Thorium-doping induced superconductivity up to 56 K in Gd1-xThxFeAsO

Abstract: Following the discovery of superconductivity in an iron-based arsenide LaO1-xFxFeAs with a superconducting transition temperature (Tc) of 26 K[1], Tc was pushed up surprisingly to above 40 K by either applying pressure[2] or replacing La with Sm[3], Ce[4], Nd[5] and Pr[6]. The maximum Tc has climbed to 55 K, observed in SmO1-xFxFeAs[7, 8] and SmFeAsO1-x[9]. The value of Tc was found to increase with decreasing lattice parameters in LnFeAsO1-xFx (Ln stands for the lanthanide elements) at an apparently optimal doping level. However, the F- doping in GdFeAsO is particularly difficult[10,11] due to the lattice mismatch between the Gd2O2 layers and Fe2As2 layers. Here we report observation of superconductivity with Tc as high as 56 K by the Th4+ substitution for Gd3+ in GdFeAsO. The incorporation of relatively large Th4+ ions relaxes the lattice mismatch, hence induces the high temperature superconductivity.

• The paper demonstrates that thorium doping in GdFeAsO increases the superconducting transition temperature to 56 K by enabling effective electron doping.
• The methodology employs solid-state reactions along with X-ray diffraction and EDX analysis to confirm Th incorporation into the lattice.
• The results indicate that improved lattice matching between Gd2O2 and Fe2As2 layers overcomes doping challenges and paves the way for further high-Tc research.

Thorium-Doping Induced Superconductivity in Gd$_{1-x}$Th$_{x}$FeAsO

The paper addresses the advancement of superconductivity in the GdFeAsO compound by introducing thorium (Th) doping, resulting in a significant modification of its electronic and structural properties. The authors have presented a comprehensive study demonstrating the enhancement of the superconducting transition temperature ($T_c$) to 56 K, achieved through the partial substitution of gadolinium (Gd) with thorium in Gd$_{1-x}$Th$_{x}$FeAsO.

The study builds on prior research that established the potential for superconductivity in iron-based arsenide compounds, such as LaO$_{1-x}$F$_{x}$FeAs, which initially exhibited a $T_c$ of 26 K. Advances in the field have incrementally increased this transition temperature through various methods like pressure application or substitution of lanthanides, with $T_c$ values previously reported as high as 55 K in similar compounds.

The incorporation of Th$^{4+}$ ions for Gd$^{3+}$ specifically aims to address the lattice mismatch issue prevalent in F$^-$-doped GdFeAsO, which hinders the effective electron-doping required for superconductivity. Thorium, due to its large tetravalence, allows for a more seamless integration into the lattice structure, specifically alleviating the incompatibility between the Gd$_2$O$_2$ and Fe$_2$As$_2$ layers. This mitigation fosters chemical stability and enables higher doping levels compared to traditional methods.

Experimental results evidence this structural change via X-ray diffraction and energy-dispersive X-ray (EDX) analysis, confirming thorium's incorporation into the lattice. The Th substitution alters lattice parameters, expanding within the $ab$-plane and contracting along the $c$-axis, which corroborates the achievement of effective electron doping.

Furthermore, electrical and magnetic measurements substantiate the superconducting nature of the Th-doped samples. A precise pattern in resistivity and susceptibility indicative of high-temperature superconductivity is observed, notably disappearing resistivity anomalies typically attributable to antiferromagnetic spin-density-wave transitions, suggesting a shift toward superconducting order as electron doping increases.

From a methodological standpoint, the synthesis of the Th-doped samples was conducted through solid-state reactions, affirming the feasibility of using Th for enhancing superconducting properties in iron-based materials. This strategy not only opens pathways for new electron doping methods in the LnFeAsO family but also hints at broader implications for other high $T_c$ iron-based compounds.

The findings presented in this paper propose a promising synthesis approach as an alternative to charge carrier doping, overcoming intrinsic lattice hurdles that limit the superconductivity potential in similar compounds. Future research could explore further optimizing doping compounds or variations, potentially increasing $T_c$ or improving other material properties. This Th-doping method offers a solid framework to examine other less-explored elements or methodologies in similar layered superconducting materials, signifying an advancing understanding of structural and electronic correlations in high temperature superconductors.