Critical-field slope reduction and upward curvature of the phase-transition lines of thin disordered superconducting YBa2Cu3O7−x films in strong magnetic fields

• An unconventional critical-field slope reduction of the phase-transition line Hc2(T) was experimentally observed for thin YBCO films by gradual increasing of ion implantation dose. • An upward curvature of temperature dependence of the upper critical field was experimentally found out for thin YBCO films by gradual increasing of ion implantation dose. • Theoretical interpretation of the experimental data was developed in the framework of linearized Ginzburg–Landau theory with an inhomogeneous superconducting coherence length. We report the effect of disorder on superconducting phase transition of YBa 2 Cu 3 O 7 − x epitaxial thin films in external magnetic fields. The disorder was produced by several successive acts of oxygen ion implantation. Controlling a total accumulated dose of implanted ions n D , we carried out transport measurements in ab -plane for temperatures T below 91 K and external magnetic fields H up to 11 T. Temperature-field dependencies of in-plane resistivity allow us to analyze H − T phase diagrams for primary compound as well as for disordered structure. Considering the upper critical field H c 2 as the magnetic field which corresponds to a local resistivity drop at the onset of superconducting transition, we found out the following results. By gradual increasing of n D , the phase-transition line H c 2 ( T ) suffers an unconventional critical-field slope reduction, while larger defect concentrations usually enhance the upper critical field in the vicinity of T c 0 . Besides, for rather large n D , the curvature of H c 2 ( T ) becomes upward for temperatures close to T c 0 . Theoretical interpretation of the experimental data is developed in the framework of linearized Ginzburg-Landau theory with an inhomogeneous superconducting coherence length. A simple expression for the critical temperature T c is obtained: T c = T c 0 ( 1 − h + α h 3 / 2 ) , where h is the dimensionless magnetic field and α is a constant which describes the defects in a specimen. The formula nicely fits our experimental results.