We measure the flux flow resistivity in FeSe0.5Te0.5 epitaxial films using a microwave dual-frequency technique (16 GHz and 27 GHz), in the range 5 K- Tc, in static magnetic fields up to 1.2 T. By applying a temperature scaling procedure, we extract from flux flow measurements the temperature dependence of the orbital upper critical field, that shows features of multiband superconductors. The reduced orbital upper critical field is then fitted with a two-band model with strong intraband and weak interband coupling, as expected in 11 Fe-based systems. We derive the vortex viscosity and we estimate the bands-averaged vortex core quasi-particle (QP) reduced scattering time within the Bardeen-Stephen framework. Our data suggest that, for our epitaxial FeSe0.5Te0.5 films, the QP scattering rate values are at the upper edge for the dirty regime. Finally, tentative numerical values of the orbital upper critical field and coherence length are provided.

Intrinsic properties of Fe(Se,Te), such as the high value of the upper critical field as well as the weaker dependence of the critical current density (Jc) on the grain misalignment than in cuprate superconductors, make this compound a promising candidate for the fabrication of high-field superconducting magnets. On the other hand, irradiation was demonstrated to be a powerful tool for tuning the pinning properties in superconducting materials. In this paper, we investigate the effect of 230 MeV Au-ion irradiation on Jc and pinning force (Fp) of biaxially-oriented Fe(Se,Te) films grown on YSZ substrate buffered with a thin Zr-doped CeO2 epitaxial layer. This structure is interesting as it can be considered a precursor template for Fe(Se,Te) coated conductors. The irradiation produces correlated defects, which - on transmission electron microscope analysis - appear as slightly meandering tracks composed of dislocation chains placed parallel to the c-axis of the crystal lattice. The Jc measurements as a function of the applied magnetic field evidence an improvement at low temperatures, with a maximum modulated in both position and height by the irradiation fluence. In the same range of temperature, the development of the irradiation tracks strongly reduces the anisotropy of the critical current density by varying the applied field orientation. Likewise, the Fp analysis highlights that irradiation defects behave as twodimensional extended defects, which strengthen the point-pinning landscape active before irradiation. Conversely, approaching the transition temperature, the effectiveness of the irradiation-induced defects decreases and a widespread Jc and Fp worsening occurs.

In this work, we have grown ∼100nm thick pristine FeSe films by pulsed laser deposition. The films were structurally characterized with X-ray diffraction and their surface morphology checked through atomic force microscopy. Microwave measurements, performed with a dielectric loaded resonator tuned at the frequency of 8GHz, allowed the characterization of the samples surface resistance, in view of potential applications in microwave haloscopes for dark matter search. Here, we report the comparison of the microwave properties of FeSe with Fe(Se,Te) thin films, as the temperature is swept from 4K to 20K. By applying a constant static magnetic field of 12T, it was also possible to discern the magnetic field resilience of the two samples. FeSe showed a larger critical temperature drift as the field is applied and a small broadening, while the opposite appears in Fe(Se,Te). A preliminary analysis of vortex pinning shows margins for optimizing pinning in FeSe.

Iron-based superconducting (IBS) wires fabricated through the powder-in-tube (PIT) method are prospective candidate materials for high magnetic field applications. In order to meet the requirements of such applications, it is important to obtain practical conductors with high transport critical currents at a competitive cost. Among the several IBS families, the (Ba,K)Fe2As2 (Ba-122) phase has the best potential to become a cost-effective high-field superconductor. However, two aspects of the fabrication procedure present issues which still hinder the development towards the affirmation of such superconductor in the industrial applications field: on one side the powder purity, especially when developing a route for the fabrication of large amounts, and on the other side the application of techniques scalable to obtain long length conductors with the same transport properties of the short samples. We here focus on an innovative and scalable fabrication process we have developed to fabricate pure Ba1-xKxFe2As2 powders in batches up to the order of 25 g, a quantity which allows us to fabricate at least 20 meters of monfilamentary conductor.

In this article, we investigate the influence of powder granulometry on the superconducting properties of (Ba0.6K0.4)Fe2As2 tapes manufactured using the Powder-In-Tube (PIT) technique. Variations in particle size distribution, which we successfully modulated through planetary ball milling and meticulous powder synthesis process, can affect the packing of the powders inside the metallic sheath and the mechanical deformation process of the samples, impacting their transport properties, as known for other practical conductors. The comparison among PIT tapes realized with a consistent fabrication route made with powders with different granulometries is reported, highlighting the effects on the transport property performances.

