The David and Edith Harris Physics Colloquium Archives

SPRING 2013 Schedule

February 7, 2013
Bristol University, UK
Hosted by PGSC

“Variations on a theme of Aharonov and Bohm”

The partial anticipation of the AB effect by Ehrenberg and Siday was an approximation whose wavefunction was not singlevalued; its connection with the singlevalued AB wave involves topology: ‘whirling waves’ winding round the flux. AB is a fine illustration of idealization in physics. There are four AB effects, depending on whether the waves and the flux are classical or quantum; in the classical-classical case, fine details of the AB wavefunction have been explored experimentally in ripples scattered by a water vortex. The AB wave possesses a phase singularity, and there is a similar phenomenon in general interferometers. There are connections between the AB wave and the Cornu spiral describing edge diffraction. For bound systems, the interplay of AB and geometric phases exemplifies general aspects of degeneracies induced by varying parameters.

February 14, 2013
Brandeis University
Hosted by Mehran Kardar

“Marginal Matter”

Granular materials such as sand or rice grains behave in ways that are often counterintuitive.  An example is “footprints on sand” which owe their origin to a phenomenon known as dilatancy.  Our intuition often fails because dry granular materials are non-cohesive, and live at zero temperature.  The distinction between gases, liquids and solids is ill understood.  These materials can solidify via non-equilibrium pathways in which applied stresses or boundary constraints play a crucial role.  A striking example of this is shear-jamming, where an amorphous granular solid is created through the application of shear.  This is in sharp contrast to our usual experience of shearing leading to flow.  In this talk, I will illustrate some of the fascinating properties of dry grains, and describe a theoretical framework for understanding how these materials “solidify”.

February 21, 2013
Princeton University, Co-chair, International Panel on Fissile Materials and MIT ‘59
Hosted by Aron Bernstein

Physics in the Interest of Society Colloquium

“Physicists and Nuclear Arms Control: Still Plenty of Work to be Done”

The Cold War is over and the danger of nuclear annihilation has receded somewhat – but not far enough. Russia and the United States still each have about 1000 nuclear warheads ready to launch at each other on 15 minutes notice; there is a dangerous nuclear arms race in South Asia; North Korea has acquired the bomb; there is fear that Iran might also; and we worry about the possibility of nuclear terrorism. It seems a miracle that a nuclear weapon has not been used against a city for almost seven decades.

The common denominator of all these problems is nuclear-weapon-usable material – most importantly, highly enriched uranium (HEU) and separated plutonium. Although the world has reduced from over 60,000 to about 10,000 nuclear weapons, there are still 1400 tons of HEU and 500 tons of separated plutonium globally– enough for more than 100,000. Major non-weapons uses in some countries are HEU for naval-reactor fuel and plutonium recycle in power reactor fuel. Nuclear-weapon-useable materials could be replaced in virtually all of these uses.

Physicists have contributed in important ways to ending the Cold War arms race and in securing and eliminating nuclear-weapons materials around the world. MIT’s Tom Neff, for example, proposed the 1993 HEU-deal whereby the U.S. will have bought by the end of this year 500 tons (20,000 bombs worth) of excess Russia Cold War highly-enriched uranium for blend-down to low-enriched uranium fuel for U.S. power reactors. More physicist policy-analyst/activists are required, however, to deal with the huge nuclear dangers that remain.

February 28, 2013
California Institute of Technology
Hosted by Nevin Weinberg

"The Restless Universe (Palomar Transient Factory)"

That occasionally new sources ("Stella Nova") would pop up in the heavens was noted more than a thousand years ago. The earnest study of cosmic explosions began in earnest less than a hundred years ago.  Stella Novae are now divided into two major families, novae and supernovae (with real distinct classes in each). Equally the variable stars have a rich phenomenology.  Together, supernovae and variable stars have contributed richly to key problems in modern astrophysics: distances to galaxies, cosmography and build up of elements in the Universe.

The Palomar Transient Factory (PTF), an innovative 2-telescope system, was designed to explicitly chart the transient sky with a  particular focus on events which lie in the nova-supernova gap. PTF is now finding an extragalactic transient every 20 minutes and a Galactic (strong) variable every 10 minutes.  The results so far: classification of 2000 supernovae, identification of an emerging class of ultra-luminous supernovae, the earliest discovery of a Ia supernovae, discovery luminous red novae, the most comprehensive UV spectroscopy of Ia supernovae, discovery low energy budget supernovae, clarification of sub-classes of core collapse and thermo-nuclear explosions, mapping of the systematics of core collapse supernovae, identification of a trove of eclipsing binaries and the curious AM CVns.

