Superconductivity in Fe-Based Compound EuAFe₄As₄ (A = Rb and Cs)

Microscopic Modeling of Correlated Systems Under Pressure: Representative Examples

“…This kind of spin fluctuations were later on confirmed for CaKFe 4 As 4 by NMR measurements . For the case of superconducting EuRbFe 4 As 4 and EuCsFe 4 As 4 , experimental signatures of long‐range Eu magnetic order are found below 15 K at zero pressure . Similarly to EuFe 2 As 2 , Eu order in 1144 systems is expected to be sensitive to pressure, as suggested by a recent enthalpy analysis and high‐pressure measurements that reveal the persistence of Eu order and a significant raise of ordering temperature up to 40–45 K around 20 GPa .…”

“…Very recently a new AeA Fe 4 As 4 family (1144) of FeBSC (space group , No. 123) with mixed alkaline‐earth ( Ae ) and alkali ( A ) spacer layers has been synthesized and a few of its members have been studied in the context of superconductivity and structural transitions under pressure . Compared to the 122‐type systems, these new 1144‐type materials show a perfect periodic arrangement of two different spacer layers in an alternating order, e.g., Ca and K in CaKFe 4 As 4 , as shown in Figure b.…”

“…Hydrostatic pressure influences the degree of charge transfer between the donor TTF and acceptor CA molecules and modifies the ferroelectric properties of the material. ii) The Fe‐based superconductor AeA Fe 4 As 4 (1144) with an alternating order of the alkaline‐earth ( Ae ) and alkali ( A ) spacer layers with a ThCr 2 Si 2 structure which undergoes two half‐collapsed tetragonal transitions under pressure, as predicted by some of the authors of this review and shows unusual magnetic states upon doping . iii) The frustrated triangular‐lattice antiferromagnet Cs 2 CuCl 4 for which we provide an overview of the sensibility of the exchange coupling constants to structural details and the predicted changes in coupling constants as a function of pressure .…”

“…Also inorganic materials can show extreme sensitivity to pressure due to formation and breaking of certain covalent bonds in their crystal structures upon pressurizing. Some examples are systems with layered ThCr 2 Si 2 (so‐called “122”) structure where the formation of a XX covalent bond (X = Si, As, Se, S, P) across the cation spacer layer at a critical pressure is accompanied by a drastic volume collapse, as is the case for the Fe‐based Ae Fe 2 As 2 and the recently discovered AeA Fe 4 As 4 families of superconductors (both structures are shown in Figure 3 where Ae and A stand for alkaline‐earth and alkali elements) where the collapse transition preserves the tetragonal lattice symmetry. A further example of pressure‐sensitive inorganic materials are layered frustrated magnets such as triangular‐lattice‐based Mott insulators Cs 2 CuCl 4 , Cs 2 CuBr 4 , and honeycomb‐lattice‐based spin‐orbit‐coupled Mott insulators α‐RuCl 3 and A 2 IrO 3 , with A = Li, Na.…”

Spin Resonance in the New-Structure-Type Iron-Based Superconductor CaKFe4As4

“…5,6) Recently, new-structure-type iron-based superconducting materials AeAFe 4 As 4 (where Ae = Ca, Sr, or Eu and A = K, Rb, or Cs) were reported. [7][8][9] The AeAFe 4 As 4 family exhibits T c = 32-37 K, which is relatively high compared with other stoichiometric iron-based superconductors. [10][11][12][13][14] As Ae and A layers stack alternatively between Fe 2 As 2 layers owing to the large difference between their ionic radii, AeAFe 4 As 4 belongs to the P4/mmm space group, which is different from I4/mmm in hole-doped (Ba 1−x K x )Fe 2 As 2 systems.…”