Neil Overend

Reaching the unreachable: critical scaling at the superconducting transition

The large coherence length of conventional superconductors causes the critical region near Tc to be unobservably small, leading to the impression that mean-field theory can adequately explain properties near the transition. The high-temperature superconductors, however, have such strongly type-II character that the critical, or at least an intermediate critical, region is experimentally accessible. We demonstrate the applicability of XY-like critical scaling to magnetization, heat capacity and conductivity data and through the fitting parameters, make contact with a strong-coupling calculation of the critical temperature. Further, the scaled conductivity data show clear evidence for a thermodynamic transition along the freezing line Hm(T)∝(1 − T/Tc)43.

The specific heat of YBa2Cu3O7−δ single crystals

The specific heat of twinned and detwinned single crystal samples of YBa2Cu3O7−δ have been measured with magnetic fields up to 8 tesla applied parallel to the c-axes of the crystals. The samples measured had transition temperatures around 90K and transition widths from 0.3 K to 2 K. The data exhibits scaling behaviour characteristic of the 3-dimensional-X-Y model with critical exponents consistent with those observed in superfluid4He. We then compare the scaling properties of the specific heat in the different samples and look for the ‘universal’ nature of the scaling function.

3d X-Y scaling of the specific heat of YBa2Cu3O7−δ single crystals

Abstract The specific heat of a single crystal sample of YBa2Cu3O7−δ (YBCO) has been measured with magnetic fields up to 8 Tesla applied parallel to the c-axis of the crystal. We analyse the data in terms of the Lowest Landau Level (LLL) and critical scaling theories. Recent measurements of magnetisation and resistivity have been unable to distinguish between the critical scaling and LLL regimes. We find that the data does not scale using the LLL theory and therefore conclude that the LLL theory is not applicable in magnetic fields less than 8 Tesla. However, we find that the data does scale using the 3-dimensional XY model. This provides strong evidence for the existence of a critical regime within which there is scaling behaviour characteristic of the 3-dimensional XY model with critical exponents consistent with those observed in superfluid 4He.