Title: Tunable valley ordering and Fermi surface reconstructions in a moiré magnet

Speaker: Prof. Saibal Bhowmik, IISc, Bangalore

Abstract: Interlayer interactions in van der Waals materials are pivotal in stabilizing electronic phases. Notably, the 'magic' rotation of two graphene layers has recently emerged as a versatile platform for manipulating interlayer hybridization. This significantly modifies the band structure, resulting in a superlattice with extremely flat bands at low energies. Such flat bands, characterized by a high density of states and strong Coulomb repulsion, give rise to diverse correlated phases in magic-angle twisted bilayer graphene (MATBG), ranging from insulating to superconducting and magnetic states. Despite its unprecedented tunability, a complete understanding of the observed phases in MATBG remains elusive. In this talk, I will briefly overview various twisted graphene-based moiré superlattices developed in the years following the discovery of MATBG. Subsequently, I will present our recent findings from magneto-transport measurements on MATBG proximitized by a tungsten diselenide layer, introducing finite spin-orbit coupling. Our primary results reveal an anomalous Hall effect near half-filling (ν = 2), coupled with an abrupt switching of magnetization tunable by carrier density. Near ν = 2, we observe Fermi surface reconstructions in the zero-magnetic field limit, evolving into a quantized Chern insulator precisely at ν = 2 with increasing magnetic field. These intriguing results collectively suggest that the bands are valley-polarized and malleable near ν = 2, stabilized by spin-orbit coupling. In the end, I will briefly mention our ongoing work, exploring the malleability of the bands through time-reversal symmetric nonlinear transport measurements.

References: 1. Bhowmik et al., Spin-orbit coupling-enhanced valley ordering of malleable bands in twisted bilayer graphene on WSe2. Nat Commun 14, 4055 (2023). 2. Bhowmik, Ghosh, and Chandni. "Emergent phases in graphene flat bands" Preprint at https://arxiv.org/abs/2309.08938.