Comprehensive Eliashberg analysis of microwave conductivity and penetration depth of K-, Co-, and P-substituted BaFe2As2

Abstract

We report on the combined experimental and theoretical analysis of microwave-frequency electromagnetic response of BaFe2As2 single crystals with different substitutions: K in the site of Ba (hole doping), Co in the Fe site (electron doping) and P in the As site (isovalent substitution). Measurements by a coplanar resonator technique lead to the experimental determination of the penetration depth and microwave conductivity as a function of temperature. The whole set of data is analyzed within a self-consistent three-band s±-wave Eliashberg approach, able to account for all the main observed features in the different properties. Beside the validation of the model itself, the comparison between experiment and theory allows discussing the possible role of the Fe-As planes in defining the superconducting properties of these compounds, the relevance of coherence effects, and the presence of nodes in the superconducting order parameter.

Tuning the Intrinsic Anisotropy with Disorder in the CaKFe4As4 Superconductor

“…The Eliashberg theory has been proven very helpful in the interpretation of experimental data for IBSs due to the possibility of taking into account the spin-fluctuation pairing mechanism and the multiband nature of these materials [24,25]. The London penetration depth can easily be calculated from the solution of the imaginary-axis version of the Eliashberg equations.…”

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

“…In particular, it has been shown that magnetic moments in the FeAs layers of iron-based pnictides have a tendency to order antiferromagnetically and it has been theorized that AFM spin fluctuations can induce the s ± pairing at the origin of superconductivity in these compounds [1]. This picture, although not completely accepted, has been suggested for several compounds of the so-called 122 family, i.e., BaFe 2 As 2 with various substitutions, isovalent or inducing electron or hole doping [2][3][4]. Within this framework, systems also containing magnetic rare-earth-metal elements are of interest, since at low temperatures they develop additional magnetic ordering of local moments.…”

Analysis of the London penetration depth in Ni-doped CaKFe4As4

“…The complete characterization of the London penetration depth (absolute value and temperature dependence) can also be carried out by means of a microwave resonator (MWR) technique that has already been applied to other IBS crystals [20][21][22]28,29]. In this case the measurement system consists of an YBa 2 Cu 3 O 7−x coplanar waveguide resonator (with resonance frequency f 0 of about 8 GHz) to which the sample is coupled.…”

“…In the IBS it has been shown that electron-boson coupling is mainly provided by antiferromagnetic spin fluctuations [11], therefore we neglect the phononic contribution and we consider the shape of the spectral functions α 2 i j F sf ( ) discussed in detail in Refs. [20,28]. However, since the two-band model is an effective one, it is not possible to set to zero the intraband coupling, hence the resulting electron-boson coupling-constant matrix .…”

“…Measurements of λ L (0) as a function of doping reveal a peak deep in the superconducting state due to the effective mass enhancement approaching a quantum phase transition [15][16][17][18]. Theoretically, London penetration depth can be computed on quite general grounds using the Eliashberg theory and, reproducing experimental data, one can discuss intrinsic quantities, such as the gap values and the coupling matrix coefficients [19][20][21][22].…”