Joseph A Stroscio

Title: Unraveling Quantum Geometry and Orbital Magnetism Contributions to Landau Levels in Moiré Superlattices

Abstract:

Flat and narrow band physics in moiré quantum matter (MQM) has proven to be extremely rich with new emergent quantum phases which can be tuned with applied electric and magnetic fields. The topological properties of the eigenstates of the moiré Hamiltonian are critical for establishing the quantum phase of the system. While the emergence of non-trivial Chern numbers has been observed, it is important to characterize the quantum geometry in detail including the Berry curvature and the less known quantum metric effects which give rise to orbital magnetism in these systems. For almost a century, magnetic oscillations have been a powerful “quantum ruler” for measuring Fermi surface topology. In this presentation, I show the use of a quantum ruler of Landau levels via scanning tunneling spectroscopy to probe quantum geometry and topology in twisted double bilayer graphene (TDBG) [1]. The high-resolution Landau level spectra reveal tunable electron- and hole-like pockets that deviate significantly in their magnetic response from the semiclassical Onsager relation. A quantitative analysis of the Landau levels demonstrates the significance of quantum geometry contributions to the magnetic energy levels, highlighting the effect of the tunable Berry curvature and local orbital magnetism. The deviation from semiclassical behavior is due to the higher order magnetic field response of TDBG, which in turn is related to the quantum geometry of the electronic structure. The first-order correction in magnetic field, interpreted as an orbital magnetic moment, manifests as a valley splitting of the Landau levels. The second-order correction—orbital magnetic suscepti