Title: TBA

Abstract: TBA

Dr. Claudio Mazzoli, Brookhaven National Laboratory

Title: Coherent REXS: a journey across space AND time of electronic orderings in some correlated material

Abstract: The investigation of electronic structures in the space domain is crucial for understanding their hierarchical relationships and functional behavior. On the other hand, the study in the time-frequency domain discloses the underlying energetics, thus paving the way to their fundamental modeling. As a result, any comprehensive understanding of the complexity expressed by correlated systems has to deal with both these domains. From an experimental point of view, however, often drastic compromises are needed to access and retrieve relevant information in space or in time. As a matter of fact, a gain in one domain typically corresponds to a loss in the other.

In the first part of my talk, I will introduce the specific point of view oCered by Resonant Elastic X-ray Scattering (REXS) under coherent illumination and present some relevant examples in either of the two domains. In the second part of my talk, I will concentrate on advances allowed by a frontier use of coherence, showcasing extremes where previously inaccessible scientific information can now be retrieved in space (fullfield lensless), time (direct space imaging and its comparison to XPCS), or both together (correlative imaging). The conclusions will highlight the potential impact of these advanced methods to a variety of systems, sources and techniques, allowing to deepen our understanding of this broad class of materials, and hopefully inspire other innovative approaches for the exploitation of coherence.

Title: From Rhombohedral Graphene to Anomalous Hall Crystals

Abstract: Recent experiments on rhombohedral multilayer graphene (RMG) with a substrate-induced moire potential have identified both Chern insulators and fractional Quantum Hall states at zero magnetic field, whose origin is mysterious. The operative degrees of freedom are in the valence band minima that feature strong correlations and nontrivial quantum geometry. The first part of this talk will study a microscopic model of RMG. 

I will show that, even without a moire potential, interactions can spontaneously break continuous translation symmetry and time-reversal symmetry at the mean-field level to produce an electron crystal with finite Chern number. This new state of matter is called an anomalous Hall crystal. Many-body numerics at fractional fillings then reveal fractionalized ground states, consistent with experiments. I will show this result holds robustly for four-to-six layers for a range of displacement fields, and I will discuss the role of hBN alignment orientation. 

The second part of the talk will introduce λ-jellium, a minimal extension of the jellium model whose interaction strength and Berry curvature are independently tunable. I will show that it hosts an anomalous Hall crystal phase that is stable to quantum fluctuations with crystallization at much lower interaction strengths than in Wigner crystals.

Title: Topological Electron Crystals in a Mass-Asymmetric Electron-Hole Bilayer

Abstract: Moiré superlattices have become a standard route to realizing correlated and crystalline electronic phases in two-dimensional materials by quenching kinetic energy. In this talk, I will describe a different mechanism for generating topological electron crystals based on interlayer charge transfer rather than moiré engineering.

Our platform is a heterostructure consisting of bilayer graphene and a Mott insulator. Charge transfer between the layers leads to a charge-neutral, mass-asymmetric electron-hole bilayer, where itinerant carriers in bilayer graphene are attractively coupled to heavy, localized carriers in a flat Hubbard band. In the dilute heavy-fermion limit, this system supports a remarkably rich set of electron crystal phases, including triangular, honeycomb, and Kagome crystals.

A key result is that the nonlocal nature of bilayer graphene wave functions strongly modifies the real-space charge profile, which in turn stabilizes these unconventional crystalline orders at intermediate interlayer attraction. The resulting phases carry distinct Hall responses and topological characteristics, opening a route to crystalline topology beyond the conventional moiré setting. I will present the phase diagram, explain the role of quantum geometry, and discuss possible experimental platforms.

Dr. Itamar Kimchi, Georgia Tech

Title: Lattice defects in quantum magnets and topological systems

Abstract: Defects are always present in solid state materials. I will present our group’s recent theoretical results showing how quantum-entangled or topological systems can enable local defects to produce surprising global effects:

• First, in the Kitaev honeycomb quantum spin liquid (QSL), non-magnetic “Stone-Wales” crystallographic defects become imbued with a fluctuating magnetic chirality. The emergent Ising model for their chiralities is ferromagnetic and long ranged (~1/r^2.7), producing an instability to a topological chiral QSL at a finite critical temperature set by the defect density. 
• Second, in the 1/3 magnetization plateau of triangular lattice magnet KCSO, IR spectroscopy observed unusual satellite lines. We show that though these lines are sharp, they arise from disorder and enable its characterization as dilute vacancies. 
• Third, in the Dirac cones of the honeycomb lattice, magnetic impurities induce circulating currents with an associated topological Chern number. Surprisingly for isolated impurities, this induced magnetization and topology is reversed above a critical impurity strength with a global phase transition generated by the local defect physics.

Title: Quantum Twisting Microscopy of Moiré Flat Bands: A Theoretical Perspective

Abstract: Moiré materials with tunable flat bands provide an exceptional platform for realizing strongly correlated and topological phases. Achieving a detailed understanding of their electronic structure, however, remains a major challenge. The recently developed quantum twisting microscope introduces a new tunneling probe for van der Waals materials. In this talk, I will present a theoretical framework for both elastic and inelastic tunneling spectroscopy of moiré flat bands using the quantum twisting microscope. I will discuss how this technique can enable momentum-resolved measurements of electronic spectral functions, neutral collective excitations and superconducting gap structures in magic-angle twisted bilayer graphene.

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.