Van der Waals Engineering of Ultrafast Carrier Dynamics in Magnetic Heterostructures

Heterostructures composed of the intrinsic magnetic topological insulator MnBi₂Te₄ and its nonmagnetic counterpart Bi₂Te₃ host distinct surface electronic band structures depending on the stacking order and exposed termination. Here, we probe the ultrafast dynamical response of MnBi₂Te₄ and MnBi₄Te₇ following near-infrared optical excitation using time- and angle-resolved photoemission spectroscopy and disentangle surface from bulk dynamics based on density functional theory slab calculations of the surface-projected electronic structure. We gain access to the out-of-equilibrium charge carrier populations of both MnBi₂Te₄ and Bi₂Te₃ surface terminations of MnBi₄Te₇, revealing an instantaneous occupation of states associated with the Bi₂Te₃ surface layer followed by carrier extraction into the adjacent MnBi₂Te₄ layers with a laser fluence-tunable delay of up to 350 fs. The ensuing thermal relaxation processes are driven by phonon scattering with significantly slower relaxation times in the magnetic MnBi₂Te₄ septuple layers. The observed competition between interlayer charge transfer and intralayer phonon scattering demonstrates a method to control ultrafast charge transfer processes in MnBi₂Te₄-based van der Waals compounds.