C-polarized vibrational modes in La2CuO4 and La1.87Sr0.13CuO4 determined by ellipsometry

We have measured the c-axis linear optical response of large, high-quality single crystals of La2CuO4 and La1.87Sr0.13CuO4 between 30 and 700 cm(-1) in the 10-300 K temperature range with a far-infrared Fourier ellipsometer. The response is, like that of an ionic insulator, mainly of phononic origin. Temperature dependent phonon parameters, i.e., TO frequency, oscillator strength, and line width are discussed for all observed modes. The parameters obtained for La2CuO4 at 300 K are compared with different phonon calculations for the tetragonal phase. The eigenvectors resulting from these calculations agree for the 235 cm(-1) and 500 cm(-1) mode whereas discrepancies exist for the weak low frequency mode, which has not been observed by far infrared spectroscopy so far. The analysis of the measured oscillator strengths in terms of effective charges based on the different proposed eigenvectors favor the assignment of this mode to the motion of Cu together with the in-plane oxygen ions against La.