Evolution of precipitate morphology during heat treatment and its implications for the superconductivity in KxFe1.6+ySe2 single crystals

Abstract

We study the relationship between precipitate morphology and superconductivity in KxFe1.6+ySe2 single crystals grown by self-flux method. Scanning electron microscopy (SEM) measurements revealed that superconducting phase forms a network in the samples quenched above iron vacancy order-disorder transition temperature Ts. For the samples obtained by natural cooling down to room temperature in the furnace, referred to as furnace cooling, superconducting phase aggregates into micrometer-sized rectangular bars and aligns as disconnected chains. Accompanying this change in morphology the superconducting shielding fraction is strongly reduced in the furnace-cooling samples. By post-annealing above Ts followed by quenching in room temperature water, the network recovers with superconducting shielding fraction approaching 80%. A reversible change from network to bar chains was realized by a secondary heat treatment in annealed samples showing large shielding fraction, i.e., heating above Ts followed by slow cooling across Ts. The large shielding fraction observed in KxFe1.6+ySe2 single crystals actually results from a uniform and contiguous distribution of superconducting phase. Through the measurements of temperature dependent x-ray diffraction, it is found that the reflection corresponding to superconducting phase merges into that from iron vacancy ordered phase upon warming. It is a solid solution above Ts, where iron atoms randomly occupy the both Fe1 and Fe2 sites in iron vacancy disordering status. By cooling across Ts, superconducting phase precipitates while iron vacancy ordered phase forms together, suggesting that phase separation in KxFe1.6+ySe2 single crystals is driven by the iron vacancy order-disorder transition.

to be published in PRB

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Cited by in corpus (28)

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• Spin fluctuations in iron pnictides and chalcogenides: From antiferromagnetism to superconductivity
• Optical conductivity of iron-based superconductors
• ARPES measurements of the superconducting gap of Fe-based superconductors and their implications to the pairing mechanism
• Spectromicroscopy of electronic phase separation in K${}_x$Fe${}_{2-y}$Se${}_2$ superconductor
• Structure and composition of the superconducting phase in alkali iron selenide K${}_y$Fe${}_{1.6+x}$Se${}_2$
• Strong and nonmonotonic temperature dependence of Hall coefficient in superconducting K${}_x$Fe${}_{2-y}$Se${}_2$ single crystals
• Critical current density and vortex pinning in tetragonal FeS${}_{1-x}$Se${}_x$ ($x=0,0.06$)
• High resolution characterisation of microstructural evolution in Rb${}_x$Fe${}_{2-y}$Se${}_2$ crystals on annealing
• Archimedean solid-like superconducting framework in phase-separated K0.8Fe1.6+xSe2 (0<=x<=0.15)
• Disordered Fe vacancies and superconductivity in potassium-intercalated iron selenide (K2-xFe4+ySe5)
• Significant enhancement of critical current density in H+-intercalated FeSe single crystal
• Origin of the Higher-$T_\rm{c}$ Phase in the K${}_x$Fe${}_{2-y}$Se${}_2$ System
• High-pressure single-crystal neutron scattering study of magnetic and Fe vacancy orders in (Tl,Rb)2Fe4Se5 superconductor
• Elucidating the magnetic and superconducting phases in the alkali metal intercalated iron chalcogenides
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