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

TL;DR

This study shows that post-annealing improves superconducting properties of SrFe2(As0.65P0.35)2 by increasing Tc, altering crystal structure, and changing electronic states from dirty to clean superconductors.

Contribution

It demonstrates how post-annealing modifies crystal and electronic structures, leading to enhanced Tc and a transition from dirty to clean superconducting states in SrFe2(As0.65P0.35)2.

Findings

Tc increased from 26 K to 33 K after annealing.

Annealing changed the electronic specific heat coefficient and magnetic field dependence.

Structural parameters such as lattice axes and pnictogen height were altered by annealing.

Abstract

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, $\gamma_r$, and changed the magnetic field ($H$) dependence from sub-linear $\gamma_r\propto H^{0.7}$ to $H$-linear $\gamma_r\propto H$. This can be attributed the electronic change from dirty to clean superconductors with $s_{\pm}$ gap.