3D-XY critical fluctuations of the thermal expansivity in detwinned   YBa2Cu3O7-d single crystals near optimal doping
  The strong coupling of superconductivity to the orthorhombic distortion in YBa2Cu3O7-d 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 (+/- 10 K) around Tc. The values of the amplitude ratio A+/A-=0.9-1.1 and the leading scaling exponent |alpha|<0.018, determined via a least-squares fit of the data, are consistent with the 3D-XY universality class. Small deviations from pure 3D-XY behavior are discussed. 
Volker Pasler, Peter Schweiss, Christoph Meingast, Bernhard Obst,   Helmut Wuehl, Alexandre I. Rykov and Setsuko Tajima