Colloquia are Wednesdays at 4:00 p.m. in DUAN G1B20, unless otherwise noted.

Coffee, tea and cookies will be available before regular colloquia beginning at 3:45 p.m. in DUAN G1B31.

January 15 — "Quantum Clocks With Ultracold Molecules"

  • Presenter:  Tanya Zelevinsky, Columbia University
  • Host: David Leibrandt
  • Abstract: As quantum science moves beyond controlling atoms, diatomic molecules can be created and optically trapped at ultracold temperatures.  This allows exquisite control over the quantum states of the molecules.  Using this platform, we can generate long-lived coherent superpositions of far-separated molecular quantum states.  The capability to create these superpositions results in a precise clock that is based on molecular vibrations.  The vibrational molecular clock allows interatomic force measurements with resolution exceeding a part per trillion, with a potential to improve the constraints on new short-range mass-dependent forces.  The full control of the molecular quantum states also helps to shed light on ultracold chemical reactions that proceed as matter waves, and to enable a range of new experiments in fundamental physics and chemistry.

January 22 — "High performance Single Photon Detectors using superconducting devices"

  • Presenter: Sae Woo Nam, NIST
  • Host: David Leibrandt
  • Abstract: Single-photon detectors are increasingly becoming an essential tool for a wide range of applications in physics, chemistry, biology, communications, medicine, and remote sensing. Ideally, a single photon detector generates a measurable signal only when a single photon is absorbed. Furthermore, the ideal detector would have 100% detection efficiency, no false positive (dark counts), and transform-limited timing resolution. Recently, there has been tremendous progress in the development of superconducting devices (Superconducting Nanowire Single Photon Detector (SNSPD or nSSPD) and superconducting Transition-Edge Sensor (TES) that nearly achieve this ideal performance.   Superconducting detectors such as the SNSPD and TES offer unprecedented performance not achievable using photomultiplier tubes and semiconducting devices.  Yet, there is still tremendous room for improvement to these devices to make them more accessible for even more applications.  I will describe recent advances in new materials, new optical packaging, new cryogenic packaging, and new readout techniques should enable megapixel scale arrays of high performance superconducting detectors.

January 29 — "Mapping the Milky Way's Dark Matter Halo with Gaia"

  • Presenter: Mariangela Lisanti, Princeton University
  • Host: Keith Ulmer
  • Abstract: The Gaia mission is in the process of mapping nearly 1% of the Milky Way’s stars. This data set is unprecedented and provides a unique view into the formation history of our Galaxy and its associated dark matter halo. I will review results based on the most recent Gaia data release, demonstrating how the evolution of the Galaxy can be deciphered from the stellar remnants of massive satellite galaxies that merged with the Milky Way early on. The recent advancements in our understanding of the Galaxy's evolution suggest that a component of the local dark matter is not in equilibrium, as typically assumed, and instead exhibits distinctive dynamics. The updated dark matter map built from the Gaia data has ramifications for direct detection experiments, which search for the interactions of these particles in terrestrial targets.

February 5 "Making Sound Matter"

  • Presenter: Konrad Lehnert, JILA, University of Colorado, Boulder
  • Host: Rahul Nandkishore
  • Abstract: The mechanical vibrations of man-made and macroscopic objects have recently come under quantum control. It is now possible to resolve the individual quantum particles of sound and to measure, prepare, and manipulate the quantum state of acoustical resonators. In my colloquium, I will describe how these fragile objects were tamed and how this new capability might be applied. Although there are some intriguing possibilities for exploiting quantum sound, our primary motivation is simply that sound is different than light. Unlike electrical and optical systems, which are governed by fundamental equations of electromagnetism, acoustical phenomena are described by the equations of elastic waves in solid bodies. They are subject to different limitations and can reach different regimes of behavior. The speed of sound in a solid material is 100,000-fold slower than light, elastic waves do not propagate through vacuum, they are fundamentally a tensor rather than vector phenomenon, and they couple to atom-like defects through strain rather than electrical or magnetic dipole interactions. These facts have consequences for quantum information science that we have yet to fully understand. 

