Electronic specific heat of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ from 1.8 to 300 K

We have determined for the first time the ‘‘electronic’’ specific heat coefficient γ(x,T) of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$ for 0.16≤x≤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≥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 indicaed by the magnitude and T dependence of S(x,T).