Absence of a Loss of In-Plane Infrared Spectral Weight in the Pseudogap Regime ofBi2Sr2CaCu2O8+δ

The ab-plane reflectance of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ (Bi-2212) thin films was measured in the $30–25000{\mathrm{cm}}^{-1\mathrm{}}$ range for one underdoped ( $T_c=70\mathrm{}\mathrm{K}$), and one overdoped sample ( $T_c=63\mathrm{}\mathrm{K}$) down to 10 K. We find similar behaviors in the temperature dependence of the normal-state infrared response of both samples. Above $T_c$, the effective spectral weight, obtained from the integrated conductivity, does not decrease when $T$ decreases, so that no opening of an optical pseudogap is seen. We suggest that these are consequences of the pseudogap opening in the $\mathbf{k}=(0,\pi )$ direction and of the in-plane infrared conductivity being mostly sensitive to the $\mathbf{k}=(\pi ,\pi )$ direction.