THICKNESS DEPENDENCE OF RESISTANCE-TEMPERATURE CHARACTERISTICS OF C-AXIS AND A-AXIS ORIENTED YBA2CU3O7-X ULTRATHIN FILMS

We have investigated the thickness dependence of the resistance-temper ature (R-T) characteristics of c-axis acid a-axis oriented YBa2Cu3O7-x (YBCO) ultrathin films with thicknesses ranging from 5 nm to 100 nm. As the film thickness decreased, the c-axis oriented films showed pers istently a metallic behavior in the normal-state R-T characteristic an d their zero-resistance temperature (T-c,T-0) decreased. However, the a-axis oriented films showed a transition from metallic to semiconduct ing behavior in the normal-state R-T characteristic and their T-c was not changed with decreasing film thickness. We discussed these differe nt R-T characteristics for both films in relation with the film strain due to the lattice misfit and thermal-expansion mismatch between film and substrate. We concluded that the strain in the CuO2 planes is an important factor to determine the T-c, whereas the strain in CuO chain layers is responsible for the normal-state resistance behavior of YBC O ultrathin films.