Role of the Zhang-Rice-like exciton in optical absorption spectra ofCuGeO3andCuGe1−

Abstract

The optical excitations in pure and Si-substituted CuGeO 3 are investigated in the 15-300 K temperature range. A structure, detected for the E field parallel to the c axis of the crystals at about 3.2 eV, is due to the formation of a Zhang-Rice-like ͑ZR͒ exciton. The weak oscillator strength is due to the low mobility of the ZR-like exciton between the edge-sharing CuO 4 plaquettes of CuGeO 3 . The temperature dependence of the ZR-like energy is explained by a renormalization effect involving phonon-electron interactions or changes in spin ordering, while the increase of ZR-like exciton intensity with the Si content is argued to arise from a different hybridization of the Si-O bond with respect to Ge-O.

Disentangling thermal and nonthermal excited states in a charge-transfer insulator by time- and frequency-resolved pump-probe spectroscopy

“…30 Assuming α=500 cm −1 and α=250 cm −1 in the 1.55 eV-c-polarized and 3.14 eV-bpolarized configurations, 15 we can estimate δT=3.6 K and δT=1.8 K, respectively. While the ratio of the estimated temperature increase (3.6 K/1.8 K≃2), related to the difference in the absorbed density of energy, is compatible with measured ratio (23 K/14 K≃1.6), the discrepancy of the absolute values can be attributed to the fact that the energy absorbed by the electronic transitions is delivered to a subset of optical lattice modes, i.e.…”

“…The shoulder in the absorption coefficient at 3.1-3-2 eV (see Fig. 1c) is attributed to a superposition of plasmons related to transitions from the ground state, being 72% of the hole density localized on Cu atoms, into a delocalized final state with a high occupation probability of the neighbor plaquettes, 13,15 as shown in Fig. 1d.…”

Ultrafast time-resolved spectroscopy of the spin-Peierls compound CuGeO3

“…The ZR singlet consists of the Cu3d hole coupled with the O2p hole, in a CuO 4 planar cluster. In the absorption spectrum of CuGeO 3 , a shoulder structure appears at 3.2 eV for the electric field parallel to the one-dimensional spin chain (c-axis of the crystals), which has been assigned to the formation of a ZR exciton [3]. The ZR exciton is a bound state between the ZR singlet of holes and an excited electron in the UHB.…”

“…However, dynamics in the excited states of CuGeO 3 is poorly studied. Inelastic X-ray scattering studies of the charge-transfer-insulator CuGeO 3 have revealed that the electronic excitation at 3.7 eV corresponds to the excitation from the so-called Zhang-Rice (ZR) singlet to the upper Hubbard band (UHB) [2]. The ZR singlet consists of the Cu3d hole coupled with the O2p hole, in a CuO 4 planar cluster.…”

“…Center photon energies, pulse widths, and beam powers were 3.14 eV, 12 fs, and 4 mW for the pump, and 1.57 eV, 12 fs, and 1 mW for the probe, respectively. The ZR-like exciton band is excited and the photon-energy region of the d-d transitions of Cu ions is probed [3]. The d-d transitions would be modified as a result of the local formation of the ZR singlet.…”