An extended infrared study of the (p,T) phase diagram of the p-doped Cu-O plane

Understand

The ab-plane optical conductivity of eleven 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), that 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.