February 12 — "Quantum supremacy using a programmable superconducting processor"

  • Presenter: John Martinis, Google and UC Santa Barbara
  • Host: Rahul Nandkishore
  • Abstract: The promise of quantum computers is that certain computational tasks might be executed exponentially faster on a quantum processor than on a classical processor. A fundamental challenge is to build a high-fidelity processor capable of running quantum algorithms in an exponentially large computational space. Here we report the use of a processor with programmable superconducting qubits to create quantum states on 53 qubits, corresponding to a computational state-space of dimension 2^53  (about 10^16). Measurements from repeated experiments sample the resulting probability distribution, which we verify using classical simulations. Our Sycamore processor takes about 200 seconds to sample one instance of a quantum circuit a million times—our benchmarks currently indicate that the equivalent task for a state-of-the-art classical supercomputer would take approximately 10,000 years. This dramatic increase in speed compared to all known classical algorithms is an experimental realization of quantum supremacy for this specific computational task, heralding a much-anticipated computing paradigm.

February 19 — "Quantum speed limits on spin transport in Fermi gases"

  • Presenter: Joseph Thywissen, University of Toronto
  • Host: Ana Maria Rey
  • Abstract: The world around us is not in equilibrium, but slowly (or quickly) getting there through transport of conserved quantities such as energy, charge, and momentum. However transport is challenging to calculate ab initio, leaving many open questions (such as high-temperature superconductivity) and room for new paradigms (such as holographic duality). Ultracold atoms provides an ideal platform for the study of non-equilibrium quantum physics, since samples are isolated from the environment, and the strength of interactions can be tuned. We have observed a kind of quantum "speed limit" on the rate of spin diffusion in a Fermi gas. I will discuss how this is measured, and its relation to various conjectured bounds. 

February 26 — "Towards an Optical Clock with Highly Charged Ions"

  • Presenter: Piet Schmidt, Leibniz Universität Hannover
  • Host: David Leibrandt
  • Abstract: Highly charged ions (HCI) have many favorable properties for tests of fundamental physics and as potential next-generation optical atomic frequency standards [1]. For example, narrow optical fine-structure transitions have smaller polarizabilities and electric quadrupole moments, but much stronger relativistic, QED and nuclear size contributions to their binding energy compared to their (near) neutral counterparts. Therefore, HCI have been found to be among the most sensitive atomic species to probe for a possible variation of the fine-structure constant or dark matter coupling.
    HCI can readily be produced and stored in an electron beam ion trap (EBIT). There, the most accurate laser spectroscopy on any HCI was performed on the 17 Hz wide fine-structure transition in Ar13+ with 400 MHz resolution, lagging almost twelve orders of magnitude behind state-of-the-art optical clocks. This was primarily limited by Doppler broadening of the megakelvin hot ion plasma in the EBIT [2]. The lack of a suitable optical transition for laser cooling and detection can be overcome through sympathetic cooling with a co-trapped Be+ ion [3]. Techniques developed for quantum information processing with trapped ions can be used to perform quantum logic spectroscopy [4]: A series of laser pulses transfers the internal state information of the Ar13+ ion after spectroscopy onto the Be+ ion for efficient readout.
    We present the first coherent laser spectroscopy of an HCI. Ar13+ are extracted from a compact EBIT [5], charge-to-mass selected and injected into a cryogenic Paul trap containing a crystal of laser-cooled Be+ ions [6]. By removing excess Be+ ions, a crystal composed of a Be+/Ar13+ ion pair is obtained. Results on sympathetic ground state cooling and quantum logic spectroscopy of the Ar13+ P1/2-P3/2 fine-structure transition at 441 nm will be presented, improving the precision of the observed line center by more than eight orders of magnitude. Furthermore, excited state lifetimes and the first high-accuracy measurement of excited state g-factor demonstrate the versatility of the technique to access all relevant atomic parameters [7]. Finally, we have started to perform frequency measurements of this transition, including first estimates of systematic uncertainties.
    [1]    M. G. Kozlov et al., Rev. Mod. Phys. 90, 045005 (2018).
    [2]    I. Draganić et al., Phys. Rev. Lett. 91, (2003).
    [3]    L. Schmöger et al., Science 347, 1233–1236 (2015).
    [4]    P. O. Schmidt et al., Science 309, 749–752 (2005).
    [5]    P. Micke et al., Rev. Sci. Instrum. 89, 063109 (2018).
    [6]    T. Leopold et al., Rev. Sci. Instrum. 90, 073201 (2019).
    [7]    P. Micke et al., Nature, in print.

