[Paper Review] Superconductivity induced by cobalt doping in iron-based oxyarsenides

Guang‐Han Cao, Cao Wang+7 more|ArXiv.org|Jul 8, 2008

Iron-based superconductors research25 references3 citations

TL;DR

This study demonstrates that cobalt doping on the Fe2As2 conducting layers of LaFeAsO induces superconductivity at Tc ~10 K with as little as 2.5% Co doping, despite cobalt's magnetic nature—challenging the conventional view that magnetic dopants destroy superconductivity. The superconducting transition temperature peaks at 13 K near x = 0.075 before declining, indicating a dome-shaped Tc(x) curve similar to cuprates but with distinct differences, including semiconducting normal state behavior and suppression of spin-density wave order via double exchange, suggesting unconventional, itinerant electron-driven superconductivity.

ABSTRACT

Chemical doping has recently become a very important strategy to induce superconductivity especially in complex compounds. Distinguished examples include Ba-doped La$_2$CuO$_4$ (the first high temperature superconductor), K-doped BaBiO$_3$, K-doped C$_{60}$ and Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O. The most recent example is F-doped LaFeAsO, which leads to a new class of high temperature superconductors. One notes that all the above dopants are non-magnetic, because magnetic atoms generally break superconducting Cooper pairs. In addition, the doping site was out of the (super)conducting structural unit (layer or framework). Here we report that superconductivity was realized by doping magnetic element cobalt into the (super)conducting-active Fe$_2$As$_2$ layers in LaFe$_{1-x}$Co$_{x}$AsO. At surprisingly small Co-doping level of $x$=0.025, the antiferromagnetic spin-density-wave transition in the parent compound is completely suppressed, and superconductivity with $T_c\sim $ 10 K emerges. With increasing Co content, $T_c$ shows a maximum of 13 K at $x\sim 0.075$, and then drops to below 2 K at $x$=0.15. This result suggests essential differences between previous cuprate superconductor and the present iron-based arsenide one.

Motivation & Objective

• To investigate whether magnetic cobalt doping on the Fe2As2 conducting layers of LaFeAsO can induce superconductivity, challenging the long-standing belief that magnetic ions destroy superconducting pairing.
• To examine the electronic phase diagram of LaFe1-xCoxAsO and compare it with F-doped analogs, particularly focusing on the suppression of spin-density wave (SDW) order and emergence of superconductivity.
• To determine the role of Co doping in modifying the magnetic and electronic structure, especially whether double exchange between Fe and Co disrupts the frustrated antiferromagnetic order in the parent compound.
• To clarify whether superconductivity in this system arises from electron doping into Fe2As2 layers and whether the normal state exhibits metallic or semiconducting behavior above Tc.

Proposed method

• Polycrystalline LaFe1-xCoxAsO samples were synthesized via solid-state reaction in vacuum using stoichiometric mixtures of LaAs, La2O3, FeAs, Fe2As, and Co3O4, with synthesis conducted in an argon-filled glovebox to prevent oxidation.
• X-ray diffraction (XRD) was used to confirm phase purity and structural integrity, with lattice parameters refined using a tetragonal P4/nmm space group and Vegard’s law to verify Co incorporation.
• Electrical resistivity and magnetic susceptibility (χ) were measured using a Quantum Design MPMS-5 system to identify superconducting transitions, structural transitions, and magnetic anomalies.
• High-pressure synthesis was employed to rule out oxygen deficiency as a cause of superconductivity, as oxygen-deficient samples typically show reduced a- and c-lattice parameters, which was not observed here.
• The electronic phase diagram was constructed from resistivity, susceptibility, and XRD data, with Tc determined from resistivity drop and χ from field-cooling protocols at 10 Oe.
• Theoretical analysis based on superexchange and double exchange mechanisms was used to explain the suppression of spin-density wave order and the role of Co in disrupting the frustrated antiferromagnetic state.

Experimental results

Research questions

• RQ1 Can magnetic cobalt doping on the Fe2As2 layers of LaFeAsO induce superconductivity, despite the conventional expectation that magnetic ions break Cooper pairs?
• RQ2 How does Co doping affect the suppression of the spin-density wave (SDW) order in the parent compound, and what is the mechanism behind this suppression?
• RQ3 Why does the superconducting transition temperature Tc peak at x ≈ 0.075 and then decrease with further Co doping, and how does this compare to the F-doped system?
• RQ4 Does the normal state of Co-doped LaFe1-xCoxAsO exhibit metallic or semiconducting behavior above Tc, and what does this imply about the electronic state?
• RQ5 What is the role of double exchange between Fe and Co in disrupting the frustrated antiferromagnetic order in the Fe2As2 planes?

Key findings

• Superconductivity emerges at Tc ≈ 10 K with only 2.5% Co doping (x = 0.025), completely suppressing the parent compound’s antiferromagnetic spin-density wave (SDW) transition at 137 K.
• The superconducting transition temperature Tc reaches a maximum of 13 K at x ≈ 0.075, forming a dome-shaped Tc(x) curve, similar to cuprates but with a narrower superconducting region.
• The normal state above Tc exhibits semiconducting behavior, despite metallic conduction at high temperatures, indicating a complex electronic state distinct from conventional metals.
• The lattice parameters show a linear decrease in cell volume with Co doping, with the c-axis shrinking significantly while the a-axis remains nearly constant, confirming successful Co substitution into the Fe2As2 layer.
• The absence of significant a-axis contraction rules out oxygen deficiency as the cause of superconductivity, as oxygen-deficient samples typically show reduced a- and c-lattices.
• Theoretical analysis indicates that Co doping induces double exchange between Fe and Co, which disrupts the frustrated antiferromagnetic order by introducing ferromagnetic coupling, explaining the rapid suppression of SDW order.