Optical spectroscopy of pure and dopedCuGeO3

Abstract

We investigated in detail the optical properties of several Cu 1−δ Mg δ GeO3 (with δ = 0, 0.01), and CuGe1−xBxO3 with B=Si (x = 0, 0.007, 0.05, 0.1), and Al (x = 0, 0.01) single crystals, in the frequency range 20 -32 000 cm −1 . We report temperature dependent reflectivity and transmission measurements, performed with polarized light in order to probe the anisotropy of the crystals along the b and c axes, and optical conductivity spectra obtained by Kramers-Kronig transformation or direct inversion of the Fresnel formula. Special emphasis is given to the far-infrared phonon spectra. The temperature dependence of the phonon parameters is presented and discussed in relation to the soft mode issue in CuGeO3. For T < TSP we could detect zone boundary folded modes activated by the spin-Peierls phase transition. Following the temperature dependence of these modes, which shows the second order character of the phase transition, we were able to study the effect of doping on TSP. Moreover, in transmission experiments we detected a direct singlet-triplet excitation at 44 cm −1 , across the magnetic gap, which is not understandable on the basis of the magnetic excitation spectrum so far assumed for CuGeO3. The optical activity of this excitation and its polarization dependence confirm the existence of a second (optical) magnetic branch, recently suggested on the basis of inelastic neutron scattering data. The anisotropy in the magnetic exchange constants along the b axis, necessary for the optical triplet mode to gain a finite intensity, and the strong effect of Si substitution on the phonon spectra are discussed in relation to the alternative space group P 212121, recently proposed for CuGeO3 in the high temperature uniform phase.

Role of the Zhang-Rice-like exciton in optical absorption spectra ofCuGeO3andCuGe1−Pagliara1, Parmigiani2, Galinetto3 et al. 2002Phys. Rev. BSelf Cite171130View full textAdd to dashboardCite

“…A similar structure, followed at high photon energies by a region of strong absorption ascribed to transitions to the empty states above the charge transfer gap, is also present in transmission EELS spectra, 10 in the optical spectra, 11 and in x-ray Raman scattering spectra. 12 The energy position and the strong dependence of this peak on light polarization suggest that this structure can be related to the Zhang-Rice-like exciton.…”

Three-dimensional dispersion induced by extreme tensile strain inLa2−xSrxCuO4films

“…For the fit, we have chosen an inner potential 13 ͑including the work function͒ V 0 = + ͉E 0 ͉ in the interval 10-14 eV, in agreement with the values found in the literature for similar oxides. 14,16 Both V 0 and m * were adjusted during the fitting procedure, converging to 13 eV and 1.38m e , respectively.…”

Crystal-field and Zhang-Rice-singlet excitations inCuGe1−xSixO3(Bondino1, Zangrando2, Zacchigna3 et al. 2007Phys. Rev. BSelf Cite5040View full textAdd to dashboardCite

“…The spin-Peierls transition at low temperature is suppressed, and ferromagnetic coupling between Cu first neighbors occurs in CuGeO 3 when Ge is partially substituted by Si. 15,16 The crystals were cleaved along their bc plane just before inserting them in the UHV chamber. Resonant emission spectra were acquired with the ComIXS fluorescence spectrometer equipped with two variable-line-spacing spherical gratings with central groove density 4800 lines/ mm ͑optimized for the 20-200 eV region͒ and 19 200 lines/ mm ͑optimized for 150-1200 eV͒, and a 20ϫ 20 m 2 pixel chargecoupled device detector.…”