Optical Properties of the Cu-O Plane in the Bi2Sr2-xLaxCuO6 Family

The ab-plane optical conductivity of seven single crystals, belonging to the family Bi2Sr2-x La (x) CuO6, has been measured for hole concentrations per Cu site 0.03a parts per thousand currency signpa parts per thousand currency sign0.18, and for 6 Ka parts per thousand currency signTa parts per thousand currency sign300 K (500 K for p=0.16). At low doping, ten phonon lines are detected, which are due to the removal of the degeneracy of five E (u) modes (out of the predicted six). They are superimposed to a far-infrared band, which as doping increases, closes the insulating gap thus building up the Drude term. The insulator-to-metal transition occurs between p=0.7 and p=0.10 consistently with a Mott mechanism. In the metallic phase, a multiband analysis identifies a Drude term plus a mid-infrared band, which weakly depends on temperature and softens as p increases, like in other cuprates. The optical response of the crystal at optimum doping has been analyzed also in the superconducting phase. The Ferrel-Glover-Tinkham sum rule requires an integration up to Omega a parts per thousand << 6 Delta and the penetration depth is 290 nm. The bosonic spectral function includes a strong peak around 50 meV, which survives up to 500 K and, therefore, might be assigned to an electron-phonon interaction.