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Condensed Matter/Biophysics Seminars
Spring 2020

Condensed Matter/Biophyics Seminars are in Room NPB 2205
on Mondays @ 4:05 pm t0 4:55 pm

Contact: Yasu Takano or Dmitrii Maslov

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January 6      

 

Speaker

Thomas Searles (Howard Univ.)

 

Title

Tunable strong coupling in terahertz metasurfaces

 

Abstract

Recently, the study of ultrastrong light-matter coupling has gained increased interest due to its potential application in optoelectronics, plasmonics and circuit quantum electrodynamics. One common way to achieve strong coupling is to place an emitter near or inside an optical cavity. In this case, the emitter?cavity system, the spatial overlap between the emitters and the cavity is often the key factor that limits the light?matter coupling strength. The cavities can be either photonic microcavities, which can have very high quality factors, or surface plasmon resonators, whose mode volume can be in the deep subwavelength regime. In contrast to microcavities, where the light field is confined by two metallic layers or dielectric mirrors, an alternative approach to achieve strong coupling is provided by metamaterials (MMs) in which the confinement is provided by the evanescent field of localized plasmons. This has led to the demonstration of strong-coupling regime with a number of quantum systems including phonons, intersubband transitions and cyclotron resonances. In addition, resonant coupling leads to light?matter hybridization into two normal modes with an energy separation known as the vacuum Rabi splitting (VRS).

In this work, we investigate a THz planar metamaterial and observe the excitation of a polaritonic state as well as a VRS with a coupling strength of ~21%. Strong splitting results in the formation of a forbidden frequency gap that can be evaluated as a transparency window caused by the hybridization of two eigenmodes. The physics of the transparency window is analogous to the lattice induced transparency effect in which there are limited demonstrations in the literature of strong coupling due to cavity-cavity interactions. Further, we show that by increasing the capacitive gap width of the MM unit cell, we increase the overall capacitance of the MM and demonstrate an anti-crossing behavior; a key signature to strong-light matter coupling. Lastly, we present graphene micro-ribbons and a nanohole array in a carbon nanotube film as two tunable platforms for actively tuned strong coupling in hybrid metasurface devices.

 

Host

Chris Stanton


January 13       

 

Speaker

Juan Guan (UF)

 

Title

Seeing is believing: Challenges and opportunities in experimental biophysics

 

Abstract

Decades of biophysics theories have developed from experimental observations which were often based on indirect ensemble-averaged and low-resolution measurements. In this talk, I will use two vignettes to show you that recent advances in high-resolution fluorescence imaging have led to surprises in experimental biophysics that have challenged conventional wisdom. In one example, in sharp contrast to the theoretical expectation of a continuous smooth process, we show that driven transport of macromolecules instead displays prominent intermittency with strong coupling between center-of-mass motion and intra-molecular conformation fluctuation. In another example, direct imaging with high spatiotemporal resolution reveals how the biophysical state of certain proteins is both necessary and sufficient to promote a model case of cancer signaling. Departing from the textbook view that signaling is mediated from the cell membrane, this novel cancer mechanism via protein phase separation serves as a paradigm shift in how we think of self-assembly mechanisms and biophysical impact of protein granules.

 

Host

Dmitrii Maslov


January 20 (No seminar - Martin Luther King Jr. Day)      

 

Speaker

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Title

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Abstract

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January 28, Tuesday (Note special date)    

 

Speaker

Mahmut Demir (Yale Univ.), biophysics faculty candidate

 

Title

Sensing & navigating naturalistic odor plumes

 

