Raman scattering study of the electronic and vibrational excitations in CeCu2Si2

The vibrational and crystal-field excitation spectrum is examined in CeCu2Si2 using polarized Raman scattering. These results are compared to spectra from the isostructural compounds RCu2Si2 (R=La,Gd,Yb) in the temperature range 2T390 K. The observed vibrational spectrum of CeCu2Si2 consists partly of a B1g and an A1g mode involving only Cu and Si atoms, respectively. Two Eg modes, involving both Cu and Si atoms, are also seen. An additional A1g phonon peak is observed, in defiance of the phonon spectrum allowed by the space-group symmetry of CeCu2Si2. This peak is believed due to disorder and is attributed to an A1g breathing mode of Cu atoms sitting at Si sites. Electronic crystal-field excitations are also evident in our spectra, displaying a broad peak centered roughly at 290 cm-1. This excitation is observed only in the purely antisymmetric A2g (depolarized) spectrum, and is seen to narrow and intensify with decreasing temperature. No other crystal-field transition is observed. The temperature dependence of the crystal-field excitation linewidth is discussed.