Eric Sembrat's Test Bonanza

 Abstract

The frustration that stems from Gauss’ Theorema Egregium is familiar to anybody who attempted to wrap a ball with a paper. This fundamental theorem states that one cannot change Gaussian curvature of a surface without straining it. Thin sheets, however, are nearly inextensible and do not tolerate stress. Often, they accommodate geometrically incompatible confinement by wrinkling.

In this talk, we discuss the behavior of a thin sheet on a spherical substrate and demonstrate how Gaussian curvature of the substrate gives rise to a “non-monochromatic” wrinkle pattern.

 Abstract

In natural settings, microbes tend to grow in dense populations where they need to push against their surroundings to accommodate space for new cells. The associated contact forces play a critical role in a variety of population-level processes, including biofilm formation, the colonization of porous media, and the invasion of biological tissues. Although mechanical forces have been characterized at the single cell level, it remains elusive how single-cell forces combine to generate population-level patterns.

I present a synthesis of theory and microbial experiments that show that contacted forces generated by microbial populations can become very large due to a self-driven jamming mechanism. These forces feed back on the physiology of the cells and can strongly perturb the mechanical integrity of the environment, thereby promoting microbial invasion. Finally, I highlight that the cooperative nature of microbial force generation induces a screening effect that reduces the selection against slower growing mutant types. These results underscore that, in crowded microbial populations, collective phenomena often have a mechanical basis.

Abstract: Is life most likely to emerge at the present cosmic time around a star like the Sun? Loeb will review the habitability throughout cosmic history from the birth of the first stars 30 million years after the Big Bang to the death of the last stars in ten trillion years. Unless habitability around low mass stars is suppressed, life is most likely to emerge near stars with a tenth of a solar mass ten trillion years from now. Forthcoming searches for bio-signatures in the atmospheres of transiting Earth-mass planets around nearby low-mass stars will determine whether present-day life is indeed premature from a cosmic perspective.

Abstract: The discovery of merging black hole (BH) binaries by the LIGO and Virgo experiments has initiated the era of gravitational wave (GW) astrophysics. Detailed comparisons between theoretical and observed waveforms have already tested classical general relativity, and placed stringent constraints on alternative theories of gravity. However, the zeroth-order astrophysical question implied by these GW detections remains unanswered: what is the astrophysical process, and what is the astronomical environment, responsible for producing the bulk of observed BH binary inspirals? A number of different formation pathways have been proposed, and can be classified as either "isolated binary star evolution" or "dynamical assembly." After broadly overviewing these two formation channels, I will focus on my recent theoretical work to better understand dynamical assembly processes in dense stellar clusters or disks. The best-studied such process is binary-single scatterings in dense star clusters, where repeated three-body interactions create metastable triples that lead to partner swaps and the hardening of binary orbits. The chaotic evolution of these three-body systems is usually studied numerically, but I will present a new analytic formalism that employs the ergodic hypothesis to derive general, closed form statistical solutions for the non-hierarchical three-body problem. This statistical mechanics formalism appears to give reasonable agreement with ensembles of numerical scattering experiments, and I will discuss applications to GW source formation. I will also briefly introduce a novel BH binary formation scenario, the "AGN channel" my collaborators and I have recently proposed. In this scenario, hydrodynamic torques in the gas disks of active galactic nuclei may accelerate the inspiral of wide BH binaries, or pair up singleton BHs into tight orbits. Given ~100 detections, the fractional contribution of this scenario to the total population of GW sources can be deduced from LIGO sky localization volumes alone.

Abstract:

The discovery of the Higgs boson reinforces the possibility that other similar, scalar particles may exist in nature. In this talk, I will begin by discussing one such dark matter and sometime inflaton candidate, the axion.

I will describe the claim that dark matter axions form an exotic state of matter called a Bose-Einstein condensate and my work on this exciting prospect. Along the way I will draw connections with the interesting relaxion idea and possible phenomenologies for the proposed NASA STROBE-X experiment.