**Special Physics Colloquium** Friday, February 28 — "Scientific espionage, open exchange, and American competitiveness"

  • Note Special Location: DUAN G130
  • Presenter: Professor Xiaoxing Xi, Temple University
  • Host: Gang Cao
  • Abstract: As the federal government warns universities and colleges about the risk of China to academia in the United States, professors, scientists, and students of Chinese ethnic origin as well as those engaging in academic collaborations with Chinese colleagues are under heightened scrutiny. In 2015, I became a casualty of this campaign despite being innocent. This experience gave me insights into the challenges Chinese scientists face and the immediate threat to the open environment in fundamental research on university campuses. Based on my personal experience and the recent events involving Chinese scientists in the US and China’s talent programs, I urge the audience to join an increasing number of scientists, university administrators, and professional societies in speaking up to defend liberty, support reaffirmation of NSDD-189 on open fundamental research, and safeguard America's research enterprise.
  • Bio: Xiaoxing Xi is Laura H. Carnell Professor of Physics at Temple University, and former chair of the Physics Department. Prior to joining Temple in 2009, he was Professor of Physics and Materials Science and Engineering at the Pennsylvania State University. He received his B.Sc. in physics from Peking University in 1982, and his PhD degree in physics from Peking University and Institute of Physics, Chinese Academy of Sciences, in 1987. After several years of research at Karlsruhe Nuclear Research Center, Germany, Bell Communication Research/Rutgers University, and University of Maryland, he joined the Physics faculty at Penn State in 1995. His research focuses on the materials physics of oxide, boride, and 2-dimensional dichalcogenide thin films. His notable contributions include early work on epitaxial thin films, heterostructures, and electric-field effect in high temperature superconductors, pioneering work on lattice dynamics in ferroelectric thin films and nanostructures, invention of hybrid physical-chemical vapor deposition for magnesium diboride, and development of atomic layer-by-layer laser molecular beam epitaxy for oxide thin films and interfaces. He is author of over 340 refereed journal articles and 3 U.S. patents in the area of superconductivity. He is a Fellow of the American Physical Society.
    Since 2015, he has actively advocated for open fundamental research and against racial profiling. He was recently awarded the 2020 Andrei Sakharov Prize by the American Physical Society.

March 4 — "Controlling trapped ions without lasers"

  • Presenter: Daniel Slichter, NIST
  • Host: David Leibrandt
  • Abstract: Trapped atomic ions are a leading platform for quantum computing and sensing experiments.  These experiments require manipulation of the ions' quantum states and the creation of entanglement between ions, both of which are typically accomplished using laser beams.  However, there are drawbacks to this approach, including errors due to photon scattering and the complexity of the required laser sources.  Our group performs quantum state control using oscillating radiofrequency and microwave-frequency magnetic and electric fields, and their near-field gradients, instead of laser beams.  A critical element is the use of a microfabricated surface-electrode ion trap, which holds the ions roughly 30 µm above the electrodes generating the control fields.  We use low-power resonant laser beams for cooling, optical pumping, and readout.  I will give a general introduction to trapped ion quantum computing, and then describe several of our recent results, including the creation of high-fidelity entangled states of two ions using only microwave and radio-frequency control fields, and the use of radio-frequency electric fields to generate squeezed states of ion motion for sensing applications and to amplify phonon-mediated ion-ion interactions. 

