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Physics Colloquium

 

Fall 2014 Colloquium Schedule

Date Speaker Title Host
August 25 Bob Svoboda
UC Davis
WATCHMAN: Neutrino Physics and Nuclear Non-Proliferation – Oh . . . Really? Tom Handler
September 8 Jaideep Singh
Michigan State University
Trapping Atoms the "Old-Fashioned" Way: New Results & Opportunities Nadia Fomin
September 15 Peng Xiong
Florida State University
Ultrathin Pb Films in a Magnetic Field: New Physics from an Old Superconductor Haidong Zhou
September 22 Randy Fishman
Oak Ridge National Laboratory
Using Inelastic Scattering Measurements to Determine the Complex Spin States of Multiferroic Materials Adolfo Eguiluz
September 29 Michael Duncan
University of Georgia
Infrared Spectroscopy of Cold Ions and their Clusters Robert Compton
October 6 Sean Sun
Johns Hopkins University
The Role of Water in Cell Mechanics, Cell Shape and Cell Motility Jaan Mannik
October 13 No Colloquium No Colloquium  
October 20 Yasu Takano
University of Florida
Spin-1/2 Heisenberg Antiferromagnets in One Dimension Haidong Zhou
October 27 K. Ernst Rehm
Argonne National Laboratory
Studies of X-ray Bursts – Bringing Neutron Stars into the Laboratory Alfredo Galindo-Uribarri
November 3 Jeremy Levy
University of Pittsburgh
Etch-a-Sketch Nanoelectronics Steven Johnston
November 10 Takeshi Egami
University of Tennessee
Liquid-State-Physics Adolfo Eguiluz
November 17 Thomas Maier
Oak Ridge National Laboratory
High-Temperature Superconductivity from Repulsive Interactions: Unconventional Pairing in Hubbard Models Adolfo Eguiluz
November 24 Alan Tennant
Oak Ridge National Laboratory
Twists in Quantum Magnets Hanno Weitering
December 1 Silas R. Beane
University of Washington
TBA Lucas Platter

Abstracts

August 25, 2014

Bob Svoboda
University of California, Davis

WATCHMAN: Neutrino Physics and Nuclear Non-Proliferation – Oh . . . Really?

Monitoring nuclear non-proliferation treaties is a challenging technological problem. Since the “fast lane” to nuclear weapons is the production of plutonium in small nuclear reactors, an important (but difficult) problem is how to monitor reactors built for research to ensure they are not being used for weapons production. Recent experiments in neutrino physics and dedicated demonstration projects at commercial nuclear plants have shown that it may be possible to use neutrinos to determine the operational cycle of such reactors from moderately distant locations. At the same time, the particle physics community is developing technology that may make such monitoring both feasible and cost effective. The WATCHMAN Project is a collaborative effort between scientists and engineers that will not only advance neutrino detector technology, but also demonstrate the ability to effectively monitor nuclear reactors for weapons production.


September 8, 2014

Jaideep Singh
Michigan State University

Trapping Atoms the "Old-Fashioned" Way: New Results & Opportunities

Inert gases frozen at cryogenic temperatures have been used to trap and study atoms and molecules for over 60 years. In particular, noble gas solids (NGS) are a promising medium for the capture, detection, and manipulation of atoms and nuclear spins. They provide stable, chemically inert, & efficient confinement for a wide variety of guest species. Because NGS are transparent at optical wavelengths, the guest species can be probed using lasers. Longitudinal and transverse nuclear spin relaxation times of a guest species can be made very long under well understood and feasible conditions. Potential applications include measurements of rare nuclear reactions and tests of fundamental symmetries.

In this talk, I will present the results of our optical spectroscopic study of ytterbium atoms embedded in a frozen neon matrix, which includes the first experimental determination of the 23 second lifetime of the metastable atomic state of Yb-171 that is used for next generation atomic clocks. I will conclude with our efforts to demonstrate (1) optical single atom detection for studying rare nuclear reactions and (2) optical pumping of Yb-171 nuclei in solid neon for a test of time-reversal symmetry.


September 15, 2014

Peng Xiong
Florida State University Department of Physics

Ultrathin Pb Films in a Magnetic Field: New Physics from an Old Superconductor

Even in materials where the origin of superconductivity is known to be conventional, dimensional confinement, disorder, electron correlation, external magnetic field, and magnetic impurities often combine to induce novel electronic phases and quantum phase transitions, many of which remain poorly understood and controversial. We have carried out a detailed examination of the superconductivity and superconductor-insulator transitions in ultrathin amorphous Pb films as functions of disorder, magnetic field, and paramagnetic pair-breaking. The Pb films are grown via quench-condensation in a modified dilution refrigerator under ultrahigh vacuum at low temperature, and all the electrical measurements are performed in situ. Here I describe and discuss two intriguing observations from these experiments: i) A perpendicular magnetic field induces features suggestive of mesoscale phase separation near the critical field and an insulating state with localized superconductivity.1 ii) In the same films, a parallel magnetic field is found to enhance superconductivity, increasing the mean-field Tc by as much as 13% in field as high as 8 T. The Tc enhancement is progressively suppressed, eventually eliminated, by incremental deposition of magnetic impurity on the film.2

1. J.S. Parker, D. Read, A. Kumar, and P. Xiong, Europhys. Lett. 75, 950 (2006).
2. H.J. Gardner, A.S. Kumar, L. Yu, P. Xiong, M. Warusawithana, L. Wang, O. Vafek, D.G. Schlom, Nature Physics 7, 895 (2011).


