A criterion for determination of the upper critical fields $H_{c2}$ in YBCO thin films with different ion irradiation doses

Magnetotransport measurements were carried out for YBa$_2$Cu$_3$O$_{7-x}$ (YBCO) thin films in external perpendicular magnetic fields of $H$. The studies were performed both for the virgin samples and for the irradiated ones. Xenon ions were used as an external irradiation. Thus we studied features of the broadening of superconducting transition in YBCO films (virgin and irradiated). The broadening of superconducting drop was analyzed depending on an external magnetic field $H$, as well as on an irradiation dose $n_D$. When processing the experimental data $R(H,T)$, we studied a criterion for determination of temperature dependence of the upper critical field $H_{c2}(T)$. The criterion was analyzed depending on the defect concentration in the film corresponding to a certain value of $n_D$. It was found out that for a virgin sample, $H_{c2}$ should be determined by the resistance level $R$ = 0.4$R_N$ inside the superconducting transition, where $R_N$ = $R$ ($T$ = 100 K). With a gradual increase in $n_D$, this resistance level decreases. At sufficiently high radiation doses $n_D>$ 7 $\cdot$ 10$^{12}$ cm$^{-2}$, the $H_{c2}(T)$ phase transition line should be determined by the level $R\approx$ 0. Keywords: thin films, HTSC, xenon ion irradiation, magnetotransport studies, broadening of the superconducting transition, upper critical field, phase transition line, Tinkham's formula, Abrikosov vortices.