A Frontiers in Science Panel Discussion

Panelists: Professors Laura Cadonati, Nepomuk Otte, and Ignacio Taboada. 

Moderator: School Chair and Professor Pablo Laguna

August 17, 2017, is a milestone date for astrophysics. For the first time, the LIGO and Virgo gravitational-wave observatories detected signals from the collision of two neutron stars. The powerful event shook space-time and produced a fireball of light and radiation from the formation of heavy elements.

Satellites and observatories all around the world observed the light produced by this event. For the first time, we have measured gravitational waves and light produced in the same astrophysical event.

What this discovery means for astrophysics is equivalent to the difference between looking at a black-and-white photo and watching a 3-D IMAX movie! 

The combined information of gravitational waves and light is greater than the sum of its parts. The combination allows us to learn new things about physics, the universe, and what we are made of – and perhaps explain mysteries that continue to emerge.  No one has ever been able to do this before!

The historic detection of a cataclysmic celestial collision using signals from multiple messengers signals the era of multi-messenger astrophysics. Discussing the milestone and its implications are School of Physics Professors Laura Cadonati, Nepomuk Otte, and Ignacio Taboada. School of Physics Chair and Professor Pablo Laguna will moderate the discussion. The panel discussion is part of the College of Sciences' Frontiers in Science Lecture Series. 

About Frontiers in Science Lectures

Lectures in this series are intended to inform, engage, and inspire students, faculty, staff, and the public on developments, breakthroughs, and topics of general interest in the sciences and mathematics. Lecturers tailor their talks for nonexpert audiences.

Abstract

Many magnetic materials exhibit resonance phenomena in the far infrared frequency range, between 10- 100 cm-1 (or 0.3-3 THz). The experimental studies of these ecitations allows the determination of basic magnetic parameters, like exchange constants, magnetic anisotropy, g-factors, etc. The application of the magnetic field significantly enhances such characterization by analyzing the two-dimensional map of resonance frequencies vs magnetic field. Here I am going to show several examples of the studies:

-         Zero-field splitting in single ion magnets.

-         Magnetic structure of skyrmion material Cu2OSeO3.

-         Antiferromagnetic resonance in multi- sublattices antiferromagnet Ca2Fe2O5.

Also, I will report on the status and recent developments of high-field infrared magneto-spectroscopy instrumentation at the National High Magnetic Field Laboratory.

 Abstract

A three-dimensional (3D) topological insulator (TI) is a crystalline solid, which is an insulator in the bulk but features spin-polarized Dirac electron states on its surface. In 2007, the first 3D TI was discovered in a bismuth-based compound. The discovery of the first TI tremendously accelerated research into phases of matter characterized by nontrivial topological invariants. Not only did the 3D TI itself attract great research interest, it also inspired the prediction of a range of new topological phases of matter. The primary examples are the topological Kondo insulator, the topological 3D Dirac, Weyl and nodal-line semimetals, the topological crystalline insulator and the topological superconductor. Each of these phases was predicted to exhibit surface states with unique properties protected by a non-trivial topological invariant.

In this talk, I will discuss the experimental realizations of these new phases of matter in real materials through momentum-, spin- and time-resolved photoemission spectroscopy. The unusual properties of protected topological surface states can lead to future applications in spintronics and quantum computation.

BIO

Dr. Madhab Neupane received his Ph.D. in Physics from Boston College, Boston, MA in 2010. He spent four years (2011-2014) as a postdoctoral research associate at Princeton University, Princeton, NJ and one year (2015-2016) as a Director’s Fellow at Los Alamos National Laboratory, Los Alamos, NM. He joined University of Central Florida in 2016 as an Assistant Professor. His research focuses on the electronic and spin properties of new quantum materials such as Dirac and Weyl materials. He utilizes various spectroscopic techniques to reveal interesting electronic and spin properties as well as the momentum resolved dynamical properties of the bulk and symmetry-protected properties of the surface of these quantum materials.