MARCH 7, 2013
ETH Zurich
Hosted by Vladan Vuletic

“Bringing Toy Models to Reality: Graphene and Quantum Magnetism”

Fermionic quantum gases in optical lattices make it possible to physically construct and study key models of quantum physics. The riddle of high temperature superconductivity, or the beauty of graphene, are becoming accessible to experiments, in which the Hamiltonian is a direct result of the optical lattice potential created by interfering laser fields and short-ranged collisional interaction between ultracold atoms. Using a tunable geometry optical lattice we have recently been able to create a honeycomb lattice structure and identify Dirac points, move them within the Brillouin zone and make them appear or disappear. In an effort to reach the regime of quantum magnetism in an atomic Fermi Hubbard model, we use a dimerized and an anisotropic cubic lattice to locally engineer the entropy distribution. This allowed us to observe short-range antiferromagnetic correlations.

MARCH 14, 2013
Hosted by Society of Physics Students

"Quantum Spin Liquids: a New Kind of Magnetism"

I will describe recent progress in the development of a new kind of magnetism rooted in the phenomenon of entanglement. This new state of matter, called the quantum spin liquid, appears when quantum effects and frustration conspire to prevent the ground state from achieving classical order. This year marks the 40th anniversary of the original proposal for such a state; however, they have only recently been realized in experiments. A recent breakthrough in crystal growth has led to the discovery of spin liquid physics in a material based on the frustrated kagomé lattice. Inelastic neutron scattering measurements reveal that the spin excitations are fractionalized into emergent quasiparticles called spinons, a remarkable first. Spin liquids may underlie the phenomenon of high temperature superconductivity, and their entanglement properties may eventually find use in quantum information processing.

MARCH 21, 2013
Columbia University
Hosted by William Detmold

"The Weak Interactions of Strange Quarks and Physics at 1000 TeV"

In the standard model, processes which violate CP symmetry or those in which a strange quark decays directly into a down quark are highly suppressed, making it possible to detect new interactions which allow these phenomena but which occur at a much higher energy scale.  Encouraged by the ability to perform lattice QCD calculations at the physical value of the light quark mass, the RBC and UKQCD collaborations are now attempting to calculate such suppressed process within the standard model so that the contributions of new, ultra-high energy interactions can be recognized.  We will describe the new methods that allow lattice QCD calculation of the physical K →Π Π decay amplitudes including their CP violating phase and the KLKS mass difference, both of which are expected to be sensitive to new physics at very high energies.  The present state and future prospects for these calculations will be presented.

APRIL 4, 2013
Hosted by Jesse Thaler

“The Mysteries of Mass Unveiled?”

On July 4, 2012, the discovery of a new boson with mass around 125 GeV was announced at CERN. A significant excess of events was observed above the expected background, signaling the production of a new particle. First measurements of its properties are compatible with those of the long sought standard-model Higgs boson. I will present the latest results on the Higgs boson from the LHC.

APRIL 11, 2013
Yale University
Hosted by Graduate Women In Physics

"The Cosmic Growth of Supermassive Black Holes and their Co-Evolution with Host Galaxies"

The growth of black holes over billions of years releases energy that may quench star formation and strongly affect galaxy evolution (“feedback”). We designed multiwavelength surveys to trace the cosmic history of this black hole growth at the centers of galaxies. We found that most Active Galactic Nuclei are heavily obscured, and thus are not found in large area optical surveys like the Sloan Digital Sky Survey, and that obscuration is more common in the young Universe and in moderate luminosity AGN. Most black hole growth takes place in these moderate luminosity AGN rather than in their higher luminosity counterparts (“quasars”), and feedback in such systems affects far more galaxies than do quasars. At the peak epoch of black hole growth and star formation (>5 billion years ago), AGN may help quench star formation (which is not the case in the local Universe). Perhaps surprisingly, most moderate luminosity AGN are hosted in galaxies with significant disks, even at the peak epoch, suggesting that major mergers do not trigger most black hole growth. Finally, we find an intriguing dependence of AGN activity on host galaxy morphology, which is not yet fully explained.

APRIL 18, 2013
Princeton University
Hosted by Nuh Gedik

"Visualizing and Manipulating Topological Quantum States in Novel Materials and Nanostructures"

Soon after the discovery of quantum mechanics it was realized why some solids are insulating (like diamond) and others are highly conducting (like graphite), even though they could be comprised of the same element. Now, 80 years later, the concept of insulators and metals is again being fundamentally revised. During the last few years, it has become apparent that there can be a distinct type of insulator, which can occur because of the topology of electronic wave functions in materials. The key consequence of this topological characteristic (and the way to distinguish a topological insulator from an ordinary one) is the presence of metallic electrons with helical spin texture at their surfaces. I will describe experiments that directly visualize these novel quantum states of matter and demonstrate their unusual properties through spectroscopic mapping with the scanning tunneling microscope (STM). These experiments show that the spin texture of these states protects them against back scattering and localization. In fact, these states appear to penetrate through barriers that stop other electronic states. I will also describe more ongoing efforts focused on manipulating these states with magnetism and experiments on new class of topological states that are protected by crystalline symmetry. Finally, I will describe efforts in which atomically fabricated nano structures are being used to create new class of topological states involving superconductors and the hunt for novel topological edge modes that behave like Majorana Fermions.

