Anomalous temperature dependence of charge-transfer excitation in the undoped cuprate Sr 2 CuO 2 Cl 2 H. S. Choi, Y. S. Lee, and T. W. Noh Department of Physics and Condensed Matter Research Center, Seoul National University, Seoul 151-742, Korea E. J. Choi Department of Physics, The University of Seoul, Seoul 130-742, Korea Yunkyu Bang Department of Physics, Chonnam National University, Kwangju 500-757, Korea Y. J. Kim Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received 3 February 1999 We investigated the temperature dependence of optical excitations of Sr 2 CuO 2 Cl 2 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 non- bonding 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 calcu- lations are also made. S0163-18299911431-0 I. INTRODUCTION The electronic structures of undoped cuprates and their evolution with hole doping are important issues in under- standing high-T c superconductivity. 1,2 In spite of a vast num- ber of works, a clear understanding on these problems is still lacking. Recently, the anomalous dynamics of a single hole in the CuO 2 plane has been heavily investigated. In particular, us- ing angle resolved photoemission experiments which create a photoexcited quasihole, its ‘‘band’’ dispersion in the mo- mentum ( k) space has been measured for an undoped insu- lating antiferromagnet Sr 2 CuO 2 Cl 2 , 3 and diverse Bi 2 Sr 2 CaCu 2 O 8 + crystals from underdoped to slightly overdoped. 4,5 In an undoped insulator, the lowest binding- energy band occurs around 1 eV below the Fermi energy E F . The band has a maximum at k=(  /2, /2), and its (  ,0) state is found below the top by about 0.3 eV. 3 As doping increases, the states near (  ,0) dramatically shift up and, at an optimal doping, become nearly degenerate with E F , forming an extended flat region with a Van Hove singularity. 5 Such a substantial band deformation with dop- ing is in contrast with previous theoretical predictions such as the t -J model or the Hubbard model. Recently, it is argued that an extended t -J model the t -t ' -t  -J model, which in- cludes longer-range hopping terms, resolves such a discrep- ancy in the band dispersion. 6 However, the physical origin of this anomalous quasihole band deformation still remains un- clear, and should be investigated more systematically. Optical spectroscopy is one of useful techniques to inves- tigate the electronic structures of solids. In this work, we have measured optical transition peaks of Sr 2 CuO 2 Cl 2 over a wide temperature range from 10 to 523 K. We focus on charge-transfer excitations whose peaks are located around 2.0 and 2.5 eV. As the temperature increases, both peaks show redshifts. However, comparing quantitatively with external-pressure-dependent energy shifts, we show that the temperature dependence of the 2.0-eV peak cannot be ex- plained by the lattice expansion effect alone. We propose magnetism as a possible source for the anomalous shift, and suggest that the shift might be closely associated with doping-dependent quasihole band deformation. We compare our results with many-body numerical calculations which consider the effect of magnetic disorder on the hole band dispersion, and find some agreement between them. II. EXPERIMENTS A. Optical measurements Sr 2 CuO 2 Cl 2 single crystals were grown by cooling a sto- ichiometric melt in an alumina crucible. 7,8 Transmittance and reflectance spectra were measured using a grating spectro- photometer between 0.4 and 6.5 eV. The surface of the sample was parallel to the CuO 2 plane. The incident angle for reflectance measurements was 7°. Thus the electric field of the radiation was nearly parallel to the CuO 2 plane. In the temperature range between 10 and 300 K, a continuous-flow helium cryostat was used. For higher temperature measure- ments up to 523 K, a home-made sample holder with a heat- ing system was adopted. 9 Temperature was controlled within 0.2 K. PHYSICAL REVIEW B 15 AUGUST 1999-I VOLUME 60, NUMBER 7 PRB 60 0163-1829/99/607/46467/$15.00 4646 ©1999 The American Physical Society