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This Week at Physics

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Events on Thursday, January 26th, 2017

R. G. Herb Condensed Matter Seminar
Observation of new fractional quantum Hall states in graphene heterostructures
Time: 10:00 am
Place: 5310 Chamberlin hall
Speaker: Andrea Young, University of California Santa Barbara
Abstract: I will describe recent experiments probing the many body ground states of a new generation of ultra-clean graphene heterostructures. By replacing conventional gate metallization based on deposition of amorphous films with single crystal graphite flakes, we find that the electronic quality of graphene heterostructures can be substantially improved, rivaling that of long-studied semiconductor quantum wells. I will discuss two recent results that have emerged from these ultra-clean electronic devices. In the first part of the talk, I will describe the fate of the half filled Landau level in bilayer graphene, where different orbital Landau levels host composite fermion liquid (for N=0) and a gapped Pfaffian quantum Hall phase (for N=1). By controllably moving occupation between these two orbitals, we find that a new phase emerges, characterized by charge gap but large inter-orbital polarization. Numerical modeling suggests that the resulting phase hosts a Fermi surface of neutral excitons, and I will discuss experimental schemes to directly probe these unusual particles.

In the second part of the talk, I will describe the effects of a superlattice on the nature of the fractional quantum Hall effect. Recent theoretical work, largely motivated by efforts to engineer fractional quantum Hall states in optical lattice systems, has suggested that new kinds of fractional quantum Hall states--termed fractional Chern insulators--can exist in lattice systems with intrinsically finite bandwidth. At high magnetic field in our devices, a substrate-induced moire superlattice gives rise to a variety of Hofstadter bands with different Chern numbers. We find a wide variety of incompressible states at fractional filling of these bands characterized by fractionally quantized Hall conductance. These results demonstrate that fractional Chern insulators are indeed a generic phenomenon. Going forward, we anticipate being able to place approximate limits on the required interaction strength and bandwidth required to realize these phases.
Host: Levchenko
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Atomic Physics Seminar
From fundamental physics to aspects of photosynthesis: Controlling and studying complex quantum systems
Time: 12:00 pm
Place: 5280 Chamberlin Hall
Speaker: Boerge Hemmerling, UC Berkeley
Abstract: The answer to many scientific questions ranging from fundamental physics to aspects of photosynthesis lie in the study of quantum systems. A requirement for such studies is often to initialize the systems, manipulate them and read them out. However, many of the systems with interesting applications tend to have a complex level structure rendering these requirements difficult to meet.

In this talk, I will discuss experimental strategies to control complex ions and molecules for which standard trapping, cooling and state manipulation methods fail. In particular, I will discuss how complex ions, such as Ti+ or Fe+, can be studied and used to place limits on the temporal variation of fundamental constants. Moreover, I will present a strategy to laser cool the diatomic molecule calcium monofluoride, a precursor to produce a degenerate dipolar quantum gas. Finally, I will show how strings of ions can be used to emulate processes relevant for transport phenomena in light harvesting processes.

I will conclude with a discussion on how to control and study two further quantum systems: electrons and aluminum chloride. Electrons can be stored in a novel two-frequency Paul trap, constituting the first step towards electron quantum computing; such a trap has, moreover, the potential to advance studies on matter-antimatter asymmetries by improving antihydrogen production. Furthermore, I will explain a laser cooling scheme for aluminum chloride, a molecule with excellent prospects for generating high phase-space density clouds at ultracold temperatures to study the physics of degenerate dipolar quantum gases.
Host: thad Walker
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Multi-messenger searches for astrophysical sources at ultra-high energies
Time: 2:30 pm
Place: 5280 Chamberlin Hall
Speaker: Foteini Oikonomou, Penn State University
Abstract: The question of the origin of ultra high energy, > 10^19 eV, cosmic rays (UHECRs) remains unanswered, although experimental searches in the last decade have yielded important results, and insights about the universe at ultra-high energies. I will discuss the interpretation of the most recent measurements of the extensive air-showers produced by UHECRs, and outline current strategies aiming to answer the question of UHECR origin. Emphasis will be given to studies of UHECR arrival directions, searches for single UHECR sources through their secondary gamma-ray and neutrino signatures, and the nascent program, by the Pierre Auger Observatory, of real-time searches for transient UHE emission, as part of multi-messenger monitoring networks.
Host: Stefan Westerhoff
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Astronomy Colloquium
An X-ray View of the Dusty Universe
Time: 3:30 pm
Place: 4421 Sterling Hall, Coffee and Cookies 3:30 pm, Talk at 3:45 pm
Speaker: Lia Corrales, Einstein Fello, UW Madison Astronomy Department
Abstract: A significant fraction of the heavy elements produced by stars spend some time in the interstellar medium as dust grains. These heavy metal transporters influence gas cooling during star formation, eventually becoming the seeds for planet formation. Much like quasar spectra are used to probe intergalactic gas, observations of X-ray bright Galactic compact objects can yield key insights to the mineralogy and evolution of dust grains in the Milky Way. With high resolution X-ray spectroscopy, we can directly measure the state of metals and the mineral composition of dust in the interstellar medium. In addition, dust scattering produces a diffuse halo image around bright X-ray objects, revealing information about dust grain sizes and their spatial distribution. I will review the most recent exciting dust scattering discoveries, which draw on multi-wavelength observations. Finally, I will discuss open questions regarding our X-ray view of the ISM that can be addressed with lab experiments and future X-ray observatories.<br>
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Discovering a new approach to cosmology with the Dark Energy Survey and Gravitational Waves
Time: 4:00 pm
Place: 4274 Chamberlin Hall
Speaker: Marcelle Soares-Santos, Fermi Lab
Abstract: Motivated by the exciting prospect of new wealth of information that will arise from observations of gravitational and electromagnetic radiation from the same astrophysical phenomena, the Dark Energy Survey (DES) Collaboration has performed a broad range follow-up program for LIGO/Virgo events using its Camera (DECam). In this talk, I present an overview of this effort, including results of searches for signatures of the first two LIGO-triggered binary black hole mergers in the 2015-2016 observing campaign and status of the ongoing 2016-2017 campaign. I will also discuss plans for upcoming seasons and long term prospects for this exciting emerging field: multi-messenger cosmology with gravitational waves and optical data.
Host: Sridhara Dasu
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