Infrared reflectivity versus doping in YBa2Cu3O6+x and Nd1+yBa2-yCu3O6+x ceramics: Relationship with the t-J model

We have measured the room-temperature reflectivity spectra in the infrared and visible range (500-25 000 cm^-1) of YBa₂Cu₃O_6+x (0<=x<=1) and Nd_1+yBa_2-yCu₃O_6+x (0<=y<=0.5) polished ceramics as a function of doping (x and y). We show that the intensity of the entire infrared spectrum increases with the hole doping level (x or y). When separated into a Drude contribution and a mid-infrared band, both contributions exhibit a correlated increase as holes are transferred into the conducting CuO₂ planes and are therefore argued to be related to free carriers. We note that recent simulations of the t-J model, studied as a function of hole doping, provide a consistent description of such a behavior.