[en] CaKFe₄As₄ (1144) is a unique stoichiometric iron-based superconductor which harbors high upper critical fields and large critical current densities. In this work, we describe a study to optimize the synthesis conditions of stoichiometric polycrystalline samples of CaKFe₄As₄ and assess their structural, magnetic and transport properties. The samples were prepared over a wide temperature range (900 °C–1100 °C) and the pure phase formation is centered around 955 °C. Outside this temperature region, impurity phases of KFe₂As₂ and CaFe₂As₂ can also form. Magnetic susceptibility and resistivity measurements establish that the critical temperature reaches ∼34 K for the optimum synthesis conditions and the critical current reaches 2 × 10⁴ A cm⁻². The post-annealing process demonstrates the stability of the 1144 phase up to 500 °C, however, under higher temperature annealing, phase degradation occurs. Our study indicates that the formation of phase-pure 1144 occurs over a much narrower window and its highly prone to multi-phase formation as compared with the 122 family. As a result, the superconducting properties are enhanced for the pure 1144 phase but they are likely to be affected by the inter- and intra-granular behavior originating from the microstructural nature of polycrystalline CaKFe₄As₄, similar to other iron-based superconductors. Based on our study, we construct the phase diagram for polycrystalline 1144 and compared it with that reported for 1144 single crystal. (paper)

[en] Single crystalline samples with starting compositions Fe_xTe_0.60Se_0.40 (x = 0.970, 0.985, 1, 1.015 and 1.030) have been grown using the self-flux method to study the effect of Fe variation (x) on the structural and superconducting properties of the grown single crystalline samples. All the grown samples are found superconducting and among all, the FeTe_0.60Se_0.40 (x = 1) sample shows the highest superconducting transition temperature (T_C). For the samples with x other than 1, a decrease in the superconducting properties, such as T_C, activation energy (U₀) and upper critical field (H_C2(0)), has been observed. The thermal activation energy (TAE) of the flux flow has been calculated using the conventional method and modified thermally activated flux flow (TAFF) model. The values of U₀(H) show transition from the single vortex pinning regime to the collective vortex pinning regime at 2T. Also, the presence of planer defects has been estimated via both models. Furthermore, the modified TAFF model suggests three dimentional (3D) behaviour for all the grown single crystals. The vortex phase diagrams have been established by analysing the magnetoresistance data. The analysis reveals the transitions from unpinned to pinned vortex liquid region and also from the pinned vortex liquid state to vortex glass state. In the vortex phase diagram, below the critical temperature (T*), the critical exponent (s) is in a good agreement with the ‘q’ values for the 3D behaviour of vortex states for all the grown samples. (paper)

[en] Among the Iron Based Superconductors, the so-called 1144 family attracts significant interest due to its high critical fields and critical current densities and to the stoichiometric nature of the compounds, disentangling the superconducting properties from the compositional homogeneity. Their practical application is partly hindered by the severe and strict synthesis conditions, complicated by high temperature (T > 900 °C) treatments of volatile and toxic elements. In this work, a milder synthetic approach to produce 1144 materials is proposed. A simple one-step High Energy Ball Milling treatment of the pure elements is coupled to a low temperature (i.e. 600 °C) thermal treatment to produce the superconducting Ca/K-1144 material, characterized by a good degree of homogeneity and critical temperatures higher than 30 K. The results here reported demonstrate the previously excluded feasibility of a simple and easily scalable lower temperature synthesis route for a Ca/K-1144 compound. We suggest that the intimate mixing and dispersion of the starting elements promoted by the mechanochemical treatment constitutes a key factor for the successful lowering of the synthesis temperature. (paper)

[en] Submicron sized strip lines of NdFeAs(O,F) were fabricated and the transport properties were studied. By using laser lithography and dry etching, strip lines with a width as low as 0.84 μm were fabricated. The superconducting critical temperature was not seriously affected by the fabrication process, but a significant decrease of the critical current density (J _c) was observed for strip lines narrower than 2 μm when the sample was etched in Ar plasma. By employing a plasma free method on the other hand, the deterioration of J _c was largely prevented, and the yield of obtaining strip lines with a J _c higher than 5 MA cm⁻² rose considerably. (paper)

[en] Iron-chalcogenide FeSe_xTe_1−x (Fe(Se,Te)) superconductors have attracted a great deal of interest in both fundamental physics and potential applications. While Majorana zero-modes hosted by the Fe(Se,Te) superconductors hold great promise for topological quantum computing, low-anisotropy and very high upper critical fields make Fe(Se,Te) an excellent candidate for high-magnetic-field and energy applications. Recently, we demonstrated a route for creating structural defects by ion irradiation in Fe(Se,Te) films, leading to strong flux pinning. Ion irradiation into Fe(Se,Te) films significantly improved the critical current density J _c over a wide field range and for all orientations with respect to magnetic field. In particular, a robust enhancement of critical temperature T _c and J _c was realized simultaneously in the Fe(Se,Te) film irradiated with protons. Here, we provide a review of the ion-irradiated Fe(Se,Te) films, including the film growth and characteristics before and after irradiation, design of the irradiation process for different ion energies and species with irradiation simulations and experimental setups that are scalable for long-length Fe(Se,Te)-coated conductors. (paper)

[en] Extensive critical current density $J_{\text{c}}$ measurements are reported as a function of magnetic field $B$, temperature $T$, angle $\mathrm{\theta <!--\; ??\; -->}$ between the applied field and the surface of the tape, and strain $\mathrm{\epsilon <!--\; ??\; -->}$, on a REBCO coated conductor. The strain, ${\mathrm{\epsilon <!--\; ??\; -->}}_{\text{peak}}\left(B,T,\mathrm{\theta <!--\; ??\; -->}\right)$, at which $J_{\text{c}}\left({\mathrm{\epsilon <!--\; ??\; -->}}_{\text{app}}\right)$ is maximised, is a function of $B$, $T$, and $\mathrm{\theta <!--\; ??\; -->}$, which is consistent with weakly emergent behaviour. It is described by the chain model that considers competition between twinned domains with different crystallographic orientations and opposing responses under an applied uniaxial strain.

