Analysis of the London penetration depth in Ni-doped CaKFe4As4

Abstract

We report combined experimental and theoretical analysis of superconductivity in CaK(Fe1−xNix)4As4 (CaK1144) for x=0, 0.017, and 0.034. To obtain the superfluid density ρ=[1+ΔλL(T)/λL(0)]−2, the temperature dependence of the London penetration depth ΔλL(T) was measured by using a tunnel-diode resonator (TDR) and the results agreed with the microwave coplanar resonator (MWR) with the small differences accounted for by considering a three orders of magnitude higher frequency of MWR. The absolute value of λL(T≪Tc)≈λL(0) was measured by using MWR, λL(5K)≈170±20 nm, which agreed well with the NV centers in diamond optical magnetometry that gave λL(5K)≈196±12 nm, which agreed well with the NV centers in diamond optical magnetometry that gave λL(5K)≈196±12 nm. The experimental results are analyzed within the Eliashberg theory, showing that the superconductivity of CaK1144 is well described by the nodeless s± order parameter and that upon Ni doping the interband interaction increases.

Tuning the Intrinsic Anisotropy with Disorder in the CaKFe4As4 Superconductor

“…II B are valid. The qualitative behavior from the two experimental techniques is comparable, with higher values in the CPWR case that can be ascribed to higher sample quality or to the higher frequency of the experimental probe, which might reduce the scattering contribution [29]. As discussed above, due to the stoichiometric composition of undoped CaK1144, it has been possible to compute the bare low-temperature anisotropy parameter from first principles as well as the renormalized one with its temperature dependence on the hypothesis that the gaps are isotropic.…”

“…This experimental evidence should be interpreted in light of the fact that the substitution of Fe with Ni provides electron doping and its main consequence is to shift the Fermi level. However, these dopant ions also cause substantial scattering [29] (i.e., pair-breaking), just as does the introduction of defects through irradiation.…”

Effect of Controlled Artificial Disorder on the Magnetic Properties of EuFe2(As1−xPx)2 Ferromagnetic Superconductor

“…Another is a controlled disorder that provides an important insight into magnetism and superconductivity [36][37][38][39][40][41][42][43][44][45][46][47], in particular in IBS [24,44,45,[48][49][50][51][52][53]. Additional scattering can be induced by various means ranging from chemical substitution [54] to particle irradiation [55]. Controlled disorder has been used to study superconductors since the times of the famous Anderson theorem [37] and Abrikosov-Gor'kov theory [38], where the main attention was paid to the variation of the superconducting order parameter, hence the experimentally accessible transition temperature, T c [2,7,9,46,[56][57][58].…”

Microwave analysis of the interplay between magnetism and superconductivity in EuFe2(As1−xPx)2Ghigo1, Torsello2, Gozzelino3 et al. 2019Phys. Rev. ResearchSelf Cite241290View full textAdd to dashboardCite

“…Moreover, with respect to dc and low-frequency responses already reported, it enables the exploration of different dynamical regimes. In particular, we used a coplanar waveguide resonator (CPWR) technique, useful to detect the complex magnetic susceptibility and the penetration depth of small and thin crystals [14][15][16][17]. We clearly observed, below the superconducting transition, the onset of magnetic ordering followed, at lower temperatures, by two distinct transitions, detected as two clear peaks in the microwave signal.…”