Anomalous temperature dependence of charge-transfer excitation in the undoped cuprate Sr₂CuO₂Cl₂

We investigated the temperature dependence of optical excitations of Sr₂CuO₂Cl₂ in a temperature region of 10–523 K, using optical reflection and transmission measurements. Two peaks were found around 2.0 and 2.5 eV, and assigned as charge-transfer excitations related to a strongly correlated Zhang-Rice band and a nonbonding oxygen band, respectively. While the temperature dependence of the 2.5 eV peak can be explained by the lattice expansion effect, that of the 2.0-eV peak cannot; the latter redshifts by as much as 150 meV, while one would expect a blueshift of 10 meV from the lattice expansion. This observation is closely related to single-hole dynamics in cuprates. We suggest magnetism, or spin disordering with increasing temperature, as an origin of the anomalous temperature dependence. Comparisons with various many-body numerical calculations are also made.