[en] The era of near-room-temperature superconductivity started after experimental discovery by Drozdov et al (2015 Nature 525 73) who found that compressed H₃S exhibits superconducting transition at T _c = 203 K. To date, the record near-room-temperature superconductivity stands with another hydrogen-rich highly compressed compound, LaH₁₀ (Somayazulu et al 2019 Phys. Rev. Lett. 122 027001), which has critical temperature of $T_c><!--\; >\; -->240K.$ In this paper, we analyse available upper critical field, B _c2(T), data for LaH₁₀ (Drozdov et al 2019 Nature 569 528) and report that this compound in all considered scenarios has the ratio of T _c to the Fermi temperature, T _F, 0.009 < T _c/T _F < 0.038, which is typical range for unconventional superconductors. In attempt to extend our finding, we examined experimental B _c2(T) data for superconductors in the palladium-hydrogen system and surprisingly find that PdH_x compounds have the ratio of 0.008 < T _c/T _F < 0.012. Taking in account that H₃S has the ratio of 0.012 < T _c/T _F < 0.039 (Talantsev 2019 Modern Phys. Lett. B 33 1950195) we come to conclusion that in the Uemura plot all discovered to date hydrogen-rich superconductors, i.e. PdH_x, H₃S and LaH₁₀, lie in same band as all unconventional superconductors, particularly heavy fermions, fullerenes, pnictides, and cuprates, and former should be classified as a new class of unconventional superconductors. (paper)

[en] Less than two years after the discovery of high temperature superconductivity in oxypnictide LaFeAs(O, F) several families of superconductors based on Fe layers (1111, 122, 11, 111) are available. They share several characteristics with cuprate superconductors that compromise easy applications, such as the layered structure, the small coherence length and unconventional pairing. On the other hand, the Fe-based superconductors have metallic parent compounds and their electronic anisotropy is generally smaller and does not strongly depend on the level of doping, and the supposed order parameter symmetry is s-wave, thus in principle not so detrimental to current transmission across grain boundaries. From the application point of view, the main efforts are still devoted to investigate the superconducting properties, to distinguish intrinsic from extrinsic behaviors and to compare the different families in order to identify which one is the fittest for the quest for better and more practical superconductors. The 1111 family shows the highest T_c, huge but also the most anisotropic upper critical field and in-field, fan-shaped resistive transitions reminiscent of those of cuprates. On the other hand, the 122 family is much less anisotropic with sharper resistive transitions as in low temperature superconductors, but with about half the T_c of the 1111 compounds. An overview of the main superconducting properties relevant to applications will be presented. Upper critical field, electronic anisotropy parameter, and intragranular and intergranular critical current density will be discussed and compared, where possible, across the Fe-based superconductor families.

[en] We report our studies on the crystal structures, morphologies, and superconductivity in CeO_1-xF_xFeAs compounds which were fabricated by solid state reaction. The crystal structures were refined using Rietveld refinement. Superconducting properties such as critical temperature (T_c), critical current density (J_c), and upper critical field (H_c2) were determined using magneto-transport and magnetic measurement over a wide range of temperature below T_c, and in magnetic fields up to 13 T. J_c is 2 x 10³ A cm^-2 for the x = 0.1 sample. However, the J_c exhibited a weak dependence on magnetic field for B > 1 T and T = 5 and 10 K. A peak effect in the J_c as a function of field was observed at 20 K in the x = 0.1 sample. We estimate H_c2^ab of 185 T for CeO_0.9F_0.1FeAs compound. The broadening of the superconducting transition near T_c with increasing field can be well understood using the thermal activated flux flow model. The pinning potential scales as U₀/K_B∝B^-n with n = 0.2 for B < 3 T and n = 0.71 for B > 3 T in the x = 0.1 sample.

