Unless otherwise noted, the physics colloquia are held in Room 307 of the Science and Engineering Research Facility. Refreshments are served at 3:00 p.m. with the talk following at 3:30. The 2017 colloquia are available here, with the archives from previous semesters available Webcast archives.
Bruce D. Gaulin, McMaster University
Quantum Ground States in Real Frustrated Magnets
The pyrochlore lattice, a network of corner-sharing tetrahedra, is one of the most pervasive crystalline architectures in nature that supports geometrical frustration. We and others have been interested in a family of rare earth pyrochlore magnets, that can display quantum S=1/2 magnetism on such a lattice. The ground states for these materials may be described by a model known as "spin ice", a model with the same frustration and degeneracy as solid ice (the kind you skate on), as well as by a quantum version of this model known as "quantum spin ice" that possesses an emergent quantum electrodynamics. I'll describe how this comes about and how we can understand these materials, with an emphasis on modern neutron scattering. I'll also briefly discuss how fragile some of these quantum ground states seem to be with respect to weak quenched disorder, which is hard to avoid in real materials.
Greg Fiete, University of Texas
Searching for New Topological Phases in Correlated Materials
Recent years have seen rapid advances in the theoretical understanding of materials with strong spin-orbit coupling, and experiments have identified new classes of materials exhibiting unusual electrical properties. Many of these discoveries fall in the class of "topological materials". In this talk, I will summarize some of the recent developments in this field and highlight some of our own work based on a combination of model Hamiltonian studies and first-principles approaches to guiding experimental discovery of these phases in transition metal oxides. Some potential device applications will also be described.
Chris A. Tulk, Oak Ridge National Laboratory
Water Ices, an Age Old System, when Compressed to Ultra-High Pressures Present New Challenges for Physicists and Chemists
Water, and it’s solid form ice, has been known to be ubiquitous throughout the universe for some time. Indeed ice and it’s compounds have been studied for hundreds of years, yet only in the last 50 years or so have we began to understand fully its crystallographic diversity. The ice that we experience every day is well known to be structurally hexagonal with a negative Clausius–Clapeyron relation between the solid and the melt. However, under varying pressure and temperature conditions at least 16 different crystallographic structures, and a multitude of glassy and amorphous solid forms, of pure water ice have been experimentally shown to exist. It is likely one of the most structurally diverse molecular systems known. The pressure and temperature conditions in question range from the high vacuum of outer space to multi-Mbar (greater than 100 GPa) conditions and from below liquid nitrogen temperature up to several thousand Kelvin. Most recently the ‘holy grail’ of ice research encompasses two main themes. One is the proposed formation of a non-molecular form of ice, known as ice X, where the hydrogen atom is proposed to sit mid-way between two oxygen atoms. In such a case it is not possible to distinguish a water molecule in the system. In planetary systems where pressures reach multi-mega bar conditions, this is the most likely structure to form. The second is the physical relationship between the multitude of non-crystalline, or glassy and amorphous, forms of ice that form at cryogenic conditions. This includes transformations between distinct amorphous forms and the possibility of a second critical point of deeply super-cooled liquid water. In this presentation I will give a broad overview of the vast structural diversity of solid water, the current challenges we face in understanding this most fundamental of systems, the unique role that neutron scattering plays in understanding water and the most advanced techniques available for studying water at extreme conditions.
Samindranath Mitra, PRL Staff
Physics After the Lab and the Desk: Your work in PRL
Physics research takes place mostly at your desk, at the keyboard, in the lab. You communicate results through posters, talks, and papers -- leading to, hopefully, wide dissemination and recognition. The sequence entails interacting with journal editors, referees, conference chairs, journalists, and so on. I will focus on this post-research collaborative process in physics, primarily through the lens that is Physical Review Letters.
