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Events During the Week of February 27th through March 6th, 2022

Monday, February 28th, 2022

No events scheduled

Tuesday, March 1st, 2022

R. G. Herb Condensed Matter Seminar
Unraveling the Bulk and Surface Theories of Helical Higher-Order Topological Insulators
Time: 10:00 am - 11:00 am
Place: 5310 Chamberlin Hall
Speaker: Ben Wieder, MIT
Abstract: Solid-state materials including bismuth, MoTe2, and BiBr have been predicted to be higher-order topological insulators (HOTIs). In theoretical HOTI models, the 3D bulk and 2D surfaces are gapped, and odd numbers of 1D gapless topological modes appear bound to the hinges of finite-sized 3D samples, providing an indicator of the bulk HOTI phase in the presence of global crystal symmetries. However, the boundaries of real material samples lack the global symmetries of HOTI models, and there exist topologically trivial models with extrinsic hinge states. In HOTIs with chiral hinge states, the bulk topology has been shown to be characterized by a nontrivial axion angle, and hence chiral HOTIs can in principle be characterized experimentally through the framework of axion electrodynamics, rather than higher-order topology. For helical HOTIs, however, the bulk axion angle is trivial, and the only experimental signatures proposed to date rely on global symmetry arguments and hinge-state measurements. It is hence desirable to identify unambiguous bulk and surface signatures of helical HOTI phases analogous to - but distinct from - the axionic magnetoelectric effects present in 3D topological insulators (TIs) and chiral HOTIs. In this talk, I will present numerical and theoretical analysis of helical HOTIs demonstrating the existence of quantized bulk topological signatures beyond the axion angle, placing helical HOTIs on the same physical footing as well-understood 3D TIs and magnetic axion insulators.​
Host: Robert McDermott
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Network in Neutrinos, Nuclear Astrophysics, and Symmetries (N3AS) Seminar
Neutrinoless double beta decay in effective field theory
Time: 2:00 pm - 3:00 pm
Place: virtual -
Speaker: Wouter Dekens, UCSD
Abstract: Neutrinoless double beta decay (NLDBD) is the most sensitive probe of lepton-number violation. Its discovery would be a clear signal of physics beyond the Standard Model, confirm the Majorana nature of neutrinos, and provide insight into scenarios of baryogenesis through leptogenesis. Whenever lepton-number violation arises at a scale well above the electroweak scale, it can be described by effective interactions in an effective-field theory framework. In this talk, I will outline the steps necessary to assess the impact of these interactions on NLDBD half lives, paying special attention to the matching of the effective interactions onto Chiral Effective Theory at low energies. In addition, I will discuss how this framework can be extended to include the effects of light sterile neutrinos and give an overview of the resulting constraints on the lepton-number violating interactions.
Host: Baha Balantekin
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Wednesday, March 2nd, 2022

Plasma Physics (Physics/ECE/NE 922) Seminar
Wednesday Nite @ The Lab
Fusion Energy, Solar Flares and Black Holes in the Wisconsin Plasma Physics Lab
Time: 7:00 pm - 8:00 pm
Place: 1111 Biotech or UWBC webcam:
Speaker: Cary Forest, UW–Madison Physics
Abstract: Plasma Physics is the overarching discipline describing plasma, the hot and energetic state of matter that makes up 99% of the visible universe. In my talk I will introduce you the exciting world of experimental plasma physics in which we build devices, here on Earth that replicate and mimic what we see in the Universe: from fusion energy powered stars; planetary and stellar magnetic fields spontaneously created by flows of plasma and liquid metals; spontaneous explosive bursts of plasma in solar flares that hammer our planet, satellites, and astronauts; and accretion of plasma onto supermassive black holes that gives rise to the galaxy sized radio jets that accelerate cosmic rays in the Universe. Each of these systems is built up from plasmas and have processes that can be studied terrestrially, which is what we do in the Wisconsin Plasma Lab. Experiments consist of big rooms, heavy equipment like large vacuum chambers, intense amounts of electric energy in the form of magnetic fields, high voltage power, and microwave heating, and specialized diagnostics to measure properties of plasma at temperatures greater than 100000 degrees. I will tell two stories in my talk. The first will describe a recent experiment we carried out to investigate how plasma might break away from the magnetosphere of our Sun and give rise to the Solar Wind that fills our solar system. This experiment complements a recently launched NASA mission called Parker Solar Probe that is a satellite that is now probing close to the sun. The second will be about revisiting an old idea called the magnetic mirror with new technology for making fusion in a simpler and more useful way than currently envision in reactors along the path that Iter is going. We are now building a new experiment called the Wisconsin High-Temperature-Superconductor Axisymmetric Mirror (WHAM) at the Physical Sciences Lab to test our ideas.

