Iron-vacancy superstructure and possible room-temperature antiferromagnetic order in superconducting CsyFe2−xSe

Abstract

Neutron and x-ray powder and single crystal synchrotron diffraction of CsyFe2−xSe2 show the presence of superstructure reflections with propagation vector k=[ 2 5 , 1 5 , 1] with respect to the average crystal structure I4/mmm (a = 4, c = 15Å). The propagation vector star corresponds to the 5 times bigger unit cell given by transformation A=2a+b, B= -a+2b, C= c. A solution for the atomic structure is found in the space groups P 42/n and I4/m with an ordered pattern of iron vacancies corresponding to the iron deficiency x = 0.29 and Cs stoichiometry y = 0.83. The superstructure satellites are more pronounced in the neutron diffraction patterns suggesting that they can have some magnetic contribution. We have sorted out possible symmetry adapted magnetic configurations and found that the presence of AFM ordering with the ordered magnetic moment of Fe with 2µB does not contradict to the experimental data. However, the solutions space is highly degenerate and we cannot choose a specific solution. Instead we propose possible magnetic configurations with the Fe magnetic moments in (ab)-plane or along c-axis. The superstructure is destroyed above Ts 500 K by a first-order-like transition.PACS numbers: 75.50. Ee, 75.25.-j, 61.05.C-, 74.90.+n arXiv:1102

Room temperature antiferromagnetic order in superconducting XyFe2 −xSe2(X = Rb, K): a neutron powder diffraction study

“…Similar to the analysis of Cs y Fe 2−x Se 2 we have sorted out all irreps (Table 3 of Ref. [8]) and found that the best fit of our data is achieved for the τ 2 and τ 5 /τ 7 irreps for the basis vectors parallel to c-axis (similar as we have found before for Cs y Fe 2−x Se 2 ). The configuration for τ 7 is equivalent to τ 5 after π/2 rotation around c-axis.…”

“…For the crystal structure we use the vacancy ordered superstructure model I4/m stated in Ref. [8], for the magnetic structure analysis we use symmetry adapted basis functions of eight irreducible representations (irreps) of I4/m space group and propagation vector k = 0 [8]. The description of the structure model and the magnetic and structure parameters and the details of the refinements are summarized in Table 1.…”

“…The description of the structure model and the magnetic and structure parameters and the details of the refinements are summarized in Table 1. Interestingly that in Cs y Fe 2−x Se 2 the lattice constants a and c have the ratio (c/a) 2 3 resulting in a complete overlap of the most important Bragg peaks with the magnetic and nuclear contribution and disentangling the magnetic and crystal structures were possible only using both x-ray and neutron data [8]. For X y Fe 2−x Se 2 (X=K, Rb) this is not the case as illustrated in Fig.…”

“…Muon-spin rotation/relaxation (µSR) experiments evidence that the superconducting state observed in Cs y Fe 2−x Se 2 below 28.5(2) K is microscopically coexisting with a magnetic phase with a transition temperature at T m = 478.5(3) K [6]. Recently the AFM order was reported in superconducting K 0.8 Fe 1.6 Se 2 with T N = 560 K with the iron magnetic moment 3.31 µ B [7] and Cs y Fe 2−x Se 2 with the magnetic moment about 2µ B per iron site [8]. In the latter paper several possible symmetry adapted magnetic configurations have been proposed, including the one with non-constant moment configuration.…”

Vacancy-induced nanoscale phase separation in KxFe2−ySe2Lazarević1, Abeykoon2, Stephens3 et al. 2012Phys. Rev. B487620View full textAdd to dashboardCite

“…When Raman scattering is measured at the (001) plane of the K x Fe 2−y Se 2 sample, two Raman-active phonons (A 1g +B 1g ) of the I4/mmm phase and 18 Raman-active phonons (9A g +9B g ) of the I4/m phase, can be observed. Here, one must have in mind that the x' and y' axes of the I4/m phase are rotated by an angle α (-α for the twin domain), 18 to the corresponding axes (x and y) of the I4/mmm phase, see Fig. 2.…”