Special CM Seminar - 1/25 - 1PM - Ipsita Das "Magnetic Field Driven Quantum Phases in Magic Angle Twisted Bilayer Graphene"

Jan 25, 2024, 1:00 pm2:00 pm
Faculty, post docs, grads



Event Description

The discovery of magic angle twisted bilayer graphene (MATBG), where two sheets of monolayer graphene are precisely stacked at a specific angle, has opened a plethora of grand new opportunities in the field of topology, superconductivity, and other strongly correlated effect. In twisted van der Waals materials, lattice mismatch can generate moiré patterns, which act as an additional periodicity that has a length scale order of magnitude larger than the underlying atomic lattice scale. For MATBG with a small twist angle close to � = 1.1°, the electronic bands are flattened by the periodic potential of the moiré bands and isolated from higher-energy dispersive bands. In this talk, we present a rich sequence of wedge-like regions of quantized Hall conductance with Chern numbers C = ±1, ±2, ±3 and ±4, which nucleate from integer fillings of the moiré unit cell n = ±3, ±2, ±1 and 0, respectively. The exact sequence and correspondence of the Chern numbers and filling factors suggest that these states are directly driven by electronic interactions, which specifically break the time-reversal symmetry in the system. The analysis of Landaulevel crossings from higher energy bands enables a parameter-free comparison to a newly derived ‘magic series’ of level crossings in a magnetic field and provides constraints on the parameters of the Bistritzer–MacDonald MATBG Hamiltonian [1]. Additionally, we present the detailed magneto-transport behaviour of the Hofstadter spectrum of MATBG. We observed the re-entrance of insulating states at n = +2, ±3 of the moiré unit cell of MATBG upon applying an external magnetic field close to the full flux quantum Φ/Φ0 = 1 of the superlattice unit cell (B = 25q2 T) and interaction-driven Fermi-surface reconstructions at other fillings, which are identified by new sets of Landau levels originating from these. These experimental observations are supplemented by theoretical work that predicts a new set of eight well-isolated flat bands at Φ0, of comparable band width, but with different topology than in zero field [2].