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Events During the Week of April 5th through April 12th, 2020

Monday, April 6th, 2020

No events scheduled

Tuesday, April 7th, 2020

Council Meeting
Time: 4:00 pm
Place: Online
Host: Sridhara Dasu
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Wednesday, April 8th, 2020

Department Meeting
Time: 12:15 pm
Place: Online
Speaker: Sridhara Dasu, Department Chair
Host: Sridhara Dasu
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Thursday, April 9th, 2020

R. G. Herb Condensed Matter Seminar
*CANCELLED* Electron Hydrodynamics in Graphene: Introduction and Status
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Denis Bandurin, MIT
Abstract: Transport in systems with many particles experiencing frequent mutual collisions (such as gases or liquids) has been studied for more than two centuries and is accurately described by the theory of hydrodynamics. It has been argued theoretically for a long time that the collective behaviour of charge carriers in solids can also be treated by the hydrodynamic approach. However, despite attempts, until recently very little evidence of hydrodynamic electron transport has been found. Graphene encapsulated between hexagonal boron nitride (hBN) offers an ideal platform to study electron hydrodynamics as it hosts an ultra-clean electronic system with electron-electron collisions being the dominant scattering source above liquid nitrogen temperatures. In the first part of my talk we will discuss why electron hydrodynamics has not been observed before and how it manifests itself in graphene. It will be shown that electrons in graphene can behave as a very viscous fluid forming vortices of applied electron current [1,2]. In the second part, we will discuss methods which can be applied to measure electron viscosity and talk about superballistic flow of viscous electron fluids through graphene point contacts [3]. Then we will talk about the behaviour of electron fluids in the presence of magnetic field where I will report the experimental measurements of the odd (Hall) viscosity in two dimensions [4]. This dissipationless transport coefficient has been widely discussed in theoretical literature on fluid mechanics, plasma physics and condensed matter physics, yet, until now, any experimental evidence has been lacking, making the phenomenon truly a unicorn. Last but not least, we will discuss how electron hydrodynamics can motivate the development of resonant terahertz detectors and I will report some recent progress in this direction [5]. [1] Negative Local Resistance Caused by Viscous Electron Backflow in Graphene, D. A. Bandurin, A. Principi, G.H. Auton, E. Khestanova, K.S. Novoselov, I. V Grigorieva, L.A. Ponomarenko, A.K. Geim, and M. Polini, Science 351, 1055 (2016). [2] Fluidity Onset in Graphene, D. A. Bandurin, A. Shytov, L. S. Levitov, R. Krishna Kumar, A. I. Berdyugin, M. Ben Shalom, I. V. Grigorieva. A. K. Geim and G. Falkovich, Nat. Comm. 9, 4533 (2018). [3] Superballistic Flow of Viscous Electron Fluid through Graphene Constrictions, R. Krishna Kumar, D.A. Bandurin, F.M.D. Pellegrino, Y. Cao, A. Principi, H. Guo, G.H. Auton, M. Ben Shalom, L.A. Ponomarenko, G. Falkovich, I. V. Grigorieva, L.S. Levitov, M. Polini, and A.K. Geim, Nat. Phys. 13, 1182 (2017). [4] Measuring Hall viscosity of Graphene’s Electron Fluid, I. Berdyugin, S. G. Xu, F. M. D. Pellegrino, R. Krishna Kumar, A. Principi, I. Torre, M. Ben Shalom, T. Taniguchi, K. Watanabe, I. V. Grigorieva, M. Polini, A. K. Geim and D. A. Bandurin, Science 364, 6436, 162-165 (2019). [5] Resonant Terahertz Detection Using Graphene Plasmons, D. A. Bandurin, D. Svintsov, I. Gayduchenko, S. G. Xu, A. Principi, M. Moskotin, I. Tretyakov, D. Yagodkin, S. Zhukov, T. Taniguchi, K. Watanabe, I. V. Grigorieva, M. Polini, G. Goltsman, A. K. Geim and G. Fedorov, Nat. Comm. 9, 5392 (2018).
Host: Alex Levchenko
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Friday, April 10th, 2020

PGSC Professional Development Seminar
Git workflows for physicists: Using version control in your research
Time: 11:00 am
Place: https://us04web.zoom.us/j/350335216
Speaker: Greg Holdman, Physics PhD Graduate Student
Abstract: Version control is a great method for documenting your progress while avoiding mistakes and backing up your work. We’ll focus mainly on git, and you’ll learn the essentials for maintaining code repositories, tracking changes in papers, and more all while avoiding the dense and scary stuff.
Host: Rob Morgan, graduate student
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Atomic Physics Seminar
Virtual AMO Seminar: Transport and spectral properties of hot Fermi-Hubbard systems
Time: 2:00 pm
Place: https://cuboulder.zoom.us/webinar/register/WN_F46RnreyTe6xzFtiIytrdg
Speaker: Waseem Bakr, Princeton University
Abstract: The normal state of high-temperature superconductors exhibits anomalous transport and spectral properties that are poorly understood. Cold atoms in optical lattices have been used to realize the celebrated Fermi-Hubbard model, widely believed to capture the essential physics of these materials. The recent development of fermionic quantum gas microscopes has enabled studying Hubbard systems with single-site resolution and extracting equilibrium charge and spin correlations. In this talk, I will report on using a quantum gas microscope to probe the transport and spectral properties of atomic Fermi-Hubbard systems. First, I will describe the development of a technique to measure microscopic charge diffusion, and hence resistivity, in doped Mott insulators. We have found that this resistivity exhibits a linear dependence on temperature and violates the Mott-Ioffe-Regel limit, two signatures of strange metallic behavior [1]. Next, I will discuss how we used the same technique to observe sub- diffusive charge transport in tilted Hubbard systems and present a hydrodynamic model that explains this observation in terms of an interplay of charge and heat transport, allowing the extraction of the infinite temperature heat diffusivity of the system [2]. Finally, I will describe the development of angle-resolved photoemission spectroscopy (ARPES) for Hubbard systems and its application to studying pseudogap physics in an attractive Hubbard system across the BEC-BCS crossover [3], setting the stage for future studies of the pseudogap regime in repulsive Hubbard systems.

[1] P. Brown et. al., Science 363, 379 (2019)
[2] E. Guardado-Sanchez et. al., PRX 10, 011043 (2020)
[3] P. Brown et. al., Nature Physics 16, 26 (2020)
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Physics Department Colloquium
*CANCELLED* TBD
Time: 3:30 pm
Place: 2241 Chamberlin Hall
Speaker: Elizabeth Simmons, UCSD
Host: Tulika Bose
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