Optical conductivity of c axis oriented ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.70}$: Evidence for a pseudogap

The c axis reflectance of high-quality single crystals of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$ has been measured over a wide range of temperatures and frequencies. The x=0.70 crystal has a pseudogap in the conductivity at 200 ${\mathrm{cm}}^{\mathrm{-}1}$ which is present at room temperature. The gap deepens gradually as the temperature is lowered to 10 K with no discontinuity at ${\mathit{T}}_{\mathit{c}}$. In contrast, crystals with x=0.95 show no gap in the normal state and their superconducting state is characterized by low-lying states. These observations are consistent with phase diagrams proposed to explain NMR and neutron data.