Abrupt change reflected by the van Hove singularity in the optical spectra of CaMn₂P₂

The trigonal CaAl₂Si₂-type structure CaMn₂P₂has been reported undergoing an exotic first order phase transition at the critical temperatureTN=69.5K. In this paper, we present the optical spectra for theab-plane of CaMn₂P₂single crystal from 300 K to 10 K for the first time. In the real part of the optical conductivity spectraσ₁(ω), a direct gap could be determined at all temperatures without any Drude term visible, i.e., the sample goes through the first order phase transition from one insulator state to the other insulator state. At higher energy, an asymmetric sharp interband transition peak appears in allσ₁(ω) spectra, which indicates a divergence of the joint density of states. This sharp peak could be well described by the two dimensional van Hove singularity function. In particular, this peak is very sensitive to the first order phase transition, especially the peak positionEtwhose the most prominent blue shift occurs only when the first order transition happens. Our data and analysis reveal that the first order phase transition leads to a weak partial re-normalization of the band structure. Our study will be useful in further investigations about the mechanism of the first order phase transition in the insulator.