[Paper Review] Superconductivity in Fluorine-Arsenide Sr_{1-x}La_xFeAsF

Xiyu Zhu, Fei Han+4 more|arXiv (Cornell University)|Oct 14, 2008

Iron-based superconductors research3 citations

TL;DR

This paper reports the discovery of superconductivity in fluorine-arsenide Sr1-xLa x FeAsF with a Tc of 32 K, achieved by electron doping via La substitution in the SrFeAsF parent compound. The material crystallizes in the ZrCuSiAs structure with a DvF (divalent metal fluoride) block replacing the REO layer, enabling a new family of FeAs-based superconductors where superconductivity emerges from a suppressed spin-density wave state, offering a new route to higher-Tc iron pnictides.

ABSTRACT

Since the discovery of superconductivity\cite{1} at 26 K in oxy-pnictide $LaFeAsO_{1-x}F_x$, enormous interests have been stimulated in the fields of condensed matter physics and material sciences. Among the five different structures in this broad type of superconductors\cite{2,3,4,5,6}, the ZrCuSiAs structure has received special attention since the $T_c$ has been quickly promoted to 55-56 K\cite{7,8,9,10,11} in fluorine doped oxy-pnictides REFeAsO (RE = rare earth elements). The superconductivity can also be induced by applying a high pressure to the undoped samples\cite{12,13}. The mechanism of superconductivity in the FeAs-based system remains unclear yet, but it turns out to be clear that any change to the structure or the building blocks will lead to a change of the superconducting transition temperatures. In this Letter, we report the fabrication of the new family of compounds, namely fluorine-arsenides DvFeAsF (Dv = divalent metals) with the ZrCuSiAs structure and with the new building block DvF instead of the REO (both the layers DvF and REO have the combined cation state of "+1"). The undoped parent phase has a Spin-Density-Wave like transition at about 173 K for SrFeAsF, 118 K for CaFeAsF and 153 K for EuFeAsF. By doping electrons into the system the resistivity anomaly associated with this SDW transition is suppressed and superconductivity appears at 32 K in the fluorine-arsenide Sr$_{1-x}$La$_x$FeAsF (x = 0.4). Our discovery here initiates a new method to obtain superconductors in the FeAs-based system.

Motivation & Objective

• To explore new FeAs-based superconductors by replacing the REO layer in ZrCuSiAs-structured compounds with alternative building blocks of equal net cationic charge (+1).
• To investigate whether divalent metal fluorides (DvF, Dv = Sr, Ca, Eu) can form stable parent compounds with the same structure as REFeAsO and host superconductivity upon electron doping.
• To determine if electron doping via La substitution in SrFeAsF can induce superconductivity with higher transition temperatures than previously observed in similar systems.
• To confirm the superconducting nature and rule out contamination from other phases such as SrFe2As2 or LaFeAsO.
• To establish the role of structural and electronic tuning via cation substitution in modulating superconducting transition temperatures.

Proposed method

• Synthesized polycrystalline DvFeAsF (Dv = Sr, Ca, Eu) and Sr1-xLa x FeAsF (x = 0–0.4) via a two-step solid-state reaction under argon atmosphere in a glove box with O2 and H2O < 0.1 ppm.
• Used FeF3 and Fe metal as sources of Fe and F, combined with DvAs and LaAs precursors synthesized from elemental powders.
• Performed high-temperature solid-state sintering at 950 °C for 60 hours, followed by a second sintering at 1000 °C to improve phase purity.
• Characterized crystal structure using X-ray diffraction (XRD), confirming tetragonal ZrCuSiAs structure with lattice parameters a ≈ 3.98 Å and c ≈ 8.96 Å for doped samples.
• Measured resistivity using a six-probe technique in a physical property measurement system (PPMS), with current direction reversed to eliminate thermal EMF.
• Conducted zero-field-cooled magnetic measurements using a SQUID magnetometer to confirm bulk superconductivity via diamagnetic transition.

Experimental results

Research questions

• RQ1 Can the REO layer in REFeAsO superconductors be replaced by a DvF (divalent metal fluoride) block with equivalent net cationic charge (+1) to form new FeAs-based superconductors?
• RQ2 Does electron doping via La substitution in SrFeAsF suppress the spin-density wave (SDW) transition and induce superconductivity, and at what Tc?
• RQ3 How does the superconducting transition temperature in Sr1-xLa x FeAsF compare to that of Co-doped SrFeAsF or other FeAs-based systems?
• RQ4 Can the observed superconductivity be attributed to the formation of secondary phases such as SrFe2As2 or LaFeAsO, and how is this ruled out?
• RQ5 What is the relationship between the structural/SDW transition temperature in the parent phase and the onset of superconductivity upon electron doping?

Key findings

• The parent compound SrFeAsF exhibits a resistivity anomaly at 173 K, corresponding to a spin-density wave (SDW) or structural transition, confirmed by magnetization and magnetoresistance measurements.
• Electron doping via La substitution in SrFeAsF suppresses the SDW transition, which shifts from 173 K to ~60 K at x = 0.2 and disappears at x = 0.4.
• A superconducting transition is observed at 32 K (onset) in Sr0.6La0.4FeAsF, with zero resistance achieved at 24.6 K, marking a significant Tc enhancement over previously reported SrFe1-xCo x AsF (Tc = 4 K).
• The resistivity of the x = 0.4 sample shows linear temperature dependence in the normal state, resembling electron-doped LaFeAsO0.9F0.1, contrasting with hole-doped systems.
• Zero-field-cooled SQUID measurements confirm bulk superconductivity with a diamagnetic transition at ~27 K, consistent with the resistivity onset.
• XRD patterns show no evidence of SrFe2As2 or LaFeAsO phases, and the absence of oxygen (below 0.1 ppm) rules out contamination from LaFeAsO, confirming the superconductivity is intrinsic to Sr1-xLa x FeAsF.