Upward curvature of the upper critical field in the Boson--Fermion model

We report on a non-conventional temperature behavior of the upper critical field ($H_{c2}(T)$) which is found for the Boson-Fermion (BF) model. We show that the BF model properly reproduces two crucial features of the experimental data obtained for high-$T_c$ superconductors: $H_{c2}(T)$ does not saturate at low temperatures and has an upward curvature. Moreover, the calculated upper critical field fits very well the experimental results. This agreement holds also for overdoped compounds, where a purely bosonic approach is not applicable.