Trends in pressure-induced layer-selective half-collapsed tetragonal phases in the iron-based superconductor family AeAFe4As4

Abstract

By performing pressure simulations within density functional theory for the family of iron-based superconductors AeAFe4As4 with Ae = Ca, Sr, Ba and A = K, Rb, Cs we predict in these systems the appearance of two consecutive half-collapsed tetragonal transitions at pressures Pc 1 and Pc 2 , which have a different character in terms of their effect on the electronic structure. We find that, similarly to previous studies for CaKFe4As4, spin-vortex magnetic fluctuations on the Fe sublattice play a key role for an accurate structure prediction in these materials at zero pressure. We identify clear trends of critical pressures and discuss the relevance of the collapsed phases in connection to magnetism and superconductivity. Finally, the intriguing cases of EuRbFe4As4 and EuCsFe4As4, where Eu magnetism coexists with superconductivity, are discussed as well in the context of halfcollapsed phases. PACS numbers:Introduction.-The so-called 122 Fe-based pnictides ( Fig. 1b) (AFe 2 As 2 , AeFe 2 As 2 , EuFe 2 As 2 ) with A alkali and Ae alkaline-earth cations crystallize at room temperature in a body-centered tetragonal ThCr 2 Si 2 structure (I4/mmm) 1,2 where As sites from the neighboring Fe-As blocks face each other across the A or Ae plane. The As-As interlayer distance in these systems can be then tuned by either mechanical or chemical pressure down to sufficiently small values allowing the formation of As-As p z bonds. This is accompanied by a structural phase transition to a collapsed tetragonal (cT) phase where the c/a ratio is significantly reduced due to a dramatic contraction of the c-lattice parameter and a slight expansion of the a-lattice parameter. This process is known to suppress superconductivity or/and long-range "stripe" magnetic order ( Fig. 1d) due to the crossover to a more three-dimensional structure and the loss of spin fluctuations and local Fe moments caused by a compression of Fe-As bonds. 3 In the 122 materials, the transition to a cT phase affects the whole structure leading to As-As p z bond formation across each cation spacer layer.In contrast, a half-collapsed tetragonal (hcT) phase transition was recently reported for the 1144 material CaKFe 4 As 4 4 ( Fig. 1c) where the periodic arrangement of Ca and K spacer layers produces two different kinds of As sites 5-7 and the tetragonal structure (P 4/mmm) shows a layer-selective collapse upon application of pressure. First, at 4 GPa the As-As p z bonding across the Ca layer induces a collapsed tetragonal transition with disappearance of superconductivity while a second collapsed transition across the K layer was predicted around ∼12 GPa. Furthermore, Ref. 4 showed that "hedgehog" (spin-vortex, Fig. 1e) magnetism had to be invoked in the pressure-dependent density functional theory (DFT) simulations in order to predict the observed structural transitions. This magnetic order has been recently measured upon electron-doping CaKFe 4 As 4 . 8 FIG. 1: (a) Cations of alkali (1+) and alkaline-earth (2+) elements, as well as divalent Eu, together with their...

Microscopic Modeling of Correlated Systems Under Pressure: Representative Examples

“…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 .…”

“…123) are superconducting and do not show any long‐range magnetic order of Fe, which preserves the tetragonal ( C 4 ) symmetry of the lattice. First ab initio DFT pressure simulations for these systems were performed using the “frozen” Fe spin‐vortex order (see Figure 4b), which simulates the effect of spin fluctuations . This kind of spin fluctuations were later on confirmed for CaKFe 4 As 4 by NMR measurements .…”

“…Electronic bands related to As‐derived molecular orbitals (a) before and (b) after the half‐collapse transition across the Ae = Ba cation layer, shown on the example of BaCsFe 4 As 4 . Reproduced with permission . Copyright 2018, American Physical Society.…”

“…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 . Since the localized 4 f Eu states are strongly correlated, it is necessary to include in the simulations the GGA+ U corrections for these states …”

“…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.…”