Imaging broken inversion symmetry state in magic angle twisted bilayer graphene.

Magic angle twisted bilayer graphene has been realized as a new platform to study strongly correlated quantum states of matter such as superconductors, correlated insulators and topological orders hosted by its native flat bands. Recent observations of anomalous Hall effect in MATBG revealed a non-zero Berry-curvature-induced orbital ferromagnetism and has been followed by extensive experimental and theoretical studies of its origin  [1,2]. Initial electronic transport works demonstrated anomalous Hall effect at ν=+3 electrons per moiré unit cell in the samples aligned with the adjacent insulating hBN layer, which breaks MATBG moiré inversion symmetry thus assisting in opening non-trivial insulating gaps with finite Chern numbers. Here we report on photovoltage scanning near-field imaging at cryogenic temperatures (10 K) of a MATBG device structurally aligned with hBN layer. We reveal a complex pattern of the quasi-local photothermoelectric effect (PTE) fringes attributed to the second-order superlattice (supermoiré) potential variation through the sample bulk. We assume that predominantly the photocurrent originates from PTE induced by the proximity of a hot spot formed on the metallic atomic force microscopy tip radiated by mid-infrared photons. Strikingly, we find a clear change of the PTE fringe real-space orientation when the sample doping changes between the valence and conductance flat bands which may signal a detection of a nematic ordered state in a highly doping-dependent low-energy spectrum of MATBG. Our observation sheds a new light on the microscopic mechanisms of the inversion symmetry breaking on a mesoscale length.

[1]  M. Serlin, et. al. Intrinsic Quantized Anomalous Hall Effect in a Moiré Heterostructure, Science 367, 900 (2020).

[2]  A. Sharpe, et. al. Emergent Ferromagnetism near Three-Quarters Filling in Twisted Bilayer Graphene, Science 365, 605 (2019).