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.

Title: Hybrid quantum phononics with superconducting qubits*

Abstract: Superconducting qubits, and the experimental architecture of circuit quantum
electrodynamics (cQED), have emerged as not only a promising platform for quantum
computation but also for investigating fundamental and applied aspects of
synthetic/hybrid quantum systems composed of qubits coupled to other quantum systems
or degrees of freedom. In particular, the ability to leverage the properties of
superconducting qubits to investigate and manipulate phononic degrees of freedom opens
the door to exploring new regimes of circuit quantum optics using high-frequency sound.
Due to the intrinsically strong nonlinearity provided by the qubit, these types of hybrid
“quantum acoustic” systems have the potential to access a broad class of quantum states
of motion beyond what is achievable with effectively linear optomechanical or
electromechanical interactions.
In this talk I will describe some of our recent experimental results investigating the
fundamental physics of hybrid systems based on superconducting qubits coupled to
piezoelectric surface and bulk acoustic wave devices and how these systems can be used
to develop next-generation technologies for quantum sensing, computation, and
communication. As I will describe, these engineered systems, in which quantum
information stored in the qubit can be controllably coupled to the microscopic surface
and bulk phonon modes of a piezoelectric crystal, are an ideal platform for investigating
the exotic behavior of synthetic open quantum systems and phononic interference in the
quantum regime. Additionally, I will describe how these devices pave the way to exciting
new technologies ranging from quantum-limited surface sensing to phonon-based
bosonic quantum memories.

*This work was supported by the National Science Foundation via Grant No. ECCS-2142846 (CAREER)

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.

Speaker: Leonid Levitov (M.I.T)

Title: Spin chirality, quasiparticle dynamics and signatures of exotic superconductivity

Speaker: Cyprian Lewandowski, Florida State University