Physics and Astronomy Calendar

Week of Monday, March 25th 2019


Monday, March 25th 2019
12:15 pm:
There will be no seminar this week.
2:00 pm:
Thesis Defense in Tate 201-20
Speaker: Peter Martin, University of Minnesota
Subject: Measuring and Simulating Protein Electron Paramagnetic Resonance Spectra
This is the public portion of Mr. Martin's Thesis Defense. His advisor is David Thomas from the Biochemistry, Molecular Biology, and Biophysics Department.

Tuesday, March 26th 2019
1:25 pm:
Space Physics Seminar in Tate 301-20
Speaker: Trevor Knuth
Subject: Fast time variations in solar flare X-ray flux - A probe for particle acceleration
Speaker: Vipin Kumar
Subject: Physics Guided Machine Learning: A New Paradigm for Modeling Dynamical Systems

Physics-based models of dynamical systems are often used to study engineering and environmental systems. Despite their extensive use, these models have several well-known limitations due to incomplete or inaccurate representations of the physical processes being modeled. Given rapid data growth due to advances in sensor technologies, there is a tremendous opportunity to systematically advance modeling in these domains by using machine learning (ML) methods. However, capturing this opportunity is contingent on a paradigm shift in data-intensive scientific discovery since the “black box” use of ML often leads to serious false discoveries in scientific applications. Because the hypothesis space of scientific applications is often complex and exponentially large, an uninformed data-driven search can easily select a highly complex model that is neither generalizable nor physically interpretable, resulting in the discovery of spurious relationships, predictors, and patterns. This problem becomes worse when there is a scarcity of labeled samples, which is quite common in science and engineering domains.

This talk makes a case that in a real-world systems that are governed by physical processes, there is an opportunity to take advantage of fundamental physical principles to inform the search of a physically meaningful and accurate ML model. Even though this will be illustrated in the context of modeling water temperature, the paradigm has the potential to greatly advance the pace of discovery in a number of scientific and engineering disciplines where physics-based models are used, e.g., power engineering, climate science, weather forecasting, materials science, and biomedicine.

Faculty Host: Vuk Mandic
4:40 pm:
CM Journal Club in Tate 201
Speaker: Yiming Wu
Subject: Transition Metal Dichalcogenide Moire Band

Please find the reading material in the link below:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.121.026402


Wednesday, March 27th 2019
1:25 pm:
Speaker: Itamar Kimchi, University of Colorado - Boulder
Subject: Dirty Entangled Quantum Magnets
Faculty Co-Host: Alex Kamenev

Studying quantum entanglement over the past 1--2 decades has allowed us to make remarkable theoretical progress in understanding correlated many-body quantum systems. However electrons in real materials experience random heterogeneities ("dirt") whose theoretical treatment, including strong correlations, has been a challenge. I will describe how synthesizing ideas from quantum information theory, statistical mechanics, and quantum field theory gives us new insights into the role of randomness in 2D correlated quantum spin systems. First I will outline our results on weak bond-randomness in two theoretically controlled cases (valence-bond-solids and classical dimer models) and apply them to random quantum magnets to show that topological defects with free spins necessarily nucleate and control the low energy physics. Second I will describe how the results lead us to conjectures in 2D, and a proved theorem in 1D, of Lieb-Schultz-Mattis-type constraints on all possible low-energy fates of quantum magnets, that hold even with randomness. Third I will describe how the theory predicts a scaling collapse of the temperature and magnetic-field dependence of thermodynamic quantities that is consistent with experimental observations from multiple materials, suggesting that these materials exhibit randomness-driven long range entanglement.

Faculty Host: Fiona Burnell

Thursday, March 28th 2019
10:10 am:
Biophysics Seminar in 120 PAN
Speaker: Jerome C. Mertz, College of Engineering, Boston University
Subject: Fast, volumetric imaging with microscopes

Fast, volumetric imaging over large scales has been a long-standing challenge in biological microscopy. Camera-based microscopes are typically hampered by the problem of out-of-focus background which undermines image contrast. This background must be reduced, or eliminated, to achieve volumetric imaging. Alternatively, scanning techniques such as confocal and multiphoton microscopy can provide high contrast and high speed, but their generalization to volumetric imaging requires an axial scanning mechanism, which, in general, drastically reduces speed. I will describe a variety of strategies we have developed to enable fast, high-contrast, volumetric imaging over large length scales. These strategies include targeted-illumination widefield microscopy, multi-z confocal microscopy and reverberation multiphoton microscopy. I will discuss the principles of these strategies and present experimental validations.

