Events During the Week of February 20th through February 27th, 2022
Monday, February 21st, 2022
- No events scheduled
Tuesday, February 22nd, 2022
- R. G. Herb Condensed Matter Seminar
- Embarking on a Thermal Journey in Low Dimensions with a 21st century thermometer: Graphene Nonlocal Noise
- Time: 10:00 am - 11:00 am
- Place: 5310 Chamberlin Hall
- Speaker: Jonah Waissman, Harvard University
- Abstract: Low-dimensional materials, such as 2D monolayers, 1D nanowires, and 0D quantum dots and molecules, are rich with many-body quantum phenomena. The reduced dimensionality, strong interactions, and topological effects lead to new emergent degrees of freedom of fundamental interest and promise for future applications, such as energy-efficient computation and quantum information. Thermal transport, which is sensitive to all energy-carrying degrees of freedom and their interactions, provides a discriminating probe to identify these emergent excitations. However, thermal measurement in low dimensions is dominated by lattice contributions, requiring an approach to isolate the electronic thermal conductance. In this talk, I will discuss how the measurement of nonlocal voltage fluctuations in a multiterminal device can reveal the electronic heat transported across a mesoscopic, low-dimensional bridge. We use 2D graphene as an electronic noise thermometer, demonstrating quantitative electronic thermal conductance measurement over a wide temperature range in an array of dimensionalities: 2D graphene, 1D nanotubes, 0D localized electron chains, and 3D, microscale bulk materials. I will discuss ongoing work revealing electron hydrodynamics, interaction-mediated plasmon hopping, spin waves in a magnetic insulator, and an electron-phonon crossover in a bulk spin liquid candidate material.
- Host: Robert McDermott
Wednesday, February 23rd, 2022
- No events scheduled
Thursday, February 24th, 2022
- R. G. Herb Condensed Matter Seminar
- Spintronics with 2D and Topological Materials: Outstanding Opportunities in van der Waals Heterostructures
- Time: 10:00 am - 11:00 am
- Place: 5310 Chamberlin Hall
- Speaker: Yunqiu Kelly Luo, Cornell University
- Abstract: Advances in quantum science and technology rely on ever-improving control over quantum degrees of freedom including electron spin, photons, and magnons, which can store, transmit, and exchange information. Understanding their mutual interactions, propagation, and non-equilibrium dynamics within solid-state systems is a crucial challenge at the forefront of this effort. My research broadly investigates the dynamics and effective couplings among these quantum degrees of freedom. Specifically, my talk will center around two themes: 1. developing multifunctional vdW heterostructures for spin-based quantum information processing; 2. accelerating next-generation spin-orbit torque MRAM based on 2D magnets and topological materials. Led by the development of optical-valleytronic spin injection, ultrafast optical scanning microscopy, and Sagnac interferometry, I will present our experimental efforts in harnessing the outstanding opportunities enabled by the pristine van der Waals interfaces of 2D materials with unparalleled Kerr sensitivity, temporal, and spatial resolutions. Our work aims to bridge the urgent needs of the microelectronics industry by enabling new forms of nonvolatile magnetic memory, low-power computing, optical interconnects, and multifunctional hybrid materials with widespread societal impact.
- Host: Robert McDermott
Friday, February 25th, 2022
- Theory Seminar (High Energy/Cosmology)
- Underdamped Axionic Blue Isocurvature Perturbations
- Time: 1:00 pm
- Place: Chamberlin 5280
- Speaker: Sai Chaitanya Tadepalli, UW Madison
- Abstract: Previous computations of strongly blue tilted axionic isocurvature spectra were computed in the parametric region in which the lightest time-dependent mass is smaller than the Hubble expansion rate during inflation, leading to an overdamped time evolution. In this talk, we present the strongly blue tilted axionic isocurvature spectrum in an underdamped time evolution parametric regime. Somewhat surprisingly, there exist parametric regions with a strong resonant spectral behavior that leads to rich isocurvature spectral shapes and large amplitude enhancements. We focus on computing this resonant spectrum analytically in parametric regions amenable to such computations. Because the spectrum is sensitive to nonperturbative classical field dynamics, we will discuss a wide variety of analytic techniques that are used like decoupling, nonlinear field redefinition, a time-space effective potential obtained by integrating out high-frequency fluctuations, and a piecewise mass-model.
- Physics Department Colloquium
- Magnetic Reconnection, a Celestial Phenomenon in the Laboratory
- Time: 3:30 pm
- Place: 2241 Chamberlin Hall
- Speaker: Jan Egedal and the WiPPL team , UW Madison
- Abstract: The Earth is embedded in the Sun's extended atmosphere, where the Earth's magnetic field acts as a shield against the incoming solar wind plasma. However, this shielding is not perfect. Through the process of magnetic reconnection solar particles can penetrate the magnetosphere and greatly influence the conditions in space that affect the Earth and its technological systems---magnetic reconnection controls space weather. Notable aspects of reconnection include its ability to convert magnetic energy into particle energy while changing the topology of the magnetic field lines. Although reconnection occurs in microscopic diffusion regions, it often governs the macroscopic properties and behavior of the systems. It controls the evolution of explosive events such as solar flares, coronal mass ejections and magnetic storms in the Earth's magnetotail. The latter drives the auroral phenomena.
The phenomenon is explored on the Terrestrial Reconnection Experiment (TREX) at UW-Madison, where the absolute rate of reconnection is set by an external drive. A shock interface between the supersonically driven plasma inflow and a region of magnetic flux pileup permits the normalized reconnection rate to self-regulate to a fixed value. In agreement with numerical and theoretical results, the width of the electron diffusion regions is characterized by the kinetic length scale of the electrons. While the reconnection layers are modulated by a current-driven instability, their characteristics remain consistent with a 3D simulation for which off-diagonal stress in the electron pressure tensor is responsible for fast reconnection.
- Host: Cary Forest