Abstract
The temperature dependence of the optical spectra of twoLa2−xSrxCuO4 single crystals was investigated for both in-plane and out-of-plane directions. For the underdoped(x = 0.08) single crystal, the in-plane optical conductivity that was analysed by ageneralized Drude formalism shows a suppression of the scattering rate1/τ(ω) and an increasedeffective mass m* as the temperature decreases at low frequencies. Since this behaviour can be explained by thepseudogap effect, it is concluded that the pseudogap is not present at the optimal doping(x = 0.15). This result isconfirmed by the c-axis optical conductivity, which decreases only for the underdoped single crystal. Theabsence of the pseudogap at the optimal composition is in accord with the quantum criticalpoint model and it can explain the phase diagram of high-temperature superconductors.
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