Past Events

  • Physics Department Colloquium: "Orientational Transitions: From Liquid Crystals to Viral Capsids"

    David Rittenhouse Laboratory - A8

    Robijn Bruinsma (UCLA) hosted by Douglas Durian

    Lars Onsager showed in 1948 that there could be a new type of phase transition where a liquid loses rotational symmetry but retains its translational symmetry, unlike the freezing transition where a liquid loses both types of symmetry operations. He proposed that this is what takes place when a liquid becomes a nematic liquid crystal, a material now extensively used in displays.

  • High Energy Theory Seminar: "Spinning Cosmology"

    David Rittenhouse Laboratory, 2N36

    Andrei Khmelnitsky (ICTP)

    We study the signatures left by new light particles with a spin on the primordial cosmological fluctuations. In distinction from the pure de Sitter background, where fields with spin have to be massive enough to satisfy the Higuchi bound, the inflation allows to host spinning particles that are parametrically lighter than the expansion rate. I introduce the description of the particles with spin in the context of the Effective Field Theory of Inflation and discuss the main signatures left by such particles on the primordial fluctuations.

  • 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.

  • WIP: "A Cosmic Perspective: Searching for Aliens, Finding Ourselves"

    Penn Museum, Harrison Auditorium University of Pennsylvania

    Dr. Jill Tarter (Bernard Oliver Chair for Search for Extra-Terrestrial Intelligence (SETI) Institute)

    Are we alone? Humans have been asking this question throughout history. We want to know where we came from, how we fit into the cosmos, and where we are going. We want to know whether there is life beyond the Earth and whether any of it is intelligent.  

  • 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