) and non-polar SrTiO3 (STO) can be controlled using conductive atomic force microscopy (c-AFM) lithography. Here we report extreme nanoscale control over the CaZrO3/SrTiO3 (CZO/STO) interface, a nonpolar 2DEG system which is formed by strain-induced polarization. Ultra-thin (1 to 3 nm wide) nanostructures can be realized at the CZO/STO interface at room temperatures, and superconducting quasi-1D and 2D structures of CZO/STO are investigated at cryogenic temperatures. Combining the versatility of c-AFM lithography with the rich physics at complex oxide interfaces, reconfigurable nanoelectronics paves new ways to investigate the correlated spin, charge and orbital degree of freedoms. |
Thursday, March 8, 2018 1:03PM - 1:15PM |
S09.00008: Nanomechanical Probes of Sketched LaAlO3/SrTiO3 Single-Electron Transistors Jessica Montone, Feng Bi, Mengchen Huang, Jung-Woo Lee, Hyungwoo Lee, Chang-Beom Eom, Patrick Irvin, Jeremy Levy 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 LaAlO3/SrTiO3, we create single-electron transistors using conducting atomic force microscope (c-AFM) lithography. The piezoelectric nature of LaAlO3/SrTiO3 gives way to an expected coupling between mechanical motion and electric charge within the device. We test this effect by applying pressure to the device using an insulating AFM tip while measuring changes in electron density. I describe our efforts to perform nanomechanical imaging of conductive structures on LaAlO3/SrTiO3. I describe our method of performing imaging in a cryogenic AFM system, as many interesting properties of single-electron devices are observed at low temperatures. |
Thursday, March 8, 2018 1:15PM - 1:27PM |
S09.00009: Tuning Spin Relaxations in Ultrathin Epitaxial SrIrO3 Thin Films via Ferroelectric Gating Le Zhang, Myung-Geun Han, Xuanyuan Jiang, Xiaoshan Xu, Yimei Zhu, Xia Hong SrIrO3 (SIO) is paramagnetic, semimetallic and with nontrivial topological properties. Its strongly correlated nature and large spin-orbital coupling (SOC) make SIO a promising material candidate for spintronic applications. As a feature of strong SOC, weak anti-localization, manifested as negative quantum interference correction to the conductance, has been observed at low temperatures. We have extracted inelastic scattering and spin precession length, which show distinct temperature dependences. Moreover, the linear mobility dependence of spin precession time points to Elliott-Yafet mechanism as the dominant spin relaxation mechanism. By fabricating the PbZr0.2Ti0.8O3 (PZT)/SIO (∼2 nm) heterostructures, we have demonstrated nonvolatile resistance change in SIO via switching the polarization of PZT. The onset temperature of resistance upturn due to weak localization is also modulated by ferroelectric field effect, with a 10 K shift observed. The modulation of the magnetoresistance indicates the change of SOC strength in SIO. Our results demonstrate a feasible way to manipulating SOC at the interface of oxides heterostructures. |
Thursday, March 8, 2018 1:27PM - 1:39PM |
S09.00010: Anomalous Dielectric Response at Oxide Heterointerfaces Valentino Cooper, Houlong Zhuang, Lipeng Zhang, Panchapakesan Ganesh, Haixuan Xu, Paul Kent Two-dimensional charge carrier accumulation at oxide heterointerfaces presents a paradigm shift for oxide electronics. Like a capacitor, interfacial charge buildup couples to an electric field across the dielectric. To prevent the so-called polar catastrophe, several charge screening mechanisms emerge, including polar distortions and interfacial intermixing which reduce the sharpness of the interface. Here, I discuss our efforts to understand how atomic intermixing at oxide interfaces affects the balance between polar distortions and electric potentials across the dielectric. Using first principles calculations, we find that intermixing moves the peak charge distribution away from the interface; thereby changing the direction of polar distortions away from this boundary with minimal effect on the electric field. This opposing electric field and polar distortions is equivalent to the transient phase transition tipping point observed in double well ferroelectrics; resulting in anomalous dielectric responses; a possible signature of local negative differential capacitance with implications for designing dissipationless oxide electronics. |
Thursday, March 8, 2018 1:39PM - 1:51PM |
S09.00011: Intertwining ferroelectricity and two-dimensional electron gas at oxide interfaces Ariando Ariando, W. X. Zhou, H. J. Wu, J. Zhou, Thirumalai Venkatesan, J. S. Chen, Yuan Feng, S. J. Pennycook We report the coexistence of ferroelectricity and two-dimensional electron gas (2DEG) in ferroelectric BaxSr1-xTiO3 (BST) thin films at the LaAlO3/BaxSr1-xTiO3 (LAO/BST) interface using piezoresponse force microscopy (PFM), electrical and optical measurements, scanning transmission electron microscopy (STEM) as well as first-principles calculations. The 2DEG is introduced into ferroelectric BST thin films by the polar discontinuity at LAO/BST interface similar to that of LAO/STO. The ferroelectricity of BST thin film of ~3.2 nm is found to persist with a two-dimensional carrier density of ~0.05 electron per unit cell. Our results demonstrate the coexistence of ferroelectricity and two-dimensional electron gas, which are conventionally regarded as mutually exclusive, in oxide heterostructures. |
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