Van der Waals Engineering of Ultrafast Carrier Dynamics in Magnetic Heterostructures.

Heterostructures composed of the intrinsic magnetic topological insulator MnBiTe and its nonmagnetic counterpart BiTe host distinct surface electronic band structures depending on the stacking order and exposed termination. Here, we probe the ultrafast dynamical response of MnBiTe and MnBiTe 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 MnBiTe and BiTe surface terminations of MnBiTe, revealing an instantaneous occupation of states associated with the BiTe surface layer followed by carrier extraction into the adjacent MnBiTe 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 MnBiTe septuple layers. The observed competition between interlayer charge transfer and intralayer phonon scattering demonstrates a method to control ultrafast charge transfer processes in MnBiTe-based van der Waals compounds.