ABSENCE OF A KOSTERLITZ-THOULESS TRANSITION IN ULTRATHIN YBA2CU3O7-DELTA FILMS

We have performed nanovolt-level dc measurements on single-unit-cell f ilms of YBa2Cu3O7-delta. When only the low-sensitivity data are analyz ed, they are roughly consistent with the occurrence of a Kosterlitz-Th ouless transition. At the lowest currents and voltages, however, the s amples are ohmic at temperatures quite far below the nominal Kosterlit z-Thouless transition temperature, contrary to the theory. The ohmic r esponse at low currents implies that there are unbound vortices presen t well below the nominal Kosterlitz-Thouless transition temperature.