APRIL 25, 2013
Leiden University
Hosted by Hong Liu

"The String Theory – Condensed Matter Flirtation: An Eyewitness Account"

A quake is rumbling through the core of physics: the empiricisms of condensed matter physics and the mathematics of string theory appear to have some deep relations. For the initiated this has an unusually strong allure, but since this cocktail involves some of the most impenetrable areas of physics it is not easy to communicate the excitement to the community at large.  I will attempt to get some of it across by telling the story from the perspective of a veteran condensed matter theorist who learned string theory only quite recently. How string theory evolved from a reductionist’s enterprise into some modern incarnation of statistical physics, equipped with general relativity turbo’s and quantum information boosters in the form of the “AdS/CFT” holographic duality. How the universality of general relativity turned into a classification method for phases of matter, including new forms of “quantum” matter characterized by dense quantum entanglements on the macroscopic scale. How the latter reveal highly unusual traits having eerie resemblances with the mysterious experimental observations in the strange metals as discovered in high Tc superconductors and other strongly interacting electron systems.  How by turning this machine in reverse gear the idea came up very recently that the movie The Matrix might have guessed something right, be it with a paradoxal twist: it appears that Einsteinian space-time can be precisely encoded in the dense quantum information “familiar” from the quantum matter. 

MAY 2, 2013
Paul Scherrer Institute (PSI)
Hosted by Markus Klute

“Computational Particle Accelerator Physics - Multidisciplinary Science for next generation Accelerators”

With the aim to discover physics beyond the standard model, the IsoDAR experiment is an excellent example to describe the sort of multidisciplinary science which is necessary for this definitive search of 1 or 2 sterile neutrinos at the 5σ level.

In the proposed IsoDAR experiment a 600 kW (CW) beam of protons will impinge on a Lithium target to generate copious Li-8. The Li-8 then decays at rest to yield a powerful source of anti-neutrinos that can be located ≈ 20 m from a hydrogenous detector.

I will use this challenging example to demonstrate how the combination of physics modeling, advanced numerical techniques and the efficient use of high performance computing are of utmost importance for projects like IsoDAR.

In more detail I will explain how non-linear dynamics and forefront computer science techniques will lead to modeling capabilities, with the aim to precisely predict tails of particle distributions at the 4 to 5σ level. This is a central accelerator physics challenge in the design and operation of all accelerators for intensity frontier physics. Results of model calculations are compared with data from the PSI 1.4 MW (CW) proton facility.

I will close with challenges for the field and highlight many opportunities for generations of accelerator scientists.

MAY 9, 2013
Rutgers University
Hosted by Pablo Jarillo-Herrero

"Graphene: a Physicist’s View of the 'Wonder Material'"

Within the eight years since its first scotch-tape extraction from graphite, Graphene – a one atom-thick crystal of carbon - has metamorphosed from the poor relative of diamond into a “wonder material” whose appeal crossed disciplinary boundaries from physics and engineering to biology and medicine. At the beginning of 2013 the European Commission pledged one billion € to seed a European “Graphene valley”, designed to bring graphene and related layered materials from academic laboratories into the public domain to revolutionize industries and stimulate economic growth.

In this talk I will review the physics of graphene with emphasis on the unusual electronic properties which stem from its ultra-relativistic charge carriers - Dirac fermions. I will describe experiments which employed transport, scanning tunneling microscopy and spectroscopy to gain access to the fascinating world of two-dimensional Dirac fermions, their interactions with each other and with the environment.

MAY 16, 2013
Harvard University
Hosted by Jeff Gore

"Attempts at Disentangling Biological Circuits by Controlling Them"

Biological circuits (biochemical circuits within a cell and neural circuits) appear to be complicated. The work in my lab has been motivated by the following questions: how many degrees of freedom do these circuits have? By attempting to control the dynamics of different parts of such circuits, can we control animals behavioral decision or a cell's developmental choice? And through such attempts at control, can we gain a better understanding of the architecture of the underlying circuits? In this talk I will describe our attempts at controlling the behavior the nematode Caenorhabditis elegans and ask if we can trigger neural activity patterns in the animal to make it  believe that there is a chemo-attractive signal gradient in its environment. In the process, I will also describe novel hardware and software tools that make our experiments possible.