Angular-dependent upper critical field of overdoped Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$

In-plane resistivity measurements as a function of temperature, magnetic field and its orientation with respect to the crystallographic $ab-$plane were used to study the upper critical field, $H_{c2}$, of two overdoped compositions of the iron-based superconductor Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$, $x=$0.054 and $x$=0.072. Measurements were performed using precise alignment (with accuracy less than 0.1$^o$) of magnetic field with respect to the Fe-As-plane. The dependence of the $H_{c2}$ on angle $θ$ between the field and the $ab$-plane was measured in isothermal conditions in a broad temperature range. We found that the shape of $H_{c2}(θ)$, clearly deviates from Ginzburg-Landau functional form.

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