Polarization-dependent infrared phonon spectra of quasi-one-dimensional Sr₂CuO₃ and SrCuO₂

We measured polarization-dependent infrared-active phonon spectra of Sr₂CuO₃ and SrCuO₂ single crystals, which have one-dimensional Cu-O single and double chains, respectively. For each sample, the far-IR conductivity spectra show three phonon peaks for a given direction, in agreement with a group theory analysis. Interestingly, in both materials, we observed splitting of the stretching mode in the Cu-O chain direction. This splitting is more clearly observed in Sr₂CuO₃ than in SrCuO₂. These observed behaviors are discussed in terms of intriguing features of the one-dimensional Cu-O chains.