Cesium vacancy ordering in phase-separatedCsxFe2−ySe2

Abstract

By simultaneously displaying magnetism and superconductivity in a single phase, the iron based superconductors provide a model system for the study of magnetism's role in superconductivity.The class of intercalated iron selenide superconductors is unique amongst these in having the additional property of phase separation and coexistence of two distinct phases -one majority phase with iron vacancy ordering and strong antiferromagnetism and the other a poorly understood minority microscopic phase with a contested structure. Adding to the intrigue, the majority phase has never been found to show superconductivity on its own while the minority phase has never been successfully synthesized separate from the majority phase. In order to better understand this minority phase, a series of high quality Cs x Fe 2-y Se 2 single crystals with (0.8 ≤ x ≤ 1; 0 ≤ y ≤ 0.3) were grown and studied. Neutron and x-ray powder diffraction performed on ground crystals show that the average I4/mmm structure of the minority phase is distinctly different from the high temperature I4/mmm parent structure. Moreover, single crystal diffraction reveals the presence of previously unobserved discrete superlattice reflections that remove the degeneracy of the Cs sites in both the majority and minority phases and reduce their 2 structural symmetries from body-centered to primitive. Group theoretical analysis in conjunction with structural modeling shows that the observed superlattice reflections originate from threedimensional Cs vacancy ordering. This model predicts a 25% vacancy of the Cs site in the minority phase which is consistent with the site's refined occupancy. Magnetization measurements performed in tandem with neutron single crystal diffraction provide evidence that the minority phase is the host of superconductivity. Our results also reveal a superconducting dome in which the superconducting transition temperature varies as a function of the nominal valence of iron.

Coupling of structure to magnetic and superconducting orders in quasi-one-dimensional K2Cr3As3

“…This is in accord with reports on Rb 2 Cr 3 As 3 and Cs 2 Cr 3 As 3 as well as pressure studies both of which found the a-axis more sensitive to external or chemical pressure 4,5,30,31 . These values of α are similar to the relatively high values reported for paramagnetic (PM) states of the 11 and various 122 members of the FBS family where strong spin-fluctuations are thought to contribute to the thermal expansion [32][33][34][35][36][37][38][39] . As we will show similar spin-fluctuations exist in K 2 Cr 3 As 3 and may be responsible for the large α values.…”

Detailed magnetic and structural analysis mapping a robust magneticC4dome inSr1−xNaxFe2Taddei1, Allred2, Bugaris3 et al. 2016Phys. Rev. BSelf Cite419501View full textAdd to dashboardCite

“…Recently, similar behavior was observed in the related intercalated iron selenide '122' family of superconductors (A x Fe 1−y Se 2 with A = Na, K, Rb, or Cs) where it was suggested that the strength and ordering temperature of the magnetic phase was dependent on the size of the intercalating ion and consequently the spacing between the tetrahedral Fe 2 Se 2 layers. 29 For the hole-doped iron arsenide systems being considered here a similar dependence is seen where T N decreases with increasing ionic radius (r A ) as monitored by the a lattice parameter, with T N = 205, 140K and a = 3.9243(1), 3.9625(1)Å for A = Sr and Ba respectively. sition in the iron selenides is not strongly coupled to a structural transition, the strong magneto-elastic coupling in the hole-doped 122 iron arsenides, where magnetism is the primary order parameter, suggests it is likely that the smaller lattice of the Sr system allows for larger magnetic interactions between neighboring iron sites and so enhances the behavior of the structural and magnetic phase transitions.…”

Intertwined Magnetic and Nematic Orders in Semiconducting KFe0.8Ag1.2Te2

“…The onset of nematic order results in the differentiation of lattice spacings along the Fe-Fe bond directions, characterized by an orthorhombicity [δ = (a−b)/(a+b)] of a few tenths of a percent. In the magnetically ordered state, spins are antiferromagnetically aligned along the longer axis and ferromagnetically aligned along the shorter axis [5,6].Although a multitude of different magnetic orders have been uncovered in related materials [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23], observations of intertwined stripe-type magnetic order and nematic order are so far limited to iron pnictides such as LaFeAsO and BaFe 2 As 2 [2, 5, 6]. Upon doping, these intertwined orders are suppressed, giving way to superconductivity near the corresponding quantum critical points [24], evidencing their intimate roles in iron-based superconductivity.…”