Congjun Wu (UCSD)
Symmetry and Correlation Aspects of Quantum DynamicsSymmetry and correlation are fundamental aspects of condensed matter physics. A solid state textbook typically starts with crystalline symmetries as classified by space group, and proceeds with the Bloch theorem which sets up the framework of electron’s quantum behavior under crystalline symmetries. We have generalized these concepts to dynamic systems by proposing “dynamic crystal” and updating the Bloch theorem. A new structure of “space-time group” is constructed for describing dynamic symmetries, including the space-time intertwined symmetries of “time-screw-rotation” and “time-glide-reflection”. Dynamic crystal applies to a large class of systems including laser-driven solid state crystals, dynamic photonic crystals and optical lattices. On the other hand, the real frequency responses at high energies is a hardcore problem of strong correlation physics. Our new progress is to employ integrable methods to investigate spin dynamics arising from the Bethe string states, which are exotic many-body excitations of high energy magnon anti-bound states. In particular, the 3-string excitations, i.e., the 3-body anti-bound states, are identified for the first time by comparing the characteristic spectra lines in the electron-spin-resonance spectroscopy measurement on SrCo2V2O8 with the theory calculations.
Josh Vura-Weis, Department of Chemistry - UIUC
What Did the Metal Know, and When Did She Know It? Ultrafast XUV Spectroscopy Reveals Short-lived States in Transition Metal Complexes and Organohalide PerovskitesX-ray absorption near edge spectroscopy (XANES or NEXAFS) is a powerful technique for electronic structure determination. However, widespread use of XANES is limited by the need for synchrotron light sources with tunable x-ray energy. Recent developments in extreme ultraviolet (XUV) light sources using the laser-based technique of high-harmonic generation have enabled core-level spectroscopy to be performed on femtosecond to attosecond timescales. We have extended the scope of tabletop XUV spectroscopy and demonstrated that M2,3-edge XANES, corresponding to 3p→3d transitions, can reliably measure the electronic structure of first-row transition metal coordination complexes with femtosecond time resolution. We use this ability to track the excited-state relaxation pathways of photocatalysts and spin crossover complexes. In semiconductors such as CH3NH3PbI3, distinct signals are observed for photoinduced electrons and holes, allowing the dynamics of each carrier to be tracked independently. This work establishes extreme ultraviolet spectroscopy as a useful tool for mainstream research in inorganic, organometallic, and materials chemistry.Host: Andrei Tokmakoff, 4-7696 or via email to email@example.com. Persons with a disability who may need assistance please contact Brenda Thomas at 2-7156 or by email at firstname.lastname@example.org.
Xiaoming Mao, University of Michigan
Topological floppy modes in aperiodic networks and a mechanical duality theoremTopological states of matter have been intensively studied in crystals, leading to fascinating phenomena such as scattering-free edge current in topological insulators. However, the power of topological protection goes well beyond ordered crystal lattices. In this talk we explore how topology protects mechanical edge modes in messy, noncrystalline, systems. We will use disordered fiber networks and quasicrystals as our examples, to demonstrate how topological edge floppy modes can be induced in these structures by controlling their geometry. Fiber networks are ubiquitous in nature and especially important in bio-related materials. Establishing topological mechanics in fiber networks may shed light on understanding robust processes in mechanobiology. Quasicrystals show unusual orientational order with quasiperiodic translational order. We found that a bulk topological polarization can be defined for mechanics of quasicrystals that is unique to their non-crystallographic orientational symmetry. References: (1) Di Zhou, Leyou Zhang, Xiaoming Mao, “Topological Edge Floppy Modes in Disordered Fiber Networks”, Phys. Rev. Lett. 120, 068003 (2018); (2) Di Zhou, Leyou Zhang, Xiaoming Mao, “Topological Boundary Floppy Modes in Quasicrystals”, arXiv:1809.09188 (2018).