Gail McLaughlin, North Carolina State University
Stellar Explosions and Element Synthesis
The astrophysical origin of many of the heaviest elements is an unsettled question. About half of the elements with mass number greater than 100 are thought to be made by a rapid neutron capture process (r-process) that requires many neutrons. A definitive determination of the astrophysical site in which the r-process elements are produced has presented a challenge, although rapid progress is underway. I will review the leading suggestions and discuss new approaches to this problem.
Thomas Corbitt, Louisiana State University
Gravitational Wave Astronomy and Quantum Noise
In 1915, Albert Einstein published his theory of general relativity, which relates gravity to the curvature of spacetime. In 1916, Einstein predicted the existence of gravitational waves, or ripples in the fabric of spacetime, as a consequence of GR. The predicted magnitude of these waves was so small that it appeared unlikely they would ever be detected. On September 14, 2015, the LIGO (Laser Interferometric Gravitational-wave Observatory) detected a gravitational wave signal that originated from the coalescence of two merging black holes approximately 1.3 billion light years away. This detection confirms Einstein’s prediction, and it marks the beginning of a new branch of astronomy. Detection of gravitational wave signals will allow us to learn about some of the most exotic objects in the universe, such as black holes and neutron stars, in a way that is impossible with electromagnetic based observations. A second binary black hole coalescence was seen on December 26, 2015. The LIGO Scientific Collaboration operates two detectors, one in Hanford, Washington, and the other in Livingston, Louisiana. These detectors are among the most sensitive devices ever built, and they detect gravitational waves by using laser interferometry to measure relative changes in the position of two test masses, separated by a distance of 2.5 miles, at a level better than one thousandth the diameter of a proton. I will discuss how quantum noise limits the sensitivity.
Wouter Deconinck, College of William and Mary
The Qweak Experiment at Jefferson Lab: Searching for TeV Scale Physics by Measuring the Weak Charge of the Proton
In analogy to the electromagnetic charge, the proton carries a weak charge that describes the strength of its interactions with Z bosons. Thanks to an accidental suppression in the Standard Model, the weak charge happens to be nearly zero and is therefore sensitive to effects from physics beyond the Standard Model. However, measuring the weak charge is all but easy. Its effect in elastic electron scattering from protons is at the level of parts per billions compared to the electromagnetic interaction. However, the tiny effect of Z boson exchange diagrams can be accessed through parity-violating electron scattering.
In the Qweak experiment at Jefferson Lab we scattered alternatively positive and negative helicity electron beams from the proton, and the scattering cross section difference is proportional to the weak charge of the proton. Using only 4% of the data collected between 2010 and 2012, the Qweak experiment made the first determination of the weak charge of the proton in 2013, in agreement with Standard Model predictions. Now, we are putting the finishing touches on our high precision result based on the full data. In the next months we will be able to compare our precision measurement of the weak charge with the Standard Model prediction, with sensitivity to new particles out of reach of any current accelerator. Regardless whether agreement or disagreement, the result will have an impact on our confidence in the Standard Model.
Julia Velkovska, Vanderbilt University
The Tiniest Perfect-Liquid Droplets
The quark-gluon plasma (QGP) produced in ultra-relativistic collisions between large nuclei, such as Au+Au or Pb+Pb, is a state of nuclear matter with extremely high temperature and energy density. The particles produced in these collisions exhibit collective behavior that indicate that QGP is a liquid with extremely low specific viscosity, which makes it the most perfect liquid in nature. In the quest of understanding how the perfect fluid emerges, experiments at the Large Hadron Collider (CERN, Switzerland) and the Relativistic Heavy Ion Collider (BNL) studied collisions between protons or other small nuclei with large nuclei, which were not expected to produce QGP. To our surprise, we found that collective behavior is also present in a fraction of these collisions, i.e. – the most violent ones that produce a large number of particles. How small can a system be and still behave as a liquid? This talk will focus on the world’s tiniest perfect-liquid droplets.
