ELECTRONIC SPECIFIC-HEAT OF YBA2CU3O6+X FROM 1.8 TO 300-K

We have determined for the first time the ''electronic'' specific heat coefficient gamma(x, T) of YBa2Cu3O6+x for 0.16 less-than-or-equal-to x less-than-or-equal-to 0.97 between 1.8 and 300 K. Weakly superconducting behavior between x=0.43 and 0.8 progresses rapidly to BCS-like superconducting and metallic normal state behavior for x greater-than-or-equal-to 0.9. However, the continuous development of the entropy S(x, T) with x and T across the entire series suggests a progressive modification of the low energy spin spectrum with hole doping rather than a simple band model. Fermi statistics and k-space pairing are indicated by the magnitude and T dependence of S(x, T).