We investigate the neutron damage expected in high-temperature superconducting tapes that will be employed in compact fusion reactors. Monte Carlo simulations yield the expected neutron spectrum and fluence at the magnet position, from which the primary knock-on atom energy distributions can be computed for each atomic species comprising the superconductor. This information is then employed to characterize the displacement cascades, in terms of size and morphology, through molecular dynamics simulations. The expected radiation environment is then compared with the neutron spectrum and fluences achievable at the facilities currently available for experimental investigation in order to highlight similarities and differences that could be relevant to the understanding of the radiation hardness of these materials in real fusion conditions. We find that the different neutron spectra result in different damage regimes, the irradiation temperature influences the number of generated defects, and the interaction of the neutrons with the superconductor results in a local increase in temperature. These observations suggest that further experimental investigations are needed in different regimes and that some neutron shielding will be necessary in compact fusion reactors.

We report on the anisotropy of the London penetration depth of ${\mathrm{Ca}\mathrm{K}\mathrm{Fe}}_4{\mathrm{As}}_4$, discussing how it relates to its electronic structure and how it modifies under introduction of disorder, both chemically induced (by $\mathrm{Ni}$ substitution) and irradiation induced (by 3.5-MeV protons). Indeed, ${\mathrm{Ca}\mathrm{K}\mathrm{Fe}}_4{\mathrm{As}}_4$ is particularly suitable for the study of fundamental superconducting properties due to its stoichiometric composition, exhibiting clean-limit behavior in the pristine samples and having a fairly high critical temperature, $T_c\approx 35$ K. The London penetration depth ${\lambda }_L$ is measured with a microwave-coplanar-resonator technique that allows us to deconvolve the anisotropic contributions ${\lambda }_{L,ab}$ and ${\lambda }_{L,c}$ and obtain the anisotropy parameter ${\gamma }_{\lambda }={\lambda }_{L,c}/{\lambda }_{L,ab}$. The ${\gamma }_{\lambda }(T)$ found for the undoped pristine sample is in good agreement with previous literature and is here compared to ab initio density-functional-theory and Eliashberg calculations. The dependence of ${\gamma }_{\lambda }(T)$ on both chemical and irradiation-induced disorder is discussed to highlight which method is more suitable to decrease the direction dependence of the electromagnetic properties while maintaining a high critical temperature. Lastly, the relevance of an intrinsic anisotropy such as ${\gamma }_{\lambda }$ on application-related anisotropic parameters (critical current, pinning) is discussed in light of the recent employment of ${\mathrm{Ca}\mathrm{K}\mathrm{Fe}}_4{\mathrm{As}}_4$ in the production of wires.

In this paper we explore the effects of 3.5 MeV proton irradiation on Fe(Se,Te) thin films grown on CaF ₂ . In particular, we carry out an experimental investigation with different irradiation fluences up to 7.30 · 10 ¹⁶ cm ⁻² and different proton implantation depths, in order to clarify whether and to what extent the critical current is enhanced or suppressed, what are the effects of irradiation on the critical temperature, resistivity, and critical magnetic fields, and finally what is the role played by the substrate in this context. We find that the effect of irradiation on superconducting properties is generally small compared to the case of other iron-based superconductors. The irradiation effect is more evident on the critical current density J _c , while it is minor on the transition temperature T _c , normal state resistivity ρ , and on the upper critical field H _c2 up to the highest fluences explored in this work. In more detail, our analysis shows that when protons implant in the substrate far from the superconducting film, the critical current can be enhanced up to 50% of the pristine value at 7 T and 12 K; meanwhile, there is no appreciable effect on critical temperature and critical fields together with a slight decrease in resistivity. On the contrary, when the implantation layer is closer to the film–substrate interface, both critical current and temperature show a decrease accompanied by an enhancement of the resistivity and lattice strain. This result evidences that possible modifications induced by irradiation in the substrate may affect the superconducting properties of the film via lattice strain. The robustness of the Fe(Se,Te) system to irradiation-induced damage makes it a promising compound for the fabrication of magnets in high-energy accelerators.

The microwave properties of polycrystalline $\mathrm{Mg}{\mathrm{B}}_2$ thin films prepared by the so-called in situ method are investigated. The characterization of the films at microwave frequencies was obtained by a coplanar resonator technique. The analysis of the experimental data results in the determination of penetration depth, surface impedance, and complex conductivity. The aim of this work is to set the experimental results in a consistent framework, involving the two-band model in the presence of impurity scattering. The energy gaps are calculated and the contribution of intra- and interband scattering is considered. From the comparison between the calculated gap values and the experimental data it turns out that the temperature dependence of the penetration depth can be accounted for by an effective mean energy gap, in agreement with the predictions of V. G. Kogan and N. V. Zhelezina [Phys. Rev. B 69, 132506 (2004)]. On the other hand, the temperature dependence of the real part of the microwave conductivity and of the surface resistance is accounted for by the single smaller gap, in agreement with the work of B. B. Jin, T. Dahm, A. I. Gubin, E.-M. Choi, H. J. Kim, S.-I. Lee, W. N. Kang, and N. Klein [Phys. Rev. Lett. 91, 127006 (2003)]. Since these findings rely on the same calculated gap structure, the required consistency is fulfilled.