Phase diagrams of thin disordered films based on HTSC YBa$_{2}$Cu$_{3}$O$_{7-x}$ in external magnetic fields

An unusual decrease in the slope of the upper critical field near $T_{c0}$ at a gradual increase in the ion implantation dose has been experimentally observed in narrow bridges formed on the base of thin HTSC YBa$_{2}$Cu$_{3}$O$_{7-x}$ films, while an increase in the defect concentration usually leads to an increase in the local slope of the phase transition line $H_{c 2}(T)$. In addition, it has been found that the temperature dependence of the upper critical field has a positive curvature near $T_{c0}$. A possible theoretical interpretation of the results is proposed. It is based on the modified Ginzburg–Landau theory with a nonuniform length of superconducting correlations.