Raman-scattering evidence for free spinons in the one-dimensional spin-1/2 chains of Sr2CuO3 and SrCuO2

The Raman scattering of single crystals Sr2CuO3 and SrCuO2 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.