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Colloquium Schedule

Spring 2023

Frustrated Magnetism, Spin Liquids, and the Tensor Network Toolbox

Aaron Szasz, Lawrence Berkeley National Laboratory
Thu., January 26th at 4:30

At a macroscopic scale, we identify a liquid by its ability to flow and change shape; at the microscopic level, a liquid is characterized by the randomness or disorder in the locations of its constituent atoms and molecules.  In the same way, the quantum spins of electrons can be disordered even when the electrons themselves are rigidly fixed in place in a solid crystal.  This behavior describes a new and exotic phase of matter, called a “spin liquid.”  Spin liquids can arise due to “geometric frustration,” where due to the specific geometry of a crystal, competing forces on the electrons prevent the otherwise expected formation of magnetic order. 

Many materials that have been identified in experiments as possible spin liquids are approximately described by a particular theoretical model of frustrated electrons, the triangular lattice Hubbard model.  Using state-of-the-art computer simulations based on so-called tensor networks, I show that this model gives rise to a particular type of spin liquid, the chiral spin liquid, alongside metallic and magnetic phases.  I likewise show that small modifications that make the model more realistic lead to additional, distinct types of spin liquids.

Decomposing the Links Between Clouds and the Large-scale Tropical Circulation

Levi Silvers, Stony Brook University
Mon., January 23 at 4:30

Climate models have become a critical tool in our quest to better understand the climate of Earth and how we might expect the climate to change in the future.  Climate models are built by using numerical methods to solve the governing dynamic and thermodynamic equations of the Earth system.  I will give a brief introduction to climate models before describing my research.

The large-scale overturning tropical circulations are intertwined with multiple cloud types including deep convection, cirrus anvil clouds, and low-level stratus cloud decks.  Meridionally oriented overturning circulations are often described in the context of the Hadley circulation and zonally oriented circulations are often described in the context of the Walker circulation.  A simple but common method of defining the Hadley and Walker circulations relies on zonal and meridional spatial averages over the regions of interest.  This method results in substantial overlap between the circulations and complicates the interpretation of each.  

In this work, I partition the horizontal divergent wind field into two independent and orthogonal circulations, an alternate definition of the Hadley and Walker circulations.  Results are then sampled into particular dynamical regimes.  I then combine the Hadley and Walker Circulations with the cloud radiative effect to calculate the cloud characteristics for the Hadley and Walker circulations. With the goal of connecting idealized tropical models such as radiative convective equilibrium (RCE) to more realistic global climate models, we analyze a wide range of model configurations that include simple models of the tropical atmosphere, models of planets with no land, and so-called comprehensive Earth system models.  I will present the range of results across the different types of models.

Journey to the Center of a Neutron Star: Using Astrophysical Observables to Study Ultra-Dense Matter

Carolyn Raithel, Princeton University
Thu., January 19 at 4:30

Neutron stars are extreme objects. Formed in the core-collapse supernovae that mark the deaths of massive stars, they contain the densest matter found anywhere in the Universe, the strongest magnetic fields known to naturally occur, and are the progenitors of the most energetic electromagnetic phenomena observed. As a result, they provide a unique laboratory for studying matter under extreme conditions. In this talk, I will discuss a computational framework for connecting astrophysical observations of neutron stars to the physics governing their interior structure. I will take a multi-messenger perspective, highlighting recent results from X-ray observations, radio pulsar timing, and, especially, gravitational wave (GW) astronomy. I will discuss what we have already learned about neutron star structure from the first few detections of GWs emitted by merging neutron stars, and what we hope to learn from the next generation of proposed experiments. Along the way, I will present recent results from numerical simulations of neutron star mergers to highlight some of the key open questions and challenges that lie ahead.