Lecture Videos

Martin Zwierlein (M.I.T.) talks about strongly interacting fermi gases of atoms and molecules. He introduces NaK, a chemically stable fermionic molecule studied in his group and explains the process of generating this molecule in the ground state with a high conversion efficiency and reasonable lifetime. He also describes the first studies of the hyperfine states of this new quantum system and how with two such states, they can build a molecular clock.

Martin Zwierlein (M.I.T.) talks about strongly interacting fermi gases of atoms and molecules. He introduces NaK, a chemically stable fermionic molecule studied in his group and explains the process of generating this molecule in the ground state with a high conversion efficiency and reasonable lifetime. He also describes the first studies of the hyperfine states of this new quantum system and how with two such states, they can build a molecular clock.

Doug Natelson (Rice University) talks about heating and vibrations at the molecular level. Non equilibrium heating is of much interest in physics although it is still difficult to obtain information about the local electronic or vibrational distribution. He therefore proceeds to describe the approach developed by his group towards simultaneous electron transport and Surface Enhanced Raman Spectroscopy with plasmons at the molecular-scale.

Xiao Yang Zhu (Columbia University) talks about many body coherent processes in semiconductors brought about by light-matter interactions with applications in solar energy conversion. He describes the recent experimental detection of phonon-assisted Auger recombination as well as the nature of the multi-exciton state and the role of vibronic coherent and incoherent rate processes in singlet fission.

Strong repulsive interactions and potential in homogeneities both tend to localize Fermions on a lattice and lead to loss of superconductivity. The natural question that comes up is whether they compete or complement each other when both are present in a system at the same time. In this talk, we will use a effective Hamiltonian approach which treats both interactions and in homogeneities on the same footing to look at two systems: (a) the Ionic Hubbard Model at half-filling, where a staggered potential on a bipartite lattice competes with interactions to delocalize charge and give birth to a novel superconductor. The superconducting Tc scales with the bandwidth of the system and shows a non-monotonic behaviour with the staggered potential, (b) the disordered Hubbard model away from half-filling, where weak disorder competes with strong interaction to preserve superconductivity, but strong disorder complements interactions leading to sudden death of superconductivity in this system.

Jeremy Levy and Aude Marjolin present the Pittsburgh Quantum Institute to the Swanson School of Engineering.

March 3, 2016

Exascale HPC, Big Data, and Quantum Computing in Rocket Science

Peyman Givi, PhD
Distinguished Professor of Mechanical Engineering and Materials Science
Swanson School Of Engineering
University of Pittsburgh

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.

 

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.

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