Upper critical field of isoelectron substituted SrFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$

Abstract

The upper critical field $H_{c2}$ of optimally doped iron-based superconductor SrFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$ ($x$ = 0.35, $T_c$ = 25 K) was measured as a function of temperature down to 1.6 K for two principal directions of magnetic field $H\parallel c$ and $H\parallel a$. Measurements were performed in pulsed magnetic fields up to 65 T using a tunnel-diode resonator technique on as-grown and heavy-ion irradiated single crystals, with columnar defect density corresponding to a matching field $B\varphi$ = 25 T. The $H_{c2,c}(T)$ is close to $T$-linear, while clear saturation is observed for $H_{c2,a}(T)$, leading to a strongly temperature dependent anisotropy parameter $\gamma$. The linear shape of $H_{c2,c}(T)$ is very similar to that observed in nodal KFe${}_2$As${}_2$ but very different from full-gap LiFeAs. Irradiation does not introduce any additional features on $H_{c2}(T)$ line corresponding to the matching field. Instead, it suppresses uniformly both $T_c$ and $H_{c2}$, keeping their ratio constant.