Quantum Hall systems were the first discovered examples of Topological Insulators. They have a gap to charged excitations in the bulk, but possess gapless modes at the edges. These gapless modes are chiral, which means they travel only in one direction at a given edge, are robust against disorder, and are responsible for all the transport properties of quantum Hall states. The nature of the bulk constrains the number and chiralities of the edge modes. However, within these constraints, phase transitions called edge reconstructions can occur to minimize the electrostatic energy as the potential that confines the electrons to the sample is varied. I will describe a new class of edge reconstructions which are primarily driven by exchange rather than electrostatics, and are expected to occur generically for partially polarized quantum Hall states. I will also discuss some very striking experimental signatures expected of such reconstructed edges. 

How to modify Kelvin's theory (1887) of the ship wake to describe the effect of an obstacle on a current sheet in a two-dimensional conductor.

Host: Ganpathy Murthy

While nuclear structure is dominated by the strong interaction, life as we

experience it happens at the electromagnetic scale of hierarchy.  I discuss

my own electromagnetic wanderings in experiments to characterize the effects

of the weak interaction on hadronic structure, nuclear deacy, and static

electromagnetic moments, and possibly discover new physics at higher energy

scales.

Host: Brad Plaster

In quantum field theory, massless fermions must segregate into left- and right-handed populations that do not ever mix. Chiral symmetry (handedness) is a protected global symmetry. However, coupling of the fermion fields to a vector gauge field ruins the symmetry. The resulting axial current that flows is known as the chiral anomaly. The first example appeared in the theory of the ultrafast decay of neutral pions into photons (Adler Bell Jackiw anomaly). In the ensuing 5 decades, anomalies have appeared at every energy scale, from QCD to gravitation physics. In 1983, Nielsen and Ninomiya predicted that the chiral anomaly should be observable as well in bulk semimetals that feature protected 3D Dirac cones. The axial current appears as an enhanced conductance in parallel electric and magnetic fields. I will describe experiments on Na3Bi and GdPtBi which show the dramatic emergence of the anomaly when carriers are confined to the lowest Landau level. Tests to distinguish this quantum effect from (classical) artifacts caused by “current jetting” will be described.

Host: Joe Brill

The deep gravitational potential wells of clusters of galaxies should capture fair samples of the total baryon fraction of the Universe, unless other physical processes drive baryons out of clusters. Thus precision measurements of the baryon fraction, particularly as a function of cluster mass, can reveal the history of baryon flux into and out of clusters. How those baryons are then apportioned between stars and intracluster gas---the star formation efficiency---informs models of cluster assembly and massive galaxy evolution, as well as efforts to use the cluster gas fraction to constrain the mass density and dark energy equation of state parameters. Even the partitioning of the stellar baryons alone, in and out of galaxies, tests models of cluster galaxy evolution, as intracluster stars are the final, unambiguous signature of stars stripped from cluster galaxies during tidal encounters. We have discovered that intracluster stars are a significant part of the stellar baryons in clusters and poorer groups of galaxies. I will present new work characterizing the properties of this previously unexplored component, as well as the consequences for the cluster baryon budget and its relationship to the Universal value.

Ironically, our understanding of dark matter is hampered in large part by the presence of baryonic matter. The measured mass profile of pristine dark matter halos would provide clear indications of whether the dark matter is truly cold or warm or interacting. Finding such halos has, in part, motivated the search for the lowest mass galaxies because these are the most dark matter dominated systems known. I will describe a new way to find even lower mass halos than those identified so far, as well as a new class of massive galaxy that may provide massive analogs of nearly pure dark matter halos.

The proton and neutron, known as nucleons, are the fundamental building blocks of all atomic nuclei and make up essentially all the visible matter in the universe, including the stars, the planets, and us. The nucleon is not static but has complex internal structure, and both theory and technology have now reached a point where human is capable of exploring the inner dynamics and structure of nucleons and nuclei at the sub-femtometer distance, which is expected to lead to a new emerging science of nuclear femtography.  In this talk, I will demonstrate that the newly proposed Electron-Ion Collider (EIC) with its unique capability to collide polarized electrons with polarized protons and light ions at unprecedented luminosity, and with almost all elements of heavy nuclei at high energy, will be the most powerful tomographic scanner able to precisely image quarks and gluons inside the proton and nuclei. It is also a precision microscope that allows us to “see” and explore the dynamics binding quarks and gluons together to form hadrons. The EIC will address the most compelling unanswered questions about the elementary building blocks of the visible world to take us to the next frontier of the Standard Model of physics

In the last several decades neutrino oscillation experiments have given us a consistent picture of neutrino mass and mixing among three neutrino flavors. However, a series of longstanding and more recent anomalies hint at the existence of additional “sterile” neutrino flavors and complicates this simple picture. In order to improve on previous short baseline sterile neutrino searches, new detector technologies are required.  Liquid Argon time projection chambers (LArTPCs) promise to have the sensitivity needed by current and next generation neutrino oscillation experiments looking for the appearance of electron-flavor neutrinos in a predominantly muon-flavored accelerator-based neutrino beam.  MicroBooNE is the first of three LArTPC detectors planned for the newly re-established Short Baseline Neutrino program at Fermilab built to address the sterile neutrino hypothesis and to develop the technologies and expertise necessary to deploy a kiloton-scale LArTPC for future long baseline neutrino oscillation experiments.  Latest results from the MicroBooNE experiment will be presented along with the prospects and status of the Fermilab neutrino program

Topological crystalline insulators (TCI's) are a class of materials which support non-trivial topology in their electronic structure, "protected" by an underlying crystal symmetry.   We will discuss how certain aspects of this topology can be uncovered at surfaces of such crystals when they are magnetically doped, and show that the system hosts a surprisingly rich variety of ferromagnetic 

states and phase transitions.  After discussing how band topology can be characterized, and the unavoidable presence of conducting surface states even as the bulk is insulating when the topology is non-trivial, we examine the impact of magnetic dopants on the surface states. By breaking the symmetry that protects the topology, the system can become ferromagnetic, but the number of degenerate groundstates is dependent on both the symmetry of the surface as well as the density of electrons there. Moreover, the same surface may support a very "stiff" ferromagnet or a rather "floppy" one. The nature of the ferromagnet realized is in principle externally controllable, and for different cases it disorders at finite temperature through phase transitions of different universality classes. The type of system realized for a specific set of circumstances can be probed via the unique properties of domain walls which appear when the system is thermally excited.