Abstract

Insects find food, mates, and egg-laying sites by tracking odor plumes swept from their sources by complex wind patterns. Previous studies have shown that insects navigate odor signals to their sources by surging upwind when they detect odor and casting cross- or downwind when the signal is lost. These strategies lead to reliable odor encounters in relatively regular plumes, such as streaming odor ribbons. Much less is understood about behavioral strategies in intermittent plumes, where the location and timing of odor encounters (whiffs) are less predictable. The timing and history dependence of whiffs could provide important information to the navigator, but it has been challenging to connect behaviors to individual odor encounters because it is difficult to visualize odors and behavior simultaneously. Here, I imaged for the first time both intermittent odor plumes and the movements of freely-walking flies navigating them, allowing us to quantify behavior in response to whiffs. We found that walking flies navigate these unpredictable plumes by using whiff timing to modulate the rate of stochastic transitions between turns, stops, and walks. Turns are random, stereotyped saccades whose direction is biased upwind by whiff frequency. Stop decisions depend on the time since the most recent whiff, while walk decisions are made by accumulating evidence from whiffs over time. Drosophila and other animals precisely encode whiff timing within their olfactory circuit, and our findings reveal how they leverage this sensory precision to navigate the natural world.

 

Host

Steve Hagen


January 30, Thursday (Note special date)    

 

Speaker

En Cai (UC San Francisco), biophysics faculty candidate

 

Title

How immune cells sense danger: Visualizing T cell antigen detection using high-precision microscopy

 

Abstract

The immune system protects us from disease by detecting pathogens and distinguishing them from the body’s own healthy tissue. T cells, a type of immune cells, can detect and destroy infected cells or cancer cells. To do so, T cells scan the surface of target cells through physical contact, looking for “danger signals” — abnormal protein fragments presented on the cell surface, called antigen. T cells can rapidly scan many cells and are very sensitive in detecting antigen presented on them. How T cells solve the classic trade-off between speed and sensitivity in the process of antigen detection is unknown. I used lattice light-sheet microscopy and quantum dot-enabled synaptic contact mapping microscopy to visualize in real-time the dynamic process of T cell antigen detection. I showed that small T cell membrane protrusions, called microvilli, dynamically scan the opposing cell before and during antigen recognition. I found that microvilli survey the majority of opposing surfaces within one minute through anomalous diffusion. Prior to antigen recognition, T cell receptors (TCRs) are non-homogenously distributed into high-density patches on the cell membrane. These patch-like distributions are highly dynamic and are transiently associated with microvilli. Upon antigen recognition, TCR-occupied microvilli are selectively stabilized on the interface of cell-cell contact, known as the immunological synapse. My work defines the efficient cellular search mechanism utilized by T cells and provides a promising strategy to enhance tumor-specific T cell responses in cancer immunotherapy.

 

Host

John Yelton


February 3   

 

Speaker

Zachary Frentz (Columbia Univ.), biophysics faculty candidate

 

Title

Matter, energy, and time in living systems

 

Abstract

Living systems appear to be highly complex, consisting of a large number of components that interact in idiosyncratic ways. In addition, their dynamics are influenced by noise at multiple scales, and thus may exhibit nontrivial history dependence. However, from the point of view of thermodynamics, the boundary conditions of living systems are quite simple, as they are determined by the flux of matter and energy. This simple perspective allows us to investigate phenomena that are unique to living systems, without getting lost in molecular details. I will discuss this approach through two examples: ecosystems that are closed to the exchange of matter with their surroundings, and spores that survive for centuries without energy from their environments.

 

Host

Steve Hagen


February 10      

 

Speaker

Andrew Harris (UC Berkeley), biophysics faculty candidate

 

Title

Cell & tissue mechanics: From millimeter-scale fracture to nanometer-scale mechanosensitivity

 

Abstract

Exposure to mechanical stresses is a normal part of physiology for cells and tissues. For example, intestinal epithelia are stretched during peristaltic movements in the gut, lung alveoli deform during breathing, and endothelia are exposed to pulsatile fluid shear stresses in blood flow. Consequently, maintaining mechanical properties that give cells and tissues a combination of strength and flexibility is critical for their proper function. This is most apparent in diseased states, where genetic mutations to cytoskeletal and intercellular adhesion proteins alter mechanical properties and result in phenomena associated with cell and tissue mechanical failure. In this talk, I will explore how new experimental tools for characterizing cell and tissue rheology at the millimeter and micron scale are providing new biophysical insights into this unique form of active matter. I will trace these properties to the nanometer scale, where I will present new work that has uncovered how proteins associated with the actin cytoskeleton dynamically organize in a mechanosensitive manner into distinct networks that underly cell and tissue mechanical properties. This combination of experimental tools and analytical techniques from the physical sciences, with classical molecular biology and biochemistry, is now poised to enable a fundamental understanding of cell mechanics that is rich in complex biophysical phenomena and will guide novel therapeutic strategies.

