Far-Infrared Absorption by Bulk YBa2Cu3O6.89 Superconductor

We have developed a Far-Infrared-Laser Calorimetric method to investigate far-infrared absorption by high-Tc superconductors using an optically pumped CH3OH laser. We obtained information on both transition temperature and superconducting energy gap. The sintered YBa2Cu3O6.89 specimen absorbed photons below the gap energy even in the superconducting state. We found that this absorption would have an onset near 5 meV and consider that the existence of the onset will support a model of the low-energy excitation.