Rajdeep Sensarma (TIFR): Potential Inhomogeneities in Presence of Strong Interactions: Birth and Death of Superconductors
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
Inaugural Lecture Peyman Givi (University of Pittsburgh): Exascale HPC, Big Data, and Quantum Computing in Rocket Science
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
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.
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.
We set out to study a system which couples a nanomechanical harmonic oscillator to a single spin. While individual spins are intrinsically quantum objects, mechanical resonators are usually observed as classical systems. Not coincidentally, spins are usually largely isolated from their environment, while nanomechanical devices excel at coupling to almost everything. In our system, a spin and a nanomechanical resonator interact such that they perform quantum non-demolition (QND) measurements on each other, enabling a bridge between the quantum and classical worlds. The strength of the coupling is enhanced by utilizing an avoided level crossing of the coupled spin-resonator system. The sensitivity is maximized by minimizing the mass of the oscillator, leading us to explore graphene resonator and trap-based implementations. Diamond nitrogen vacancy centers are chosen as the source of a spin due to their exceptional spin state coherence times, large zero-field splitting, and optical addressability. Progress towards an experimental realization of this system has further lead us to improve graphene growth techniques, develop novel fabrication methods, and create magnetic traps for diamond nanocrystals.
Mark A. Novotny (Mississippi State University): How Ubiquitous is the Total Electron Transmission Through Nanostructures?
In transport through nanostructures connected to two semi-infinite leads, the transmission probability calT (E) as a function of the energy E of the incoming electron plays a central role in the Landauer calculation of the electrical conductance G. A quantum dragon nanostructure is one which when connected to appropriate leads has total electron transmission for all energies, calT (E) =1. In two-lead measurements of single-channel quantum dragons, the quantum of conductance, G0 = 2e2 / h , should be observed. A quantum dragon may have strong scattering. In the disorder was along the axis of electron propagation, the z axis. We show that quantum dragon nanostructures can be found for strong disorder perpendicular to the z axis. In select types of nanostructures, we find the ratio of the dimension of the parameter space where quantum dragons exist to that of the complete parameter space. The results use the single-band tight-binding model, and are for the case with only one open channel and homogeneous leads. One type of nanostructure with calT (E) =1 has completely disordered slices perpendicular to the z axis, but identical slices along the z direction.