Temperature evolution of the pseudogap state in the infrared response of underdoped La2-xSrxCuO4

The ab-plane optical spectra of two single crystals of underdoped La2-xSrxC uO4 were investigated. The reflectivity of La1.87Sr0.13CuO4 has been measur ed in the frequency range 30-9000 cm(-1) (0.004-1 eV) both parallel and per pendicular to the CuO2 planes, whereas La1.86Sr0.14CuO4 was studied only in the ab plane. The extended Drude model shows that the frequency-dependent effective scattering rate 1/tau(omega,T) is strongly suppressed below the h igh-frequency straight-line extrapolation, a signature of the pseudogap sta te. This suppression can be seen from temperatures below the superconductin g 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 t here is a strong temperature dependence of the high-frequency 1/tau(omega,T ). The out-of-plane direction is also examined for evidence of the pseudoga p state. [S0163-1829(99)08009-1].