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 YBa2Cu3O6+x (0 less-than-or-equal-to x less-than-or-equal-to 1) and Nd1+yBa2-yCu3O6+x (0 less-than-or-equal-to y less-than-or-equal-to 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.