Marina Putti

Irradiating a superconductor with ions is a powerful tool to create a controlled distribution of defects in it, with a morphology depending on the energy and the mass of the chosen particle. In this study, high-energy Pb-ion irradiation (1.15 GeV) was performed on Fe(Se,Te) thin films grown on CaF ₂ substrates, to introduce columnar defects and to enhance flux pinning capability and critical current density ( J _c ). The employed fluence was 2.9 × 10 ¹¹ cm ⁻² , corresponding to a dose equivalent field of 6 T. X-ray diffraction and scanning transmission electron microscopy analyses confirmed an increase of the defect density after irradiation, and the formation of columnar tracks. Despite a slight reduction in the critical temperature (around 1 K), the irradiated samples showed an increase of J _c up to 40%, in magnetic fields close to the dose equivalent field. Irradiation also induces a kink in the irreversibility line, which is consistent with a transition from a low-field single-vortex pinning regime provided by irradiation columnar defects to a high-field collective pinning regime. Accordingly, the analysis of the pinning force evidenced a shift in the peak position after irradiation, which can be associated to the active role of the irradiation tracks. These results demonstrate the effectiveness of irradiation in optimizing the performance of iron-based superconducting films.

The efficacy of MgB₂-based cables for DC power transmission applications is well-established due to several advantageous properties such as the natural abundance and cost-effectiveness of precursor powders and the innate compatibility with liquid hydrogen, which has a boiling point of 20 K. Notably, MgB₂ has a critical temperature T c of 39 K. However, transitioning this technology to alternating current (AC) applications, like electric motors or AC power transmission, poses challenges primarily due to the material’s susceptibility to AC losses when subjected to alternating fields. Therefore, it is crucial to conduct a thorough investigation into the dissipative phenomena affecting MgB₂ wires under AC conditions to develop design strategies for optimizing these wires for such applications. A detailed study involving both numerical simulations and experimental analyses has been conducted to understand the complex electrodynamics of MgB₂ wires under sinusoidal time-varying external field conditions. Remarkably, a comprehensive numerical model has been developed, incorporating all dissipative contributions in a multifilamentary wire in a single numerical tool. This model also includes the ferromagnetic loss contributions due to the presence of ferromagnetic materials like Nickel and Monel in the stabilizing matrix. The monolithic nature of this model allows for a general treatment of AC losses in multifilamentary wires, accounting for the influence of ferromagnetic materials within the stabilizing matrix.

Layered van der Waals tellurides reveal topologically nontrivial properties that give rise to unconventional magneto-transport phenomena. Additionally, their semimetallic character with high mobility makes them promising candidates for large magneto-thermoelectric effects. Remarkable studies on the very large and unconventional Nernst effect in WTe 2 have been reported, raising questions about whether this property is shared across the entire family of van der Waals tellurides. In this study, systematic measurements of the Nernst effect in telluride van der Waals Weyl semimetals are presented. Large linear Nernst coefficients in WTe 2 and MoTe 2 are identified, and moderate Nernst coefficients with nonlinear behavior in magnetic fields are observed in W 0.65 Mo 0.35 Te 2 , TaIrTe 4 , and TaRhTe 4 . Within this sample set, a correlation between the dominant linear-in-magnetic-field component of the Nernst coefficient and mobility is established, aligning with the established Nernst scaling framework, although with a different scaling factor compared with existing literature. This enhancement might be caused by the shared favorable electronic band structure of this family of materials. Conversely, the nonlinear component of the Nernst effect in a magnetic field could not be correlated with mobility. This nonlinear term is almost absent in the binary compounds, suggesting a multiband origin and strong compensation between electron-like and hole-like carriers. This comprehensive study highlights the potential of van der Waals tellurides for thermoelectric conversion.

Iron-based superconductors are under study for their potential for high-field applications due to their excellent superconducting properties such as low structural anisotropy, large upper critical fields and low field dependence of the critical current density. Between them, Fe(Se,Te) is simple to be synthesized and can be fabricated as a coated conductor through laser ablation on simple metallic templates. In order to make all the steps simple and fast, we have applied the spark plasma sintering technique to synthesize bulk Fe(Se,Te) to obtain quite dense polycrystals in a very short time. The resulting polycrystals are very well connected and show excellent superconducting properties, with a critical temperature onset of about 16 K. In addition, when used as targets for pulsed laser ablation, good thin films are obtained with a critical current density above 10⁵ A cm⁻² up to 16 T.

