PQI Seminar Guen Prawiro-Atmodjio (University of Copenhagen): Pair Tunneling with Negative U at LAO/STO Complex Oxide Interface
The conducting interface at the LaAlO3/SrTiO3 (LAO/STO) interface has sparked large interest due to its many coexisting properties. Interfaces based on non-crystalline top layers show similar characteristics to their crystalline equivalent. Recently, we reported superconductivity in patterned devices of the disordered-LAO/STO interface where strong evidence was found for a superconducting phase originating from a Josephson-coupled array of superconducting puddles, possibly induced by inhomogeneous doping. We studied this system with nano-patterned gates to form a nano-scale constriction in a Hall bar, in which we were able to pinch off current flow. Close to pinch-off, a dot regime is formed with peaks of conductivity, that form diamonds of suppressed current when applying a bias voltage across the constriction. The conductance peaks split up when applying a large in-plane or out-of-plane magnetic field, indicating pairing of electrons without superconductivity (as reported in Cheng et al., Nature 2015). We study the system as a function of gate voltage, bias voltage, magnetic field and temperature and compare with a negative-U model that describes the behavior of pair tunneling, which differs from the conventional Coulomb-blockade scenario.
The current in a two-dimensional topological insulator (2D TI) is expected to be carried by edge channels, which so far is only evidenced by transport measurements. At Stanford we have imaged the current density in HgTe quantum well as well as in InAs/GaSb quantum wells, the first and second experimentally realized TIs, and directly observed that current flows in the edges. The measurement principle is shown in the cartoon. A current (blue) flows at the edge of a device (green), producing a magnetic field (red) that is measured by detecting the flux through the SQUID’s pickup loop (white). For a two-dimensional current density we can reconstruct the current distribution from the flux image. An example is shown on the right. The normalized x-component of the current density when the Fermi level in the device is tuned into the bulk energy gap. shows that the current flow along the edge of the device.
Edward Beall is a graduate student in in the Department of Chemistry at Pitt.
He works in the Waldeck lab and observes charge transport through molecular bridges by studying the electrical conductance of a single molecule. This will aid in miniaturizing circuitry, hopefully to the nanoscale.
Edward won a travel award at the Science 2015 poster session for his poster on "Scanning Tunneling Microscope Break Junction Method with Continuous Bias Modulation”.
Megan Kirkendall is a graduate student in the Department of Physics and Astronomy at Pitt.
She works in the Levy lab where she researches quantum simulation at the lanthanum aluminate strontium titanate interface. Her research involves engineering a lattice interface on the nanometer scale, and then using that information to simulate a quantum system that can be studied. This process provides insight into quantum systems that cannot be simulated with a normal computer.
Megan won the grand prize at the Science 2014 poster session for her poster on “Experimental Quantum Simulation Using 1D LaAlO3/SrTiO3”.
Devashish Gopalan is a graduate student in the Department of Physics of Carnegie Mellon University.
He works in the Feenstra lab where he normally researches the large-scale synthesis of 2D heterotructures. He has recently shifted his interest in the direction of studying physical phenomena within the 2-dimensional limit.
Devashish won a travel award at the Science 2015 poster session for his poster on “Formation of Hexagonal Boron Nitride On Graphene-Covered Copper Surfaces”.
Mitch Groenenboom is a graduate student in the Department of Chemical and Petroleum Engineering at Pitt.
He works in the Keith group on molecular-promoted CO2 reduction, anti-corrosion coatings, and multiscale modeling of metal nanoparticles.
Mitch won a travel award at the Science 2015 poster session for his poster on "Aqueous Phase CO2 Reduction with Sodium Borohydride: an Ab Initio Molecular Dynamics and Nudged-Elastic Band Mechanistic Study."
Peng Ji is a graduate student in the Department of Physics and Astronomy at Pitt.
He works in the Dutt lab on the optical trapping of nanodiamonds in the air and collect the emitted photoluminescence.
Peng won a travel award at the Science 2015 poster session for his poster on "Towards a Quantum Interface between Diamond Spin Qubits and Phonons in an Optical Trap".
Adam Argondizzo is a graduate student in the Department of Physics and Astronomy at Pitt.
He works in the Petek lab where he focuses on Multiphoton Photoemission. His research will aid in increasing efficiency of photocatalytic processes towards producing clean energy.
Adam won a travel award at the Science 2015 poster session for his poster on “Multi-Photon Photoemission Excitation in TiO2”.
The development of metallic alloys is arguably one of the oldest of sciences, dating back at least 3,000 years. It is therefore very surprising when a new class of metallic alloys is discovered. High Entropy Alloys (HEA) appear to be such a class; furthermore, one that is receiving a great deal of attention in terms of the underlying physics responsible for their formation as well as unusual combinations of mechanical and physical properties that make them candidates for technological applications. The term HEA typically refers to alloys that are comprised of 5, 6, 7… elements, each in in equal proportion, that condense onto simple underlying crystalline lattices but where the different atomic species are distributed randomly on the different sites - face centered cubic (fcc) Cr0.2Mn0.2Fe0.2Co0.2Ni0.2 and body-centered-cubic (bcc) V0.2Nb0.2Mo0.2Ta0.2W0.2 being textbook examples. The naming of these alloys originates from an early conjecture that these unusual systems are stabilized as disordered solid solutions alloys by the high entropy of mixing associated with the large number of components - a conjecture that has since proved insufficient. In the first part of the presentation I will provide a general introduction to these materials and how they differ from conventional alloys that underpin much of our energy generation and transportation technologies. In addition I will describe a model that allows us to predict which combinations of N elements taken from the periodic table are most likely to yield a HEA that is based on the results of modern high-throughput ab initio electronic structure computations. In the second part I will broaden the discussion to a wider class of equiatomic fcc concentrated solid solution alloys that is based on the 3d- and 4d-transition metal elements Cr, Mn, Fe, Co, Ni, Pd that range from simple binary alloys, such as Ni0.5Co0.5 and Ni0.5Fe0.5, to the quinary high entropy alloys Cr0.2Mn0.2Fe0.2Co0.2Ni0.2 an Cr0.2Pd0.2Fe0.2Co0.2Ni0.2 themselves. Here I will discuss the role that increasing chemical complexity and disorder has on the underlying electronic structure and the magnetic and transport properties. Finally, I will argue that the manipulation of chemical complexity may offer a new design principle for more radiation tolerant structural materials for energy applications.
Eric Stach, Brookhaven National Laboratory: Using In-Situ and Operando Methods to Characterize Working Catalysts
The field of electron microscopy has seen dramatic advances in the past decade, with the development of advanced electron optics such as aberration-correctors and source monchromators, new detector modalities and advances in sample manipulation and probing. In this talk, I will detail activities within the Electron Microscopy Group at Brookhaven National Laboratory’s Center for Functional Nanomaterials, with a specific focus on understanding the structure, composition and performance of heterogenous catalysts in-situ and in- operando. Specifically, I will describe how environmental transmission electron microscopy can be used to create real time movies of the nucleation, growth and growth termination of carbon nanotubes. Thereafter, I will detail new methods that are being developed to characterize working catalysts in-operando using a closed-cell micro reactor which allows imaging at atmospheric pressure. Finally, I will describe how this same micro reactor allows correlated measurements to be obtained from both electron microscopy and x-ray spectroscopy and diffraction and will present a vision for the integration of this capability into the facilities that are soon to come online at the National Synchrotron Light Source-II at BNL.