ANOMALOUS BEHAVIOR OF THE COMPLEX CONDUCTIVITY OF Y1-XPRXBA2CU3O7 OBSERVED WITH THZ SPECTROSCOPY

We have measured the complex conductivity of Y1-xPrxBa2Cu3O7 single-crystal thin films using coherent THz spectroscopy. We obtain directly the London penetration depth, the plasma frequency omega(p), and the quasiparticle scattering rate 1/tau. We find that 1/tau drops rapidly with temperature below T(c) in all the superconducting samples, implying that this is a signature of high-T(c) superconductivity. omega(p) decreases with increasing chi, indicating that Pr takes mobile carriers from the CuO planes. Both the THz and dc conductivity yield evidence for the opening of a spin gap above T(c).