Synthesis and Characterization of Superconducting Ca₁-xNaxFFeAs.

Klaus Wolff,Markus Bischoff,Larysa Shlyk,Rainer Niewa,Eva Rose,Thomas Schleid

doi:10.3390/ma7031984

Abstract

A representative of the fluoride-containing iron pnictide high-temperature superconductors, namely CaFFeAs, was doped with sodium up to the composition Ca0.86Na0.14FFeAs for the first time. Single crystals with an edge length in the range of 0.1 – 2.0 mm were obtained via solid-state and flux syntheses, respectively. The composition of the crystals was verified by means of single crystal X-ray diffractometry and energy dispersive X-ray spectroscopy (EDX). Measurements of the electrical resistivity, as well as the magnetization on a crystal of Ca0.89Na0.11FFeAs both show a transition to the superconducting state on cooling to 34.5 K. Investigations of the upper critical fields reveal an anisotropy ratio of about five. The lattice parameters and molar volumes increase with rising sodium content. This effect is clearly observable for the c-axis and the volume, whereas the increase of the a-axis is rather minor.

Highlights

With the detection of superconductivity in iron pnictides in 2008 [1,2], a new class of high-temperature superconductors [3] was found 22 years after the discovery of this phenomenon in oxocuprates [4]
Within the 1111 compounds, superconductivity is only observed if a partial reduction or oxidation in the [FeAs]− layers occurs [11]
Larger single crystals with a composition of Ca0.89Na0.11FFeAs and a size up to 2 × 2 mm2 were grown in a flux of sodium chloride for physical measurements

Summary

Introduction

With the detection of superconductivity in iron pnictides in 2008 [1,2], a new class of high-temperature superconductors [3] was found 22 years after the discovery of this phenomenon in oxocuprates [4]. After the discovery of superconductivity in 1111 phases, hole doping on LaOFeAs with Ca2+ was already investigated without the detection of any superconducting phases [2]; the reason why electron doping was claimed as a critical factor for the generation of superconductivity in this system at that time. This view was disproven by hole-doped La1−xSrxOFeAs with Tc = 25 K for x = 0.13 three months later [15]

Results and Discussion

Experimental Section

Conclusions