THE ELECTRONIC SPECIFIC-HEAT OF OVER AND UNDERDOPED Y0.9CAO0.1BA2CU3O6+X

The electronic specific heat coefficient gamma(T) of Y0.9Ca0.1Ba2Cu3O6+x has been determined between 1.8K and 300K as a function of x. Ca substitution for Y increases the planar hole concentration, overdoping the fully oxygenated material and lowering x(opt) for optimum T-c to 0.78 for 10% Ca doping. A dramatic collapse of the specific heat jump for x<0.86 provides dear evidence for the opening of a normal state pseudogap in the underdoped material. From the shift to lower x by similar to 0.1 relative to corresponding values for YBa2Cu3O6+x we conclude that both x(opt) and the onset of a pseudogap are determined by the planar hole concentration and not decoupling of the planes or chain disorder due to oxygen vacancies.