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Superstructures in Iron Superconductors

Two best-fit magnetic configurations in a projection of one layer of Fe on the ab-plane. Iron vacancy positions are shown by crosses. Open blue and filled red circles show Fe down and up spins. Large red unit cell (A,B) corresponds to the supercell with vacancy ordered structure.
Two best-fit magnetic configurations in a projection of one layer of Fe on the ab-plane. Iron vacancy positions are shown by crosses. Open blue and filled red circles show Fe down and up spins. Large red unit cell (A,B) corresponds to the supercell with vacancy ordered structure.
The discovery of Fe-based superconductors in early 2008 has renewed interest in finding new routes to high-temperature superconductivity. As observed in the cuprates, the iron-based superconductors exhibit interplay between magnetism and superconductivity, suggesting the possible occurrence of unconventional superconducting states. A unique feature of the new alkali-metal intercalated iron selenides XyFe2-xSe2 (X = K, Cs, Rb) discovered towards the end of 2010 is the presence of robust antiferromagnetism (Fig.1,2) with an extraordinary high Néel temperature above 500 K and large iron magnetic moment, and high-temperature superconductivity with a critical temperature at around 30 K [1-4]. Another interesting specific feature of this new class of magnetic superconductors is the presence of an iron vacancy superstructure. Concomitantly with the vacancy ordering, the ordering of the rest of the iron atom spins have the same propagation wave vector at almost the same temperature. It is believed that the iron vacancy and spin ordering may be key to the superconducting pairing mechanism in this system. A pronounced reversible phase separation revealed recently in 122 single crystals [4], as well as controversies regarding the origin of superconductivity and the exact stoichiometry of the superconducting phase are still in the forefront of scientific activity. Further, investigations on alternative synthesis routes of superconducting 122 phases have been undertaken. A new iron selenide superconductor with a Tc onset of 45 K and the nominal composition Lix(C5H5N)yFe2−zSe2, have been synthesized via intercalation of dissolved alkaline metal in anhydrous pyridine at room temperature [5].


In our attempt to explore ternary iron chalcogenides we gave attention to different alkaline earth metal families. The alkaline earth intercalated iron selenide BaFe2Se3 (Ba123) have been successfully grown and studied by neutron diffraction [6]. The crystal structure is similar to that of the 122 alkali metal intercalated chalcogenides, with the difference being the one-dimensional arrangement of the FeSe4 edge connected tetrahedra creating double chains running along the b-axis within the Fe2Se3 bc-layers. AFM long range order is observed below TN ≈ 240 K with the structure containing four-spin ferromagnetic blocks oriented antiparallel along the Fe2Se3 chains with Fe moments of 2.1 μB along the a-axis.
The Rietveld refinement pattern and difference plot of the neutron diffraction data for KyFe2-xSe2 at room temperature measured at the HRPT with the wavelength λ=1.886Å.
The Rietveld refinement pattern and difference plot of the neutron diffraction data for KyFe2-xSe2 at room temperature measured at the HRPT with the wavelength λ=1.886Å.
  • [1] A Krzton-Maziopa, Z Shermadini, E Pomjakushina, V Pomjakushin, M Bendele, A Amato, R Khasanov, H Luetkens and K Conder, "Synthesis and crystal growth of Cs0.8(FeSe0.98)2: a new iron-based superconductor with Tc = 27 K", J. Phys.: Condens. Matter 23 (2011) 052203.
  • [2] V. Yu. Pomjakushin, D. V. Sheptyakov, E. V. Pomjakushina, A. Krzton- Maziopa, K. Conder, D. Chernyshov, V. Svitlyk, and Z. Shermadini, "Iron-vacancy superstructure and possible room-temperature antiferromagnetic order in superconducting CsyFe2-xSe2", PHYSICAL REVIEW B 83, 144410 (2011).
  • [3] V Yu Pomjakushin, E V Pomjakushina, A Krzton-Maziopa, K Conder and Z Shermadini, "Room temperature antiferromagnetic order in superconducting XyFe2-xSe2 (X = Rb, K): a neutron powder diffraction study", J. Phys.: Condens. Matter 23 (2011) 156003.
  • [4] Pomjakushin VYU, Krzton-Maziopa A, Pomjakushina EV, Conder K, Chernyshov D, Svitlyk V, Bosak A, "Intrinsic crystal phase separation in the antiferromagnetic superconductor RbyFe2-xSe2: a diffraction study", JOURNAL OF PHYSICS-CONDENSED MATTER 24, 435701 (2012); Bosak A, Svitlyk V, Krzton-Maziopa A, Pomjakushina E, Conder K, Pomjakushin V, Popov A, de Sanctis D, Chernyshov D, "Phase coexistence in Cs0.8Fe1.6Se2 as seen by x-ray mapping of reciprocal space", PHYSICAL REVIEW B 86, 174107 (2012).
  • [5] Krzton-Maziopa A, Pomjakushina EV, Pomjakushin VYU, von Rohr F, Schilling A, Conder K, "Synthesis of a new alkali metal-organic solvent intercalated iron selenide superconductor with Tc=45K", JOURNAL OF PHYSICS-CONDENSED MATTER 24, 382202 (2012).
  • [6] Krzton-Maziopa A, Pomjakushina E, Pomjakushin V, Sheptyakov D, Chernyshov D, Svitlyk V, Conder K, "The synthesis, and crystal and magnetic structure of the iron selenide BaFe2Se3 with possible superconductivity at Tc=11K", JOURNAL OF PHYSICS-CONDENSED MATTER 23, 402201 (2011) .

Funding:

Contact:
Vladimir Pomjakushin
vladimir.pomjakushin@psi.ch
Denis Cheptiakov
denis.cheptiakov@psi.ch,