Infrared properties of La_2-x(Ca,Sr)_xCaCu₂O_6+δ single crystals

The in-plane optical properties of two crystals of the bilayer cuprate La_2-x(Ca,Sr)_xCaCu₂O_6+δ, one with excess Ca and x=0.10 and the other with Sr and x=0.15, were investigated over the frequency range of 45–25000cm^-1. The optical conductivity has been derived from Kramers-Kronig transformation. Each crystal exhibits a peak at around 15000cm^-1, which corresponds to the charge-transfer (CT) excitation of the parent insulator. With increasing carrier density, spectral weight shifts from the CT excitation to the low-ω region. For the superconducting sample (x=0.15), the optical conductivity displays a peak in the far-infrared region, which shifts toward zero frequency with decreasing temperature. The temperature-dependent behavior favors a dynamical localization picture. A “pseudogap” feature is observed in the low-frequency reflectance and the scattering rate spectra. Both the energy scale and the temperature dependence of the pseudogap are similar to other bilayer cuprates.