Nicoletti, D.; Calvani, P.; Di Pietro, P.; Limaj, O.; Schade, U.; Ono, S.; Ando, Y.; Lupi, S.: Optical Properties of the Cu-O Plane in the Bi2Sr2-xLaxCuO6 Family. Journal of Superconductivity and Novel Magnetism 26 (2013), p. 969-977
10.1007/s10948-012-2055-6

Abstract:
The ab-plane optical conductivity of seven single crystals, belonging to the family Bi2Sr2−xLaxCuO6, has been measured for hole concentrations per Cu site 0.03 ≤ p ≤ 0.18, and for 6 K ≤ T ≤ 300 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 Eu 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 Ω >> 6Δ 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.