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	NA
	Title	NA
	Abstract	NA
	Host	NA

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-T_C 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 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 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	NA
	Title	NA
	Abstract	NA
	Host	NA

March 9
	Speaker	Jed Pixley (Rutgers Univ.)
	Title	Twistronics in solid-state devices and beyond
	Abstract	The ability to control and manipulate the strength of correlations in quantum matter is one of the central questions in condensed matter physics today. While pressure, chemical doping, or magnetic field have served as conventional tuning knobs for a wide class of correlated systems, the ability to twist van der Waals materials has recently emerged as a novel scheme to engineer strong correlations and tune electronic properties. In the case of twisting two sheets of graphene, at a particular “magic-angle,” the kinetic energy of electronic degrees of freedom is expected to vanish, and as a result, interaction effects should dominate. This has now been demonstrated experimentally following the recent discovery of superconductivity in close proximity to correlated insulating phases in magic-angle graphene. These results have now been reproduced by a number of experimental groups and extended to other two-dimensional van der Waals materials. In this talk, I will discuss our theory to describe the universal properties that give rise to strong correlations in twisted bilayer graphene and related systems. As a result, we will generalize this phenomenon to a wide class of simpler models and distinct physical settings, such as ultra-cold atomic gases, trapped ions, and metamaterial experiments. We will show that the “magic-angle” in twisted bilayer graphene is, in fact, a single particle quantum phase transition that can be described by a delocalization transition in momentum space: the incommensurate potential, that is generated by the twist, eventually destabilizes the ballistic plane-wave eigenstates. Lastly, we will demonstrate the effects of correlations by constructing effective Hubbard models with dramatically enhanced interactions due to this novel quantum phase transition.
	Host	Peter Hirschfeld

March 16
	Speaker	Valentin Stanev (Univ. Maryland)
	Title	TBA
	Abstract
	Host	Peter Hirschfeld

March 23
	Speaker	Se Kwon Kim (Univ. Missouri)
	Title	Unconventional spin transport in quantum materials
	Abstract	Recent advancements in spintronic techniques originally developed for spin-based devices now enable us to study fundamental spin physics of various quantum materials with unprecedented spin-current control and measurement, opening a new area of theoretical and experimental investigation of quantum systems. In this talk, we will introduce this emerging research area of spin transport in quantum materials which is fueled by the global interest in quantum information science. As examples, we will discuss our discovery of a new magnonic topological insulator realized by a 2D honeycomb ferromagnet [1], which shows how spintronic techniques can be used for probing elusive quantum materials, and our prediction of long-range spin transport mediated by a vortex liquid in superconductors [2], which shows that quantum materials can provide novel platforms for efficient spin-transport devices. We will conclude the talk by offering a future outlook on quantum spintronics. [1] S. K. Kim, H. Ochoa, R. Zarzuela, and Y. Tserkovnyak, Phys. Rev. Lett. 117, 227201 (2016). [2] S. K. Kim, R. Myers, and Y. Tserkovnyak, Phys. Rev. Lett. 121, 187203 (2018)
	Host	Yoonseok Lee

March 30
	Speaker	Andrey Chubukov (Univ. Minnesota)
	Titl	TBA
	Abstract
	Host	Dmitrii Maslov

April 6
	Speaker	Alimamy Bangura (NHMFL, Tallahassee)
	Title	TBA
	Abstract
	Host	Mark Meisel

April 13
	Speaker	Kater Murch (Washington Univ.)
	Title	TBA
	Abstract
	Host	James Hamlin

April 20
	Speaker	Yi Li (Johns Hopkins Univ.)
	Title	TBA
	Abstract
	Host	Yuxuan Wang