Electronic specific heat of YBa2Cu3O6+x from 1.8 to 300 K

We have determined for the first time the electronic specific heatγ(x, T) of YBa₂Cu₃O_6+x for 0.16≤x≤0.97 from 1.8 to 300 K. Weakly superconducting behavior betweenx=0.4 and 0.8 progresses rapidly to strong coupling BCS-like superconducting and metallic normal state behavior forx> 0.9. However, the continuous development of the entropyS(x, T) withx andT 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 andk-space pairing for allx is indicated by the magnitude andT-dependence ofS(x, T). Pseudogap behavior inS(x, T) is observed over a temperature region aboveT _c , which increases rapidly with oxygen depletion to around 200 K forx～0.7. This loss of entropy reflects normal-state correlations apparently unrelated to the superconducting pairing.