The zero-field superconducting phase transition obscured by finite-size effects in thick $\mathrm{\bf{YBa_{2}Cu_{3}O_{7-\delta}}}$ films

TL;DR

This study investigates the normal-superconducting phase transition in thick YBa2Cu3O7−δ films, revealing finite-size effects that obscure the true transition temperature and critical exponents, and clarifying the transition's three-dimensional nature.

Contribution

It demonstrates that finite-size effects in thick films can mask the true superconducting transition, providing more accurate critical parameters and explaining discrepancies in previous literature.

Findings

Finite-size effects are significant even in thick films.

The true critical exponent z is approximately 2.1.

Crossover to 2D behavior in thinner films leads to incorrect critical parameters.

Abstract

We report on the normal-superconducting phase transition in thick $\mathrm{YBa_{2}Cu_{3}O_{7-\delta}}$ films in zero magnetic field. We find significant finite-size effects at low currents even in our thickest films ($d = 3200$ \AA). Using data at higher currents, we can unambiguously find $T_c$ and $z$, and show $z = 2.1 \pm 0.15$, as expected for the three-dimensional XY model with diffusive dynamics. The crossover to two-dimensional behavior, seen by other researchers in thinner films ($d \leq 500$ \AA), obscures the three-dimensional transition in both zero field and the vortex-glass transition in field, leading to incorrect values of $T_c$ (or $T_g$), $\nu$, and $z$. The finite-size effects, usually ignored in thick films, are an explanation for the wide range of critical exponents found in the literature.