Abrupt change reflected by the van Hove singularity in the optical spectra of CaMn 2 P 2 .

The trigonal CaAl2Si2-type structure CaMn2P2 has been reported undergoing an exotic first
order phase transition at TN= 69.5 K. In this paper, we present the optical spectra for the ab-plane
of CaMn2P2 single crystal from 300 K to 10 K for the first time. In the real part of the optical
conductivity spectra σ1(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 all σ1(ω) spectra, which indicates a divergence of the joint density of states(JDOS). This sharp
peak could be well described by the 2D Van Hove singularity function. In particular, this peak is
very sensitive to the first order phase transition, especially the peak position Et whose 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 tranition in the insulator.