Narrow superconducting window in Ni-doped LaFeAsO

TL;DR

This study investigates Ni-doping in LaFeAsO, revealing a narrow superconducting window with a maximum Tc of 6.5 K, suppressed resistivity anomalies, and an upper critical field exceeding the Pauli limit.

Contribution

It demonstrates the effects of Ni substitution on superconductivity and magnetic properties in LaFeAsO, highlighting a very limited doping range for superconductivity.

Findings

Superconductivity appears only in a narrow Ni doping range (0.03 to 0.06).

Maximum Tc of 6.5 K occurs at x=0.04.

Upper critical field exceeds the Pauli paramagnetic limit.

Abstract

We have studied Ni-substitution effect in LaFe$_{1-x}$Ni$_{x}$AsO ($0\leq x \leq0.1$) by the measurements of x-ray diffraction, electrical resistivity, magnetic susceptibility, and heat capacity. The nickel doping drastically suppresses the resistivity anomaly associated with spin-density-wave ordering in the parent compound. Superconductivity emerges in a narrow region of $0.03\leq x \leq0.06$ with the maximum $T_c$ of 6.5 K at $x$=0.04, where enhanced magnetic susceptibility shows up. The upper critical field at zero temperature is estimated to exceed the Pauli paramagnetic limit. The much lowered $T_c$ in comparison with LaFeAsO$_{1-x}$F$_{x}$ system is discussed.