Far-infrared sphere resonance in $T^{*}$-phase ${\mathrm{SmLa}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{CuO}}_{4-\delta }$ powder samples is measured down to $7{\mathrm{cm}}^{-1\mathrm{}}$. Below $T_c$, two peaks are observed at $11$ and $30{\mathrm{cm}}^{-1\mathrm{}}$ for $\delta =0.05\mathrm{}$. Based on a comparison with the $\dots {S/I/S/I}^{\prime }{/S/I/S/I}^{\prime }/S\dots$ ( $\dots$superconductor/insulator1/superconductor/insulator2/superconductor $\dots$)-type Josephson junction array model discussed by van der Marel and Tsvetkov [(Czech. J. Phys. 46, 3165 (1996)], these peaks are attributed to the Josephson plasma resonances of the intrinsic Josephson junctions at the fluorite-type ${\mathrm{Sm}}_2{\mathrm{O}}_2$ block layer and the rocksalt-type $(\mathrm{La},\mathrm{Sr}{)}_2{\mathrm{O}}_{2-\delta }$ block layer, respectively. The relation between the multiple Josephson plasma and the crystal structure of high- $T_c$ cuprates is also discussed.

• Received 6 March 1998

DOI:https://doi.org/10.1103/PhysRevLett.81.3519