Host: WN@TL
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Thursday, March 3rd, 2022

R. G. Herb Condensed Matter Seminar
How to create and leverage many-body entanglement for near-term quantum networks and simulation
Time: 10:00 am - 11:00 am
Place: 5310 Chamberlin Hall
Speaker: Sophia Economou, Virginia Tech
Abstract: Quantum information science and related technologies include quantum computers, which will be able to solve important problems beyond the reach of classical computers, as well as the ‘quantum internet’, an inherently secure network for communication and for accessing remote quantum computers. I will discuss these technologies, focusing on the question of how to enable them by creating and leveraging multipartite entangled states using near-term quantum systems. In the case of quantum networks, I will describe our protocols for the generation of logically encoded photonic graph states from quantum emitters. For quantum simulation, I will present our recent work on efficient variational quantum algorithms. Bio: Sophia Economou is a Professor of Physics and the Hassinger Senior Fellow of Physics at Virginia Tech. She focuses on theoretical research in quantum information science, including quantum computing with solid-state and photonic qubits, quantum communications, and quantum simulation algorithms.
Host: Mark Saffman
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R. G. Herb Condensed Matter Seminar
Quantum computing with semiconductor spins
Time: 4:00 pm - 5:00 pm
Place: 5310 Chamberlin Hall
Speaker: Edwin Barnes, Virginia Tech
Abstract: Recent years have witnessed enormous progress toward the development of quantum computers---novel devices that exploit quantum mechanics to perform tasks far beyond the reach of the world’s best supercomputers. Qubits based on semiconductor spins are particularly promising because of their long coherence times and prospects for scaling up to large processors by leveraging the existing semiconductor electronics infrastructure. However, many fundamental challenges related to decoherence, controllability, and device architecture remain. I will describe our efforts to address these challenges on multiple fronts using smart control schemes, dynamical stabilization with Floquet physics, and entanglement generation between remote spins.
Host: Mark Saffman
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Friday, March 4th, 2022

Theory Seminar (High Energy/Cosmology)
Corrections to the LARGE-Volume Scenario
Time: 1:00 pm
Place: Chamberlin 5280
Speaker: Daniel Junghans, Harvard University
Abstract: We argue that de Sitter vacua in the LARGE-volume scenario of type IIB string theory are vulnerable to various unsuppressed curvature, warping and g_s corrections. We discuss how these corrections affect the moduli vevs, the vacuum energy and the moduli masses. Our analysis reveals that the corrections are parametrically larger in the relevant expressions than one might have guessed from their suppression in the off-shell potential. Some corrections appear without any parametric suppression at all, which makes them particularly dangerous for candidate de Sitter vacua. Other types of corrections can in principle be made small for appropriate parameter choices. However, we show in an explicit model that this is never possible for all corrections at the same time when the vacuum energy is positive.
Host: George Wojcik
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Physics Department Colloquium
Extreme Plasma Astrophysics
Time: 3:30 pm - 4:30 pm
Place: 2103 Chamberlin Hall
Speaker: Dmitri Uzdensky, University of Colorado
Abstract: Physical conditions in plasma environments of exotic relativistic objects like neutron stars and black holes can be extreme and very different from those in more familiar, traditional heliospheric and laboratory plasmas. The richer physics of these extreme astrophysical plasmas includes the effects of special and general relativity, pair-plasma composition, strong interaction between plasma particles and high-energy photons, and, in the most extreme cases, QED effects like pair production and annihilation. Understanding how these “exotic” physical effects modify fundamental collective plasma processes — such as waves, instabilities, magnetic reconnection, shocks, turbulence — is the scope of Extreme Plasma Astrophysics — an challenging and exciting frontier of modern physics. I will review the recent rapid progress in exploring this frontier, motivated by spectacular astrophysical discoveries and enabled by recent computational advances like the development of novel kinetic plasma codes incorporating radiation and pair-creation effects, in combination with vigorous, concerted theoretical efforts. Examples include new breakthroughs in our understanding of radiative relativistic magnetic reconnection and turbulence, with applications to accreting black holes and neutron star magnetospheres. I will also outline the key future directions of this burgeoning field, including for laboratory studies.
Host: Stas Boldyrev & Cary Forest
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