Past Events

  • Condensed Matter Seminar: "A change in stripes for cholesteric shells via anchoring in moderation"

    David Rittenhouse Laboratory, A4

    Lisa Tran (University of Pennsylvania)

    Chirality, ubiquitous in complex biological systems, can be controlled and quantified in synthetic materials such as cholesteric liquid crystal (CLC) systems. In this work, we study spherical shells of CLC under weak anchoring conditions. We induce anchoring transitions at the inner and outer boundaries using two independent methods: by changing the surfactant concentration or by raising the temperature close to the clearing point.

  • High Energy Theory Seminar: "Equivalence Principle in Scalar-tensor Theories"

    David Rittenhouse Laboratory, 2N36

    Lasma Alberte (ICTP)

    We study the question of whether the equivalence principle holds for extended objects moving on cosmological backgrounds in modified gravity theories. We do so within the framework of effective field theory of dark energy focusing in particular on the subclass corresponding to the Horndeski theories. These are the most general second order scalar-tensor theories with non-linear derivative self-interactions allowing the Vainshtein screening mechanism to operate on small scales.

  • Condensed Matter Seminar: "The Structure and Function of Organic-Inorganic Hybrid Perovskites"

    David Rittenhouse Laboratory, A4

    Cherie Kagan (University of Pennsylvania)

    Organic-inorganic hybrid perovskites represent a class of materials composed of corner sharing metal halide octahedra charge balanced by organic cations. The hybrid perovskites may be tailored in their composition and in their dimensionality. Research exploring hybrid perovskites skyrocketed in the past few years because of the remarkable, >20% power conversion efficiency demonstrated for the three-dimensional (3D) lead halide structures in solar cells, which currently rivals that of commercial silicon-based solar cells.

  • Astro Seminar: "Stellar Spin-orbit Misalignment"

    David Rittenhouse Laboratory, A4

    Joshua Winn (Princeton)

    In the Solar system, the planets follow orbits that are aligned with the Sun’s equatorial plane to within a few degrees.  But what about planets around other stars?  Many techniques are now available to measure the obliquities of planet-hosting stars, ranging from an effect predicted in the 19th century to several techniques relying on data from the Kepler space telescope.  Many exoplanetary systems show good alignment, as in the Solar system. We have also found planets on highly inclined orbits and even retrograde orbits.

  • High Energy Theory Seminar: "Topological Sectors, Supersymmetric Localization, and Holography"

    David Rittenhouse Laboratory, 2N36

    Silviu Pufu (Princeton University)

  • *Special* HET Seminar: "Many-body Localization: Breakdown of Thermalization in Quantum Matter"

    David Rittenhouse Laboratory, 4N12

    Arijeet Pal (Oxford)

    Until recently interacting many-particle systems governed by the laws of quantum mechanics were assumed to eventually reach thermal equilibrium, being described by equilibrium statistical physics. Rapid developments in theory and experiments in the last decade have established a phase of matter where this assumption is false, due to a phenomenon known as many-body localization (MBL). In this phase, the system undergoing unitary time dynamics retains the memory of the initial state in local observables for infinitely long times.

  • Condensed Matter Seminar: "Magnetism in Amorphous Alloys"

    David Rittenhouse Laboratory, A4

    Frances Hellman (University of California at Berkeley)

    Most condensed matter textbooks start by introducing crystal symmetries and the periodic lattice as foundational to the field.  Yet, it has long been known that the amorphous structure supports ferromagnetism, superconductivity, and a host of other condensed matter properties.  Superconductivity theory was famously expanded from the original Bloch wave pairing to be described as pairing of electrons with time-reversed wavefunctions to enable explanation of superconductivity in amorphous systems where electrons in the normal state have a mean free path of approximately an

  • Astro Seminar: "Fundamental Physics from the Non-linear Universe"

    David Rittenhouse Laboratory, A4

    Marcel Schmittfull (IAS)

    The cosmology community is running a large program of galaxy surveys over the next 10 years. Our goal is to map out the properties of dark energy as a function of time, measure the sum of neutrino masses, and study the origin of the Universe with unprecedented precision. A key limitation for these efforts is how to extract cosmological information from small scales that are affected by nonlinear dynamics.

  • High Energy Theory Seminar: "A Toy Model of Axion Gauge Field Inflation"

    David Rittenhouse Laboratory, 2N36

    Robert Caldwell (Dartmouth College)

    We present a toy model of an axion gauge field inflation scenario that yields viable density and gravitational wave spectra. The scenario consists of an axionic inflaton in a steep potential that is effectively flattened by a coupling to a collection of non-Abelian gauge fields. The model predicts a blue-tilted gravitational wave spectrum that is dominated by one circular polarization, resulting in unique observational targets for cosmic microwave background (CMB) and gravitational wave experiments.

  • *Special Condensed Matter Seminar*: "Self-organization and Self-assembly in Bio-molecular Systems"

    David Rittenhouse Laboratory, A2

    Prof. Erwin Frey (University of Munich)

    Active matter is a fascinating new field in soft matter physics aiming to understand how interacting active particles self-organize into an intriguing set of patterns and collective non-equilibrium states. Superficially, flocks of animals, self-propelled microorganisms or cytoskeletal systems appear to display similar phenomenologies, hinting towards universal organizing principles.