Physics & Astronomy Colloquium


Thursdays, 4:00-5:00 pm

1-434 Physics and Astronomy (map)

Reception from 3:15-3:30
(unless otherwise posted)

For more information, contact Yaroslav Tserkovnyak


Fall 2019
Thursday, October 3, 2019

Saxon Lecture
How many numbers does it take to determine our Universe?
Michael S. Turner
Kavli Foundation and University of Chicago

Over the past three decades our understanding of the Universe has deepened.  The WMAP and Planck teams have asserted that just six numbers are needed to describe the whole Universe (fewer than the ten digits in a phone number), based upon their high-precision, all-sky maps of the Cosmic Microwave Background. Others have different opinions: one, two, a different six, and nine to determine our Universe. As I will discuss, the choice of numbers reveals much about what we know, our aspirations, and how we think about the Universe. After exploring the landscape, I will advocate for zero!


Thursday, October 10, 2019

On Ising's model of ferromagnetism
Peter Armitage
Johns Hopkins University

The 1D Ising model is a classical model of great historical significance for both classical and quantum statistical mechanics. Developments in the understanding of the Ising model have fundamentally impacted our knowledge of thermodynamics, critical phenomena, magnetism, conformal quantum field theories, particle physics, and emergence in many-body systems. Despite the theoretical impact of the Ising model there have been very few good 1D realizations of it in actual real material systems. However, it has been pointed out recently, that the material CoNb2O6, has a number of features that may make it the most ideal realization we have of the Ising model in one dimension.   In this talk I will discuss the surprisingly complex physics resulting in this simple model and review the history of "Ising’s model” from both a scientific and human perspective.  In the modern context I will review recent experiments by my group and others on CoNb2O6.  In particular I will show how low frequency light in the THz range gives unique insight into the tremendous zoo of phenomena arising in this simple model system.


Thursday, October 17, 2019

Building a quantum computer with neutral atoms
David Weiss
Penn State University

I will describe our work towards making a quantum computer using ultra-cold atoms trapped in a 3D optical lattice. In particular, I will explain: how we change the quantum state of individual atoms, even in the middle of the array, without affecting the quantum states of other atoms; how we sort atoms by realizing a Maxwell's demon; and how we reliably detect the internal states of the atoms without losing any.


Thursday, October 24, 2019

CANCELLED


Thursday, October 31, 2019

Physics and the HIV Virus
Robijn Bruinsma
University of California, Los Angeles

The intense research effort dedicated to the HIV virus and to other retroviruses has revealed our fundamental lack of understanding how the HIV virus "works". The colloquium will discuss how a combination of the physics of soft matter and of numerical simulations provides us with important insights into the different stages of the life-cycle of HIV. Conversely, the study of the operation of HIV provides statistical physics with interesting challenges, such as the apparent violation of the Second Law of Thermodynamics during the assembly process.


Thursday, November 7, 2019

The Physics of Active Matter
M. Cristina Marchetti
University of California, Santa Barbara

In two-dimensional systems, such as thin films of superfluids, crystals, liquid crystals and magnets, topological defects are key to understanding the transition between ordered and disordered states.  Almost fifty years ago, Berezinskii, Kosterlitz and Thouless showed that these systems disorder through a topological phase transition associated with the proliferation of unbound pairs of vortices of opposite charge.  The essence of this transition relies on the mapping of the statistical physics of defects onto a Coulomb gas. In active liquid crystals, topological defects become motile particles and drive the transition from spontaneous laminar flow to self-sustained turbulent-like motion. In this talk I will outline the statistical physics of defects in active nematics and their possible role in materials science and biology. By viewing the active nematic as a collection of swarming and interacting active defects, the onset of active turbulence can be described as an activity-driven defect unbinding transition. A hydrodynamic theory of a gas of unbound defects captures a new state of hierarchically organized active matter - a defect flock where defects themselves line up and order into a collectively flowing liquid. The hydrodynamic treatment of active defects provides a framework to address fundamental questions of defect organization in active matter and paves the way for the design of active devices with targeted transport functionalities through the controlled variation of activity.


Thursday, November 14, 2019

Probing Dark Matter Throughout Cosmic History
Vera Gluscevic
University of Southern California

I will review the status of cosmological searches for dark matter-baryon interactions, summarizing the best current limits on scattering of light particle candidates with protons derived from the cosmic-microwave-background anisotropy measurements. I will then present stringent new bounds on the same physics, inferred recently from the observed population of the Milky Way satellite galaxies. I will highlight complementarities between different observations and laboratory searches for dark matter, and discuss the prospects for unveiling the physics of dark matter in the coming decade.


