Out-of-plane electrodynamic properties of nearly optimally doped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_y$ in the superconducting state have been studied at 14.2 GHz with the microwave electric field parallel to the $c$ axis. The zero crossing of the $c$-axis dielectric constant, which arises from the Josephson plasma resonance, was observed just below $T_c.$ The $c$-axis microwave conductivity ${\sigma }_1^c\mathrm{}\left(T\right)\mathrm{}$ was found to decrease rapidly just below $T_c$ and remained suppressed even in the superconducting state. The results suggest that the strong confinement of the excited carriers in the ${\mathrm{CuO}}_2$ planes is essentially retained even through the superconducting transition.

• Received 3 November 1997

DOI:https://doi.org/10.1103/PhysRevB.57.10946