Anisotropy induced vortex lattice rearrangement in CaKFe4As4

Abstract

The magnetic penetration depth anisotropy γ λ = λc/λ ab (λ ab and λc are the in-plane and the out-of-plane components of the magnetic penetration depth) in a CaKFe4As4 single crystal sample (the critical temperature Tc ≃ 35 K) was studied by means of muon-spin rotation (µSR). γ λ is almost temperature independent for T 20 K (γ λ ≃ 1.9) and it reaches ≃ 3.0 by approaching Tc. The change of γ λ induces the corresponding rearrangement of the flux line lattice (FLL), which is clearly detected via enhanced distortions of the FLL µSR response. Comparison of γ λ with the anisotropy of the upper critical field (γH c2 ) studied in Phys. Rev B 94, 064501 (2016), reveals that γ λ is systematically higher than γH c2 at low-temperatures and approaches γH c2 for T → Tc. The anisotropic properties of λ are explained by the multi-gap nature of superconductivity in CaKFe4As4 and are caused by anisotropic contributions of various bands to the in-plane and the out-of-plane components of the superfluid density.

Tuning the Intrinsic Anisotropy with Disorder in the CaKFe4As4 Superconductor

“…(31) and (32) in Ref. [4], that both γ λ and γ H increase with increasing temperature and that the variation is stronger for H c2 than for λ L , as experimentally observed [7] [γ λ (0)/γ λ (T c ) 0.87 and γ H (0)/γ H (T c ) 0.8]. As a reference, we add to Fig.…”

“…The experimental γ λ (T) for the pristine undoped CaK1144 case is shown in Fig. 4, where it is also compared to literature data from muon spin rotation (μSR) experiments [7] and to calculated values. Experimental data are shown up to the reduced temperature at which all the approximations discussed in Secs.…”

“…If γ λ increases with increasing temperature, γ H should decrease (as in MgB 2 [3]) or vice versa (as in the 122 family of IBSs [5,6]). The recent observation of both anisotropies increasing with increasing temperature in the case of CaKFe 4 As 4 (CaK1144) [7] has challenged this view and stimulated further theoretical and experimental investigations.…”

Analysis of the London penetration depth in Ni-doped CaKFe4As4

“…III C in light of the differences between the TDR and MWR techniques. The anisotropy parameter γ λ , shown in the inset, is found to be comparable to that measured with μSR [30]. Theoretically, substantial variation of the anisotropies of the characteristic lengths with temperature is consistent with multigap superconductivity and, as recently shown, can both increase or decrease with temperature depending on the order parameter symmetry and electronic structure [31].…”

“…The deviation at high temperature is due to the fact that close to T c the deconvolution procedure of the MWR measurement loses its validity, because the assumption that λ L,ab c falls for the thinnest sample. The other deviation between the two measurements happen below T /T c = 0.3, where the λ L,ab MWR values dip lower, a feature that nicely corresponds to the increase of ρ s observed by Khasanov et al with the μSR technique [30]. This difference could be explained by looking at the probing frequencies of the two techniques: we notice that at low temperature the characteristic time for pair-breaking scattering, which can be estimated within a two-fluid model as done in [29], becomes comparable to the characteristic time of the microwave probe (∼125 ps), whereas the characteristic time of TDR is two orders of magnitude larger.…”