THICKNESS DEPENDENCE OF THE KOSTERLITZ-THOULESS TRANSITION IN ULTRATHIN YBA2CU3O7-DELTA FILMS

The resistive transition is studied systematically on ultrathin YBa2Cu3O7-delta, (YBCO) films with the number of layers n ranging from 1 to 10. The zero-resistance transition temperature, which is T(c) = 30 K for n = 1, increases rapidly with increasing n. To interpret the results, topological excitation of vortices in thin films of a layered structure is discussed. A ''vortex-string pair,'' in which vortices and antivortices piercing all n layers are pairwise bounded, is shown to be an important topological excitation in thin films. Dissociation of the vortex-string pair gives rise to the Kosterlitz-Thouless (KT) resistive transition in thin films of layered structure. The observed dependence of T(c) on n is explained quantitatively by the increase of the projected two-dimensional carrier density with n in the framework of the KT theory. The result suggests that the KT transition at T(c) = 30 K is intrinsic to the CuO2 conducting planes in one YBCO layer, and questions the earlier interpretation ascribing T(c) congruent-to 90 K in bulk YBCO to the KT transition.