ULTRATHIN YBA2CU3O7 FILMS WITH YBA2CU2.7NB0.3O7 BUFFER LAYER

A new type of buffer layer, based on Nb doped YBa2Cu3O7 material, has been used for high quality HTSC ultrathin film fabrication. This new m aterial represents a dielectric continuum of YBa2NbO6 phase with YBa2C U3O7 phase inclusions. The later serve as nuclei while growing ultrath in YBa2CU3O7 film on this buffer layer. As an example we have fabricat ed 6 unit cell YBa2Cu3O7 film showing transition into a superconductin g state with onset temperature 87K,zero resistance temperature 80K and critical current density j(c)(77K) = 2.10(5) A/cm(2).