Abstract
The ab-plane optical conductivity of 11 single crystals, belonging to the families Sr2−xCuO2Cl2, Y1−xCaxBa2Cu3O6, Bi2Sr2−xLaxCuO6 and Bi2Sr2CaCu2O8, has been measured with hole concentrations p between 0 and 0.18 and for 6 K ⩽ T ⩽ 500 K to obtain an infrared picture of the p,T phase diagram of the Cu–O plane. At extreme dilution (p = 0.005), a narrow peak is observed at 1570 cm−1 (195 meV), which we assign to a single-hole bound state. For increasing doping, that peak broadens into a far-infrared (FIR) band whose low-energy edge sets the insulating gap. The insulator-to-metal transition (IMT) occurs when the softening of the FIR band closes the gap, thus evolving into a Drude term. In the metallic phase, a multi-band analysis identifies a mid-infrared band which weakly depends on temperature and softens for increasing p, while the extended-Drude analysis leads to an optical scattering rate larger than the frequency, as was found in other cuprates. The infrared spectral weight W(T) is consistent with a Fermi liquid renormalized by strong correlations, provided that the T4 term of the Sommerfeld expansion is included above 300 K. In the superconducting phase, the optical response of single-layer Bi2Sr2−xLaxCuO6 at optimum doping is similar to that of the corresponding optimally doped bilayer Bi2Sr2CaCu2O8.
Highlights
Despite twentyfive-year-long experimental and theoretical efforts, the low-energy electrodynamics of the high-Tc cuprates still presents phenomena, both in the normal and in the superconducting phase, which are not fully understood [1, 2]
The modification with T of the filling of the Hubbard bands produces changes in σ1(ω) up to an energy U and this explains why b(Ω) = 0 for Ω >> Ωp. This relation has been quantitatively supported by Dynamical Mean Field (DMFT) calculations for LSCO, which predict the correct values of b(Ωp) only if one takes into account the effect of electronic correlations [55]
In the present work we have measured the ab-plane reflectivity of single crystals belonging to three different one-layer cuprate families
Summary
Despite twentyfive-year-long experimental and theoretical efforts, the low-energy electrodynamics of the high-Tc cuprates still presents phenomena, both in the normal and in the superconducting phase, which are not fully understood [1, 2]. Among them we can cite the pseudogap, which in the ab plane does not manifest itself in the optical conductivity but only in the scattering rate, the bosonic spectral feature at 41 meV, the anomalous dependence of the spectral weight on frequency and temperature, the effect of the superconducting transition up to energies larger than the optical gap by one-two orders of magnitude, and so on The understanding of those intriguing phenomena can be improved by collecting further spectroscopic data on the different high-Tc families, in order to enucleate the common features of the Cu-O plane electrodynamics at low energy.
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