The Raman scattering of single crystals ${\mathrm{Sr}}_2$Cu${\mathrm{O}}_3$ and SrCu${\mathrm{O}}_2$ is investigated under resonant conditions. Of all polarizations, only the Raman spectrum with the electrical vector along the CuO chains shows additional peaks superimposed on a strong continuum. Three of the peaks and the continuum demonstrate unconventional temperature dependence. These additional features are ascribed to either the phonons coupled to a resonating valence bond, or to the pairs of unbound spinons that resonate as the excitation energy approaches the charge-transfer gap. The additional scattering cannot be explained by excitations out of a long-range ordered state but is consistent with that expected for either free spinon pairs or spin-liquid-state excitations.

• Received 21 February 1996

DOI:https://doi.org/10.1103/PhysRevB.53.R14733