Floquet-Bloch manipulation of the Dirac gap in a topological antiferromagnet

Floquet-Bloch manipulation, achieved by driving a material periodically with a laser pulse, is a method that enables the engineering of electronic and magnetic phases in solids by effectively modifying the structure of their electronic bands. However, the application of Floquet-Bloch manipulation in topological magnetic systems, especially those with inherent disorder, remains largely unexplored. In this work, we realize Floquet-Bloch manipulation of the Dirac surface-state mass of the topological antiferromagnet MnBi$_2$Te$_4$. Using time- and angle-resolved photoemission spectroscopy, we show that opposite helicities of mid-infrared circularly polarized light result in substantially different Dirac mass gaps in the antiferromagnetic phase, despite the equilibrium Dirac cone being massless. We explain our findings in terms of a Dirac fermion with a random mass. Our results underscore Floquet-Bloch manipulation as a powerful tool for controlling topology even in the presence of disorder, and for uncovering properties of materials that may elude conventional probes.