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 CaF2 substrates, to introduce columnar defects and to enhance flux pinning capability and critical current density (Jc). The employed fluence was 2.9 x 1011 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 Jc 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 spontaneous Hall effect (SHE), a finite voltage occurring transversal to the electrical current in zero-magnetic field, has been observed in both conventional and unconventional superconductors, appearing as a peak near the superconducting transition temperature. The origin of SHE is strongly debated, with proposed explanations ranging from intrinsic and extrinsic mechanisms such as spontaneous symmetry breaking and time-reversal symmetry breaking (BTRS), Abrikosov vortex motion, or extrinsic factors like material inhomogeneities, such as non-uniform critical temperature (Tc) distributions or structural asymmetries. This work is an experimental study of the SHE in various superconducting materials. We focused on conventional, low-Tc, sharp transition Nb and unconventional, intermediate-Tc, smeared transition Fe(Se,Te). Our findings show distinct SHE peaks around the superconducting transition, with variations in height, sign and shape, indicating a possible common mechanism independent of the specific material. We propose that spatial inhomogeneities in the critical temperature, caused by local chemical composition variations, disorder, or other forms of electronic spatial inhomogeneities could explain the appearance of the SHE. This hypothesis is supported by comprehensive finite elements simulations of randomly distributed Tc's by varying Tc-distribution, spatial scale of disorder and amplitude of the superconducting transition. The comparison between experimental results and simulations suggests a unified origin for the SHE in different superconductors, whereas different phenomenology can be explained in terms of amplitude of the transition temperature with respect to Tc-distribution.

[This corrects the article DOI: 10.1016/j.isci.2024.111032.].

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.

Iron-based superconductors (IBSs) are promising for high-field applications due to their exceptional characteristics, like ultrahigh upper critical field and minimal electromagnetic anisotropy. Creating multifilamentary superconducting wires with elevated transport critical current density is essential for practical use. The Powder in Tube (PIT) technique is commonly used for this purpose, but achieving optimal results requires careful exploration of powder microstructural properties. This is particularly crucial for superconductors like (Ba,K)122, the IBS most promising from an applicative point of view, where factors such as reactivity, volatility, and toxicity of constituent elements affect phase formation. Potassium volatility often leads to nonstoichiometric conditions, introducing excess potassium in the formulation. This study focuses on the impact of potassium excess delta on the microstructural properties of the 'optimally doped' (Ba0.6K0.4+delta)Fe2As2 phase (0 <= delta <= 0.08). Using techniques like Scanning Electron Microscopy, x-ray diffraction, and temperature-dependent magnetization measurements, we demonstrate the ability to produce nearly pure powders of the superconducting phase with controlled grain size. Our findings are relevant for PIT wire fabrication, where grain size strongly affects mechanical deformation. Grain size also influences transport properties, as observed in previous studies, where reducing grain size enhanced current-carrying capability at high magnetic fields.

The design of iron-based coated conductors (IBS-CC) with a simplified architecture is possible thanks to the material properties that allow for milder requirements on the template crystalline quality. With respect to the state-of-the-art multilayered layout, it is possible to use a single buffer layer that remains necessary for protection and to promote the oriented growth of the superconducting film. In this work, Fe(Se,Te) films are grown via pulsed laser deposition (PLD) on commercial tapes using a single, chemically deposited, CeO2-based buffer layer, and interesting properties are obtained. In detail, the preparation and characterization of the buffer layer is presented, along with the detailed analysis of the Fe(Se,Te) current transport properties. The samples show superconducting transitions with T c 0 around 12 K and critical current densities of ∼0.1 MA cm -2 at 4.2 K at zero field. These results show that the design of a low-cost IBS-CC with a single chemical buffer layer is possible.

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 CeO2 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.

