Influence of microstructure on superconductivity in K${}_x$Fe${}_{2-y}$Se${}_2$ and evidence for a new parent phase K${}_2$Fe${}_7$Se${}_8$

Abstract

The search for new superconducting materials has been spurred on by the discovery of iron-based superconductors whose structure and composition is qualitatively different from the cuprates. The study of one such material, K${}_x$Fe${}_{2-y}$Se${}_2$ with a critical temperature of 32 K, is made more difficult by the fact that it separates into two phases: a dominant antiferromagnetic insulating phase with a K${}_2$Fe${}_4$Se${}_5$ structure, and a minority superconducting phase whose precise structure is as yet unclear. Here we perform electrical and magnetization measurements, scanning electron microscopy and microanalysis, X-ray diffraction, and scanning tunnelling microscopy on K${}_x$Fe${}_{2-y}$Se${}_2$ crystals prepared under different quenching processes to better understand the relationship between its microstructure and its superconducting phase. We identify a 3D network of superconducting filaments within this material and present evidence to suggest that the superconducting phase consists of a single Fe vacancy for every eight Fe-sites arranged in a $\sqrt{10}\times \sqrt{8}$ parallelogram structure.

26 pages, 7 figures including the main text and the Supplementary as corrections for the original Figure 4

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• Fe-vacancy order and superconductivity in PbO-type tetragonal β-Fe1-xSe
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• Experimental Demonstration of the Sign Reversal of the Order Parameter in (Li1-xFex)OHFe1-yZnySe
• Dumbbell Defects in FeSe Films: A Scanning Tunneling Microscopy and First-Principles Investigation
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• The preparation and phase diagrams of (${}^7$Li${}_{1-x}$Fe${}_x$OD)FeSe and (Li${}_{1-x}$Fe${}_x$OH)FeSe superconductors
• Structure and composition of the superconducting phase in alkali iron selenide K${}_y$Fe${}_{1.6+x}$Se${}_2$
• The role of 245 phase in alkaline iron selenide superconductors revealed by high pressure studies
• What is the minimal model of magnetic interactions in Fe-based superconductors?
• Strong and nonmonotonic temperature dependence of Hall coefficient in superconducting K${}_x$Fe${}_{2-y}$Se${}_2$ single crystals
• Understanding doping, vacancy, lattice stability and superconductivity in KxFe2-ySe2
• Orbital-dependent effects of electron correlations in microscopic models for iron-based superconductors
• High resolution characterisation of microstructural evolution in Rb${}_x$Fe${}_{2-y}$Se${}_2$ crystals on annealing
• Reversible Non-Volatile Electronic Switching in a Near Room Temperature van der Waals Ferromagnet
• Study of multiband disordered systems using the typical medium dynamical cluster approximation
• Two spatially separated phases in semiconducting Rb${}_{0.8}$Fe${}_{1.5}$S${}_2$
• Origin of Pressure-induced Superconducting Phase in K${}_x$Fe${}_{2-y}$Se${}_2$ studied by Synchrotron X-ray Diffraction and Spectroscopy
• Tunable Fe-vacancy disorder-order transition in FeSe thin films
• 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
• Superconductivity at 44.4 K achieved by intercalating EMIM+ into FeSe
• Correlated disorder to order crossover in the local structure of K${}_x$Fe${}_{2-y}$Se${}_{2-z}$S${}_z$
• Elucidating the magnetic and superconducting phases in the alkali metal intercalated iron chalcogenides
• Pair-checkerboard antiferromagnetic order in $\beta$-Fe${}_4$Se${}_5$ with $\sqrt{5}\times \sqrt{5}$ ordered Fe vacancies
• Spin glass in semiconducting KFe${}_{1.05}$Ag${}_{0.88}$Te${}_2$ single crystals
• Superconductivity at the vacancy disorder boundary in K${}_x$Fe${}_{2-y}$Se${}_2$
• Impurity-induced sub-gap bound gap states in alkali doped iron chalcogenide superconductors
• Evidence of superconductivity-induced phonon spectra renormalization in alkali-doped iron selenides
• Sustained phase separation and spin glass in Co-doped K${}_x$Fe${}_{2-y}$Se${}_2$ single crystals
• Phase-separation control of K${}_x$Fe${}_{2-y}$Se${}_2$ superconductor through rapid-quenching process
• Raman spectroscopy of K_xCo_{2-y}Se_2 single crystals near the ferromagnet-paramagnet transition
• Low-energy bound states at interfaces between superconducting and block antiferromagnet regions in KxFe{2-y}Se2
• Nonmetallic Low-Temperature Normal State of K0.70Fe1.46Se1.85Te0.15
• Suppression of the antiferromagnetic order when approaching the superconducting state in a phase-separated crystal of K${}_x$Fe${}_{2-y}$Se${}_2$
• Delicate Superconductivity in NaAlGe Single Crystal
• Annealing Effect on Microstructure and Superconductivity of KxFeySe2 Synthesized by an Easy One-step Method
• Magnetization distribution and orbital moment in the non-Superconducting Chalcogenide Compound K0.8Fe1.6Se2
• Near-field microwave imaging of inhomogeneous K${}_x$Fe${}_y$Se${}_2$: separation of topographic and electric features
• Superconductivity in the bilayer Hubbard model: Are two Fermi surfaces better than one?