Superconducting and magnetic phase diagram of RbEuFe4As4 and CsEuFe4As4 at high pressure

Abstract

The recently discovered (Rb,Cs)EuFe 4 As 4 compounds exhibit an unusual combination of superconductivity (T c ∼ 35 K) and ferromagnetism (T m ∼ 15 K). We have performed a series of x-ray diffraction, ac magnetic susceptibility, dc magnetization, and electrical resistivity measurements on both RbEuFe 4 As 4 and CsEuFe 4 As 4 to pressures as high as ∼ 30 GPa. We find that the superconductivity onset is suppressed monotonically by pressure while the magnetic transition is enhanced at initial rates of dT m /dP ∼ 1.7 K/GPa and 1.5 K/GPa for RbEuFe 4 As 4 and CsEuFe 4 As 4 respectively. Near 7 GPa, T c onset and T m become comparable. At higher pressures, signatures of bulk superconductivity gradually disappear. Room temperature x-ray diffraction measurements suggest the onset of a transition from tetragonal (T) to a half collapsed-tetragonal (hcT) phase at ∼ 10 GPa (RbEuFe 4 As 4) and ∼ 12 GPa (CsEuFe 4 As 4). The ability to tune T c and T m into coincidence with relatively modest pressures highlights (Rb,Cs)EuFe 4 As 4 compounds as ideal systems to study the interplay of superconductivity and ferromagnetism.

Microscopic Modeling of Correlated Systems Under Pressure: Representative Examples

“…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.…”

“…Further below, we discuss the results of pressure simulations and analyze the electronic structure of CaKFe 4 As 4 .In the next step, we review the general trends in the whole 1144 family with a focus on the hcT transitions . In addition, we discuss the magnetic behavior of Eu‐based 1144 systems that are attracting increasing attention due to the coexistence and complex interplay of ferromagnetism and superconductivity …”

“…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 .…”

“…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.…”