Upper critical field of isoelectron substituted SrFe2(As1−xPx)2

Abstract

The upper critical field ${H}_{c2}$ of optimally doped, iron-based superconductor SrFe${}_{2}$(As${}_{1\ensuremath{-}x}$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 $H\ensuremath{\parallel}c$ and $H\ensuremath{\parallel}a$. 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 ${B}_{\ensuremath{\phi}}$ $=$ 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 $\ensuremath{\gamma}$. 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 the ${H}_{c2}(T)$ line corresponding to the matching field. Instead, it suppresses uniformly both ${T}_{c}$ and ${H}_{c2}$, keeping their ratio constant.