Charge dynamics across the ${\mathrm{CuO}}_2$ planes in ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$

Infrared reflectivity measurements on high-quality single crystals of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ were performed with polarization perpendicular to the ${\mathrm{CuO}}_2$ planes (E∥c). The c-axis spectrum shows a dramatic change at ${\mathit{T}}_{\mathit{c}}$, from a featureless spectrum above ${\mathit{T}}_{\mathit{c}}$ to the one with a sharp reflectivity edge below ${\mathit{T}}_{\mathit{c}}$. We find that the spectrum below ${\mathit{T}}_{\mathit{c}}$ is dominated by the plasma edge arising from the carriers condensed in the superconducting state, not by a superconductong gap excitation, and that the superconducting transition is accompanied by the onset of coherent charge transport along the c axis.