Complex conductivity of $\rm Y_{1-x}Pr_xBa_2Cu_3O_7$ thin films measured   by coherent terahertz spectroscopy
  The complex transmission of Y_{1-x}Pr_xBa_2Cu_3O_7 single crystal thin films has been measured in the range 0.2-1.0 THz using time domain spectroscopy. The complex conductivity is calculated without using a Kramers-Kronig analysis. All of the superconducting samples show a peak in $\sigma_1(T)$ below $T_c$. The underdoped samples show a deviation from $1/(\alpha+\beta T)$ behavior above $T_c$ that may be linked with the onset of a spin gap. 
J.O. White, R. Buhleier, S.D. Brorson, I.E. Trofimov, H.-U.   Habermeier, and J. Kuhl (Max-Planck-Institut f\"ur Festk\"orpe...