Thursday, November 21, 2019

Quantum decorating: Imaging novel electronic states in defect-engineered 2D materials
Christopher Gutierrez
University of California, Los Angeles

Two-dimensional (2D) quantum materials have attracted much excitement due to the many interesting physical properties that emerge when they are thinned down to single atomic layers. Graphene, for instance, is a single layer of the ordinary graphite found in your pencil lead, yet its electrons behave like relativistic massless Dirac fermions. This has allowed graphene to act as a tabletop testbed for exploring novel forms of symmetry breaking and for verifying longstanding theoretical predictions in relativistic quantum mechanics.

Importantly, owing to its exposed surface, graphene’s electronic properties can be precisely tailored by the presence of atomic defects. In this talk I will present atomic scanning probe and photoemission spectroscopy experiments that highlight how the spatial arrangement of such defects can be harnessed to create novel electronic states in graphene. In the first part, I will show how different types of atomic scatterers peppered above (or below) graphene can self-assemble and drive the formation of new and topologically distinct collective density wave phases in graphene. In the second part, I will show that when substrate defects instead form large, amorphous clusters, they can create local potentials that can trap graphene’s quasi-relativistic electrons into quantized atomic-like orbitals, opening the door to studying 2D analogs of large, relativistic “Dirac atoms.”


Thursday, November 28, 2019

Thanksgiving Holiday

No Colloquium


Thursday, December 5, 2019, 4:00-5:00 p.m.

Kicking as a Route to Discovery: Condensed Matter Systems Away From Equilibrium
Anshul Kogar
University of California, Los Angeles

Traditionally, we have studied condensed matter systems at or near equilibrium using a variety of thermodynamic and spectroscopic measurements. Recently, through the advent of laser technology that has enabled intense ultra-short pulses, we have been able to gain access to material properties in a far-from-equilibrium regime. In this talk, I will describe what we have been able to learn using these technologies and how we went from studying phases of matter to being able to manipulate their properties with these intense laser pulses. Specifically, I will focus on a particular phase of matter called a charge density wave, where we have been able to visualize phase competition away from equilibrium and the writing and erasing of domain walls in these materials using light.


Winter 2020
Thursday, January 9, 2020, 4:00-5:00 p.m.

Gender, Equity, Power Structures and Implicit Bias in STEM
Elizabeth Simmons (University of California, San Diego)
University of California, San Diego

The presentation will start by reviewing data on the current status of gender equity in science, technology, engineering and mathematics (STEM) disciplines and summarizing social science research that illuminates some causes of gender disparities in STEM. With this context established, the focus will shift to how women enter into leadership roles in academic settings, what they experience and how gender impacts the way they exercise their authority. The final part of the talk will discuss how we can all contribute to changing the face of leadership for the future, to the benefit of all of us in STEM.


Thursday, January 16, 2020, 4:00-5:00 p.m.

CANCELLED: "Twisting 2d materials: the magic and the mystery"
Leon Balents
University of California, Santa Barbara

Graphene, a single atom thick lattice of pure carbon, is an ideal material to study the physics of electrons in a two dimensional "flatland". A new twist - literally - on graphene physics arose in the last two years. Driven by a theoretical prediction from 2011, experiments in 2018 confirmed thats placing one layer of graphene atop another rotated by a tiny angle of about 1 degree completely changes the behavior of the electrons. The rotation forms a moiré pattern, which acts as a new artificial lattice within which electrons move. At this "magic" angle, this motion is highly correlated and very different from what occurs in the parent graphene. The resulting insulating, magnetic, topological, and superconducting states are the subject of intense current research. This talk will review this active area and describe some of the latest results and theoretical predictions for twisted graphene and beyond.


Thursday, January 23, 2020, 4:00-5:00 p.m.

Hunting dark particles at colliders
Stefania Gori
University of California, Santa Cruz

Abstract: Dark matter is believed to make up most of the matter of our Universe, but its particle origin remains a mystery. So far experimental searches for dark matter particles have largely focused on the mass window at around the Hoggs boson mass. At the same time, lighter dark matter candidates in a dark sector are theoretically well-motivated and arise generically in many theories beyond the Standard Model. In this colloquium, I will first present an overview of the most recent progress exploring light dark matter candidates at high energy and high intensity colliders, highlighting the role of the Higgs boson in this endeavor. Then I will motivate new searches and new collider experiments that will have a unique opportunity to broadly explore viable light dark matter models.

Location: PAB 4-330

For more information, contact Yaroslav Tserkovnyak.

We thank the following people for their contributions to the wine fund for the post-colloquium reception: Professors Katsushi Arisaka, Andrea Ghez, Karoly Holczer, Huan Huang, HongWen Jiang, Per Kraus, Alexander Kusenko, Matthew Malkan, Mayank Mehta, Warren Mori, Ni Ni, Seth Putterman, Yaroslav Tserkovnyak, Vladimir Vassiliev, Shenshen Wang, and Nathan Whitehorn.


