Is there a Kosterlitz-Thouless transition in YBCO films?

Several investigators reported a Kosterlitz-Thouless (KT) transition in Yba₂Cu₃O_7-δ films. Since such films are often in the 3D limit, such a finding is surprising. We performed sensitive measurements on the thinnest possible specimens, single-unit-cell films. Low sensitivity data are roughly consistent with a KT transition. However, IVs at the lowest currents are ohmic at temperatures far below the nominal KT temperature, contrary to the theory. An ohmic response at low currents and temperatures implies the existence of unbound vortices, most likely due to finite size effects.