Title: Quantum Twist Microscope

Abstract: TBA

Speaker: Dr. Xiaomeng Liu (Cornell)

Title: Superconductivity and Ferroelectric Orbital Magnetism in Semimetallic Rhombohedral Hexalayer Graphene

Abstract: Rhombohedral multilayer graphene has emerged as a promising platform for exploring correlated and topological quantum phases, enabled by its Berry-curvature-bearing flat bands. While prior work has focused on separated conduction and valence bands, we probe the semimetallic regime of rhombohedral hexalayer graphene. We uncovered a rich phase diagram dominated by flavor-symmetry breaking and an electric-field-driven band inversion. Near this inversion, we find a superconducting-like state confined to a region with emergent electron and hole Fermi surfaces. In addition, two multiferroic orbital-magnetic phases are observed: a ferrovalley state near zero field and a ferroelectric state at large fields around charge neutrality. The latter shows electric-field-reversible magnetic hysteresis, consistent with a multiferroic order parameter.

Title: Enhanced and Extended Strange Metallicity due to Coulomb Repulsion and Disorder

Abstract: I will discuss the problem of strange metals, where the traditional notion of Fermi liquid quasiparticles ceases to apply. I will view the problem through the lens of a model of electrons with Hubbard-U Coulomb repulsion and a disordered Yukawa coupling to a two-dimensional bosonic bath, which can be solved in an extended dynamical mean field theory scheme. The model exhibits a quantum critical point, at which the repulsive component of the electron interactions strongly enhances the effects of the quantum critical bosonic fluctuations on the electrons, leading to a breakdown of Fermi liquid physics and the formation of a strange metal with `Planckian' quasiparticle decay rates at low temperatures, although with no holographic dual. Furthermore, the eventual Mott transition that occurs as the repulsion is increased seemingly bounds the maximum decay rate in the strange metal. I will also discuss some applications and collaborations based on this work to the iron-based superconductors and moire materials. Time permitting, I will conclude with future directions to include nonlocal effects.  

Title: Strongly correlated topological phenomena in graphene multilayers

Abstract: Multilayer rhombohedral graphene has recently been experimentally demonstrated to host a panoply of strongly correlated and topological phenomena. In the presence of alignment to hBN, this platform exhibits Chern insulators and the fractional quantum anomalous Hall effect. On the other hand, signatures of unconventional (chiral) superconductivity arise in pristine multilayers. From a theoretical standpoint, several key issues are under active debate. In particular, what is the nature of the moire effect in this setting? How can we understand the emergence of these exotic topological states? I will discuss our progress towards resolving these questions, and highlight broader implications for other material platforms.

Speaker: Dr. Pontus Laurell, University of Missouri

Title: Witnessing quantum correlations and entanglement in materials

Abstract: Entanglement and other nonclassical correlations are ubiquitous in quantum many-body systems. This is well-established in quantum information applications, where they represent resources to be harnessed for quantum operations. However, they also play a prominent role in theories of important condensed matter phenomena, such as novel phases of matter. Yet there has been a distinct lack of viable methods to detect these correlations in the solid state, impeding our ability to identify suitable materials and to unravel their secrets. In this talk I will describe the rapid progress made in recent years towards finding useful measures of these properties, which can both be modeled theoretically and measured experimentally in a model-independent fashion, by making use of information “hidden” in spectroscopic data. By employing entanglement witnesses—quantities that are akin to order parameters for certain classes of entangled states—multipartite entanglement has now been observed in quantum spin systems and strongly correlated electron systems. Such quantum information-informed approaches offer new quantitative insights into many-body states and can provide hints for modeling of enigmatic states in quantum materials.