Raman and infrared (IR) spectra of an underdoped ${\mathrm{La}}_{1.90}{\mathrm{Sr}}_{0.10}{\mathrm{CuO}}_4$ single crystal have been measured as a function of temperature. Both techniques provide unconventional low-energy spectra. The IR conductivity exhibits features peaked at finite frequencies which do not have a counterpart in the Raman response. Below approximately 100 K a transfer of both Raman and IR spectral weight towards lower energies is found and a new component in the Raman response builds up being characterized by a very long lifetime of electrons propagating along the Cu-O bonds.

• Received 8 March 2002

DOI:https://doi.org/10.1103/PhysRevB.66.060502