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.
Migratory birds travel spectacular distances, navigating and orienting by a variety of means, most of which are poorly understood. Among them is a remarkable ability to perceive the intensity and direction of the Earth's magnetic field. Biologically credible mechanisms for the sensing of such weak fields (25-65 microtesla) are scarce and in recent years just two proposals have emerged as frontrunners. One involves biogenic iron-containing nanoparticles; the other relies on the magnetic sensitivity of short-lived photochemical intermediates known as radical pairs. The latter began to attract attention following the proposal 15 years ago that the necessary physics and chemistry could take place in the bird's retina in specialised photoactive proteins called cryptochromes. The coherent dynamics of the electron-nuclear spin systems of pairs of photo-induced radicals is conjectured to form the basis of the sensing mechanism even though the interaction of an electron spin with the geomagnetic field is six orders of magnitude smaller than the thermal energy. The possibility that slowing decohering, entangled electron spins could form the basis of an important sensory mechanism has qualified radical pair magnetoreception for a place under the umbrella of ``Quantum Biology.'' In this talk, I will introduce the radical pair mechanism, comment on the roles of entanglement and quantum coherence, outline some of the experimental evidence for the cryptochrome hypothesis, and summarize what still needs to be done to determine whether birds (and maybe other animals) really do use a chemical compass to find their way around.