c-Electron transport in Bi2Sr2CaCu2O8+δ6 and Bi2Sr2CuO6+δ probed by Josephson plasma resonance

We have measured the Josephson plasma resonance for underdoped Bi2Sr2CaCu2O8+delta and Bi2Sr2CuO6+delta by a novel scanning microwave spectrometer in a wide frequency range (21-147 GHz). The resonance frequency and the line width enable us to determine a superfluid density and quasiparticle conductivity sigma (c)(qp) in the c-axis direction accurately. In the slightly underdoped crystals, the superfluid density shows very little change at low temperature, which is consistent with a d-wave coherent tunneling model. Below T-c, sigma (c)(qp) falls to low values, then decreases gradually with decreasing T. This behavior is very different from that in the ab plane. (C) 2001 Elsevier Science B.V. All rights reserved.