Polarized far-infrared and Raman spectra of SrCuO2 single crystals

Authors measured polarized far-ir reflectivity and Raman scattering spectra of SrCuO2 single crystals. The frequencies for ir-active modes were determined using an oscillator-fitting procedure of reflectivity data. The Raman spectra were measured at different temperatures using several laser energies wL. In addition to eight of twelve Raman active modes, predicted by factor-group analysis, authors observed a complex structure in the Raman spectra for polarization parallel to the c-axis, which consists of Raman-allowed Ag symmetry modes, and B1u LO ir-active (Raman-forbidden) modes of the first and higher order as well as their combinations. The Raman-forbidden modes have a stronger intensity at higher wL than the Raman-allowed ones. In order to explain this resonance effect, authors measured the dielectric function and optical reflection spectra of SrCuO2 in the visible range. Authors show that the Raman-allowed Ag symmetry modes are resonantly enhanced when a laser energy is close to E0, while Raman-forbidden (IR-active) modes resonate strongly for laser line energies close to the electronic transition of higher energy gaps. 25 refs.