 

Host

Mark Meisel


February 11 Tuesday (Note special date) 

 

Speaker

Zhenzhong Shi (Duke Univ), low-temperature research faculty candidate

 

Title

Quantum materials in extreme conditions: From high-TC superconductors to quantum magnets

 

Abstract

A central challenge in the study of quantum materials lies in disentangling the effects of a complex landscape of competing and coexisting states of matter. To map out the phase diagrams of quantum materials, experiments need to be conducted using a variety of tuning parameters, such as temperature, magnetic field, and pressure. Scientific breakthrough is often achieved as these tuning parameters are pushed to their extremes.

In this talk, I will discuss our recent studies on underdoped cuprates and quantum magnets in high magnetic fields and low temperatures. In the case of cuprates, we studied stripe-ordered La-214, which feature very low zero-field superconducting transition temperatures. This enabled us to explore a previously inaccessible energy scale window in underdoped cuprates. Our results established a complete vortex phase diagram of the underdoped striped cuprates, provided much-needed transport signatures of an elusive state of matter called pair density wave, and revealed a novel high-field normal state characterized by the vanishing Hall coefficient. For quantum magnet, we studied the Shastry-Sutherland system SrCu2(BO3)2 (SCBO), which is a valence bond solid. Here we reported the discovery of emergent bound states and impurity pairs in doped SCBO, offering insights into the nontrivial effects of impurities in quantum spin systems. In the end, I will discuss my future research plans, thoughts on capability development, and ideas for funding proposals.

 

Host

Yasu Takano


February 17  

 

Speaker

Fangwei Si (UC San Diego), biophysics faculty candidate

 

Title

Quantitative principles of the coupling between DNA replication, cell division, and growth in bacteria

 

Abstract

The goal of microbial physiology is to understand the overarching principles of cellular reproduction. In my view, the major challenge in physiological research is to delineate the primary structure of the physiological control of the cell. As such, the goal of my research is to understand how essential cellular processes (e.g., metabolism, DNA replication, or cell division) are coupled in response to environmental changes.

In this talk, I will introduce two principles characterizing the coupling between DNA replication and cell growth, and that between cell division and cell-size control. As for the DNA replication-growth coupling, we hypothesized that the E. coli cell is composed of conceptual “unit cells.” By extensively perturbing the growth physiology in steady-state conditions using genetic and pharmacological methods, we discovered the invariant initiation mass as the basic unit of the cell size. This finding unravels how cells coordinate their biomass and DNA content by imposing a simple and robust control on DNA replication initiation. As for the division-growth coupling, we proposed a hypothesis that bacterial cell division requires the accumulation of division proteins to a fixed threshold number. By combining single-cell microfluidics, programmed genetic modulation, and automated image analysis, we confirmed this hypothesis in evolutionary divergent E. coli and B. subtilis cells. This latter result explains the mechanistic origin of cell-size homeostasis in a wide range of bacterial species that my postdoc lab and others have discovered.

I will end my talk by demonstrating the roadmap of my future research. Driving questions include how the cell coordinates the assembly of complex macromolecular machinery (e.g., divisome) with the environment-dependent growth, and how the cell spatially organizes the essential cellular processes that define the primary structure of physiological control.

