Indication of subdominant d -wave interaction in superconducting CaKFe4As4

Abstract

We report inelastic light scattering results on the stoichiometric and fully ordered superconductor CaKFe4As4 as a function of temperature and light polarization. In the energy range between 10 and 315 cm −1 (1.24 and 39.1 meV) we observe the particle-hole continuum above and below the superconducting transition temperature Tc and 7 of the 8 Raman active phonons. The main focus is placed on the analysis of the electronic excitations. Below Tc all three symmetries projected with in-plane polarizations display a redistribution of spectral weight characteristic for superconductivity. The energies of the pair-breaking peaks in A1g and B2g symmetry are in approximate agreement with the results from photoemission studies. In B1g symmetry the difference between normal and superconducting state is most pronounced, and the feature is shifted downwards with respect to those in A1g and B2g symmetry. The maximum peaking at 134 cm −1 (16.6 meV) has a substructure on the high-energy side. We interpret the peak at 134 cm −1 in terms of a collective Bardasis-Schrieffer (BS) mode and the substructure as a remainder of the pair-breaking feature on the electron bands. There is a very weak peak at 50 cm −1 (6.2 meV) which is tentatively assigned to another BS mode.

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.…”

“…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.…”

Growth, properties and magnetism of CaKFe4As4

“…Raman spectroscopy is yet another experimental probe of the superconducting gap magnitude in addition to phonon and other collective modes [4,177,178,179]. Raman measurements on CaKFe 4 As 4 crystals were made by Daniel Jost et al [174] and Weilu Zhang et al [173]. Both groups observe a redistribution of spectral weight in the spectra consistent with the superconducting gap.…”

Cavity superconductor-polaritons

“…Typically d-wave fluctuations are considered about an s-wave state, as we will consider here. These modes have long been sought experimentally but are difficult to detect because they do not directly couple to electromagnetism; their detection has only been recently reported through Raman spectroscopy in iron-based materials [20][21][22][23]. Both polariton branches are formed by significant hybridization between photons and the Bardasis-Schrieffer collective mode of the superconductor.…”