Faculty Host: Jochen Mueller
12:10 pm:
Speaker: Jin-Ah Kim and Liliya Williams
3:35 pm:
Physics and Astronomy Colloquium in Physics Tate B50
Speaker: Jason Hogan, Stanford University
Subject: Atom interferometry for fundamental physics and gravitational wave detection

In recent years, atom interferometry and atomic clocks have made impressive gains in sensitivity and time precision. The best atomic clocks have stability corresponding to a loss of less than one second in the lifetime of the universe. Matter wave interferometers have achieved record-breaking coherence times (seconds) and atomic wavepacket separations (over half a meter), resulting in a significant enhancement in accelerometer and gravity gradiometer sensitivity. Leveraging these advances, atomic sensors are now poised to become a powerful tool for discovery in fundamental physics. I will highlight ongoing efforts to test aspects of general relativity and quantum mechanics, and search for new fundamental interactions. A particularly exciting direction is gravitational wave detection. I will describe the Mid-band Atomic Gravitational wave Interferometric Sensor (MAGIS) proposal, which is targeted to detect gravitational waves in a frequency band complementary to existing detectors (0.03 Hz – 10 Hz), the optimal frequency range to support multi-messenger astronomy. Finally, I will discuss MAGIS-100, a 100-meter tall atomic sensor being constructed that will serve as a prototype of such a detector, and will also be sensitive to proposed ultra-light dark matter (scalar and vector couplings) at unprecedented levels.

Faculty Host: Roger Rusack

Friday, March 29th 2019
11:00 am:
Nuclear Physics Seminar in Physics Tate 301-20
Speaker: James Austin Harris, Oak Ridge National Lab
Subject: The multidimensional character of nucleosynthesis in core-collapse supernovae

The intrinsically multi-dimensional neutrino-driven explosion mechanism of core-collapse supernovae (CCSNe) is notoriously difficult to model self-consistently.

As a matter of either computational expediency or necessity, nuclear burning, when included at all, is traditionally constrained to a small reaction network consisting only of the (α,γ) reactions necessary in linking 4He to 56Ni.

Feedback between the evolving hydrodynamics and changing composition, and resulting energy generation, precludes the deficiencies of this simplification from being entirely resolved with post-processing calculations.

Using a much more realistic, in situ reaction network capable of accurately tracking nuclear energy generation and neutronization, we examine the nucleosynthesis in multidimensional, self-consistent, neutrino-driven supernova models.

We find differences between the in situ and post-processing approaches, indicating that such rigor in evolving the nuclear composition is needed to accurately calculate the nucleosynthesis of matter that has been ejected from the inner regions of the explosion mechanism.

This has implications for some of the most interesting nucleosynthetic processes in CCSNe, specifically α-rich and α-poor freeze-out, which produces several isotopes particularly relevant to observations (e.g.44Ti, 48Ca, and 92Mo).

12:20 pm:
Speaker: Zhen Jiang
Subject: The temperature and doping dependence of the inverse spin Hall effect in n-GaAs
12:30 pm:
Speaker: Vitaly Vanchurin (U. Minnesota, Duluth)
Subject: A quantum-classical duality and emergent space-time

We consider the quantum partition function for a system of quantum spinors and then derive an equivalent (or dual) classical partition function for some scalar degrees of freedom. The coupling between scalars is non-trivial (e.g. a model on 2-sphere configuration space), but the locality structure of the dual system is preserved, in contrast to the imaginary time formalism. We also show that the measure of integration in the classical partition function can be formally expressed through relativistic Green's functions which suggests a possible mechanism for the emergence of a classical space-time from anti-commutativity of quantum operators.

2:30 pm:
Speaker: Vuk Mandic
Subject: Observing the Universe with Gravitational Waves

Since 2015 the LIGO and Virgo gravitational-wave detectors have observed 10 collisions of black hole pairs and one merger of two neutron stars. In addition to enabling unprecedented tests of the theory of General Relativity, these discoveries introduced a new way of observing the universe. While the traditional telescopes observe the universe using electromagnetic waves (i.e. light), LIGO and Virgo do so by measuring tiny ripples in space-time produced by accelerating massive objects moving at speeds near the speed of light. Using the two types of observations together, as was done in the case of the binary neutron star merger, opens a variety of new possible studies ranging from formation mechanisms for the heaviest elements in the periodic table to novel measurements of the expansion rate of the universe. I will describe how these discoveries were made, and I will discuss their implications as well as the exciting future prospects in the field of gravitational waves.

Faculty Host: Evan Skillman
Speaker: Lynn Nyhart, History - University of Wisconsin - Madison
Subject: The Politics of Popular Physiology in Germany in the 1840s and 50s
Refreshments served at 3:15 p.m.
3:35 pm:
To be announced.
4:40 pm:
Speaker: Roger Rusack, High Energy

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