Robust and tunable Floquet altermagnets in sliding A-type antiferromagnetic bilayers

Abstract

Arranging the stacking orders of A-type antiferromagnetic (A-AFM) bilayers offers a practical pathway to realizing two-dimensional altermagnets. Previous proposals, however, rely on stringent symmetry constraints, including layer groups and stacking registries. In this work, we demonstrate that circularly polarized light (CPL) irradiation breaks time-reversal symmetry, enabling altermagnetism without these restrictions. Our comprehensive symmetry analysis reveals that inversionsymmetric A-AFM bilayer building blocks can host altermagnetism that is robust against stacking slides and variations in illumination direction, with rich and continuously tunable symmetry representations. Using bilayer MnBi$_2$Te$_4$ as a case study, we show that out-of-plane CPL stabilizes distinct $f$-wave or $p$-wave altermagnetic states in forward stacking, while reverse stacking manifests an even-odd parity conversion of altermagnetism based upon specific sliding configurations. These revealments broaden the scope of altermagnet design in inversion-symmetric A-AFM bilayers and establish CPL as a versatile control knob for engineering altermagnetic symmetries.