Dr. Petr Stepanov
University of Notre Dame
Title: Strong Electronic Correlations in Moiré Materials
Abstract: The unexpected discovery of superconductivity in magic angle twisted bilayer graphene (MATBG) immediately generated a wave of intense theoretical and experimental research attracted by its rich phase diagram, which seemingly resembles ones of copper-oxide high-temperature superconductors. Originated in low-energy ¨flat¨ electronic bands, MATBG hosts a collection of exotic phases including but not limited to superconductivity, correlated insulators, topological and magnetic orders. Compared to other strongly-correlated systems, graphene multilayers offer a unique opportunity to tune the charge carrier density in situ and adjust system properties in other ways (for example, by alternating the distance to the gate or varying the dielectric environment), thus offering a potentially faster progress in understanding the underlying microscopic mechanisms governing its strong correlations. In this talk, as an example of such tuneability, I will discuss how the dielectric environment engineering affects the strong correlations in MATBG. Under a close proximity to the graphite gate (i. e. strong Coulomb interaction screening), MATBG exhibits a quenching of correlated insulator phases, while the vacated phase space is taken over by the superconductor domes. This observation demonstrates that the correlated insulating phases in MATBG can be untied from the superconductors in contrast to the case of cuprates, where the pairing occurs in a heavily interacting environment that locally favors the insulating state. In the second part of my talk, I will present an ongoing work revealing local photovoltage generation in magic angle bilayer and trilayer graphene superlattices, studied by cryogenic near-field imaging (cryo-SNOM). Light-matter interactions probed at the nanoscale help us uncover important symmetry breaking patterns, investigate strongly-correlated phases at slightly elevated temperature above the Tc, where ¨strange¨ metal and nematic ordering have been observed, and finally reveal a complex domain structure explained by the strain and twist angle inhomogeneity inherent to the entire class of moiré materials.
Beyond BCS: An Exact Model for Superconductivity and Mottness (University of Illinois Urbana-Champaign))
Prof. Phillip Phillips
University of Illinois Urbana-Champaign
Title: Beyond BCS: An Exact Model for Superconductivity and Mottness\
Abstract: The Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity described all superconductors until the 1986 discovery of the high-temperature counterpart in the cuprate ceramic materials. This discovery has challenged conventional wisdom as these materials are well known to violate the basic tenets of the Landau Fermi liquid theory of metals, crucial to the BCS solution. Precisely what should be used to replace Landau's theory remains an open question. The natural question arises: What is the simplest model for a non-Fermi liquid that yields tractable results. Our work builds on an overlooked symmetry that is broken in the normal state of generic models for the cuprates and hence serves as a fixed point. A surprise is that this fixed point also exhibits Cooper's instability[2,3]. However, the resultant superconducting state differs drastically from that of the standard BCS theory. For example the famous Hebel-Slichter peak is absent and the elementary excitations are no longer linear combinations of particles and holes but rather are superpositions of composite excitations. Our analysis here points a way forward in computing the superconducting properties of strongly correlated electron matter.
 E. Huang, G. La Nave, P. Phillips, Nat. Phys., 18, pages511–516 (2022).
 PWP, L. Yeo, E. Huang, Nature Physics, 16, 1175-1180 (2020).
J. Zhao, L. Yeo, E. Huang, PWP, PRB, Phys. Rev. B 105, 184509 (2022).
Speaker: Geoff Greene
University of Tennessee
Title: The Life and Death of the Free Neutron
Abstract: The decay of the free neutron is the simplest example of nuclear beta decay and, as such, is the archetype for a wide variety of Weak Interaction processes. These include radioactivity, Big Bang Nucleosynthesis, and energy production in the sun. Additionally, The precise value of the free neutron lifetime, can, along with other data, be used to test the consistency of the Standard Model. Remarkably, the value of neutron lifetime can also help determine the atmospheric composition of Venus. Given the breadth of physics involved, it is disconcerting to note that, at present, measurements of the neutron lifetime by different methods are inconsistent. In this talk, I will discuss the physics of neutron decay and will review the strategies for the experimental determination of the neutron lifetime. I will discuss some of the experimental challenges and will attempt to provide some illumination of the current discrepant situation.
Title: Electrify Everything!
Abstract: Making everything run on electricity is a necessary step in the transition from fossil fuels. Starting that process immediately is also necessary, and helpful both to the process and the environment.
Title: Trapped-ion optical clocks: Telling time and testing physics at the quantum limit
Abstract: Optical transitions in trapped, laser-cooled ions can provide an extremely well-controlled frequency reference for atomic clocks. The most stable and accurate atomic clocks now make measurements with total uncertainty approaching 1×10-18. The Ion Storage Group at NIST develops optical clocks based on the 1S0-3P0 resonance in 27Al+. To perform precision spectroscopy on this atomic system we use the basic building block of a quantum computer, the two-qubit gate, which transfers information from 27Al+ to a second ion species held in the same trap. I will introduce these systems and present recent frequency comparisons between them and other optical clocks at NIST. These comparisons provide valuable data for international time/frequency standards and can test our fundamental theories including relativity and the Standard Model. I will also describe quantum metrology techniques that have allowed us to approach the quantum limit for stability in a 27Al+ single-ion clock.
Title: Dynamics at the edge: charge fractionalization and near-stationary high energy state