Conductivity peak, relaxation dynamics, and superconducting gap of YBa2Cu3O7 studied by terahertz and femtosecond optical spectroscopies
A. Frenkel, F. Gao, Y. Liu, J. F. Whitaker, C. Uher, S. Y. Hou, J. M. Phillips
DOI 10.1103/PhysRevB.54.1355 · Physical Review B
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Abstract
Recent measurements at microwave, terahertz (THz), and infrared frequencies have revealed a peak in σ1 below Tc. Based on our THz measurements, which were performed on high quality, single crystal films of YBCO (900 and 500 Å), we have found that σ1 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 relaxation time extracted from these results using the two-fluid Drude model exhibits an enhancement below Tc, the analysis may not be adequate to account for the strong frequency dependence of the conductivity peak by the competition between the drop in scattering rate and the decreasing normal fluid density with temperature. On the contrary, we were able to account for the frequency dependent σ1 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 Tc. This is consistent with the higher normal fluid density (higher than Gorter-Casimir values) from the two-fluid model interpretation of our THz results. Thus, we have found evidence of BCS coherence factors in a high-Tc superconductor with a slower than BCS gap increase below Tc. We have discussed the role of coherence factors to account for the presence of the conductivity peak and the absence of the peak in NMR relaxation rate. Furthermore, we have presented a model for the quasiparticle relaxation time measured by the femtosecond pump-probe spectroscopy. This model allowed us to find a fit to the temperature-dependent energy gap function which is also consistent with the slower gap increase below Tc. In addition, recent theoretical developments based on an anisotropic s-wave gap [A. Sudbo/ et al., Phys. Rev. B 49, 12 245 (1994)] coincide with our conclusion about the slower gap change below Tc. © 1996 The American Physical Society.
Source-reported materials — not catalogue approval
| Formula | Reported Tc (K) | Pressure (GPa) | Type |
|---|---|---|---|
| YBa2Cu3O7 Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula. | 90 | Pressure not reported | onset |
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