Discovery of iron-based superconductors paved the way to a competitor of high-temperature superconductors, easier to produce, better performing in high fields, and promising to be less expensive. Critical parameters are investigated by resistivity measurements as a function of temperature, field, and angle R(T,H,θ). This work presents a deep analysis of H-θ phase diagram of PLD-processed Fe(Se,Te) superconducting films, thus revealing material and pinning anisotropy at once. By selecting different thresholds along the R(T,H,θ) curves, all possible regimes emerge. Surprisingly, anisotropy arises moving from the upper critical field toward the irreversibility line: gradually a non-monotonous transition from 3D to 2D, and backward to 3D occurs. Although Fe(Se,Te) appears as a 3D superconductor, its anisotropic pinning landscape shows up similarities with an intrinsic layered superconductor and Fe(Se,Te) definitively mimics YBCO. We propose a general method to disentangle, in any other superconductor, material dimensionality and pinning anisotropy that are key constraints for applications.

The fabrication of a Fe-based coated conductor (CC) becomes possible when Fe(Se,Te) is grown as an epitaxial film on a metallic oriented substrate. Thanks to the material’s low structural anisotropy, less strict requirements on the template microstructure allow for the design of a simplified CC architecture with respect to the REBCO multi-layered layout. This design, though, still requires a buffer layer to promote the oriented growth of the superconducting film and avoid diffusion from the metallic template. In this work, Fe(Se,Te) films are grown on chemically-deposited, CeO 2 -based buffer layers via pulsed laser deposition, and excellent properties are obtained when a Fe(Se,Te) seed layer is used. Among all the employed characterization techniques, transmission electron microscopy proved essential to determine the actual effect of the seed layer on the final film properties. Also, systematic investigation of the full current transport properties J ( θ , H , T ) is carried out: Fe(Se,Te) samples are obtained with sharp superconducting transitions around 16 K and critical current densities exceeding 1 MA cm −2 at 4.2 K in self-field. The in-field and angular behavior of the sample are in line with data from the literature. These results are the demonstration of the feasibility of a Fe-based CC, with all the relative advantages concerning process simplification and cost reduction.

Among other Fe-based superconductors, Fe(Se,Te) is particularly interesting because of the low structural anisotropy, large upper critical fields, low field dependence of the critical current density and low toxicity. It can also be grown as an epitaxial film on a metallic oriented substrate, making the fabrication of a Fe-based coated conductor (CC) possible. Less strict requirements on the template microstructure allow for the design of a simplified design compared to REBCO CCs. This design requires a buffer layer to promote the oriented growth of the superconducting film and avoid diffusion from the metallic template. In this work, CeO 2 based buffer layers are prepared on single crystals via two chemical deposition techniques, metal organic decomposition (MOD) and polymer assisted deposition (PAD). With the design of a suitable thermal treatment, it is possible to obtain oriented buffers with large flat grains and low values of surface roughness. Fe(Se,Te) films are deposited on these templates via laser deposition, and excellent samples are obtained when a Fe(Se,Te) seed layer is used to favour chemical matching with the buffer: sharp superconducting transitions around 16 K and critical current densities exceeding 1 MA cm − 2 at 4.2 K in self-field are observed. These results are the demonstration of the feasibility of a Fe-based CC architecture, with all the relative advantages concerning process simplification and cost reduction.

Ion irradiation of superconductors allows both the establishment of their radiation hardness and the modification and optimization of their properties useful for applications. In this work, we investigate the effects of proton irradiation with different energies on Fe(Se,Te) thin films grown on CaF $_2$ and on buffered YSZ substrates. These systems allowed us to perform preliminary studies for the development of Fe(Se,Te) coated conductors. Critical temperature and critical current were measured for different levels of displacement damage, and with ion implantation happening at various depths inside the substrate, as evaluated by simulations with Monte Carlo codes. All measurements evidenced that these Fe(Se,Te) films are robust against proton induced damage, and that an increase of critical current can be achieved introducing pointlike defects in the superconductor. However, we find that damage induced in the substrate also plays a crucial role in modifying superconducting film properties through a variation of the strain exerted on the film. This strain degrades superconducting parameters and should be minimized.

We report on the anisotropy of the vortex motion surface impedance of a FeSe x Te 1-x thin film grown on a CaF 2 substrate. The dependence on the magnetic field intensity up to 1.2 T and direction, both parallel and perpendicular to the sample c-axis, was explored at fixed temperature at two distinct frequencies, ~ 16 GHz and ~ 27 GHz, by means of a bitonal dielectric resonator. The free flux flow resistivity ρ ff was obtained by exploiting standard models for the high frequency dynamics, whereas the angle dependence was studied in the framework of the well known and widely used Blatter-Geshkenbein-Larkin (BGL) scaling theory for anistropic superconductors. Excellent agreement with the scaling law prescription by the fluxon flux flow resistivity was obtained. From the scaling analysis, a low-field mass anisotropy ~ 1.8was obtained, well within the value ranges reported in literature. The angular dependence of the pinning constant suggests that pinning is dominated by random, isotropic point pins, consistently with critical current density measurements.