[en] In this paper we present the results of critical current (I _c) measurements of MgB_2 wires made with two different set-ups of the four-point probe method: current sweep type—constant magnetic field and increasing current, and field sweep type—constant current and rapidly increasing magnetic field. Results from magnet field sweep type measurements can be interpreted by a new physical concept—a jump of the electric field in low magnetic fields. This physical concept can be correlated with damages in the Nb-barrier existing in the MgB_2 wire and be employed as a detection scheme. The damage in Nb barrier reduces critical current density (J _c) and complicates the study on critical temperature (T _c), upper critical field (B _c_2), irreversible magnetic field (B _i_r_r), pinning force (F _p), and pinning centers in superconducting MgB_2 wires. Our proposed method to detect damages in Nb barrier would benefit efforts in development and applications of MgB_2 wires. (paper)

[en] Measurements of the critical parameters for {xNH₃ + (1 - x)H₂O} with x = (0.9098, 0.7757, 0.6808) were carried out by using a metal-bellows variable volumometer with an optical cell. The expanded uncertainties (k = 2) in temperature, pressure, density, and composition measurements have been estimated to be less than 3.2 mK, 3.2 kPa, 0.3 kg . m^-3, and 8.8 . 10^-4, respectively. In each mole fraction, the critical temperature T_c was first determined on the basis of the intensity of the critical opalescence. The critical pressure p_c and critical density ρ_c were then determined as the point at which the meniscus disappears on the isotherm at T = T_c. The expanded uncertainties (k = 2) in the present critical parameters have also been estimated. Comparisons of the present values with the literature data as well as the calculated values afforded using the equation of state are also presented

[en] Inhomogeneity of a high T_c superconducting sample is described by alternating weak and strong regions, which are depicted by different critical temperatures T_cw and T_cs, or different critical current densities j_cw and j_cs. Based on flux creep simulation, a step and two peaks in ac susceptibility (ACS) curves, as reported in the references, are observed. The ACS response in the two inhomogeneous models is analyzed through the calculated field distributions and their evolution with temperature. The turning point, according to the dissipation peak at low temperature, is not a critical temperature of any part of the sample, and the step in the real part χ' is induced by the slow movement of flux lines in the strong region. The different influence of ac magnetic field amplitudes B_ac and frequencies f on ACS curves for two inhomogeneous models can be used as a practical criterion to distinguish T_c inhomogeneity from j_c inhomogeneity in sintered samples.

[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] The safety analysis of Pressurized Water Reactors, in the event of LOCA, strongly depends on the ability to evaluate the discharge rate of coolant inventory through the breach. Due to the huge pressure difference between the primary system and the reactor containment, the mass flow rate is choked at the break. Under such conditions, both mechanical and thermal equilibrium between phases are not ensured. A general theory to evaluate the two-phase critical mass flow rate is not yet available. However, some models are capable of providing accurate evaluations of either critical mass flux or critical pressure and such a model is the Delayed Equilibrium Model (DEM), which is examined in this article. Here we show how to integrate the DEM system of equations coupling a standard Runge-Kutta method with the Possible-Impossible Flow algorithm. Hence a simple procedure which does not require sophisticate computational schemes. The main objective of this work is to compare DEM, Homogeneous Equilibrium Model, Moody (1965) and Henry and Fauske (1971) models to experimental data. The four models were tested and the results from experimental data containing a sample range in excess of 450 conditions compared in determining an appropriate benchmark. Each of the chosen models is representative of a particular category of critical flow models. Furthermore, two-phase critical models provide good estimations depending on the configuration or set of conditions. Consequently, the models have been individually tested incorporating long tubes, short tubes and slits. This analysis has been carried out for both critical mass flux and critical pressure evaluations. (authors)

No abstract available

[en] In the mid-1970s and early 1980s, almost 100 critical and approach-to-critical experiments using water-moderated plutonium metal arrays were performed at the Rocky Flats Critical Mass Laboratory. The experiments were performed in two programs: 2x2xN arrays conducted in the 1970s and 3x3x3 arrays conducted in the 1980s. Both sets of experiments used the same double-canned plutonium parts and were water moderated. The major differences between the two programs were the support structure and tank and the administrative limits on criticality. This paper summarizes the critical data and results of both sets of experiments. These experiments are then analyzed in a companion paper