Colloquia are Wednesdays at 4:00 p.m. in the JILA Auditorium.
Coffee, tea and cookies will be available in G1B31 (across from G1B20) from 3:30 - 3:50 p.m.
January 17— "Porphene: an Easily Tunable 2-D Conjugated Organic Polymer"
Presenter: Josef Michl, Department of Chemistry, University of Colorado Boulder
Host: Dan Dessau
- Abstract: Extending quantum effects beyond the nanoscale has been a long-standing goal of chemists and physicists. Graphene has provided one very successful avenue, and porphyrin nanostructures have been another. To date, large porphyrin nanostructures have only been realized as onedimensional tapes or rings. In Nat. Commun. 2023, 14, 6308, we described the synthesis of sheets of porphene 1, a 2-D material made of porphyrin macrocycles. If DFT predictions of metalloporphene properties are right, if our ability to reversibly insert metal ions into porphene can be perfected, if we can produce patterns lithographically or by ink printing, and if we can make and stack distinct single crystalline metalloporphene monolayer domains of sufficient size with sufficiently accurate registry, it will be possible to construct flexible monolithic electronic, photonic, and spintronic circuitry analogous to that currently available in silicon with an ultimate resolution of 0.84 nm.
January 24 — "What's new? Isn't fusion energy always 30 years away?"
- Presenter: Scott Parker, University of Colorado Boulder
- Host: Michael Litos
- Abstract: Fusion energy is a promising technology for producing clean, limitless, zero-carbon energy. Recently, there has been a paradigm shift where today, privately funded research dominates over the historic government-funded fusion program. Private research and development paths to fusion have very short timelines, and some future milestones appear speculative. I will discuss plasma and nuclear physics constraints that experiments will face as they progress toward the fusion goal. I will start with an overview of fusion energy and briefly discuss the recent demonstration of a net gain in inertial confinement fusion at the National Ignition Facility. My focus will be magnetic confinement, the space where the most promising devices exist and where the most private equity is invested.
January 31 — "Two tales about time in living (and not-so-living) transport networks"
- Presenter: Eleni Katifori, University of Pennsylvania
- Host: Andrew Lucas
- Abstract: We utilize transport systems daily to commute, e.g. via road networks, or bring energy to our houses through the power grid. Our body needs transport networks, such as the lymphatic, arterial or venous system, to distribute nutrients and remove waste. If the transported quantity is information, for example carried by an electrical signal, then even the internet and the brain can be thought of as members of this broad class of webs. Despite our daily exposure to transport networks, their function and physics can still surprise us. This is exemplified by the Braess paradox, where the addition of an extra road in a network worsens rather than improves traffic contrary to a naïve prediction.
In this talk we will explore two cases that highlight the importance of time in load transmission in transport networks. In the first problem, we will discuss how short timescale dynamics in the flow alters the topology of the network in longer timescales, and shapes its morphology. We will first present the system of phenomenological adaptation equations that govern the structural evolution of vascular networks. We will then demonstrate how implicit of explicit dynamics in the boundary conditions (the power supply, or heart) can drastically alter the network topology, and discuss the implications for the development and function of human circulation. Moving to a larger system, will provide evidence that the similar dynamical developmental rules to the ones that are thought to control vascular remodeling in humans also shape tidal delta geomorphology.
In the second problem, we consider stochastic transport in geometrically embedded graphs. Intuition suggests that providing a shortcut between a pair of nodes improves the time it takes to randomly explore the entire graph. Counterintuitively, we find a Braess' paradox analog. For regular diffusion, shortcuts can worsen the overall search efficiency of the network, although they bridge topologically distant nodes. We propose an optimization scheme under which each edge adapts its conductivity to minimize the graph's search time. The optimization reveals a relationship between the structure and diffusion exponent and a crossover from dense to sparse graphs as the exponent increases.
February 7 — "A Matter of Mystery"
- Presenter: Hirohisa Tanaka, SLAC
- Host: Eric Zimmerman
- Abstract: Neutrinos are enigmatic particles. Their properties are rather basic and yet so bizarre and surprising that at times we hardly believe them. We barely notice their presence, and yet they are everywhere and are essential to things as glaring as the sun’s energy production. The minuscule but non-zero mass of a neutrino, nearly a million times smaller than the electron (the next lightest particle), has enormous consequences for our understanding of these particles and their role in shaping the universe. It is possibly an indication of new processes and interactions that we don’t know about and may enable a matter/antimatter imbalance that allows the universe to exist. In this talk, I’ll briefly introduce this perplexing particle and discuss a quantum interference process called neutrino oscillations that allow us to probe its properties. I’ll discuss the challenges in observing and measuring this process, and conclude with where we stand in studying neutrino oscillations and our next steps.
February 14 — "Exciting 1D gases"
- Presenter: David Weiss, Pennsylvania State University
- Host: Adam Kaufman
- Abstract: 1D gases with point contact interactions are special because they are integrable many-body systems, which means that they have many extra conserved quantities, beyond the usual few (energy, momentum, etc.). I will explain how we make bundles of 1D Bose gases in the lab, the various ways we excite them out of equilibrium, and how we use them as model systems for studying quantum dynamics.