Although Fe(Se,Te) film shows limited superconducting properties compared to REBCO in terms of critical temperature and critical current density, it represents a potential low-cost alternative to REBCO-based coated conductor applications at low temperature and high field condition. In fact, due to the low deposition temperature and absence of oxygen, materials constraints are less strict and Fe(Se,Te) coated conductor can be realized using a very simplified architecture. A further process simplification is the use of cube-textured substrate combined with a single buffer layer by chemical solution deposition (CSD), which may provide a cheap, vacuum-free route for oriented template fabrication. Zr-doped CeO ₂ /Ni-W templates were realized by CSD and successfully tested with epitaxial Fe(Se,Te) film growth by pulsed laser deposition showing good superconducting properties. Further, a detailed analysis before and after buffer layer deposition using optical, atomic, and scanning electron microscopy, electron backscattered diffraction and Raman and X-ray photoelectron spectroscopy was carried out to relate Zr-doped CeO ₂ film quality to Ni-W microstructure, morphology, and composition. Results show that 30-nm-thick Zr-doped CeO ₂ film is effective to prevent Ni diffusion. Further, buffer-layer quality is mainly dependent on the characteristics of individual Ni-W grains.

In the frame of the Next Generation Europe program, the EU program to boost after-COVID recovery, the Italian Minister of University and Research has funded a project called Innovative Research Infrastructure for applied Superconductivity (IRIS). New laboratories will be built or upgraded in six poles: Milan (hub of the infrastructure), Genoa, Frascati, Naples, Salerno, and Lecce, to carry out basic research on magnetism and superconducting materials, test of wires, tapes, and large current cables, superconducting magnets construction with advanced instrumentation, power tests of magnets, and a special facility for high current-high voltage superconducting lines. The program will be executed over three years and then will operate for at least 10 years. It includes two first demonstrators: one HTS magnet to be operated at 10–20 K and a superconducting line of 1 GW (40 kA–25 kV) about 140 m long. The demonstrators anticipate the main scope of the investment in the IRIS infrastructure: to support the use of superconductivity for improving sustainability by decreasing the energy consumption without compromising performance. This article describes the global IRIS project.

Hydrogen (H) plays a key role in the near-to-room temperature superconductivity of hydrides at megabar pressures. This suggests that H doping could have similar effects on the electronic and phononic spectra of materials at ambient pressure as well. Here, we demonstrate the non-volatile control of the electronic ground state of titanium diselenide (1T-TiSe₂) via ionic liquid gating-driven H intercalation. This protonation induces a superconducting phase, observed together with a charge-density wave through most of the phase diagram, with nearly doping-independent transition temperatures. The H-induced superconducting phase is possibly gapless-like and multi-band in nature, in contrast with those induced in TiSe₂ via copper, lithium, and electrostatic doping. This unique behavior is supported by ab initio calculations showing that high concentrations of H dopants induce a full reconstruction of the bandstructure, although with little coupling between electrons and high-frequency H phonons. Our findings provide a promising approach for engineering the ground state of transition metal dichalcogenides and other layered materials via gate-controlled protonation.

Copper-based chalcogenides have emerged as promising thermoelectric materials due to their high thermoelectric performance, tunable transport properties, earth abundance and low toxicity. We have presented an overview of experimental results and first-principal calculations investigating the thermoelectric properties of various polymorphs of Cu₂SnS₃ (CTS), Cu₂ZnSnS₄ (CZTS), and Cu₂ZnSnSe₄ (CZTSe) synthesized by high-energy reactive mechanical alloying (ball milling). Of particular interest are the disordered polymorphs of these materials, which exhibit phonon-glass–electron-crystal behavior—a decoupling of electron and phonon transport properties. The interplay of cationic disorder and nanostructuring leads to ultra-low thermal conductivities while enhancing electronic transport. These beneficial transport properties are the consequence of a plethora of features, including trap states, anharmonicity, rattling, and conductive surface states, both topologically trivial and non-trivial. Based on experimental results and computational methods, this report aims to elucidate the details of the electronic and lattice transport properties, thereby confirming that the higher thermoelectric (TE) performance of disordered polymorphs is essentially due to their complex crystallographic structures. In addition, we have presented synchrotron X-ray diffraction (SR-XRD) measurements and ab initio molecular dynamics (AIMD) simulations of the root-mean-square displacement (RMSD) in these materials, confirming anharmonicity and bond inhomogeneity for disordered polymorphs.