c-Electron transport in Bi2Sr2CaCu2O8+delta 6 and Bi2Sr2CuO6+delta probed by Josephson plasma resonance

We have measured the Josephson plasma resonance for underdoped Bi2Sr2CaCu2O 8+delta and Bi2Sr2CuO6+delta by a novel scanning microwave spectrometer in a wide frequency range (21-147 GHz). The resonance frequency and the line w idth enable us to determine a superfluid density and quasiparticle conducti vity sigma (c)(qp) in the c-axis direction accurately. In the slightly unde rdoped crystals, the superfluid density shows very little change at low tem perature, which is consistent with a d-wave coherent tunneling model. Below T-c, sigma (c)(qp) falls to low values, then decreases gradually with decr easing T. This behavior is very different from that in the ab plane. (C) 20 01 Elsevier Science B.V. All rights reserved.