Hydrogen-induced modification of the local charge density in MnSb$_2$Te$_4$

We investigate the effects of ionic hydrogen (H$^+$) intake on the occupied and unoccupied states of MnSb$_2$Te$_4$, a van der Waals magnetic material. Using angle-resolved photoemission spectroscopy (ARPES) and time-resolved ARPES, combined with resonant and core level photoemission, we reveal that hydrogen binds near specific lattice sites, locally redistributing charge density and reshaping the valence band spectral weight. While the band gap and picosecond carrier dynamics show only minor changes upon hydrogenation, the conduction band exhibits a striking stoichiometry-dependent response. These findings indicate that hydrogen interacts strongly with the intrinsic defects in the lattice, acting as a selective dopant in MnSb$_2$Te$_4$. Our work provides a spectroscopic basis for using hydrogen as a tool for the control of the electronic and magnetic properties in layered 2D materials.