The Normal-Superconducting Phase Transition of YBCO in Zero Magnetic Field

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We have investigated the superconducting phase transition of YBCO in zero magnetic field. Most of our data were taken on thin films, grown by pulsed laser deposition. To ensure we are looking at intrinsic properties of the phase transition, we have endeavored to optimize our films, which we characterize using ac susceptibility, x-ray diffraction, and surface analysis using SEM and AFM.

We examined voltage vs. current measurements at temperatures close to the transition temperature, Tc. Previous work in our group by D. R. Strachan has suggested that the standard scaling analysis fails at low current, contrary to what is widely accepted, and finds evidence for the transition at higher currents. Using data at higher currents, we can unambiguously find Tc and the dynamic critical exponent z, and show z = 2.1 +- 0.15, as expected for the three-dimensional XY model with diffusive dynamics.

At lower currents, we find significant finite-size effects, due to the thickness of the films. The crossover to two-dimensional behavior has been seen by other researchers in thinner films (d < 500 A), but were considered irrelevant for thicker films. We show that even in our thickest film (d = 3200 A), the finite-size effects obscure the transition in zero field. This effect would also occur in a magnetic field, and may explain the wide range of critical exponents found in the literature.

Finally, we report on work with bulk single crystals. Our measurements of specific heat in crystals disagrees with the critical exponent nu > 1 as is widely reported in the literature. We will also discuss voltage vs. current measurements on crystals.