**Canceled** March 11 

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**Special Physics Colloquium** Thursday, March 12 — "Bacterial swimming: internal clock and fluid environment"

  • NOTE SPECIAL LOCATION: DUAN G126
  • NOTE SPECIAL TIME: 1:00 P.M.
  • Presenter: Nuris Figueroa-Morales, Pennsylvania State University
  • Host: Noel Clark
  • Abstract: Bacterial suspensions are ubiquitous in naturally occurring and man-made scenarios. Their transport determines how infections spread and microbiota organize in guts or in soils. In addition, bacteria can turn common passive materials into novel active materials with emergent properties. In this talk, I will discuss our latest and ongoing research on bacterial transport, from statistical properties of their run-and-tumble dynamics, to macroscopic migration in microfluidic channels, and swimming in non-isotropic synthetic and biological liquids. Our results unravel bacterial strategies for space exploration and provide insight into the design and control of out-of-equilibrium materials.

**Special Physics Colloquium** Monday, March 16 — "Polymers and Parkinson’s: Elucidating Protein Function through Soft Matter Paradigms and Techniques"

  • ONLINE LECTURE
  • NOTE SPECIAL TIME: 2:00 P.M.
  • Presenter: Dr. Peter J. Chung, University of Chicago 
  • Host: Noel Clark
  • Abstract: Despite being unequivocally linked to Parkinson’s disease, the function of a-synuclein remains unclear beyond transiently binding to the lipid membrane of synaptic vesicles (organelles filled with neurotransmitters). This is due, in part, to its intrinsically disordered nature; a-synuclein does not fold into a globular structure and instead behaves much like a biopolymer. While precluding traditional characterization methods, this makes a-synuclein incredibly amenable to investigation via a polymer physics framework. First, through purpose-designed membrane nanoparticles and advanced synchrotron X-ray methods I will demonstrate that a-synuclein binds to and collectively interacts to sterically-stabilize membrane surfaces, a biological manifestation of polyelectrolyte-stabilized colloids. I will then reconcile observed transient binding to synaptic vesicles by establishing that a-synuclein preferentially binds to osmotically-stressed membranes (a proxy for neurotransmitter-filled synaptic vesicles), a newly discovered biophysical function by which a-synuclein interrogates organelle contents. Utilizing these insights, I will contextualize a-synuclein as a guidepost that spatiotemporally directs non-equilibrium synaptic vesicles, a conferred function uniquely possible through its polymeric properties. 

**Special Physics Colloquium** Monday, March 16 — "Searching for the invisible: how dark forces shape our Universe"

  • NOTE SPECIAL LOCATION: DUAN G125
  • Time: 4:00 p.m.
  • ONLINE LECTURE
  • Presenter: Dr. Katelin Schutz, Massachusetts Institute of Technology 
  • Host: Ed Kinney
  • Abstract: How different would our Universe look with the addition of extra particles and forces beyond what we know? We already have ample gravitational evidence for at least one invisible new particle that has properties unlike any particle we have previously discovered. It is possible that this dark matter is made of many different kinds of particles that experience forces unlike the ones we are familiar with in day-to-day life. If these forces only act on dark particles, it may be difficult to discover them and learn more about what is happening in this dark sector. However, dark matter and visible matter do interact gravitationally at the very minimum, and this fact alone might be a good reason not to lose hope. If there are dark forces affecting the distribution of dark matter in our Universe, then that distribution will gravitationally affect the visible matter that we can see. I will discuss how the gravitational portal between dark and visible matter can constrain dark matter theories where dark matter can dissipate energy, can scatter with itself (elastically or inelastically), or can be born non-thermally in the moments after the Big Bang. I will demonstrate this constraining power by harnessing synergies between astrophysical systems including the local Milky Way, nearby dwarf galaxies, galaxy clusters, and large-scale cosmological structures. 

