Presentations

We are actively presenting our work in local, national, and international conferences and workshops. You can see a partial list of events the members of the group have participated in on our conferences page. Here is a sampling of recorded presentations in conferences, colloquiua, and seminar series given by the members of the group.

Floquet Topology and Nonequilibrium Dynamics in Quantum Matter
Babak Seradjeh
Northeastern Quantum Matter Seminar | February 6, 2025

Abstract: Periodic driving can generate new interactions in quantum systems that are absent in the static system. Such “Floquet engineering” has been employed and demonstrated in various platforms, such as cold atoms in optical lattices, trapped ions, photonic devices, and solid-state systems, promising exquisite in-situ control for realizing and probing novel quantum phases of matter. In this presentation, I will present our recent theoretical works in this area: first, on using twisted light to generate and control multiple topological phases in graphene; second, on the steady-state population of driven systems weakly coupled to a fermionic bath. I will use the latter to show a “Josephson Floquet sum rule” for driven superconductors.

Real-space multifold degeneracy in graphene irradiated by twisted light
Suman Aich
VSF Speakers' Corner Seminar | November 27, 2023
Preprint: arXiv:2311.04792

Abstract: We report the theoretical discovery of real-space multifold degenerate Floquet-Bloch states in monolayer graphene coherently driven by twisted circulalry-polarized light. Using Floquet theory, we characterize the real-space structure of quasienergies and Floquet modes in terms of the orbital angular momentum and radial vortex profile of light. We obtain the effective real-space Floquet Hamiltonian and show it supports crossings of Floquet modes, especially at high-symmetry K and Γ points of graphene, at specific radial positions from the vortex center. At specific frequencies, the vortex bound states form a multifold degenerate structure in real-space. This structure is purely dynamically generated and controlled by the frequency and intensity of twisted light. We discuss the experimental feasibility of observing and employing the real-space multifold degeneracy for coherent optoelectronic quantum state engineering.

Dirac Magic and Lifshitz Transitions in AA-Stacked Twisted Multilayer Graphene
Yantao Li
VSF Speakers' Corner Seminar | October 20, 2021
Preprint: 2107.10687

Abstract: We uncover a new type of magic-angle phenomena when an AA-stacked graphene bilayer is twisted relative to another graphene system with band touching. In the simplest case this constitutes a trilayer system formed by an AA-stacked bilayer twisted relative to a single layer of graphene. We find multiple anisotropic Dirac cones coexisting in such twisted multilayer structures at certain angles, which we call "Dirac magic." We trace the origin of Dirac magic angles to the geometric structure of the twisted AA-bilayer Dirac cones relative to the other band-touching spectrum in the moiré reciprocal lattice. The anisotropy of the Dirac cones and a concomitant cascade of saddle points induce a series of topological Lifshitz transitions that can be tuned by the twist angle and perpendicular electric field. We discuss the possibility of direct observation of Dirac magic as well as its consequences for the correlated states of electrons in this moiré system.

Floquet Gauge Pump
Abhishek Kumar
VSF Speakers' Corner Seminar | January 21, 2021
Preprint: arXiv:2012.09677

Abstract: We introduce the concept of a Floquet gauge pump whereby a dynamically engineered Floquet Hamiltonian is employed to reveal the inherent degeneracy of the ground state in interacting systems. We demonstrate this concept in a one-dimensional XY model with periodically driven couplings and transverse field. In the high frequency limit, we obtain the Floquet Hamiltonian consisting of the static XY and dynamically generated Dzyaloshinsky-Moriya interaction (DMI) terms. The dynamically generated magnetization current depends on the phases of complex coupling terms, with the XY interaction as the real and DMI as the imaginary part. As these phases are cycled, it reveals the ground state degeneracies that distinguish the ordered and disordered phases. We discuss experimental requirements needed to realize the Floquet gauge pump in a synthetic quantum spin system of interacting trapped ions.

Presentations

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