3D-XY Critical Fluctuations of the Thermal Expansivity in Detwinned YBa2Cu3O7-δ Single Crystals Near Optimal Doping

The strong coupling of superconductivity to the orthorhombic distortion in $\mathrm{YBa}{}_2\mathrm{Cu}{}_3{O}_{7-\delta }$ makes possible an analysis of the superconducting fluctuations without the necessity of subtracting any background. The present high-resolution capacitance dilatometry data unambiguously demonstrate the existence of critical, instead of Gaussian, fluctuations over a wide temperature region $(\pm 10\mathrm{K})$ around $T_c$. The values of the amplitude ratio $A^{+}{/A}^{-}=0.9\mathrm{}–1.1$ and the leading scaling exponent $|\alpha |\le 0.018$, determined via a least-squares fit to the data, are consistent with the $3\mathrm{D}-\mathrm{XY}$ universality class. Small deviations from pure $3\mathrm{D}-\mathrm{XY}$ behavior are discussed.