Resonant microwave techniques are powerful tools to investigate the properties of superconductors (SC), in particular to determine the penetration depth, superfluid density, quasiparticle conductivity, and in general the complex impedance. Our approach to the issue is the use of coplanar waveguide resonators (CPWR), in two different configurations (all details here).
Briefly: in one case we fabricate a CPWR from a thin film of the SC under study, and by analyzing the resonance curves as a function of temperature with a proper parametric model we extract the physical parameters. We used this technique to characterize the microwave linear and nonlinear response of YBa2Cu3O7-x and MgB2 thin films.
On the other hand, the microwave properties of small single crystals can be deduced by coupling them to an already characterized CPWR and by analyzing the changes they cause to the CPWR resonance frequency and quality factor, in a resonator-perturbation approach. From these changes, the complex permittivity and complex permeability of the sample (and in turn its complex ac susceptibility) can be determined. We used this technique to extensively study several families of the iron-based superconductors (IBS), mainly the Ba-122 family (BaFe2As2-based, with different kinds of chemical doping to induce superconductivity), but also 11 (FeSe1-xTex), 1144 (CaKFe4As4), 12442 (RbCa2Fe4As4F2), and the Eu-122 ferromagnetic superconductor.
This contactless technique is also reliable to study the effects of introducing artificial defects into the system - e.g. through ion irradiation - by measuring the same crystal before and after treatment, to avoid sample to sample variability. This allowed us to investigate the fundamental properties of IBSs by studying their dependence on structural disorder, e.g. in the case of their multigap structure, the symmetry of the order parameter, and the anisotropy of the penetration depth.
D. Torsello, K. Cho, K. R. Joshi, S. Ghimire, G. A. Ummarino, N. M. Nusran,
M. A. Tanatar, W. R. Meier, M. Xu, S. L. Bud’ko, P. C. Canfield, G. Ghigo, and
R. Prozorov - Analysis of the London penetration depth in Ni-doped CaKFe4As4 - Phys. Rev. B 100, 094513 (2019)
Gianluca GHIGO 
Daniele TORSELLO 
