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 12442 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.
Highlights
We report on a microwave technique based on the use of a coplanar waveguide resonator (CPWR) that is suitable for the study of very small iron-based superconductors (IBSs) single crystals, and we show how the absolute values of χ ac can be obtained for thin platelets (Section 2)
This is a consequence of the fact that the penetration depth connects, at Tc, to the skin depth δ( T ), which in this frequency range is lower than the half-thickness of the sample, resulting in shielding (χ0ac < 0) as well as dissipation (χ00ac > 0) in the normal state
An overview of results obtained using the CPWR method described in Section 2.2 is given
Summary
Since 2006, when iron-based superconductors (IBSs) were discovered [1], they raised great interest both from a fundamental point of view and for application-oriented research [2,3]. Throughout the years, new IBS “families” came into play, sharing some common intriguing features [4], such as their multiband nature [5], unconventional pairing [6,7], and nontrivial pairing symmetries [8], and for these reasons they are currently the object of an intense study effort by research groups throughout the world Within this framework, the investigation of their high-frequency (microwave) properties has proven to be extremely useful in the characterization of such novel materials, yielding crucial information about the mechanisms of superconductivity [9,10,11]. The latter case is paradigmatic, since it shows the emergence of both superconductivity and ferromagnetism, which can coexist in IBSs with magnetic rare-earth elements, as is clearly evinced by features in both the real and imaginary parts of AC susceptibility
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