Thursday, January 30, 2020, 4:00-5:00 p.m.

The Dynamic Radio Sky
Gregg Hallinan
Caltech

All the magnetized planets in our solar system, including Earth, produce bright emission at low radio frequencies, predominantly originating in high magnetic latitudes and powered by magnetospheric currents. It has long been speculated that similar radio emission may be detectable from exoplanets orbiting nearby stars, potentially providing the first direct confirmation of the presence, strength and topology of exoplanet magnetospheres, and informing on their role in shielding the atmospheres of potentially habitable exoplanets. Despite 4 decades of searching, no exoplanet radio emission has been detected. Surprisingly, however, brown dwarfs have been found to produce both radio and optical emissions that are strikingly similar to the auroral emissions from solar system planets, albeit 100,000 times more luminous. I will discuss the radio emission from exoplanets and brown dwarfs with particular focus on the OVRO-LWA, a low frequency radio astronomy array located in the Owens Valley, California, that images the entire sky every 10 seconds to search for radio emission from exoplanets, and the FARSIDE array, a proposed probe class mission to place a radio array on the lunar far side to detect habitable exoplanets orbiting M dwarfs.

Location: PAB 4-330

For more information, contact Yaroslav Tserkovnyak.

We thank the following people for their contributions to the wine fund for the post-colloquium reception: Professors Katsushi Arisaka, Andrea Ghez, Karoly Holczer, Huan Huang, HongWen Jiang, Per Kraus, Alexander Kusenko, Matthew Malkan, Mayank Mehta, Warren Mori, Ni Ni, Seth Putterman, Yaroslav Tserkovnyak, Vladimir Vassiliev, Shenshen Wang, and Nathan Whitehorn.


Thursday, February 6, 2020, 4:00-5:00 p.m.

"TBA" by Ian Shoemaker (Virginia Tech)


Thursday, February 13, 2020, 4:00-5:00 p.m.

Magic Angle Graphene: a New Platform for Strongly Correlated Physics
Pablo Jarillo-Herrero
Massachusetts Institute of Technology)

The understanding of strongly-correlated quantum matter has challenged physicists for decades. Such difficulties have stimulated new research paradigms, such as ultra-cold atom lattices for simulating quantum materials. In this talk I will present a new platform to investigate strongly correlated physics, based on graphene moiré superlattices. In particular, I will show that when two graphene sheets are twisted by an angle close to the theoretically predicted ‘magic angle’, the resulting flat band structure near the Dirac point gives rise to a strongly-correlated electronic system. These flat bands exhibit half-filling insulating phases at zero magnetic field, which we show to be a correlated insulator arising from electrons localized in the moiré superlattice. Moreover, upon doping, we find electrically tunable superconductivity in this system, with many characteristics similar to high-temperature cuprates superconductivity. These unique properties of magic-angle twisted bilayer graphene open up a new playground for exotic many-body quantum phases in a 2D platform made of pure carbon and without magnetic field. The easy accessibility of the flat bands, the electrical tunability, and the bandwidth tunability though twist angle may pave the way towards more exotic correlated systems, such as quantum spin liquids or correlated topological insulators.


Thursday, February 20, 2020, 4:00-5:00 p.m.

The Ghostly Messengers of the Universe
Irene Tamborra
Niels Bohr Institute

Neutrinos are key particles in a wide range of astrophysical sources. Neutrinos affect the stellar dynamics, drive the formation of new elements, and carry information about the physics of the most energetic events in our Universe. Recent developments on the role of neutrinos in cosmic sources will be reviewed. The detection perspectives of neutrino will be outlined.


Thursday, February 27, 2020, 2:00-3:00 p.m., CNSI

"Capturing the First Picture of a Black Hole and Beyond" by Katie Bouman (Caltech)

Joint P&A Colloquium / STROBE Seminar: This talk will present the methods and procedures used to produce the first image of a black hole from the Event Horizon Telescope, as well as future developments. It had been theorized for decades that a black hole would leave a "shadow" on a background of hot gas. Taking a picture of this black hole shadow would help to address a number of important scientific questions, both on the nature of black holes and the validity of general relativity. Unfortunately, due to its small size, traditional imaging approaches require an Earth-sized radio telescope. In this talk, I discuss techniques the Event Horizon Telescope Collaboration has developed to photograph a black hole using the Event Horizon Telescope, a network of telescopes scattered across the globe. Imaging a black hole’s structure with this computational telescope required us to reconstruct images from sparse measurements, heavily corrupted by atmospheric error. This talk will summarize how the data from the 2017 observations were calibrated and imaged, and explain some of the challenges that arise with a heterogeneous telescope array like the EHT. The talk will also discuss how we are developing machine learning methods to help design future telescope arrays.


