Observation of Out-of-Phase Bilayer Plasmons inYBa2Cu3O7−δ

The temperature dependence of the $c$-axis optical conductivity $\sigma \left(\omega \right)$ of optimally and overdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ ( $x=6.93\mathrm{}$ and 7) is reported in the far- (FIR) and midinfrared (MIR) range. Below $T_c$ we observe a transfer of spectral weight from the FIR not only to the condensate at $\omega =0\mathrm{}$, but also to a new peak in the MIR. This peak is naturally explained as a transverse out-of-phase bilayer plasmon by a model for $\sigma \left(\omega \right)$ which takes the layered crystal structure into account. With decreasing doping the plasmon shifts to lower frequencies and can be identified with the surprising and so far not understood FIR feature reported in underdoped bilayer cuprates.