Infrared signatures of the spin-Peierls transition inCuGeO3

We investigated the infrared reflectivity of several Mg- and Si-substituted ${\mathrm{CuGeO}}_3$ single crystals. The temperature-dependent $b$-axis and $c$-axis optical response is reported. For $T<\mathrm{}{T}_{\mathrm{SP}}$ we detected the activation of zone-boundary phonons along the $b$ axis of the crystal on the pure sample and for 1% Mg and 0.7% Si concentrations. From a detailed analysis of the phonon parameters the redshift of the $B_{2u}$ mode at 48 ${\mathrm{cm}}^{-1}$ is observed and discussed in relation to the soft mode expected to drive the spin-Peierls phase transition in ${\mathrm{CuGeO}}_3.$ Moreover, the polarization dependence of a magnetic excitation measured in transmission at 44 ${\mathrm{cm}}^{-1}$ has been investigated.