Temperature evolution of the pseudogap state in the infrared response of underdoped La 2 − x Sr x CuO 4

The ab-plane optical spectra of two single crystals of underdoped ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ were investigated. The reflectivity of ${\mathrm{La}}_{1.87}{\mathrm{Sr}}_{0.13}{\mathrm{CuO}}_{4}$ has been measured in the frequency range $30\char21{}9000 {\mathrm{cm}}^{\ensuremath{-}1}$ (0.004\char21{}1 eV) both parallel and perpendicular to the ${\mathrm{CuO}}_{2}$ planes, whereas ${\mathrm{La}}_{1.86}{\mathrm{Sr}}_{0.14}{\mathrm{CuO}}_{4}$ was studied only in the ab plane. The extended Drude model shows that the frequency-dependent effective scattering rate $1/\ensuremath{\tau}(\ensuremath{\omega},T)$ is strongly suppressed below the high-frequency straight-line extrapolation, a signature of the pseudogap state. This suppression can be seen from temperatures below the superconducting transition up to 400 K. In the case of underdoped LSCO the straight-line extrapolation is temperature independent below 200 K, whereas above 200 K there is a strong temperature dependence of the high-frequency $1/\ensuremath{\tau}(\ensuremath{\omega},T).$ The out-of-plane direction is also examined for evidence of the pseudogap state.