Stacking-sliding and irradiation-direction invariant Floquet altermagnets in A-type antiferromagnetic bilayers

Abstract: Arranging the stacking orders of A-type antiferromagnetic (A-AFM) bilayers offers an accessible pathway to two-dimensional altermagnets, but requires strict symmetry conditions such as layer groups, sliding positions, and twisting angles. Here, we find that circularly polarized light (CPL) irradiation breaks time-reversal symmetry, enabling the development of altermagnets beyond these constraints. Based on symmetrical analysis, our revealments indicate that A-AFM bilayer building-blocks with inversion symmetry exhibit altermagnetism robust to stacking sliding and variations of illumination directions. These bilayers can be constructed from arbitrary ferromagnetic monolayers and guided by the $d$-electron counting rule. Adopting bilayer MnBi$_2$Te$_4$ as a template, out-of-plane illumination with CPL reveals an $f$-wave altermagnetic feature at sliding positions $\left{E|\left(0,0\right)\right}$, $\left{E|\left(\frac{1}{3},\frac{2}{3}\right)\right}$ and $\left{E|\left(\frac{2}{3},\frac{1}{3}\right)\right}$, while a $p$-wave feature is predicted at other sliding positions. Our unveilings popularize the applicability of altermagnets in A-AFM bilayers with inversion symmetry, igniting a new wave of research in this field.