September 22, 2014

Randy Fishman
Oak Ridge National Laboratory, Materials Science and Technology Division

Using Inelastic Scattering Measurements to Determine the Complex Spin States of Multiferroic Materials

Because they couple magnetic and electric degrees of freedom, multiferroic materials hold tremendous technological promise and remain the subject of intense scrutiny. In practice, elastic neutron scattering alone is insufficient to determine the complex, non-collinear spin structures of these materials. But inelastic spectra provide dynamical “fingerprints” for the spin states and interactions of multiferroic materials. This is demonstrated for two materials that fall within different classes of multiferroics. Whereas BiFeO3 is a type I multiferroic with the ferroelectric transition temperature Tc higher then the Neel transition temperature TN, CuFeO2 is a type II multiferroic with Tc = TN. Although the spin states of these materials are distorted cycloids or spirals, there are important differences between the two due to the different origins of their multiferroic behavior. Research sponsored by the Division of Materials Sciences and Engineering, U.S. Department of Energy under contract with UT-Battelle, LLC.


September 29, 2014

Michael Duncan, Franklin and Regents Professor
Department of Chemistry, University of Georgia

Infrared Spectroscopy of Cold Ions and their Clusters

Cold cations of metal-molecular complexes or of small hydrocarbons (aka "carbocations") are produced in a pulsed supersonic molecular beam by laser vaporization or pulsed discharge sources. These ions are mass-selected and studied with infrared photodissociation spectroscopy. Infrared spectra are compared to the predictions of theory to elucidate the structures of these ions and, in the case of metals, their electronic states. Transition metal (Co, V, Mn, Cu) carbonyls are studied in the C-O stretching region. The spectra reveal coordination numbers and ligand vibrational shifts as a function of cluster size. Carbocations (C2H3+, C3H5+, C3H3+, protonated benzene, protonated naphthalene) are studied in the C-H stretching and fingerprint regions of the spectrum. Several of these species exhibit more than one structural isomer, allowing investigation of the multiple minima on their potential surfaces. Unusual vibrations are detected for non-classical structures with bridging hydrogens. Protonated naphthalene has spectral lines relevant for the Unassigned Infrared Bands seen in interstellar gas clouds.


October 6, 2014

Sean X. Sun, Department of Mechanical Engineering
Johns Hopkins University

The Role of Water in Cell Mechanics, Cell Shape and Cell Motility

The lipid bilayer membrane of eukaryotic cells is directly permeable to water. Careful volume measurements during osmotic shock experiments show that the cell can actively adjust its volume by adapting to external osmotic shocks. We mathematically analyze cellular pressure and volume control by considering both cytoskeletal dynamics and active regulation of cellular osmotic content. We show that water permeation across the cell membrane is a major contribution to the slow phase of cellular mechanical response. Cell shape changes during division and morphogenesis, and cellular tension homeostasis may also have significant contributions from water dynamics. We demonstrate that water permeation alone can drive cell motility in confined environments. The last finding is significant for cancer cell motility in some situations.


October 20, 2014

Yasu Takano, Department of Physics
University of Florida

Spin-1/2 Heisenberg Antiferromagnets in One Dimension

The one-dimensional spin-1/2 Heisenberg model is one of the few exactly solvable models in physics. When the interaction is antiferromagnetic, the ground state of this model is a Tomonaga-Luttinger spin liquid, in which dynamic and static properties are inextricably linked. Low-energy excitations are spinons, which are fermions, instead of bosonic magnons, with a unique gapless dispersion. These and other properties of the model have been extensively studied since the pioneering work by Bethe published in 1931. This talk describes recent experiments that finally put some of the theoretical results to tests.


October 27, 2014

K. Ernst Rehm
Argonne National Laboratory

Studies of X-ray Bursts – Bringing Neutron Stars into the Laboratory

X-ray bursts are thermonuclear explosions occurring on the surface of accreting neutron stars. Measurements from orbiting X-ray satellites during the last years have provided us with a wealth of information about the nuclear reactions thought to occur in this extreme, high-density environment. With radioactive ion beams available at first-generation facilities we have begun to study these processes in the laboratory. In this talk I will report on experiments performed with radioactive beams from the ATLAS accelerator at Argonne which are related to X-ray bursts. I will discuss the properties of X-ray bursts, the production of radioactive beams, ways to improve the purity and the energy resolution of these beams, novel detectors, optimized for experiments with weak radioactive beams, which are crucial to measure the relevant nuclear cross sections.

