Temperature Evolution of the Pseudogap State in the Infra-Red Response of Underdoped La(2-x)SrxCuO4
Abstract
The ab-plane optical spectra of two single crystals of underdoped La(2-x)SrxCuO4 were investigated. The reflectivity of La(1.87)Sr(0.13)CuO4 has been measured in the frequency range 30- 9,000 cm-1 (0.004 -- 1 eV) both parallel and perpendicular to the CuO2 planes, whereas La(1.86)Sr(0.14)CuO4 was studied only in the ab-plane. The extended Drude model shows that the frequency-dependent effective scattering rate 1/tau(omega)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/tau(omega,T). The out-of-plane direction is also examined for evidence of the pseudogap state.
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