Bound Excitons inSr2CuO3

Abstract

We investigated temperature dependent optical spectra of the one-dimensional chain compound Sr2CuO3. The charge transfer transition polarized along the chain direction shows a strongly asymmetric line shape as expected in one-dimensional extended Hubbard model. At low temperature, the charge transfer peak shows a large blueshift and reveals additional sharp peaks at the gap. Even though many spectroscopic studies suggest that this material cannot have a bound exciton based on the one-dimensional extended Hubbard model, we attribute the additional sharp peaks to excitons, which come to exist due to the long-range Coulomb interaction.

Optical excitations inSr2CuO3

“…The strongly asymmetric line shape has been understood based on 1D one band extended Hubbard model (EHM) including on site Coulomb repulsion U and the nearest neighbor interaction V , which has explained many other properties of 1D charge transfer insulators including Sr 2 CuO 3 , Ca 2 CuO 3 , and Ni-halogen bridged 1D materials. 7,9,11,12,13 However, a close look on σ(ω) of those 1D charge transfer insulators finds another common feature which is not pronounced in 1D EHM. That is, like the peak B in Sr 2 CuO 3 , they have a hump structure in the long tail above the excitonic peak.…”

“…In addition, the one dimensionality of this material enriches its physics attracting many researchers with interesting phenomena such as the spin-charge separation, a large optical nonlinear effect, and Wannier-like bound excitons. 6,7,8,9 All these factors make the excitation spectra of this system of great interest.A. S. Moskvin et al investigated the electronic structure of Sr 2 CuO 3 along and perpendicular to the chain direction by electron energy loss spectroscopy (EELS).…”

“…Because the inter-site Coulomb interaction V in 1D EHM lies close to the critical boundary of V ∼ 2t in Sr 2 CuO 3 , a small lattice and/or chemical environment change may affect the electronic structure rather strongly. 9 Even though the inter-site interaction has been considered as an important parameter in 1D cases, it has seldom been considered in 2D cuprates for the simplicity in theoretical treatments. However, the importance of the excitonic effect in the CuO 2 plane has been recognized by a simplified calculation on CuO 4 plaquettes clusters.…”

“…Comparison of the values in the gap region with other reported data suggests a rather small error in our spectra. 2,7,9 To examine the spectral weight redistribution, the spectral weight change between 300 K and 10 K normalized by energy is shown in fig. 4(b).…”

“…However, it has been noticed that σ(ω) can be simply scaled by the peak height and width right above the gap. 9 Best scaling factors expect decrease of spectral weight at low temperature. But it should be noted that the peak A could be composed of a few peaks due to electron-phonon coupling.…”