February 21 — "Turbulent Origins of the Sun's Hot Corona and the Solar Wind"
- Presenter: Steven Cranmer, Department of Astrophysical & Planetary Sciences, University of Colorado Boulder
- Host: Dmitri Uzdensky
- Abstract: The solar corona is the hot and ionized outer atmosphere of the Sun. It traces out the complex solar magnetic field and expands into interplanetary space as the supersonic solar wind. In 1958, Eugene Parker theorized that the presence of a million-degree corona necessarily requires the outward acceleration of a wind. However, despite many years of exploration of both phenomena, we still do not have a complete understanding of the processes that heat the coronal plasma to its bizarrely high temperatures. In this talk, I will discuss some new observations and theoretical concepts that are helping us get closer to an answer to this infamous coronal heating problem. We will begin by examining super-high-resolution images of the Sun's surface from the Daniel K. Inouye Solar Telescope (DKIST), zoom out to ultraviolet and X-ray images that illustrate how magnetic field lines thread their way through the corona, and then follow the Parker Solar Probe (PSP) spacecraft through its repeated dives into the innermost zones of the solar wind. I will do my best to make sense of the puzzling physical processes that must be at work in producing and maintaining this massive plasma laboratory that sits about 150 million kilometers away.
February 28 — "Exploring many-body problems with arrays of individual atoms"
- Presenter: Antoine Browaeys, Institut d'Optique
- Host: Adam Kaufman
- Abstract: Over the last twenty years, physicists have learned to manipulate individual quantum objects: atoms, ions, molecules, quantum circuits, electronic spins... It is now possible to build "atom by atom" a synthetic quantum matter. By controlling the interactions between atoms, one can study the properties of these elementary many-body systems: quantum magnetism, transport of excitations, superconductivity... and thus understand more deeply the N-body problem. More recently, it was realized that these quantum machines may find applications in the industry, such as finding the solution of combinatorial optimization problems.
This seminar will present an example of a synthetic quantum system, based on laser-cooled ensembles of individual atoms trapped in microscopic optical tweezer arrays. By exciting the atoms into Rydberg states, we make them interact, even at distances of more than ten micrometers. In this way, we study the magnetic properties of an ensemble of more than a hundred interacting ½ spins, in a regime in which simulations by usual numerical methods are already very challenging. Some aspects of this research led to the creation of a startup, Pasqal.
March 6 — "Collapse and Ejection in the N-body problem and the Formation of Rubble Pile Asteroids"
- Presenter: Daniel Scheeres, Department of Aerospace Engineering
- Host: Michael Ritzwoller
- Abstract: Rubble pile asteroids are thought to form in the aftermath of cataclysmic collisions between proto-planets. The details of how the detritus from such collisions reaccumulate to form these bodies are not well understood, yet can play a fundamental role in the subsequent evolution of these bodies in the solar system. Simple items such as how particle sizes and porosity is distributed within a body can have a significant influence on how they subsequently evolve. Current space missions are just starting to gain limited insight into these fundamental questions, but require a better theoretical understanding to fully explain their observations.
To that end, this work studies how the initial energy and angular momentum of a random collection of gravitating bodies is partitioned and redistributed between escaping components and bound multiple body systems. A generic initial distribution of N bodies will naturally lose many components due to multi-body dynamical interactions. If the bodies have finite density, some components will also form condensed distributions, becoming single, binary or multiple component rubble pile asteroids. We derive and apply rigorous results from the Full N-body problem to place limits and constraints on how the energy and angular momentum of such systems can evolve, which controls the formation of stable rubble pile asteroids.
We are able to establish some of our constraints analytically, providing unique insight into this process. Ultimately, however, we require numerical simulations to elucidate certain aspects of the ejection process. As will be shown, these gravitational ejections will always reduce the system energy yet can cause significant fluctuations in the total angular momentum of the remaining bodies. Some possible implications of these trends will be discussed.
- Presenter: Moumita Das, University of Rochester
- Host: Nuris Figueroa Morales
- Presenter: Murray Holland, JILA, University of Colorado, Boulder
- Host: James K. Thompson
March 27 — No Colloquium, Spring Break
April 3 — "Waves of Topological Origin in the Fluid Earth System and Beyond"
- Presenter: Brad Marston, Brown University
- Host: Michael Ritzwoller
- Abstract: Symmetries and topology are central to our understanding of physical systems. Topology, for instance, explains the precise quantization of the Hall effect and the protection of surface states in topological insulators against scattering from disorder or bumps. However discrete symmetries and topology have not, until recently, contributed much to our understanding of the fluid dynamics of oceans and atmospheres. In this talk I show that, as a consequence of the rotation of the Earth that breaks time reversal symmetry, equatorial Kelvin and Yanai waves emerge as topologically protected edge modes. The non-trivial topology of the bulk Poincaré
waves is revealed through their winding number in frequency - wavevector space. Bulk-interface correspondence then guarantees the existence of the two equatorial waves. I discuss our recent direct detection of the winding number in observations of Earth’s stratosphere. Thus the oceans and atmosphere of Earth naturally share basic physics with topological insulators. As equatorially trapped Kelvin waves in the Pacific ocean are an important component of El Niño Southern Oscillation, the largest climate oscillation on time scales of a few years, topology plays a surprising role in Earth’s climate system. We also predict that waves of topological origin will arise in magnetized plasmas. The waves may appear in laboratory plasma experiments, and they may also arise in the solar system and beyond.
- Presenter: Dmitri Basov, Columbia University
- Host: Andrew Lucas
- Presenter: Daniel Slichter, National Institutes of Standards and Technology
- Host: Adam Kaufman
- Presenter: Liang Fu, Massachusetts Institute of Technology
- Host: Andrew Lucas
- Presenter: Alberto Nicolis, Columbia University
- Host: Andrew Lucas
For more information about colloquia this semester, contact: Andrew Lucas.