This work was supported by the US Department of Energy, Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357.


November 3, 2014

Jeremy Levy
Distinguished Professor at the University of Pittsburgh in the Department of Physics and Astronomy, and Director of the Pittsburgh Quantum Institute

Etch-a-Sketch Nanoelectronics

Electronic confinement at nanoscale dimensions remains a central means of science and technology. I will describe a novel method for producing electronic nanostructures at the interface between two normally insulating oxides, LaAlO3 and SrTiO3. Conducting nanostructures are written, erased and reconfigured under ambient conditions at room temperature, similar to the operation of an etch-a-sketch toy. A wide variety of devices can be created, including nanowires, tunnel junctions, diodes, field-effect transistors, single-electron transistors, superconducting nanowires, and nanoscale THz emitters and detectors. After an introduction, I will focus on two recent results: the discovery of a novel phase in which electrons form pairs without becoming superconducting, and the discovery of electronically controlled ferromagnetism at room temperature. Both phenomena occur in the same family of LaAlO3/SrTiO3 heterointerfaces.


November 10, 2014

Takeshi Egami
Director, Joint Institute for Neutron Sciences, and UT/ORNL Distinguished Scientist Professor, Department of Physics and Astronomy and Department of Materials Science and Engineering

Liqid-State-Physics

Liquids are found everywhere. Our body is mostly made of water. And yet we understand so little about the liquid at the atomic level. This is because most of the theoretical tools in condensed matter physics assume lattice periodicity and translational invariance, whereas in the liquid atoms are constantly moving and are arranged in a disordered manner. The absence of translational symmetry has made it so difficult to develop sufficiently powerful and sophisticated theory of liquids. In the absence of periodicity we have to face the many-body problem head-on. For this reason today’s most advanced liquid theory is a continuum theory based upon hydrodynamics in which the atomic structure is considered only obliquely. But I argue that the atomic discreteness is crucial in understanding the liquid, and in this talk I introduce a new view of the liquid state based upon the topology of the atomic structure. For instance the elementary excitations of lattice vibration are phonons. However, in high-temperature liquids phonons are localized and marginalized. Instead, we discovered that the local topological excitations in the atomic connectivity network are the elementary excitations in the liquid [1], and govern the shear flow of the liquid [2]. In the glassy state local deformation is triggered by a very small group of atoms, five atoms in average, which starts local avalanche [3]. We also found that the condensation to discrete states, very much like quantization, forms the basis for the glass transition. These findings hopefully would seed the growth of a new discipline of the “liquid-state-physics."

[1] T. Iwashita, D. M. Nicholson and T. Egami, Phys. Rev. Lett., 110, 205504 (2013).
[2] T. Iwashita and T. Egami, Phys. Rev. Lett., 108, 196001 (2012).
[3] Y. Fan, T. Iwashita and T. Egami, Nature Communications, 5, 5083 doi: 10.1038/ncomms6083 (2014).


November 17, 2014

Thomas Maier
Computer Science and Mathematics Division and Center for Nanophase Materials Sciences, Oak Ridge National Laboratory

High-Temperature Superconductivity from Repulsive Interactions: Unconventional Pairing in Hubbard Models

Unconventional superconductors such as the copper-oxide and iron-based materials represent some of the most challenging problems in condensed matter physics, and many of their properties remain poorly understood and controversial. Driven by the physics of strong electronic interactions, they do not fit the standard BCS model of superconductivity, in which an attractive electron-phonon interaction binds electrons into pairs to mediate superconductivity. Here I will review numerical calculations of Hubbard models, in which the repulsive Coulomb interaction between the electrons is believed to give rise to pairing. These models exhibit many of the properties that are also observed in experiments, including antiferromagnetism, superconductivity and pseudogap behavior, and thus provide an excellent framework to understand the type of pairing that occurs in unconventional superconductors. I will discuss what these models tell us about the nature and origin of unconventional superconductivity and how they have to be extended to describe more detailed aspects of the physics of these systems.


November 24, 2014

Alan Tennant
Oak Ridge National Laboratory

Twists in Quantum Magnets

Neutrons provide the ability to look at quantum states of matter in unrivaled detail. Using quantum magnets in conjunction with magnetic fields exotic phases of matter can be generated that are highly quantum entangled. The wave functions in these states can be probed in space and time and in quantum critical states in particular elaborate symmetries and strange properties like fractional quantum numbers are revealed. In this talk I will show some of the remarkable physics that can be explored and how neutron scattering can be used to investigate what is going on.


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