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

Heterostructures composed of the intrinsic magnetic topological insulator MnBi 2 Te 4 and its nonmagnetic counterpart Bi 2 Te 3 host distinct surface electronic band structures depending on the stacking order and exposed termination. Here, we probe the ultrafast dynamical response of MnBi 2 Te 4 and MnBi 4 Te 7 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 MnBi 2 Te 4 and Bi 2 Te 3 surface terminations of MnBi 4 Te 7 , revealing an instantaneous occupation of states associated with the Bi 2 Te 3 surface layer followed by carrier extraction into the adjacent MnBi 2 Te 4 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 MnBi 2 Te 4 septuple layers. The observed competition between interlayer charge transfer and intralayer phonon scattering demonstrates a method to control ultrafast charge transfer processes in MnBi 2 Te 4 -based van der Waals compounds.