Alexander L. Fetter, Stanford University
Trapped Rotating Bose-Einstein Condensates
After reviewing the basic physics of Bose-Einstein condensation, I discuss the non-linear Gross-Pitaevskii equation that provides a good description of a trapped dilute condensate. Here the interest is the rotational properties of the condensate, especially the role of quantized vortex lines. For slow rotations, only a few vortices appear, but as the angular velocity increases, the vortices form a triangular lattice that is analogous to the Abrikosov vortex lattices in type-II superconductors. Eventually the system enters the lowest Landau level regime where the mean interaction energy per particle is small compared to the energy gap separating the first and second Landau levels. For very fast rotations, theorists predict a quantum phase transition to a non-superfluid highly correlated state analogous to those seen in the quantum Hall regime for two-dimensional electrons in a strong magnetic field.
Krzysztof P. Rykaczewski, ORNL Physics Division
Super Heavy Elements and Nuclei
The existence of the Island of Stability in super heavy nuclei has been predicted fifty years ago. The first nuclei at the Island have been reached nearly twenty years ago, in pioneering studies at Dubna (Russia) using radioactive actinide targets from Oak Ridge and Dmitrovgrad (Russia) irradiated by doubly magic 48Ca projectiles. As for today, we know the basic decay properties of over fifty nuclei at the Island of Stability. Recently, the discovery of four new elements has been recognized by a joint committee of both International Unions, for Pure and Applied Chemistry (IUPAC) and for Pure and Applied Physics (IUPAP). New element names and symbols have been accepted, including 117Ts tennessine. However, there are still many fundamental questions in the physics and chemistry of super heavy nuclei and elements:
- how many protons and neutrons a nucleus can hold?
- what are the effects of the strongest Coulomb fields on atomic properties?
- where are the limits of the Island of Stability?
- what is the most effective way to synthesize even heavier nuclei and new elements?
- is it possible to develop a unified description of nuclear properties across varying proton and neutron numbers?
New powerful laboratories like Dubna’s SHE Factory (Russia), RIKEN (Japan), GANIL (France) and other facilities under considerations, neutron-rich actinide targets and possibly radioactive isotope beams together with improved separation and digital detection methods can help us to continue the discoveries at the top of nuclear and atomic worlds. I’ll address above topics and present future studies focused on the heaviest isotopes and atomic elements.
Young Lee, Stanford University and SLAC National Accelerator Laboratory
Quantum Spin Liquids: A New State of Magnetism
Quantum spin liquids are new states of matter that are characterized by long-range entanglement and support exotic excitations. Unlike traditional magnetic ground states, the spin liquid state does not break conventional symmetries. Rather, they are one of the few systems believed to have topological order. After 40 years since the original theoretical proposal for such a state, these systems have only recently been realized in experiments. A material called Herbertsmithite is a leading candidate for having a quantum spin liquid ground state. Inelastic neutron scattering measurements on single crystals reveal that the spin excitations are fractionalized, a remarkable first. More recent studies of the low-energy excitations with NMR and neutron techniques reveal evidence for a spin-gap consistent with theoretical calculations, providing the strongest proof yet of the existence of this unusual ground state.
Ariel Amir, Harvard University
Controlling Cell Size and DNA Replication in Bacteria - Insights from Mathematical Modeling
Understanding how cells control and coordinate the various ongoing cellular processes, such as DNA replication, growth and division is an outstanding fundamental problem in biology. Remarkably, bacterial cells may divide faster than their chromosomes replicate, implying that cells maintain multiple rounds of chromosome replication, and that tight control over DNA replication must be in place. I will show how ideas from statistical mechanics and mathematical modeling can serve as alternative "microscopes" into this problem. Our results suggest that both cell size and chromosome replication may be simultaneously regulated by following a simple control mechanism, in which, effectively, a constant volume is added between two DNA replication initiation events. This model elucidates the experimentally observed correlations between various events in the cell cycle, and explains the exponential dependence of cell size on the growth rate, as well as recent experiments in which cell morphology is perturbed.