Thursday, March 5, 2020, 4:00-5:00 p.m.

"Formation and Compositions of Planet Interiors and Atmospheres: New Discoveries from Wide-Field Transit Surveys"
Erik Petigura
University of California, Los Angeles

The Kepler mission revolutionized our understanding of the demographics of extrasolar planets. However, because Kepler observed only 1/400 of the sky, most Kepler planets are too distant to permit detailed measurements of their masses, orbits, and atmospheres. Recently, the K2 mission helped extend Kepler's legacy by surveying 20x more sky, and TESS is on its way toward surveying the entire sky. These wide-field surveys cast a wide net for planets around nearby bright stars that are more amenable to precise characterization. In this talk, I will highlight how the planets found by these surveys have provided a window into exoplanet interior composition, orbital dynamics, and formation histories. In particular, they are helping to illuminate a mysterious class of planets between Neptune and Saturn size, that are not present in our Solar System. These planets have mean densities ranging from 2.0 g/cc (concrete) to 0.05 g/cc (Styrofoam) and offer an intriguing window into the processes that form and sculpt planetary systems.

Location: PAB 4-330

For more information, contact Yaroslav Tserkovnyak.

We thank the following people for their contributions to the wine fund for the post-colloquium reception: Professors Katsushi Arisaka, Andrea Ghez, Karoly Holczer, Huan Huang, HongWen Jiang, Per Kraus, Alexander Kusenko, Matthew Malkan, Mayank Mehta, Warren Mori, Ni Ni, Seth Putterman, Yaroslav Tserkovnyak, Vladimir Vassiliev, Shenshen Wang, and Nathan Whitehorn.


Thursday, March 12, 2020, 4:00-5:00 p.m.

CANCELLED: Shedding Light on the Nature of Dark Matter
Andrew Benson
Carnegie Institute for Science

In the 50 years since dark matter was confirmed to be a major component of our Universe, astronomical observations have reached a critical point, where they will soon have the power to either infer fundamental properties of the dark matter particle, or confirm that dark matter is essentially "cold" for most astrophysical purposes. Connecting this observations to dark matter particle properties requires careful modeling of the growth and destruction of dark matter halos. I will describe a program underway to predict key observables - such as the distribution of halo masses, and their spatial correlations with galaxies - and highlight some of the challenges that these predictions face. I will present current constraints on dark matter properties derived from this combination of observation and modeling, and discuss future prospects for furthering our understanding of the dark matter particle.


Spring 2020
Thursday, April 16, 2020, 4:00-5:00 p.m.

CANCELLED: "TBA" by Vinvenzo Vitelli
University of Chicago


Thursday, April 23, 2020, 4:00-5:00 p.m.

CANCELLED: Structure and Dynamics with Ultrafast Electron Microscopes -- or how to make atomic-level movies of molecules and materials
Bradley J. Siwick
McGill University

In this talk I will describe how combining ultrafast lasers and electron microscopes in novel ways makes it possible to directly 'watch' the time-evolving structure of condensed matter and the couplings between carrier and lattice degrees of freedom on the fastest timescales open to atomic motion [1-4]. By combining such measurements with complementary (and more conventional) spectroscopic probes we can now develop structure-property relationships for materials under even very far from equilibrium conditions [2].
I will assume no familiarity with ultrafast lasers or electron microscopes.
I will assume no familiarity with ultrafast lasers or electron microscopes.
[1] Morrison et al Science 346 (2014) 445
[2] Otto et al, PNAS, 116 (2019) 450
[3] Stern et al, Phys. Rev. B 97 (2018) 165416
[4] Rene de Cotret et al, Phys. Rev. B 100 (2019) 214115


Thursday, April 30, 2020, 4:00-5:00 p.m.

CANCELLED: "TBA" by Jianwei Qiu
Jefferson Lab


Thursday, May 7, 2020, 4:00-5:00 p.m.

CANCELLED: "TBA" by Jessica McIver
LIGO


Thursday, May 14, 2020, 4:00-5:00 p.m.

CANCELLED: "TBA" by Mark Eriksson
University of Wisconsin-Madison


Thursday, May 21, 2020, 4:00-5:00 p.m.

CANCELLED: "TBA" by Annika Peter
The Ohio State University


Thursday, 28, 2020, 4:00-5:00 p.m.

CANCELLED: "TBA" by Clifford Cheung
Caltech


Thursday, June 4, 2020, 4:00-5:00 p.m.

CANCELLED: "TBA" by Sera Markoff
University of Amsterdam