**Canceled** - March 18

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**Special Physics Colloquium** Thursday, March 19 — "Dynamics and rheology of 2-d membranes and soft complex interfaces"

  • NOTE SPECIAL TIME: 12:30 p.m.
  • ONLINE LECTURE
  • Presenter: Dr. Mehdi Molaei, University of Pennsylvania 
  • Host: Noel Clark
  • Abstract: Soft interfaces and membranes control many biological processes. They form the complex membranes of living cells, and they are hot spots to adsorb biomolecules and active colloids. In the first part of my talk, I discuss how we characterize the dynamics and mechanics of a 2-d model lipid membrane and the complex plasma membrane of liver cells. The plasma membrane displays time-dependent undulations, which are controlled by its bending modulus, excess area, and membrane tension. Together, these physical characteristics couple to and modulate vesicle trafficking, membrane adhesion, and actin-based motility. While the complex physics of lipid membranes is relatively well understood in reconstituted form, the understanding of the dynamics and mechanics of plasma membranes in living cells remains at an early stage. We have developed a new nanoscale rheology approach, based on angle resolved dark field microscopy, to probe interfacial rheology, tension, and curvature fluctuation of the plasma membrane. The goal of this study is to develop a high throughput assay for identification of cancer cells. In the second part of my talk, I present the new approach to study the mechanics of complex fluid interfaces. Adsorbed surfactant, proteins, and macromolecules on interfaces form thin layers with complex structures and with wide range of viscoelasticities. We have developed a new approach for manipulating fluid interfacial systems that enables simultaneous tensiometry to measure surface pressure and high-performance interfacial shear microrheology. We study an adsorbed layer of protein on the air-water interfaces as a model system. The power law frequency dependent shear modulus at high surface pressure reveals the soft glassy response of protein-laden interfaces. To understand complex flow fields established on fluid interfaces, which depend on interface viscoelasticity, (in)compressibility, and hydrodynamic coupling with bulk fluids, we measure displacement fields induced by the thermal motion of passive particles in interfacial films. 

    March 25 — Spring Break, No Colloquium

    **Special Physics Colloquium** Monday, March 30 — "Bridging multi-scale biology with the physics of solids, fluids and information"

    • NOTE SPECIAL TIME: 12:30 p.m.
    • ONLINE LECTURE
    • Presenter: Dr. Arnold JTM Mathijssen, Stanford University
    • Host: Noel Clark
    • Abstract: Biological systems flourish through collective functionality, by self-assembling into cells, tissues, flocks and parliaments. Understanding this multi-scale organization also lies at the heart of modern engineering and medicine: Pathologies can arise from deficiencies in collective functionality, while active and adaptive materials can be designed from controlling systems out of equilibrium. In this talk, I will overview our recent work on building first-principle theories, numerical tools, and experiments for studying the fascinating physics of life. We will first focus on reliable communication in ultra-fastbiology, exemplified by the discovery of ‘hydrodynamic trigger waves’ [1]. Second, we will discuss bacterial contamination dynamics, which is enhanced by the ability of cells to swim against flows [2]. Third, we consider the role of topology in biofunctionality, especially in ‘active carpets’ like ciliary arrays [3]. These insights open up exciting new avenues towards unravelling synthetic and biological active matter, through collective functionality, together.
      [1.] Mathijssen AJTM, Culver J, Bhamla MS, Prakash M, “Collective intercellular communication through ultra-fast hydrodynamic trigger waves,” Nature 571, 560-564 (2019)
      [2.] Mathijssen AJTM, Figueroa-Morales N, Junot G, Lindner A, Clement E, Zöttl A, “Oscillatory surface rheotaxis of swimming E. coli bacteria,” Nat. Comm. 10, 3434 (2019)
      [3.] Ramirez-San Juan G, Mathijssen AJTM, He M, Jan L, Marshall WF, Prakash M, “Multi-scale heterogeneity enhances mucus clearance in mouse airways”, Nat. Phys., accepted (2020)

