[en] Proximity-induced superconductivity in single-layer graphene (SLG) and in topological insulators represent almost ideal examples of superconductivity in two dimensions. Fundamental mechanisms governing superconductivity in the 2D limit are of central interest for modern condensed-matter physics. To deduce fundamental parameters of superconductor/graphene/superconductor and superconductor/bismuth selenide/superconductor junctions we investigate the self-field critical currents in these devices using the formalism of the Ambegaokar–Baratoff model. Our central finding is that the induced superconducting state in SLG and bismuth selenide each exhibits gapping on two superconducting bands. Based on recent results obtained on ultra-thin films of natural superconductors, including single-atomic layer of iron selenide, double and triple atomic layers of gallium, and several atomic layer tantalum disulphide, we conclude that a two-band induced superconducting state in SLG and bismuth selenide is part of a wider, more general multiple-band phenomenology of currently unknown origin. (paper)

[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] Coated conductor Roebel cables are an effective way to create a high current density, fully transposed cable. However, despite REBCO tapes being robust against transverse stress, the Roebel architecture can concentrate transverse stress in non-trivial and random patterns depending on the exact arrangement of strands. If stands are embedded in a solid media which consolidates all strands then a transverse stress concentration will not occur. We tested this idea through mechanical and thermo-cycling tests on 5 strand Roebel cables. For non-impregnated cable irreversible degradation in critical currents is initiated at transverse pressures in a range of 4–34 MPa. Optical examination of the cables shows stress concentration patterns beyond those predicted by thickness variations. For cables impregnated with epoxy filled with SiO₂ nanopowder, which has a similar thermal expansion coefficient to the metallic substrate of the strands, the irreversibility point is increased above our highest experimentally available pressure of 270 MPa. Thermo-cycling experiments confirmed a closely matched thermal expansion coefficient between the embedding media and metallic substrate is critical to avoid wire failures. (paper)

[en] Weak-links are defects that limit dissipation-free transport current flow in superconductors. Grain boundaries, nano- and micro-cracks, and planar precipitations of secondary phases are typical examples of weak-links in practical superconductors. There is an unanswered practical question: is the critical current for a given superconductor limited by weak-links, or does wire fabrication provide a weak-link-free superconductor? In this paper, we answer this question for layered quasi-two-dimensional (quasi-2D) superconductors, namely pnictides and cuprates. Our approach is based on the fact that the self-field critical current density in weak-link-free superconductors is J _c(sf, T) = A/λ ³(T), where λ(T) is the London penetration depth and A is the relevant fundamental constant. Taking into account that the transition temperature, T _c, in layered quasi-2D superconductors is limited by the phase fluctuation temperature, T _fluc = B/λ ²(0) ≥ 1.2·T _c, where B is the relevant fundamental constant, then the substitution of λ(0) deduced from the measured J _c(sf, T) gives a tool to compare the deduced T _fluc and experimentally measured T _c. This provides a simple criterion to reveal the presence or absence of weak-links which has been proven by an analysis of self-field critical currents in a variety of high-temperature superconductors, ranging from atomically thin FeSe up to commercially available tapes of RBa₂Cu₃O₇. (paper)

[en] A dissipation-free current is one of the most fascinating and practically important properties of superconductors. At self-field conditions (when no external magnetic field is applied) dissipation-free current density, J _c(sf, T), in thin weak-link-free superconductors described by the equation $J_c(sf,T)=\frac{{\mathrm{\varphi <!--\; ??\; -->}}_0}{4\cdot <!--\; ???\; -->\mathrm{\pi <!--\; ??\; -->}\cdot <!--\; ???\; -->{\mathrm{\mu <!--\; ??\; -->}}_0}\cdot <!--\; ???\; -->\frac{\mathrm{l}\mathrm{n}(1+\sqrt{2}\cdot <!--\; ???\; -->\mathrm{\kappa <!--\; ??\; -->})}{{\mathrm{\lambda <!--\; ??\; -->}}^3\left(T\right)}$ (where λ(T) is the London penetration depth, and κ is the Ginzburg–Landau parameter) was proved for more than 90 superconductors, including type-I and type-II superconductors, elementary superconductors, pnictides, cuprates, MgB₂, heavy fermions and H₃S. In addition, it was recently proposed for quasi-two dimensional superconductors (namely pnictides and cuprates), that maximum achievable critical current density, J _c(sf, T ∼ 0 K), is linked with the transition temperature, T _c, and the mean spacing between superconducting sheets, d, by the following equation: $J_c(sf,T\simeq <!--\; ???\; -->0K)\gtrsim <!--\; ???\; -->\frac{{\mathrm{\pi <!--\; ??\; -->}}^3\cdot <!--\; ???\; -->{\mathrm{\mu <!--\; ??\; -->}}_0^{1/2}}{{\mathrm{\varphi <!--\; ??\; -->}}_0^2}$ $\cdot <!--\; ???\; -->\left(\mathrm{l}\mathrm{n}(1+\sqrt{2}\cdot <!--\; ???\; -->\mathrm{\kappa <!--\; ??\; -->})\right)\cdot <!--\; ???\; -->{\left(\frac{k_B\cdot <!--\; ???\; -->T_c}{d}\right)}^{3/2}$ (Talantsev and Crump 2018 Supercond. Sci. Technol. 31 124001). In this paper, we focused on the inverse problem, i.e., to find the best candidates for the developing practical wires in terms of their self-field critical current capacity from known parameters of newly discovered superconductors (for which we draw largely on Hosono et al 2015 Sci. Technol. Adv. Mater. 16 033503). Considering that in-field critical currents of iron-based superconductors are very slow functions of applied magnetic fields, our calculations may have wider applicability outside self-field conditions. (paper)