David Sussillo, Google
Universality and individuality in neural dynamics across large populations of recurrent networksCurrently neuroscience is undergoing a data revolution, where many thousands of neurons can be measured at once. These new data are extremely complex and will require a major conceptual advance in order to infer the underlying brain computations from them. In order to handle this complexity, systems neuroscientists have begun training deep networks, in particular recurrent neural networks (RNNs), in order to make sense of these newly collected, high-dimensional data. These RNN models are often assessed by quantitatively comparing neural dynamics of the model with the brain. However, the nature of the detailed neurobiological inferences one can draw from such comparisons remains elusive. For example, to what extent does training RNNs to solve simple tasks, prevalent in neuroscientific studies, uniquely determine the low-dimensional dynamics independent of neural architectures? Or alternatively, are the learned dynamics highly sensitive to different neural architectures? Knowing the answer to these questions has strong implications on whether and how to use task-based RNN modeling to understand brain dynamics. To address these foundational questions, we study populations of thousands of RNN architectures commonly used to solve neuroscientifically motivated tasks and characterize their dynamics. We find the geometry of the dynamics can be highly sensitive to different network architectures. Moreover, we find that while the geometry of neural dynamics can vary greatly across architectures, the underlying computational scaffold: the topological structure of fixed points, transitions between them, limit cycles, and aspects of the linearized dynamics, often appears universal across all architectures. Overall, this analysis of universality and individuality across large populations of RNNs provides a much needed foundation for interpreting quantitative measures of dynamical similarity between RNN and brain dynamics.
Amos Yaron, Technion
Probing anomalous drivingI will describe two novel effects that may be observed once one drives a system whose underlying matter content generate an ’t Hooft anomaly. The effects are tied to the existence of quasi-normal modes of magnetically charged black branes at low temperatures and to features of Chern-Simons Maxwell dynamics in an asymptotically AdS geometry.
Carlos Bustamante: University of California,Berkeley
The Ribosome Modulates Nascent Protein Folding and Nascent Protein Folding can Modulate the Ribosome ActivityThe Ribosome Mandates Nascent Protein Folding and Nascent
Carlos Bustamante: University of California,Berkeley
Division of Labor Among the Subunits of a Highly Coordinated Ring ATPase
Atac Imamoglu, ETH Zurich
Many-body Optical Excitations in Solid-State SystemsTwo dimensional materials provide new avenues for synthesizing compound quantum systems. Monolayers
with vastly different electric, magnetic or optical properties can be combined in van der Waals
heterostructures which ensure the emergence of new functionalities; arguably, the most spectacular example
to date is the observation of strong correlations and low electron density superconductivity in Moire
superlattices obtained by stacking two monolayers with a finite twist angle. Optically active monolayers such
as molybdenum diselenide provide a different "twist" as they allow for investigation of nonequilibrium
dynamics in van der Waals heterostructures by means of femtosecond pump-probe measurements. Moreover,
interactions between electrons and the elementary optical excitations such as excitons or polaritons, provide
an ideal platform for investigation of quantum impurity physics, with possibilities to probe both Fermi- and
Bose-polaron physics as well as mixtures with tunable density of degenerate fermions and bosons.
After introducing the framework we use to describe many-body optical excitations in van der Waals
heterostructures, I will describe two recent developments in the field. The first experiment uses pump-probe
measurements to demonstrate how exciton-electron interactions beyond the non-self-consistent T-matrix
approximation lead to optical gain by stimulated cooling of exciton-polaron-polaritons. The second experiment shows that a tri-layer system, consisting of two semiconducting monolayers separated by an insulating layer, could lead to hybridization of intra- and inter-layer excitons. The latter has potentialapplications ranging from strongly interacting polaritons to reaching Feshbach resonance condition in exciton-electron scattering.Host: Jonathan Simon, 2-9661 or via email at email@example.com. Persons with a disability who may need assistance please contact Brenda Thomas at 2-7156 or by email at firstname.lastname@example.org.
IME Distinguished Colloquium Series - Ralph ColbyProfessor Ralph Colby from Penn State University will speak as part of the IME Distinguished Colloquium Series.
Event will be followed by a reception from 5 pm to 6 pm at ERC in IME’s 2nd floor lounge/atrium area