Events on Thursday, October 24th, 2024
- R. G. Herb Condensed Matter Seminar
- In-situ twisting and imaging of moiré superlattices
- Time: 10:00 am - 6:00 pm
- Place: 5310 Chamberlin
- Speaker: Monica Allen, UCSD
- Abstract: Moiré superlattices, which form in twisted stacks of 2D materials, constitute a versatile platform for the exploration of topological and correlated phenomena. Here we present a route to mechanically tune the twist angle of individual atomic layers with a precision of a fraction of a degree inside a scanning probe microscope, which enables continuous control of the electronic band structure in-situ. In twisted bilayer graphene, we demonstrate nanoscale control of the moiré wavelength via mechanical rotation, as revealed using piezoresponse force microscopy. We also extend this methodology to create twistable boron nitride devices, enabling dynamic control of the ferroelectric domain structure. This approach provides a route for real-time manipulation of moiré materials, which may allow for systematic investigation of the phase diagrams at multiple angles in a single device. Looking forward, we will also discuss progress on the construction of a new milliKelvin microwave impedance microscope in a dilution refrigerator, which supports spatially-resolved detection of topological states in the GHz regime. As an application, I will briefly discuss the imaging of edge modes in a Chern insulator.
- Host: Alex Levchenko
- Preliminary Exam
- Characterizing the Diffuse Astrophysical Neutrino Flux with Contained and Uncontained Cascades in IceCube
- Time: 2:00 pm - 4:00 pm
- Place: 5280 Chamberlin Hall
- Speaker: Zoe Rechav, Physics PhD Graduate Student
- Abstract: Recently, the IceCube Neutrino Observatory has reported a deviation from a single power law in the extragalactic diffuse neutrino flux. This deviation is primarily driven by the hardening of the low-energy flux below 30 TeV. However, uncertainties remain in many features of the neutrino flux across the energy spectrum, including but not limited to the characterization of the neutrino flux below 30 TeV, the flux bump at ~30 TeV, the flux dip at ~500 TeV, and the characterization of the neutrino flux above 10 PeV. A stronger constraint on the energy spectrum is needed to resolve these uncertainties.
The DNNCascades event selection is a neural network based, high statistics cascade-like dataset that was first used to detect high energy neutrinos in the Galactic plane. As well as contained cascades, the selection includes ~30% uncontained cascades – neutrinos with interaction vertices at the edge or outside of the detector instrumentation volume. The high statistics, contained and uncontained cascades event selection could be key to more tightly constraining the diffuse flux across the energy spectrum.
My work involves optimizing this unique event selection to bring into the diffuse neutrino physics space. Extensive work on updated atmospheric neutrino background modeling, systematics updates, and data/MC improvement will be discussed, as well as my intention to perform an astrophysical diffuse flux measurement that could resolve uncertain features of the astrophysical spectrum. - Host: Lu Lu
- Astronomy Colloquium
- The X-ray Sky in High Definition: Twenty-five Years of Astrophysics with the Chandra X-ray Observatory
- Time: 3:30 pm - 4:30 pm
- Place: 4421 Sterling Hall
- Speaker: Patrick Slane, Harvard Center for Astrophysics
- Abstract: X-ray astronomy is a product of the space age. Possible only from viewing platforms above the atmosphere, the first celestial X-ray source other than the Sun was discovered just over sixty years ago. Following fresh on the heels of this discovery, with X-ray astronomy just barely in its infancy, an almost unimaginably-bold proposal was put forth to build a visionary space-borne observatory based on X-ray optics whose resolution and sheer size represented leaps by orders of magnitude over any such mirrors ever built. Following a series of smaller, but ever-improving X-ray observatories, this vision was fully realized in July of 1999 with the launch and deployment of NASA's Chandra X-ray Observatory.
A quarter century later, Chandra remains the world's premier X-ray astrophysics facility - the crucial high energy component of NASA's fleet of flagship observatories. From resolving the hazy X-ray background into a speckled array dominated primarily by black holes to peering through a gravitational lens to peek at supermassive black hole formation at the edge of time, Chandra has continued to do things that no other X-ray observatory can. It is unique, it is powerful, and it is healthy - poised to help continue framing the future of high energy astrophysics for years to come. As we celebrate its ongoing success, I will provide a summary of some of the most exciting results from Chandra's ever-increasing scientific legacy, along with a look forward to its continued role as both a unique stand-alone facility and a core element for panchromatic investigations of the Universe. - Host: Melinda Soares-Furtado