Excitons in insulating cuprates

Abstract

We study the electronic excitations near the charge-transfer gap in insulating CuO 2 planes, starting from a six-band model which includes p π and d xy orbitals and Cu-O nearest-neighbor repulsion U pd . While the low lying electronic excitations in the doped system are well described by a modified t−J model, the excitonic states of the insulator include hybrid d xy − p π states of A 2g symmetry. We also obtain excitons of symmetries B 1g and E u , and eventually A 1g , which can be explained within a one-band model. The results agree with observed optical absorption and Raman excitations.

Joys and Pitfalls of Fermi Surface Mapping inBi2Sr2CaCu2Борисенко1, Golden2, Legner3 et al. 2000Phys. Rev. Lett.927460View full textAdd to dashboardCite

“…18 Naturally, the binding energy of excitons in insulating cuprates is sensitive to the magnitude of the intersite Coulomb interaction. 19 Returning to Sr 2 CuO 3 , we note that a strongly enhanced nearest-neighbor Coulomb interaction V (1B) ϭ0.8 eV as well as a moderately enhanced transfer integral t (1B) ϭ0.55 eV in the context of a 1D extended one-band ͑1B͒ Hubbard model have been deduced from recent electron energy-loss spectroscopic ͑EELS͒ data. 20 The corresponding theoretical values for the 2D CuO 2 plane derived for La 2 CuO 4 , which has even smaller Cu-O bond lengths (dϭ1.89 Å) than Sr 2 CuO 3 (d ϭ1.95 Å) are V 2D ϭ0.11 to 0.14 eV, 14,21 and t 2D ϭ0.42 to 0.48 eV.…”

Resonant Inelastic X-Ray Scattering Study of Charge Excitations inLa2CuO4

“…In the simplest picture of this insulating cuprate, the ground state consists of one hole per copper ion (Cu 2+ ), and the low-lying electronic excitations include excitons formed via a charge-transfer (CT) process, in which charge is moved from the oxygen onto the copper [4,5,6]. Specifically, an electron-hole pair, created by exciting an electron from the valence band -the ZhangRice (ZR) band [7] -to the conduction band across the CT gap, can form a bound exciton state as a result of the Coulomb interaction.…”

Charge-transfer excitons in underdopedCa2−xNaxCuO2Cl2Schuster1, Pyon2, Knupfer3 et al. 2009Phys. Rev. B8060View full textAdd to dashboardCite

“…[6][7][8] But there are considerable arguments about the precise nature of these entities in the literature. In particular, the level of delocalization and the number of orbitals, required for a sufficient theoretical description, are a matter of debate.…”

“…The processes governing the electronic spectrum in this energy range are mainly caused by charge transfer from oxygen to copper states which results in the formation of electron-hole pairs, CT excitons. [6][7][8] Electron energy-loss spectroscopy ͑EELS͒ in transmission is a valuable tool for the investigation of materials on all levels of complexity in the electronic-excitation spectrum. 9,10 The EELS cross section is basically proportional to Im͓−1 / ⑀͑ , q͔͒-called loss function ͑LF͒-where ⑀͑ , q͒ = ⑀ 1 ͑ , q͒ + i⑀ 2 ͑ , q͒ is the momentum and energy-dependent complex dielectric function.…”