Abstract
We measured polarized far-infrared reflectivity and Raman scattering spectra of SrCuO 2 single crystals. The frequencies for infrared-active modes were determined using an oscillator-fitting procedure of reflectivity data. The Raman spectra were measured at different temperatures using several laser energies ω L. In addition to eight of twelve Raman active modes, predicted by factor-group analysis, we observed a complex structure in the Raman spectra for polarization parallel to the c-axis, which consists of Raman-allowed A g symmetry modes, and B 1u LO infrared-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 ω L than the Raman-allowed ones. In order to explain this resonance effect, we measured the dielectric function and optical reflection spectra of SrCuO 2 in the visible range. We show that the Raman-allowed A g symmetry modes are resonantly enhanced when a laser energy is close to E 0, while Raman-forbidden (IR-active) modes resonate strongly for laser line energies close to the electronic transition of higher energy gaps.
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