Evidence for a Peierls distortion in La2CuO4 from vibrational spectroscopy

Two features have been observed in the infrared absorption spectrum of the new superconductors La2-xSrxCuO4. The higher-frequency feature at 677 cm-1 in La2CuO4 dominates the spectrum for x=0. However, small Sr replacements (x0.03) cause the 677-cm-1 band to be reduced dramatically in intensity and to disappear entirely by x=0.15. We propose that this is a zone-boundary phonon that is folded to the center of the zone and that the frozen-in charge-densitywave distortion proposed by Mattheiss gives rise to the strong transition moment we observe. Sr doping suppresses this distortion.