Quasiparticle excitations in the superconducting state observed in light scattering

In light scattering experiments on superconducting Y-Ba-Cu-O ceramic oxides we observe an unexpectedly intense Stokes-shifted spectrum from about 100 ${\mathrm{cm}}^{-1\mathrm{}}$ (≈ 13 meV) to 4000 ${\mathrm{cm}}^{-1\mathrm{}}$ (≈0.5 eV). We propose that this is due to light scattered from energy-density fluctuations of quasiparticle states excited above the superconducting ground state. The temperature dependence of the scattering intensity shows strong hysteresis, not observed in other experiments on the oxide superconductors.