 

Host

BingKan Xue


February 20, Thursday, Room 1002 (Note special date and place) 

 

Speaker

Mirna Mihovilovic (New York Univ.), biophysics faculty candidate

 

Title

Cracking neural circuits of a simple brain

 

Abstract

How do brains compute? The Drosophila (fruit fly) larva is a small, semi-transparent crawling organism with about 10,000 neurons, compared to 100 billion in humans and 100 million in mice. Despite this simplicity, the larva carries out information-processing tasks, including navigation – moving towards a favorable location based on information from its senses. A century of genetic work in Drosophila combined with recent innovations in protein engineering allow us to use light to directly activate specific neurons in the larva. For instance, we can engineer larvae with light-activable neurons in their “noses.” When presented with red light, these larvae perceive an odor and respond by attempting to find its source. Using sophisticated light patterns and analysis methods, we developed an assay that allowed us to quantify how the larva makes decisions based on multiple sources of sometimes conflicting information.

Advances similar to the ones that allow us to activate neurons using light allow us to measure thought patterns using light microscopes. Because the larva is almost clear, it has been a long-standing goal to use a microscope to “read the larva’s mind” as it navigates its surroundings. However, the 3D brain movements generated by the larva’s complicated locomotion have prevented optical recording of neural activity in behaving larvae. We developed a two-photon microscope capable of tracking single neurons moving rapidly in 3D while monitoring their activity in real time without motion artifacts. To record from many neurons we added a second beam that scans the volume around the tracked neuron to enable motion-corrected volumetric imaging in a freely-behaving animal. This allowed us to image correlated activity of motor and pre-motor neurons from a significant portion of larva’s “spine” in a completely unrestrained crawling animal. I will use these techniques to follow information flow through the larva’s circuits during sensory-motor transformations and achieve a neuron-level understanding of how a simple brain implements fairly complex calculations.

 

Host

Purushottam Dixit


February 24      

 

Speaker

Constantin Schrade (MIT)

 

Title

Protected qubits in superconducting circuits

 

Abstract

Superconducting circuits are the foundation for impressive progress in quantum computing technology. However, the protection of quantum information in superconducting circuits against environmental noise remains a significant obstacle towards a large-scale quantum computer. Should we keep improving device parameters for resolving this problem, or should we opt for a fundamentally new approach that relies on the same well-developed technology but enables the robust storage of quantum information?

In this talk, I will present my work and vision towards the design of a protected superconducting qubit. I will introduce the concept of a “Majorana Superconducting Qubit,” which leverages topological superconductor islands as new circuit elements for reliable quantum information storage. I will then specify the requirements for initialization, read-out, and, most importantly, a universal quantum gate set. Finally, I will identify a “parity-controlled Josephson effect” as the physical foundation of my qubit design, and discuss its implementation in a superconducting interference device.

 

Host

Dominique Laroche


March 2 (No Seminar - UF Spring Break Week, APS March Meeting in Denver)

 

Speaker

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Title

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Abstract

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Host

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March 9      

 

Speaker

Zi Yang Meng (IOP-CAS and Univ. Hong Kong)

 

Title

Lattice models and Monte Carlo solutions for quantum criticality


Abstract

I will review recent developments in a priori and a posteriori numerical strategies in dealing with quantum many-body systems. Thanks to these philosophical and numerical advancements, novel paradigms in condensed matter and high energy physics such as non-Fermi-liquid, quantum criticality, emergent gauge-field coupled with matter field and SYK holographic quantum matter can be readily accessed with large-scale numerical simulations. These results in turn inspire further analytical and numerical progress towards the complete understanding of few important quantum many-body physics problems.

References: TOPICAL REVIEW, J. Phys.: Condens. Matter 31, 463001 (2019); PNAS August 20, 2019 116 (34) 16760-16767; . Phys. Rev. X 9, 021022 (2019); arXiv:2001.06586.