    **Special Physics Colloquium** Monday, March 30 — "Quantum Error Correction with Schrödinger Cats"

    • NOTE SPECIAL TIME: 4:00 p.m.
    • ONLINE LECTURE
    • Presenter: Dr. Shruti Puri, Yale University
    • Host: Ed Kinney
    • Abstract: We are currently in the midst of a second quantum revolution where the fundamental laws of quantum mechanics are being applied to develop computers for solving otherwise intractable problems. Perhaps more remarkable than quantum computing itself is the concept of quantum error correction (QEC). In fact, with fault-tolerant QEC it is possible to implement an arbitrarily long quantum algorithm accurately even with a sequence of faulty operations on imperfect quantum hardware. However, fault-tolerant protocols come at the cost of dauntingly large overheads, pushing practical realization of useful quantum computers out of reach. In this talk I will present an elementary introduction to QEC. I will then introduce a new type of qubit called the Schrödinger cat qubit which is realized in a driven non-linear oscillator. The Schrödinger cat states are superpositions of two opposite phase coherent states. The driven-cat qubit has some in-built protection against errors in hardware and software. Finally, I will discuss how the inherent noise protection in this qubit can be exploited to significantly reduce the overheads for fault-tolerant QEC.

    **Canceled** April 1

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    **Special Physics Colloquium** Thursday, April 2 — "Ultrafast laser pulses drive novel non-equilibrium dynamics in strongly-correlated materials"

    • NOTE SPECIAL TIME: 10:00 a.m.
    • ONLINE LECTURE
    • Presenter: Dr. Phoebe Tengdin, École polytechnique fédérale de Lausanne
    • Host: Ed Kinney
    • Abstract: The interactions between electrons, spins, and phonons in magnets, superconductors and other strongly correlated electron materials hold the key to their emergent properties and provide a route to manipulating their phases. While the fundamental length- and time-scales for these phenomena are nanometers (exchange length) and femtoseconds (exchange splitting), tools that enable the exploration of their interactions and functional properties have only recently become available. Using a combination of ultrafast lasers, electron and x-ray pulses, we can go beyond merely observing the static properties, to manipulating a material as it undergoes a light-induced phase transition. In this talk I will describe advances in ultrafast X-ray and electron beam spectroscopy and imaging that allow us to directly observe and manipulate new phases of matter in magnetic and other strongly correlated electron materials. Specifically, I will show examples of how we can drive nonequilibrium phase transitions and observe new material physics in a half-metallic heusler alloy Co2MnGe, a skyrmion hosting ferromagnetic insulator Cu2OSeO3, and in a high-temperature superconductor MgB2.

    **Special Physics Colloquium** Thursday, April 2 — "Dynamic Plasmonic and Biophotonic Devices enabled through Liquid Crystal Materials"

    • NOTE SPECIAL TIME: 12:30 p.m.
    • ONLINE LECTURE
    • Presenter: Dr. Daniel Franklin, Northwestern University
    • Host: Noel Clark
    • Abstract: The next generation of displays, detectors and biosensors will be enabled by highly functional nanomaterials. Within this talk, I will summarize the fundamental and applied aspects of highly tunable plasmonic-liquid crystalline systems as applied to the visible and infrared regimes. For the visible, this results in dynamic color-changing surfaces capable of covering the entire RGB color space and which is compatible with active addressing schemes. I will then show the application of these concepts to infrared absorbers with liquid crystal and phase change materials. The later of these devices can find use in infrared data/image encoding, thermal management and camouflage. Lastly, I will demonstrate a transient/biodegradable liquid crystal laser for remote temperature sensing. Using biologically derived cholesteryl esters, I will introduce encapsulation strategies, cytotoxicity and system limits stemming from theoretical considerations. This project aims to create a new class of biocompatible NIR imaging probes with dynamic temperature sensing with additional applications including photothermal therapy with active temperature feedback. Together, these works explore the limits of liquid crystal tunable systems and the novel photonic devices they might lead to.