The role of a layered structure in superconducting pinning properties is still at a debate. The effects of the vortex shape, which can assume for example a staircase form, could influence the interplay with extrinsic pinning coming from the specific defects of the material, thus inducing an effective magnetic field dependence. To enlighten this role, we analysed the angular dependence of flux pinning energy U(H,θ) as a function of magnetic field in FeSe0.5Te0.5 thin film by considering the field components along the ab-plane of the crystal structure and the c-axis direction. U(H,θ) has been evaluated from magneto-resistivity measurements acquired at different orientations between the applied field up to 16 T and FeSe0.5Te0.5 thin films grown on a CaF2 substrate. We observed that the U(H,θ) shows an anisotropic trend as a function of both the intensity and the direction of the applied field. Such a behaviour can be correlated to the presence of extended defects elongated in the ab-planes, thus mimicking a layered superconductor, as we observed in the microstructure of the compound. The comparison of FeSe0.5Te0.5 with other superconducting materials provides a more general understanding on the flux pinning energy in layered superconductors.

The process of developing superconducting materials for large scale applications is mainly oriented to optimize flux pinning and the current carrying capability. A powerful approach to investigate pinning properties is to combine high resolution imaging with transport measurements as a function of the magnetic field orientation, supported by a pinning modelling. We carry out Transmission Electron Microscopy, Electron Energy Loss Spectroscopy and critical current measurements in fields up to 16 T varying the angle between the field and c-axis of Fe(Se,Te) epitaxial thin films deposited on CaF2 substrates. We find evidence of nanoscale domains with different Te:Se stoichiometry and/or rotated and tilted axes, as well as of lattice distortions and two-dimensional defects at the grain boundaries. These elongated domains are tens of nm in size along the in-plane axes. We establish a correlation between these observed microstructural features and the pinning properties, specifically strongly enhanced pinning for the magnetic field oriented in-plane and pinning emerging at higher fields for out-of-plane direction. These features can be accounted for within a model where pinning centers are local variations of the critical temperature and local variations of the mean free path, respectively. The identification of all these growth induced defects acting as effective pinning centers may provide useful information for the optimization of Fe(Se,Te) coated conductors.

The epitaxial growth of Zr-doped CeO2 (CZO) films by chemical solution deposition (CSD) on single crystal substrates such as Al2O3, SrTiO3 and Y2O3-stabilized ZrO2 (YSZ) and their use as buffer layers for Fe(Se,Te) film growth are reported. The growth of 30 nm thin CZO films is mostly dominated by dewetting issues, leading to highly incomplete substrate coverage due to the strong tendency of CZO grains to agglomerate. This effect is less severe on YSZ substrate and, in addition, can be fully controlled by setting the deposition temperature at 950 °C and operating under Ar–H2 reducing atmosphere. These conditions promote the growth of epitaxial and compact CZO films on YSZ, showing a flat surface morphology with root-mean-square roughness around 2.5 nm. Preliminary results on Fe(Se,Te) film deposited by pulsed laser deposition on CZO-buffered YSZ are reported. Although good epitaxial growth is achieved on CZO buffer layer, Fe(Se,Te) films exhibit poor superconducting properties. On the other hand, Fe(Se,Te) films show excellent properties when a seed layer is used, exhibiting a sharp superconducting transition at about 17 K and a critical current density exceeding 1 MA cm−2 at 4.2 K in self field. The main role of the seed layer, consisting of a non-superconducting Fe(Se,Te) film, is to favour the crystal and chemical matching between buffer layer and film, allowing for the growth of high-quality superconducting Fe(Se,Te) at deposition temperature as low as 200 °C. In perspective of applications, this result is a significant step towards process simplification and cost reduction of Fe(Se,Te)-based coated conductor technology development, being the first demonstration of superconducting Fe(Se,Te) films grown on CSD buffer layer.