Theory for optical activity in monolayer black phosphorus under external magnetic field Chenchen Liu, Feng Wu, Qingyun Jiang, Yihang Chen and Chengping Yin Eur. Phys. J. B, 93 10 (2020) 197 Published online: 14 October 2020 DOI: 10.1140/epjb/e2020-10390-0