High Pressure Synthesis of Indirectly electron-doped 122 Iron Superconductor Sr1-xLaxFe2As2 with a maximum Tc = 22 K

TL;DR

This study demonstrates the synthesis of electron-doped Sr1-xLaxFe2As2 superconductors under high pressure, achieving a maximum Tc of 22 K, and compares the effects of doping mode and carrier polarity on superconductivity.

Contribution

First experimental evidence of electron-doped superconductivity in 122 iron pnictides via aliovalent substitution at the alkaline earth site.

Findings

Maximum Tc of 22 K at x=0.4 doping level.

Superconductivity observed for x ≥ 0.2.

Doping mode has less impact on Tc than carrier polarity.

Abstract

Compounds of Sr1-xLaxFe2As2 were synthesized by solid state reaction at 1273 K, under pressures of 2 or 3 GPa. The Sr1-xLaxFe2As2 phase was dominant up to x = 0.5, and superconductivity was observed at $x \ge 0.2$. A maximum critical temperature Tc of 22 K, and a maximum shielding volume fraction of ~70 % were obtained at x = 0.4. This is the first experimental demonstration of electron-doped superconductivity in 122-type iron pnictides, where electrons were doped through aliovalent substitution at the site of the alkaline earth metal. The optimal Tc was slightly higher than that for the Co-doped (directly electron-doped) samples (19 K), and much lower than that for the hole-doped case (37 K). The bulk superconductivity range was narrower than that for the Co-substituted case, and both ranges were much narrower than that for the hole-doped case. These observations revealed that differences in the electron-doping mode (direct or indirect) did not have a prominent effect on the optimal Tc or superconductivity range, compared with differences in carrier polarity.