CONDUCTIVITY PEAK, RELAXATION DYNAMICS, AND SUPERCONDUCTING GAP OF YBA2CU3O7 STUDIED BY TERAHERTZ AND FEMTOSECOND OPTICAL SPECTROSCOPIES

Recent measurements at microwave, terahertz (THz), and infrared freque ncies have revealed a peak in sigma(1) below T-c. Based on our THz mea surements, which were performed on high quality, single crystal films of YBCO (900 and 500 Angstrom), we have found that sigma(1) features a peak which increases in amplitude and shifts to lower temperatures as frequency changes from 1.2 to 0.4 THz. Although the quasiparticle rel axation time extracted from these results using the two-fluid Drude mo del exhibits an enhancement below T-c, the analysis may not be adequat e to account for the strong frequency dependence of the conductivity p eak by the competition between the drop in scattering rate and the dec reasing normal fluid density with temperature. On the contrary, we wer e able to account for the frequency dependent sigma(1) by fitting with Mattis-Bardeen theory (modified to include scattering) using a slower average rate of increase of the anisotropic gap than for the BCS case as temperature decreases below T-c. This is consistent with the highe r normal fluid density (higher than Gorter-Casimir values) from the tw o-fluid model interpretation of our THz results. Thus, we have found e vidence of BCS coherence factors in a high-T-c superconductor with a s lower than BCS gap increase below T-c. We have discussed the role of c oherence factors to account for the presence of the conductivity peak and the absence of the peak in NMR relaxation rate. Furthermore, we ha ve presented a model for the quasiparticle relaxation time measured by the femtosecond pump-probe spectroscopy. This model allowed us to fin d a fit to the temperature-dependent energy gap function which is also consistent with the slower gap increase below T-c. In addition, recen t theoretical developments based on an anisotropic s-wave gap [A. Sudb o et at, Phys. Rev. B 49, 12 245 (1994)] coincide with our conclusion about the slower gap change below T-c.