"Photocurrents as symmetry-breaking indicators in twisted bilayer graphene"
Magic-angle twisted bilayer graphene (TBG) displays a complex phase diagram as a function of flat band filling, featuring compressibility cascade transitions and a variety of competing ground states with broken spin, valley, and point group symmetries. In this talk, I will discuss how photogalvanic effects -sensitive to the geometrical properties of the band structure- can serve as a promising tool for experimentally distinguishing these symmetry-broken phases. Focusing on TBG, I will first highlight the potential of non-linear optical responses in revealing crucial bandstructure information about its high-temperature state, such as the proximity to the magic angle and the influence of the environment on the system symmetries [1].
In the second part of the talk, I will go beyond the non-interacting limit and present our latest results on the low-temperature description of TBG. Motivated by the recent THz photocurrent spectroscopy measurements which show a dependence on the filling inconsistent with the simplest cascade picture of sequential filling of equivalent flat bands [2], we employ a self-consistent heavy fermion framework [3] to compute the non-linear optical responses associated with the different spontaneous symmetry-broken states, mainly resulting from the local interactions between the heavy electrons. We find that the magnetic injection photocurrent, which is enabled due to spontaneous time-reversal symmetry breaking, can be used to determine the spontaneous valley polarization and, therefore, to discriminate between topological anomalous Hall and trivial valley (-spin) Hall states [4].
[1] F. Peñaranda, H. Ochoa, F. de Juan, Phys. Rev. Lett. 133, 256603 (2024).
[2] R. K. Kumar et al., arXiv: 2406.16532 (2024).
[3] Z.-D. Song and B. A. Bernevig, Phys. Rev. Lett. 129, 047601 (2022).
[4] F. Peñaranda, H. Ochoa, F. de Juan, arXiv: 2501.09703 (2025).