Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session J08: Onsager and Davisson-Germer Prize SessionInvited Live Prize/Award Undergrad Friendly
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Sponsoring Units: GSNP DAMOP DCMP Chair: Natan Andrei, Rutgers University, New Brunswick |
Tuesday, March 16, 2021 3:00PM - 3:36PM Live |
J08.00001: Lars Onsager Prize (2020): Swarms, flocks and crowds Invited Speaker: Tamas Vicsek Over the past 25 years collective motion has attracted increasing interest in the physics and biology communities. Motivations include the associated novel type of the “collision rule” (for physicists) as well as major technological advances regarding resources such as computational power or the collection of data. Observations of bacteria as they – thousands of them – exhibit rich group-motion patterns under the microscope in a Petri dish inspired an interpretation in terms of statistical physics. It turned out that a simple rule could be used to interpret the behaviour and that variants of this rule could be introduced to describe group motion in a surprisingly wide variety of comoving units, from cells through animals to people, and very recently to swarms of aerial robots. The related results have been obtained by computer simulations, developing theoretical techniques and, naturally, carrying out experiments. After an introduction to the basic examples and aspects of flocking, I shall present two recent case studies related to the role of hierarchical decision-making during collective motion. |
Tuesday, March 16, 2021 3:36PM - 4:12PM Live |
J08.00002: The flocking theory: The early developments and a new perspective Invited Speaker: Yuhai Tu In this talk, I will first describe some of the "accidental" events over 25 years ago that led to the early developments of the flocking theory. I will then discuss a possible new perspective on the flocking theory by looking at the non-equilibrium thermodynamics of collective motion in active matter. |
Tuesday, March 16, 2021 4:12PM - 4:48PM Live |
J08.00003: Davisson-Germer Prize in Atomic or Surface Physics: Exploring the Atomic and Electronic Landscape of Low-Dimensional Materials Invited Speaker: Michael F Crommie When materials are probed at the smallest length scales their quantum mechanical properties become highly apparent. The scanning tunneling microscope (STM) provides an ideal tool for visualizing this behavior down to the atomic scale. I will discuss how cryogenic scanning tunneling microscopy techniques can be used to both probe the local electronic properties of materials as well as to modify them via atomic manipulation. This enables, for example, precise electron confinement structures (quantum corrals) to be constructed from individual atoms, thus providing a means to directly visualize the effects of electronic quantum interference at the nanoscale. Integration of these techniques with atomically-thin 2D materials such as graphene provides a completely new window into low-dimensional electronic behavior. I will describe how characterization of graphene via cryogenic STM has enabled ultra-relativistic behavior such as atomic collapse and Dirac fermion quantum confinement to be directly imaged in gated graphene devices. By reducing the dimensionality of graphene into 1D strips (i.e., graphene nanoribbons (GNRs)) it is possible to flexibly alter graphene’s electronic structure even further, ranging from tunable semiconducting behavior all the way to robust metallicity. I will discuss how GNRs having different atomically-precise structures can be fabricated using “bottom-up” synthesis techniques involving molecular self-assembly and then interrogated via STM. Such measurements have enabled the topological properties of GNRs to be directly visualized, thus providing a powerful new strategy for engineering the electronic properties of low-dimensional materials. |
Tuesday, March 16, 2021 4:48PM - 5:24PM Live |
J08.00004: Lars Onsager Prize (2021): Bose-Einstein condensate – a classical limit of matter waves Invited Speaker: Lev Pitaevskii In 1925 A. Einstein predicted that at low temperature macroscopically large number of atoms of an ideal Bose gas are condensed in the lowest quantum state, creating Bose-Einstein condensate (BEC). Wave function of this state is a constant for a uniform stationary gas. However for an inhomogeneous gas in non-stationary condition this function depends on coordinates and time and satisfied the non-linear Gross-Pitaevskii equation (GPE). This equation demonstrates the wave nature of the condensates and plays a role in the theory of BEC which is analogous to a role of the Maxwell equations in the theory of the electromagnetism. Initially it was suggested by E.P. Gross and L.P. Pitaevskii in 1961 for description of a quantized vortex line, which existence in rotating BEC was predicted by L. Onsager in 1949. After BEC was achieved in 1995 by Cornell, Wieman and Ketterle, GEP was used for description of numerous observed phenomena. It was used for calculation of interference between two condensates, including in the presence of a vortex line, for calculation of structure and motion of solitons and vortex rings. New possibilities arise after realization of mixtures of BEC, which is described by a system of GPE. After creation of BEC of atoms with electrical or magnetic momenta the equation was generalized for these systems. |
Tuesday, March 16, 2021 5:24PM - 6:00PM Live |
J08.00005: Title: Birth, Death, and Flight: the hydrodynamics of Malthusian flocks Invited Speaker: John J Toner I'll present the hydrodynamic theory of ``Malthusian Flocks": moving aggregates of self-propelled entities (e.g., organisms, cytoskeletal actin, microtubules in mitotic spindles) that reproduce and die. Long-ranged order (i.e., the existence of a non-zero average velocity 〈v,r,t〉 ≠ 0) is possible in these systems, even in spatial dimension =2. Their spatiotemporal scaling structure can be determined exactly in d=2; furthermore, they lack both the longitudinal sound waves and the giant number fluctuations found in immortal flocks. Number fluctuations are very {\it persistent}, and propagate along the direction of flock motion, but at a different speed. I'll also present recent results for the three dimensional version of this problem, which required the first full blown dynamical renormalization treatment of a flocking system in its ordered phase. |
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