The change of electronic state and crystal structure by post-annealing in superconducting SrFe2(As0.65P0.35)2

We investigated the annealing effects on the physical properties of SrFe$_2$(As$_{1-x}$P$_x$)$_2$ ($x=0.35$). The superconducting transition temperature ($T_c$) increased from 26 K to 33 K by annealing. The X-ray diffraction measurement suggested that the annealed crystals have the shorter/longer $a/c$-axes and the larger pnictogen height $h_\mathrm{Pn}$. This must be linked to the $T_c$-enhancement by annealing. Moreover, it was found that the post-annealing decreased the electronic specific heat coefficient at $T$=0 K, $γ_r$, and changed the magnetic field ($H$) dependence from sub-linear $γ_r\propto H^{0.7}$ to $H$-linear $γ_r\propto H$. This can be attributed the electronic change from dirty to clean superconductors with $s_{\pm}$ gap.

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