Detailed effective upper critical field $B_{\text{c}2}^{\ast <!--\; ???\; -->}\left(T,{\mathrm{\epsilon <!--\; ??\; -->}}_{\text{app}},\mathrm{\theta <!--\; ??\; -->}=0\right)$ data are presented that show universal temperature and strain scaling. They lead to an accurate flux pinning relation for the volume pinning force, $F_{\text{p}}\propto <!--\; ???\; -->F_{\text{p},\text{max}}{b}^p{\left(1-<!--\; ???\; -->b\right)}^q$, where $b=B/B_{\text{c}2}^{\ast <!--\; ???\; -->}\left(T,{\mathrm{\epsilon <!--\; ??\; -->}}_{\text{app}},\mathrm{\theta <!--\; ??\; -->}=0\right)$ and $p$ and $q$ are constants, and are used to help parameterise the scaling behaviour of the angular $J_{\text{c}}$ data more accurately in those cases where $B_{\text{c}2}^{\ast <!--\; ???\; -->}$ cannot be measured directly. We derive approximate analytic in-field expressions that explain how the fraction, $f$, of a-domains amongst the a- and b-domains affects the strain dependence of the critical parameters and conclude that in our tape, $f$ = 0.4, and the strain at which $J_{\text{c}}$ is the same in both domains is ${\mathrm{\epsilon <!--\; ??\; -->}}_{J_{\text{cA}}=J_{\text{cB}}}=0.15\mathrm{\%<!--\; \%\; -->}$. We report a sharp peak in $J_{\text{c}}$ as the applied field approaches alignment with the ab-plane and the unusual result that with it, a suppression of the index of transition N also occurs. We find that the effective upper critical field $B_{\text{c}2}^{\text{*}}$ increases as the field angle approaches the ab-plane significantly faster than any available theoretical model for the upper critical field $B_{\text{c}2}$.

In addition, we conclude that a weak-emergence description is not limited to high temperature superconductors, but also describes some low-temperature superconductors. (paper)

[en] Moving vortex lattice in type-II superconductors can result in the radiation of electromagnetic waves in the range of terahertz frequency. The vortex flow dynamics in superconductors follow the London equation. In this paper, the effects of the superconductor coherence length and the London penetration depth on the radiated power is investigated by solving the London equation in the presence of vortices. The results show that by decreasing the coherence length, the radiation power will significantly increase. Also, it is possible to obtain more radiated power with a sharp peak by increasing the penetration depth. Further investigation of the effect of Joule heating reveals that radiation power is almost independent of temperature when we are far from critical temperature. But on the other hand, near the critical temperature, the radiation power is strongly dependent on the temperature variation. Also, by increasing the bias current, the radiation power will augment, and the frequency of radiated peaks will change accordingly. (paper)

[en] This publication is the outcome of an IAEA coordinated research project (CRP) on understanding and prediction of thermohydraulic phenomena relevant to supercritical water cooled reactors (SCWRs). The publication illustrates the state of the art in SCWR research and development. It is a key supporting publication to researchers and engineers pursuing the development of SCWRs or equipment/components operating at supercritical pressures. Scientific investigators from participating institutes identified specific research objectives to improve predictive capability of key technology areas (such as heat transfer and pressure drop for SCWR fuel related geometries, parallel channel stability boundary, natural circulation flow, critical heat flux at near critical pressures, critical flow, and subchannel and plenum mixing). The publication presents the background and objectives and descriptions of the revised Canadian SCWR design concept and a new SCWR design concept being developed at the Nuclear Power Institute of China. It also presents updated information on key areas of technology, such as supercritical heat transfer in simple geometries, stability and critical flow, which have been obtained after the completion of the previous CRP. New experiments and data on supercritical heat transfer in bundles and on critical heat flux, and the application of direct numerical simulation approach for supercritical heat transfer are also detailed.

[en] The electrical transport properties of (Li,Fe)OHFeSe films have been investigated in detail. The sharply textured films, prepared by matrix-assisted hydrothermal epitaxy (MHE) on LaAlO₃, show a zero-resistance critical temperature T _c of ∼42 K, J _c values well above 1 MA cm⁻² at low temperatures, and a maximum pinning force density F _P of ∼100 GN m⁻³ at 4 K. The activation energy U ₀ for thermal depinning of flux lines has been resolved for low magnetic fields, it agrees well with literature data. The coherence lengths and penetration depth were estimated via upper critical field B _c2 and self-field J _c, respectively, to be ξ _ab ∼ 2.7 nm, ξ _c = 0.24 nm, and λ _ab ∼ 160–200 nm. The layered crystal structure leads to highly anisotropic and two-dimensional electrical properties, including trapping and lock-in of vortices. (paper)

[en] The paper presents the statistical analysis of experimental results of radiation-induced critical brittle temperature ∆TF shifts and reference temperatures ∆T₀ obtained respectively from the impact bend and fracture toughness tests of the reactor vessel metal surveillance specimens to define the possibility of their mutual application for the irradiation embrittlement coefficient to be determined more accurately. The correlation between these parameters is shown to remain up to the accumulation of over-design fast neutron fluence. (author)