 

Host

Yuxuan Wang


March 16       

 

Speaker

Valentin Stanev (Univ. Maryland)

 

Title

Machine learning modeling of superconducting critical temperature

 

Abstract

Machine learning has emerged as a powerful new research tool that can be used to answer many scientific questions in unconventional ways. In this talk I will discuss how it can help us address one of the most challenging problems in the study of quantum matter – finding connection between superconductivity (in particular critical temperature Tc) and chemical/structural properties of materials. I will present several recently developed machine learning methods for modeling Tc of the more than 12,000 known superconductors available via the SuperCon database. These models use coarse-grained predictors based only on the chemical composition of the materials. They demonstrate good performance and strong predictive power, with learned predictors offering insights into the mechanisms behind superconductivity in different families. The models can be combined into a single pipeline and employed to search for potential new superconductors. Searching the entire Inorganic Crystallographic Structure Database led to the identification of 35 compounds as candidate high-Tc materials. I will also discuss how machine learning can be used to guide and accelerate the experimental process.

 

Host

Peter Hirschfeld


March 23       

 

Speaker

Purushottam Dixit (UF Physics)

 

Title

TMI: Thermodynamic inference of data manifolds

 

Abstract

The Gibbs-Boltzmann distribution offers a physically interpretable way to massively reduce the dimensionality of high dimensional probability distributions where the extensive variables are `features' and the intensive variables are `descriptors'. However, not all probability distributions can be modeled using the Gibbs-Boltzmann form. Here, we present TMI, Thermodynamic Manifold Inference, a thermodynamic approach to approximate a collection of arbitrary distributions. TMI simultaneously learns from data intensive and extensive variables and achieves dimensionality reduction through a multiplicative, positive valued, and interpretable decomposition of the data. Importantly, the reduced dimensional space of intensive parameters is not homogeneous. The Gibbs-Boltzmann distribution defines an analytically tractable Riemannian metric on the space of intensive variables allowing us to calculate geodesics and volume elements. We discuss the applications of TMI with multiple real and artificial data sets. Possible extensions are discussed as well.

Reference: https://arxiv.org/abs/1911.09776

 

Host



March 30    

 

Speaker


 

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Abstract


 

Host



April 6      

 

Speaker

Yuxuan Wang (UF Physics)

 

Title

Pair-density-wave order and paired fractional quantum Hall fluids

 

Abstract

Pair density wave (PDW) is a nonuniform pairing order that is oscillatory in real space. In contrast to the Fulde-Ferrel-Larkin-Ovchiniski order, its formation does not require a Zeeman splitting of the Fermi surface.  In this talk I will begin by discussing the experimental evidence and phenomenology of the PDW order in recent STM experiments in underdoped cuprates. I will then shift gear and discuss the possible existence of the PDW order in the nematic quantum Hall fluid at ν =5/2 under hydrostatic pressure. The PDW order parameter has a local p+ip pairing symmetry and alternates in its overall sign. Depending on the interplay between pairing, hopping, and PDW wavelength, we show a remarkably rich phase diagram featuring gapped states of distinct topology as well as symmetry-protected gapless states. We discuss its implications on the properties of quantum Hall state, including an exotic phase conducting heat but not electricity.

Reference: L. H. Santos, Y. Wang, and E. Fradkin, Phys. Rev. X 9, 021047 (2019).

 

Host



April 13      

 

Speaker

Kater Murch (Washington Univ.)

 

Title

TBA

 

Abstract


 

Host

James Hamlin


April 20      

 

Speaker

Xiao-Xiao Zhang (UF Physics)

 

Title

Ultrafast excitation of coherent spin waves in 2D antiferromagnets

 

Abstract

The recently discovered atomically-thin magnetic crystals provide a unique playground to develop new approaches to manipulate magnetism. Rapid progresses have been made that demonstrate the potentials of utilizing 2D magnets to construct novel spintronics devices. However, their spin dynamics, which are crucial for microscopic understanding and determine the fundamental limit of spin manipulation, still remain elusive due to the difficulty to characterize these micron-sized samples with conventional microwave techniques. In this talk, we will show how we can access and probe the collective spin-wave excitations in an antiferromagnetic bilayer CrI3 which allows us to extract magnetic anisotropy and exchange energy. In particular, we will demonstrate the gate tunability of magnon frequencies, which is unique for the 2D magnet system.


Host