IS THERE A KOSTERLITZ-THOULESS TRANSITION IN YBCO FILMS

Several investigators reported a Kosterlitz-Thouless (KT) transition i n Yba(2)Cu(3)O(7-delta) films. Since such films are often in the 3D li mit, such a finding is surprising. We performed sensitive measurements on the thinnest possible specimens, single-unit-cell films. Low sensi tivity 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 curr ents and temperatures implies the existence of unbound vortices, most likely due to finite size effects.