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

Abstract

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 $\theta$ between the field and the $ab$-plane was measured in isothermal conditions in a broad temperature range. We found that the shape of $H_{c2}(\theta )$, clearly deviates from Ginzburg-Landau functional form.