High-Frequency ac Susceptibility of Iron-Based Superconductors

Abstract

A microwave technique suitable for investigating the AC magnetic susceptibility of small samples in the GHz frequency range is presented. The method—which is based on the use of a coplanar waveguide resonator, within the resonator perturbation approach—allows one to obtain the absolute value of the complex susceptibility, from which the penetration depth and the superfluid density can be determined. We report on the characterization of several iron-based superconducting systems, belonging to the 11, 122, 1144, and 12,442 families. In particular, we show the effect of different kinds of doping for the 122 family, and the effect of proton irradiation in a 122 compound. Finally, the paradigmatic case of the magnetic superconductor EuP-122 is discussed, since it shows the emergence of both superconducting and ferromagnetic transitions, marked by clear features in both the real and imaginary parts of the AC susceptibility.

Complex AC Magnetic Susceptibility as a Tool for Exploring Nonlinear Magnetic Phenomena and Pinning Properties in Superconductors

“…At higher frequencies, the time window for flux-front penetration is far more limited, resulting in an indirect improvement in the shielding ability and overall shifting of the diamagnetic transition to higher temperatures. This approach is also suitable to probe the vortex response even at very low currents or to prompt reversible vortex oscillations (inside or between the pinning centers) at ultrahigh frequencies [47]. The fundamental AC loss peak 𝜒𝜒′′ 1 marks the total dissipation (both linear and nonlinear) at the point of full penetration of the applied field.…”

“…This approach is also suitable to probe the vortex response even at very low currents or to prompt reversible vortex oscillations (inside or between the pinning centers) at ultrahigh frequencies [47]. The fundamental AC loss peak χ 1 marks the total dissipation (both linear and nonlinear) at the point of full penetration of the applied field.…”

Unusually weak irradiation effects in anisotropic iron-based superconductor RbCa2Fe4As4F2

“…Measurements were carried out as a function of the temperature for the empty resonator and with the investigated samples coupled to it (i.e., placed on the central stripline, far from the edges). The presence of the sample causes shifts of the resonance frequency and of the quality factor that are due to the electromagnetic properties of the sample: from these shifts, after a self-consistent calibration procedure, the absolute value of the penetration depth and its temperature dependence can be assessed [24].…”

Theoretical Investigation of the Interplay of Superconductivity and Magnetism in Ba1−xKxFe2As2 Superconductor in a Two-Band Model by Using the Bogoliubov Transformation Formalism

“…For the 122 ironbased superconductors family, AFe 2 An 2 , where (A indicates alkaline earth metals such as Ba, Sr or Ca and Eu) and An is a pnictide (As, P), superconductivity can be achieved with the introduction of dopants [14]. There are several ways to introduce dopants [15]. These are (1) hole doping is achieved with substituting A for monovalent B + (B ¼ Cs, K, Na) atoms partially in the blocking layer, and this substitution should add an excess hole into the system, for example, Ba 1−x K x Fe 2 As 2 [16,17] (2) partially substitute Fe for transition metals (Co, Ni, Pd, Rh) into FeAs layers and yields electrons into the system.…”