# Physics Colloquium

## Spring 2013 Colloquium Schedule

Date Speaker Title
January 14 Stephen E. Nagler
ORNL Corporate Fellow and Director, Quantum Condensed Matter Division
Good Vibrations: Quantum Oscillations of Nitrogen Atoms in a Crystal
January 21 MLK Holiday No Colloquium
January 28 Victor Yakovenko
Department of Physics, University of Maryland
Statistical Mechanics of Money, Income, Debt, and Energy Consumption
February 4 Pawel Hawrylak
Semiconductor and Graphene Quantum Dots
February 11 Yuval Grossman
Cornell University
Leptogenesis
February 18 John Donoghue
University of Massachusetts, Amherst
Is This the Best of All Possible Universes?
February 25 Steve Elliott
Los Alamos National Laboratory
Double Beta Decay: Is the Neutrino Mass within Reach?
March 4 Chris Greene
Purdue University
Clustering and Recombination of Ultracold Bosonic Atoms
Los Alamos National Laboratory
Nuclear and Particle Physics with Cold Neutrons
March 18 APS March Meeting No Colloquium
March 25 Spring Break No Colloquium
April 1 Mehran Kardar
Massachusetts Institute of Technology
Levitation by Casimir Forces in and out of Equilibrium
April 8 Peter Jacobs
Lawrence Berkeley National Laboratory
Hot QCD Matter in the Laboratory
Michigan State University
Tracking Changes in the Structure of Nuclei with Fast Beams of Rare Isotopes
April 22 Departmental Honors Day Celebration

## Abstracts

#### January 14 Stephen E. Nagler* Oak Ridge National Laboratory Quantum Condensed Matter Division Good Vibrations: Quantum Oscillations of Nitrogen Atoms in a Crystal

Next generation of time-of-flight inelastic neutron scattering instruments are now enabling the exploration of new realms of dynamics in solids. In this talk I will briefly describe spectrometers now operating at the Spallation Neutron Source, and then focus on one recent experiment where startling and beautiful results have been obtained. The particular example is a measurement of vibrational excitations in a single crystal of uranium nitride, which has a simple cubic rock-salt structure. In addition to the usual acoustic and optic phonon excitations one observes a ladder of equally spaced well-defined modes extending to energies at least as high as 500 meV. Analysis of the momentum and energy dependence of the scattering and additional insights obtained from ab-initio calculations show that each individual nitrogen atom is a nearly ideal realization of the isotropic three dimensional quantum harmonic oscillator, one of the few exactly solvable problems in quantum mechanics. This behavior arises because of the large mass difference between the nitrogen and uranium atoms. Knowledge of the oscillator modes is important for at least one potential application of uranium nitride. See Nature Communications 3, 1124 (2012); also research highlights comment in Nature Materials 11, 1002 (2012).
*Work on UN done in collaboration with A. A. Aczel, G. E. Granroth, G. J. MacDougall, W. J. L. Buyers, D. L. Abernathy, G. D. Samolyuk and G. M. Stocks. Recent work on UC also in collaboration with Yuen Yiu. Research supported by the U. S. Department of Energy, Office of Basic Energy Sciences, Scientific User Facilities Division.

#### January 28 Victor Yakovenko, University of Maryland Department of Physics Statistical Mechanics of Money, Income, Debt, and Energy Consumption

By analogy with the probability distribution of energy in statistical physics, I argue that the probability distribution of money in a closed economic system should follow the exponential Boltzmann-Gibbs law. Analysis of the empirical data shows that income distribution in the USA has a well-defined two-class structure. The majority of the population (about 97%) belongs to the lower class characterized by the exponential ("thermal") distribution. The upper class (about 3% of the population) is characterized by the Pareto power-law ("superthermal") distribution, and its share of the total income expands and contracts dramatically during bubbles and busts in financial markets. The probability distribution of energy consumption per capita around the world also follows the exponential Boltzmann-Gibbs law, which is consistent with entropy maximization. For more information, see http://physics.umd.edu/~yakovenk/econophysics/, Rev. Mod. Phys. 81, 1703 (2009), New J. Phys. 12, 075032 (2010).

#### February 4 P. Hawrylak, Quantum Theory Group, Emerging Technologies Division National Research Council of Canada Semiconductor and Graphene Quantum Dots

We review progress in theory and experiments on semiconductor and graphene quantum dots with potential applications in nanoelectronics, nanospintronics, and nanophotonics. We will describe lateral quantum dot molecules with controlled electron numbers in each dot and discuss potential use of such a molecule as building block of a field effect transistor with a macroscopic quantum state. We next turn to hybrid systems of self-assembled semiconductor quantum dots containing single magnetic impurities. Such impurities can be thought of as an atomic limit of quantum memory directly integrated into a semiconductor host. Finally, we describe one atom thick semiconductor quantum dots made of graphene. We show that their electronic, optical and magnetic properties can be engineered by the size, shape, type of edge and number of layers. We focus on their optical and magnetic properties, and their control with external gate, electric field and photons. Possibility of realizing a fully integrated carbon-only quantum circuit will be discussed.