    **Canceled** April 8

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    **Special Physics Colloquium** Thursday, April 9 — "Topologically Frustrated Dynamics of Charged Macromolecules in Crowded Media"

    • NOTE SPECIAL TIME: 12:30 p.m.
    • ONLINE LECTURE
    • Presenter: Dr. Di Jia, University of Massachusetts
    • Host: Noel Clark
    • Abstract: Movement of very long electrically charged macromolecules, such as DNA, dispersed in aqueous media, is a ubiquitous phenomenon relevant to life processes and formulations of new materials. The facts that such molecules are topologically correlated due to their chain connectivity and that the charges are correlated over large distances through Coulomb forces contribute to rich phenomenology which is difficult to fully understand. In spite of complications from long-range topological and electrostatic correlations, Einstein’s law of diffusion is operative and these large molecules are able to diffuse even when they are in a crowded solution or under strong confinement. As an example, a tagged chain inside a collection of fully entangled long flexible chains diffuses like a drunken snake in a tube-like environment (de Gennes’s reptation model). In contrast with expectations from the Einstein’s law, I will present a new dynamical regime of non-diffusive topologically frustrated state by experimentally eliciting very long-lived metastable states, under intermediate confinements. Why does the molecule diffuse under strong confinements (reptation) but not under weaker confinements? The origin of the new topologically frustrated dynamical state lies in the emergence of multiple correlated entropic barriers, which are so prohibitive for the diffusion of the macromolecule which is localized. I will also address how such localization effects are modulated by enthalpic interactions between guest macromolecules and their oppositely charged host gel matrix. The experimental conditions (essentially water and ambient temperatures) for realizing the new dynamical state are very similar to many biological environments in human body, indicating implications for a better understanding of biological and biologically relevant processes.

    **Canceled** April 15 

    April 22 — "A Ferroelectric Nematic Liquid Crystal"

    • ONLINE LECTURE: Contact Veronica Lingo to preregister.
    • Presenter: Noel Clark, University of Colorado, Boulder
    • Host: Rahul Nandkishore
    • Abstract: Water is a prime example of a "polar" liquid, a classification created by Peter Debye based on his study of the dielectric response of materials made from molecules that have permanent internal electric dipole moments. Liquid water does not exhibit bulk polarity because its dipoles are oriented in  random directions, but, as Debye noted, is "polar" in the sense that the molecules have a tendency to orient with their dipoles parallel to the direction of an applied electric field. This field-induced alignment results in the strong dielectric response of water, nearly two orders of magnitude larger than that of the vacuum.  Thus water is not polar, but readily "polarizable”.
      We have recently found a three-dimensional fluid that is actually polar (RM734), its dipoles spontaneously adopting a mutual alignment parallel to one another to form a liquid in which there is bulk polarization density [1]. This polarization can be described as a 3D continuum vector field that gives the local average dipolar, and therefore molecular orientation, making this fluid a form of uniaxial nematic liquid crystal. The magnitudes of the polarization density and molecular dipole moment combine to show that, remarkably, this polar molecular alignment is nearly complete (polar order parameter ~0.93) at the lower temperatures in its nematic range (T ~ 90 ºC). As a result the molecular orientation responds strongly and in collective elastic fashion to applied field, exhibiting "giant" field-induced electro-optical and flow effects. This reorientability of the polarization and its spontaneous appearance make this phase a ferroelectric liquid. 
      First-Principles Experimental Demonstration of Ferroelectricity in a Thermotropic Nematic Liquid Crystal: Spontaneous Polar Domains and Striking Electro-Optics,
      X. Chen, E. Korblova, D. Dong, X. Wei, R.F. Shao, L.H. Radzihovsky, M.A. Glaser,  J.E. Maclennan, D. Bedrov , D.M. Walba, N.A. Clark, http://arxiv.org/abs/2003.03020, PNAS (to appear).  

    **Canceled** April 29

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    For more information about colloquia this semester, contact: Rahul Nandkishore.