[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] We consider the superconducting transition in fermionic quantum critical systems. Assuming the validity of Migdal theorem, the gap equation can be written in terms of the retarded pair susceptibility. Instead of the usual BCS form, the pair susceptibility is now subject to scale invariance. The gap and transition temperature is thus of the algebraic form, totally different from the exponential behavior in BCS theory. Consequently, with reasonably small glue strength, we can get very large gap and transition temperature comparable to those discovered in cuprates. The ratio of the gap to retardation gets boosted by increasing retardation. We also find the upper critical field has a different scaling with the critical temperature. With a non-Lorentzian dynamical exponent, the upper critical field is greatly enhanced when approaching the critical point, though the critical temperature only changes modestly, in agreement with recent experiments on heavy fermions.

[en] The discovery of superconductivity is 100 years old but theoretical works are still necessary: the BCS theory does not apply to the new families of high temperature superconducting materials discovered after 1986. In 2001 it was discovered that MgB₂ is superconducting at 39 K, this critical temperature is not the highest but MgB₂ is easy to produce and cheap. Today's highest critical temperature under atmospheric pressure is that of the HgTlBaCaCuO compound: 138 K. The complexity and the cost of cryogenic systems restrain the applications of superconductivity. The author reviews the applications of superconducting in medical imaging, particle detectors, and in the safety systems of power networks. (A.C.)

[en] Published in summary form only

[en] We consider a Ginzburg-Landau model for superconductivity with a Chern-Simons term added. The flow diagram contains two charged fixed points corresponding to the tricritical and infrared stable fixed points. The topological coupling controls the fixed point structure and eventually the region of first order transitions disappears. We compute the critical exponents as a function of the topological coupling. We obtain that the value of the v exponent does not vary much from the XY value, v_xy = 0.67. This shows that the Chern-Simons term does not affect considerably the XY scaling of superconductors. We discuss briefly the possible phenomenological applications of this model. (author)

[en] Published in summary form only

[en] A flux pinning model is presented which predicts the maximum critical current density attainable in superconductors. That such a limit must exist comes from the realization that flux pinning is strongest in regions of weak superconductivity, but these regions cannot carry a large supercurrent. Since the same regions within the superconductor cannot be used for both pinning and supercurrent conductions, there must be an optimum mix, leading to a maximum J/sub c/. Measurements on films and multilayers of NbN have verified many details of the model including anisotropy effects and a strong reduction in J/sub c/ for defect spacings smaller than the flux core diameter. In an optimized multilayer the pinning force reached /approximately/22% of the theoretical maximum. The implications of these results on the practical applications of NbN films and on the maximum critical current density in the new high temperature superconductors are also discussed. 24 refs., 4 figs

[en] Both critical current and Shapiro steps in Josephson ladders are found to show effects of vortex exclusion below a critical magnetic field. Vortices are excluded even though the flux itself is not screened out. When current is injected perpendicular to the ladder edges, the critical current is unchanged from its f=0 value up to a penetration field of f_{c1perpendicular}≅0.12 flux quanta per plaquette. Similarly, there are only integer Shapiro steps below a critical c1 perpendicular *, where vortices first penetrate. A narrower critical current plateau also appears to form near f =1/2. No such plateaus occur when current is injected in the parallel direction. We attribute the exclusion in the perpendicular geometry to screening currents which flow along the edges of the ladder. copyright 1996 The American Physical Society

[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] Weak-coupling BCS superconductivity theory is shown to admit a more general T_c formula wherein T_c approaches zero somewhat faster than with the BCS T_c formula. This allows some recent empirical values for ceramic superconductors of the gap-to-T_c ratio smaller than the universal BCS value of 3.53 not to be inconsistent with weak electron-boson coupling. Similarly, other universal constants related to specific heat and critical magnetic field will be modified. (author)