Superconductivity induced by oxygen deficiency in Sr-doped LaOFeAs

TL;DR

This study demonstrates that inducing oxygen deficiency in Sr-doped LaOFeAs enhances superconductivity, achieving a maximum Tc of about 26 K, and reveals the transition from hole to electron-type conduction.

Contribution

It is the first systematic investigation showing how oxygen deficiency induces superconductivity in Sr-doped LaOFeAs, highlighting the role of oxygen vacancies in tuning electronic properties.

Findings

Superconductivity appears after vacuum annealing to create oxygen deficiency.

Maximum Tc reaches about 26 K in oxygen-deficient samples.

Oxygen deficiency changes thermoelectric power from positive to negative.

Abstract

We synthesized Sr-doped $La_{0.85}Sr_{0.15}OFeAs$ sample with single phase, and systematically studied the effect of oxygen deficiency in the Sr-doped LaOFeAs system. It is found that substitution of Sr for La indeed induces the hole carrier evidenced by positive thermoelectric power (TEP), but no bulk superconductivity is observed. The superconductivity can be realized by annealing the as-grown sample in vacuum to produce the oxygen deficiency. With increasing the oxygen deficiency, the superconducting transition temperature ($T_c$) increases and maximum $T_c$ reaches about 26 K the same as that in La(O,F)FeAs. TEP dramatically changes from positive to negative in the nonsuperconducting as-grown sample to the superconducting samples with oxygen deficiency. While $R_H$ is always negative for all samples (even for Sr-doped as grown sample). It suggests that the $La_{0.85}Sr_{0.15}O_{1-\delta}FeAs$ is still electron-type superconductor.