c-electron transport in Bi sub(2)Sr sub(2)CaCu sub(2)O sub(8+ delta ) and Bi sub(2)Sr sub(2 )CuO sub(6+ delta ) probed by Josephson plasma resonance

We have measured the Josephson plasma resonance for underdoped Bi sub(2)Sr sub(2 )CaCu sub(2)O sub(8+ delta ) and Bi sub(2)Sr sub(2)CuO sub(6+ 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 super(c) sub(q) sub(p) 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 sub(c), sigma super(c) sub(q) sub(p) falls to low values, then decreases gradually with decreasing T. This behavior is very different from that in the ab plane. copyright 2001 Elsevier Science B.V. All rights reserved.