Optical spectroscopy of pure and dopedCuGeO3

Abstract

We investigated the optical properties of ${\mathrm{Cu}}_{1-\delta }{\mathrm{Mg}}_{\delta }{\mathrm{GeO}}_3$ $(\delta =0,0.01)$ and ${\mathrm{CuGe}}_{1-x}{B}_x{\mathrm{O}}_3$ with $B=\mathrm{Si}$ $(x=0,0.007,0.05,0.1)$ and Al $(x=0,0.01)$ in the frequency range 20–32 000 ${\mathrm{cm}}^{-1}.$ We report temperature-dependent reflectivity and transmission measurements, performed with light polarized 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 ${\mathrm{CuGeO}}_3.$ For $T<\mathrm{}{T}_{\mathrm{SP}}$ we detected zone-boundary 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 $T_{\mathrm{SP}}.$ The optical activity and the polarization of a singlet-triplet excitation detected at 44 ${\mathrm{cm}}^{-1},$ across the magnetic gap, confirm the existence of the 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 space group ${P2\mathrm{}}_1{2}_1{2}_1$ recently proposed for ${\mathrm{CuGeO}}_3$ in the high-temperature uniform phase.