Electronic specific heat of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">YBa</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Cu</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">O</mml:mi></mml:mrow><mml:mrow><mml:mn>6</mml:mn><mml:mo>+</mml:mo><mml:mi mathvariant="italic">x</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>from 1.8 to 300 K
Abstract
We have determined for the first time the ``electronic'' specific heat coefficient \ensuremath{\gamma}(x,T) of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$ for 0.16\ensuremath{\le}x\ensuremath{\le}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\ensuremath{\ge}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).
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