#### February 11 Yuval Grossman, Cornell University Leptogenesis

There are two open questions in physics which seem unrelated. The first is why is there only matter around us? The second is how neutrinos acquire their tiny masses. It turns out that these two open questions may be related. That is, the same mechanism that gives neutrino masses can also generate a universe without anti-matter. In this talk I will explain the connection between these two issues and describe the on-going theoretical and experimental efforts in understanding them.

#### February 18 John Donoghue, University of Massachusetts, Amherst Is This the Best of All Possible Universes?

It is possible that the underlying theory could allow different regions of the universe to have different properties - an idea often called the "multiverse." This changes the way that we think about our fundamental theories. I will tour through some of the motivations and consequences of such theories.

#### February 25 Steven R. Elliott, Los Alamos National Laboratory Double Beta Decay: Is the Neutrino Mass within Reach?

The recent demonstrations of oscillations in the atmospheric and solar neutrino data convincingly indicate that neutrinos do have mass. Those data however, do not tell us the absolute mass scale but only the differences of the square of the neutrino masses. Even so, we now know that at least one neutrino has a mass of about 50 meV or larger.

Studies of double beta decay rates offer hope for determining the absolute mass scale. In particular, zero-neutrino double beta decay (ββ(0ν)) can address the issues of lepton number conservation, the particle-antiparticle nature of the neutrino, and its mass. In fact, the next generation of ββ(0ν) experiments will be sensitive to neutrino masses in the exciting range below 50 meV. An overview of ββ(0ν) and its relation to neutrino mass will be discussed followed by a brief profile of an upcoming experiment: the MAJORANA DEMONSTRATOR Project.

#### March 4 Chris H. Greene, Purdue University Department of Physics Clustering and Recombination of Ultracold Bosonic Atoms

This colloquium will describe recent theoretical progress in few-body systems with N=3, 4, or 5 particles. Such systems exhibit intriguingly simple behavior in the so-called "universality limit" where the bizarre quantal Efimov effect becomes relevant. Theoretical developments and experimental evidence demonstrate a deeper level of universality in the three-atom problem than had originally been expected. Recent theory extensions and experiments show a general pattern in the onset of cluster formation that occurs in a gas of free particles.

#### March 11 Nadia Fomin, Los Alamos National Laboratory Nuclear and Particle Physics with Cold Neutrons

Neutrons have been a useful probe in many fields of science as well an an important physical system for study in themselves. Modern neutron sources provide extraordinary opportinites to study a wide variety of physics topics. Among them is a detailed study of the weak interaction. In this talk, I will present an overview of studies of the hadronic weak (quark-quark) as well as semi-leptonic (quark-lepton) interactions. In addition, I will describe the progress of the NPDGamma experiment, which, due to the simplicity of the neutron, will provide an unambiguous measurement of the key weak coupling (often referred to as $f_{\pi}$), which will finally test the theoretical predictions.

#### April 1 Mehran Kardar, Massachusetts Institute of Technology Levitation by Casimir Forces in and out of Equilibrium

A generalization of Earnshaw's theorem constrains the possibility of levitation by Casimir forces in equilibrium. The scattering formalism, which forms the basis of this proof, can be used to study fluctuation-induced forces for different materials, diverse geometries, both in and out of equilibrium. In the off-equilibrium context, I shall discuss non-classical heat transfer, and some manifestations of the dynamical Casimir effect.

#### April 8 Peter Jacobs, Lawrence Berkeley Laboratory Hot QCD Matter in the Laboratory

Quantum Chromo-dynamics (QCD) is well-established as the correct gauge theory of the strong interaction, but exploration of the rich variety of phenomena driven by QCD is far from complete. Numerical calculations of QCD on the lattice indicate that matter at sufficiently high temperature should be in a de-confined state called the Quark-Gluon Plasma (QGP), in which quarks and gluons, rather than hadrons as in the ordinary matter around us, govern the dynamics. Experimental exploration of hot QCD matter with beams of heavy nuclei at collider energies has been carried out for over a decade at the Relativistic Heavy Ion Collider at Brookhaven, and more recently at the Large Hadron Collider at CERN. I will discuss recent progress in understanding the nature of the QGP from these experimental measurements and theoretical modeling, as well as connections to other fields of physics such as cold atomic gases and (depending on your taste) string theory.

#### April 15 Alexandra Gade, Michigan State University Tracking Changes in the Structure of Nuclei with Fast Beams of Rare Isotopes

The goal of nuclear structure physics is a comprehensive understanding of the properties of nuclei and nuclear matter from the interactions of the constituent protons and neutrons. Enormous progress has been made with measurements of properties of rare isotopes and developments in nuclear theory. Typically, the most exotic nuclei provide the most stringent guidance for nuclear models and allow identifying “missing physics.” At NSCL, rare isotopes are efficiently produced by the in-flight fragmentation of stable beams and are available for measurements as beams of fast ions. Well-established experimental techniques used for decades to study stable nuclei are not applicable at the low beam rates encountered for the most exotic isotopes. Powerful new precision techniques have been developed to enable in-beam spectroscopy studies of fast rare-isotope beams with intensities of a few ions per second. This presentation will show how in-beam experiments measure complementary observables that advance our understanding of the structure of nuclei. The interplay of experimental results and theory will be emphasized at the intersection of nuclear structure and reactions in the joined quest for a reliable model of the atomic nucleus.

Previous Physics Department Colloquia

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