Two-gap superconductivity with line nodes in CsCa2Fe4As4F2

Abstract

We report the results of a muon-spin rotation (μSR) experiment to determine the superconducting ground state of the iron-based superconductor CsCa 2 Fe 4 As 4 F 2 with T c ≈ 28.3 K. This compound is related to the fully gapped superconductor CaCsFe 4 As 4 , but here the Ca-containing spacer layer is replaced with one containing Ca 2 F 2. The temperature evolution of the penetration depth strongly suggests the presence of line nodes and is best modeled by a system consisting of both an sand a d-wave gap. We also find a potentially magnetic phase which appears below ≈10 K but does not appear to compete with the superconductivity. This compound contains the largest alkali atom in this family of superconductors, and our results yield a value for the in-plane penetration depth of λ ab (T = 0) = 244(3) nm.

Nodal multigap superconductivity in the anisotropic iron-based compound RbCa2Fe4As4F2

“…As already mentioned, a combination of multiple bands and unconventional gap symmetry in 12442 compounds was also suggested on the basis of the low-temperature linear feature in the superfluid density observed by µSR in K-and Cs-12442 polycrystals [6,7]. Fig.…”

“…This situation of almost vanishing electron pockets is close to what is found at the end of the doping series in 122 IBSs (KFe 2 As 2 and KFe 2 Se 2 at the opposite extrema) in which the typical s ± symmetry of the order parameter seems not to hold, with the emergence of nodal behavior [11][12][13][14] due to the proximity, in terms of energy, of the two instabilities. Features compatible with the presence of a d-wave gap on one of the bands, possibly coexisting with a fully gapped state on other bands, have in fact been observed by µSR superfluid density measurements (on K-and Cs-12442 polycrystals [6,7]) and low temperature specific heat (on K-12442 single crystals [8]). Recently, schemes that explain the coexistence of gapless and gapped features were proposed and shown to be suitable candidates for multiband systems [15,16].…”

“…The 12442 structure is therefore less trivial and clearly more anisotropic than those of the rest of the IBSs and, in this respect, more resembling that of cuprates such as Bi 2 Sr 2 CaCu 2 O 8+δ and YBa 2 Cu 3 O 7−δ (YBCO) [3][4][5]. This intuitive and intriguing similarity acquires even more charm in light of the several (although not uncontroversial) experimental observations suggesting the presence of a d-wave-like gap opening on the complex, multiband, Fermi surface [6][7][8]. It seems therefore that the 12442 family might be a host of several coexisting features that are key for the understanding of the physics of unconventional superconductors.…”

“…As for the gap symmetry, a guess has to be made before trying to fit the spectra. Since specific-heat [38] and µSR [6,7] measurements in similar compounds were interpreted as suggesting the coexistence of a nodal gap together with a nodeless one, we initially tried to fit the spectra by using one s-wave gap and one nodal gap, for simplicity assumed to have a d-wave symmetry, reasonably residing on different Fermi surface sheets. We found that all curves for undoped and 5% Ni doped crystals, both in ab-plane and c-axis contacts, can be satisfactorily fitted by this s−d model, with a larger s−wave gap ∆ 1 and a smaller d−wave gap ∆ 2 , but the contribution of the s gap is barely visible and the gap values extracted from different contacts are rather scattered (for x = 0, ∆ 1 = 4.3 − 6.6 meV and ∆ 2 = 1.9 − 3.5 meV) and, within the experimental uncertainty, look to be directly proportional to T c (see Supplementary Fig.…”

Effects of Pair-Hopping Coupling on Properties of Multi-Band Iron-Based Superconductors

“…Recent experiments based on the muon spin rotation (µSR) measurements suggest a realization of this state in Ba 1−x K x Fe 2 As 2 , around x ∼ 0.7 [132]. Similar measurements suggest also a realization of s+d symmetry in CsCa 2 Fe 4 As 4 F 2 [133] and KCa 2 Fe 4 As 4 F [134].…”