Dirac gap opening and modifying mechanism in an axion antiferromagnetic topological insulator MnBi$_{2}$Te$_{4}$

The angle-resolved photoelectron spectroscopy with photoexcitation by laser radiation ($h\nu$ = 6.3 eV) is used to study the modification of the gap open at the Dirac point in an axion antiferromagnetic topological insulator MnBi$_2$Te$_4$ at temperatures higher and lower the Néel temperature (24.5 K). The open gap is shown can have both anomalously high (62–67 meV) and significantly decreased (15–18 meV) values in various MnBi$_{2}$Te$_{4}$ samples or various areas of the same sample. In both the cases, the gap remains open above the Néel temperature. As a result of the studies, it is assumed that the difference in the widths of the gaps open at the Dirac point is related to the manifestation of the effect of fractionation of axion term $\theta$. The gap width (62–67 meV) that correlates with the result of theoretical calculations corresponds to the state with axion term $\theta = \pi$. In this case, the experimental observation of the gap at the Dirac point of 15–18 meV corresponds to the effective modulation of the axion term $\theta = \pi/4$ due to the generation of many-body chiral spin fluctuations and their interaction upon laser photoexcitation.