Prof. Jeanie Lau

Department of Physics

The Ohio State University

Host: Chunli Huang

Title: Flat Bands and Quantum Geometry in Flatlands

Abstract: In a flat band, the electronic interactions dominate, leading to emergent correlated phases such as superconductivity and charge density waves. The advent of flatlands, i.e. two-dimensional (2D) materials provide us with unprecedented opportunities to design and engineer flat bands via stacking, magnetic fields, and twisting. For flat bands in flatlands, quantum geometric contributions become important. To illustrate this, I will focus on superconductivity in twisted bilayer graphene, in which the slow Fermi velocity and the small charge density appear to invalidate conventional BCS equations and present a paradox. The paradox is resolved by our experimental demonstration that the superfluid stiffness is dominated by the quantum geometric contribution. We also find that the band velocity in this Dirac superconductor constitutes a new limiting mechanism for the critical current, analogous to a relativistic superfluid. Finally, I will also present on flat bands in suspended few-layer graphene, where gate tunable magnetism is observed.