Thermodynamic fluctuations in the superconductor ${\mathrm{Y}}_1{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{9-y}$: Evidence for two-dimensional superconductivity

The electrical resistance and the magnetoresistance of the ceramic superconductor ${\mathrm{Y}}_1{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{9-y}$ have been precisely measured in order to extract the temperature derivative and to analyze the fluctuation regimes. The logarithmic dependence at high temperature is followed near the transition by a two-dimensional fluctuation regime in agreement with Aslamazov-Larkin theory (critical exponent ${\lambda }_{2D}=1\mathrm{}$, and a complex behavior below $T_c$. Our findings disprove the observation of a three-dimensional fluctuation regime near and above $T_c$.