Probing the Interplay Between Exciton and Axion Order in Bilayer MBT

The unusual magnetoelectric transport present in certain topological materials can be compactly understood as manifestations of an emergent axion field, which itself is intrinsically related to the material’s underlying topology and is determined by the microscopic band structure. However, unambiguous identification of the axion collective mode is challenging due to its inherent nonlinear dynamics, as well as competing order with other quasiparticle dynamics. Past work has shown that the dynamical axion collective mode may be detected through its nonlinear coupling to externally applied fields through a time-resolved Kerr measurement. Meanwhile, it has been previously shown that examining the bulk exciton spectrum can be a means to read out a material’s underlying topology. As such, we propose a new route to study axionic phases of matter by examining the competing order between excitons and dynamical axion collective modes in the popular magnetic axionic insulator MnBi2Te4 (MBT). For a simplified model we consider the two-dimensional limit of a single MBT bilayer and carry out beyond mean field theory calculations to consider scattering events between fluctuations in exciton and axion order, which themselves are understood as collective fluctuations in the Néel order. We calculate the spectral function at finite temperature to probe the coupled dynamics and complex interplay of competing order in this topological bilayer system.