NYU Arts & Science

Physics Colloquia

April 18, 2024 Thursday 4:00 PM  +
Hybrid: 726 Broadway, 940 and Zoom
Physics Colloquia (colloquia)

John Eiler

Body Temperatures of Dinosaurs

The study of life’s origin, evolution and distribution in the universe involves many questions that seem unsolvable on first inspection; a familiar example concerns the body temperatures of the dinosaurs: Should we look at their fossilized skeletons and imagine vigorous, warm-blooded, bird-like animals, or plodding, sedentary reptiles like modern alligators? This question has often been approached through qualitative arguments based on phylogeny, histology, ecology and other loose correlatives with metabolism — disappointing if you want the kind of direct and quantitative data a veterinarian might gather with a well-aimed thermometer.
Recent advances in studies of the chemical physics of isotopes has provided surprisingly nuanced and precise answers to this question. Well-preserved tooth enamel and egg shells of dinosaurs and other ancient vertebrates contain carbonate groups (CO3-2) that were drawn from their host’s blood stream and represent fossil remnants of their metabolic chemistry. The heavy rare isotopes, 13C and 18O, are present as trace substitutions in these carbonate groups, in amounts that reflect a variety of factors, such as diet and local climate. But the state of organization of those rare isotopes — their propensity to ‘stick’ to one another with a shared chemical bond as opposed to being randomly scattered across a population of molecules — is controlled by the temperature dependent changes in vibrational energy caused by isotopic substitution. I will present the latest discoveries revealed by exceptionally sensitive and precise measurements of isotopic ordering in fossils of ancient vertebrates, revealing their body temperatures and informing inferences regarding their metabolism, physiology, lifestyle and ecology.

April 25, 2024 Thursday 4:00 PM  +
Hybrid: 726 Broadway, 940 and Zoom
Physics Colloquia (colloquia)

David Awschalom
University of Chicago

The Quantum Revolution: Emerging Technologies at the Atomic Scale

Traditional electronics are rapidly approaching the length scale of atoms and molecules. In this regime, a single atom out of place can have outsized negative consequences and so scaling down classical technologies requires ever-more perfect control of materials. Surprisingly, one of the most promising pathways out of this conundrum may emerge from current efforts to embrace these atomic ‘defects’ to construct devices that enable new information processing, communication, and sensing technologies based on the quantum nature of electrons and atomic nuclei. In addition to their charge, individual defects in semiconductors and molecules possess an electronic spin state that can be employed as a quantum bit. These qubits can be manipulated and read using a simple combination of light and microwaves with a built-in optical interface and retain their quantum properties over millisecond to second timescales. With these foundations in hand, we discuss emerging opportunities and the importance of collaborating with industry to atomically-engineer qubits for nuclear memories, entangled registers, sensors and networks for science and technology.
Note: this talk will not be taped nor put on YouTube

May 2, 2024 Thursday 4:00 PM  +
Hybrid: 726 Broadway, 940 and Zoom
Physics Colloquia (colloquia)

Kathleen Stebe
University of Pennsylvania

Defect Propelled Swimming of Nematic Colloids

Nematic liquid crystals (NLCs) are highly non-linear fluids that have elastic responses that resist nematogen rearrangement and high-energy defect sites at which nematogen order is lost. Generally, the field of nematic colloids seeks to develop control over these elastic responses and defect structures to tailor colloidal interactions. We have been studying ferromagnetic disk colloids rotated by an in-plane magnetic field in nematic liquid crystals. The disk diameter and rotation rate are sufficiently slow that colloid inertia is negligible. In Newtonian fluids, these colloids rotate without translation. However, in NLC, the colloids’ anisotropic defect structure and the NLC’s elastic response generate broken symmetries that propel colloid translation. For patchy, rough colloids, a defect loop which forms on the disk undergoes periodic defect pinning, release, and contraction. This periodic defect motion generates a swim stroke that powers colloidal swimming. Changes in defect configuration with rotation rate provide a steering mechanism. In addition to this swimming motion, colloid shape and surface chemistry generate long-ranged emergent interactions with neighboring passive colloids in quasi-static settings. Furthermore, the non-linear response of the nematic fluid host allows pair interactions among rotating disks that differ strikingly in range and form from their static counterparts. These interactions provide a rich toolkit for reconfigurable materials assembly and open important fundamental questions regarding swimming at low Reynolds number in NLC.