[en] In this study, YBCO films were fabricated on RABiTS metal substrates by metal-organic deposition of trifluoroacetates. Precursor solutions were made with different Ba concentrations (Ba/Y = 1.50, 1.70, 1.85, 2.0) with the aim of optimizing the critical current density (J_c). Our results confirmed that YBCO films with Ba/Y = 1.70(J_c = 3.6 MA/cm² at T 77 K) have significantly higher J_c than stoichiometric (Ba/Y = 2.0) YBCO (J_c 2.4 MA/cm²). Application of low-angle polishing techniques and X-ray diffraction (XRD) studies for quenched (partially reacted) films has shown that YBCO films with Ba/Y = 1.70 nucleate more rapidly than other films, but that the crystal growth rate is increased when the Ba-concentration is increased. These results provide new insights into the physical mechanisms required to achieve high J_c in YBCO.

[en] Recently, Pan et al (2017 J. Am. Chem. Soc. 139 4623) reported that randomly restacked chemically exfoliated monolayers of TaS₂ have enhanced superconducting transition temperatures of up to T _c = 3 K, compared with T _c = 0.8 K for the bulk 2 H-TaS₂ compound. Ma et al (2018 NPJ Quantum Mater. 3 34) measured the angular dependence of the upper critical field, B _c2(θ), for this material and employed several models to fit the experimental data, namely the three-dimensional Ginzburg–Landau (3D GL) model (Blatter et al 1994 Rev. Mod. Phys. 66 1125–1388), the two-dimensional Tinkham (2D Tinkham) model (Harper and Tinkham 1968 Phys. Rev. 172 441–450), and the modified 3D GL (Ma et al 2018 NPJ Quantum Mater. 3 34) model. However, differences between experimentally measured B _c2(θ) and theoretical model values are large, showing a great enhancement of experimental B _c2(θ) over a wide range of angles. The same result was obtained for 1T′-MoS₂ restacked nanosheets (Ma et al 2018 NPJ Quantum Mater. 3 34). Here we stress that the physical reason for enhanced superconductivity in these materials is the randomness in restacked monolayers (Pan et al 2017 J. Am. Chem. Soc. 139 4623; Ma et al 2018 NPJ Quantum Mater. 3 34). Based on this viewpoint (Pan et al 2017 J. Am. Chem. Soc. 139 4623; Ma et al 2018 NPJ Quantum Mater. 3 34), and despite the fact that the B _c2(θ) of each individual monolayer will obey the 2D Tinkham model, the total B _c2(θ) should mainly reflect the angular statistical distribution of the 2D nanosheets within the stack. Fits of the experimental B _c2(θ) data using a statistical distribution model for the 2D nanosheets have excellent quality and the deduced parameters have meaningful values. We also propose that the angular dependences of the lower, B _c1(θ), and thermodynamic, B _c(θ), critical fields in randomly restacked 2D nanosheets should also obey statistical distribution models. (paper)

[en] Most widely used practical superconductors (i.e., NbTi, Nb₃Sn, Bi₂Sr₂Ca₂Cu₃O_10+x) are manufactured in a form of multigranular superconducting filaments embedded in a metallic matrix. The performance of these multifilamentary conduits at different physical conditions has been a topic of extended research over the last decades. In this paper we targeted to reveal the precise onset of the electric power dissipation in one of these composite superconductors, Bi₂Sr₂Ca₂Cu₃O_10+x (so-called 1G HTS wire), at the conditions when external magnetic field, B _appl, is applied in the maximum Lorentz force geometry. As we showed earlier (Talantsev et al 2017 AIP Advances 7 125230), at self-field conditions the transition to the dissipative state in 1G wire upon increasing the transport current, I, despite a multifilamentary wire design sharply steepens at a threshold current, I _c,surfB, at which a simultaneous and abrupt crossover from a non-linear to a linear dependence of the perpendicular component of the local magnetic flux density, B _surf(I), measured at the conduit surface occurs. We found that the same transition takes place in 1G HTS wire with an applied magnetic field, B _appl, and thus, the definition of critical current in multifilamentary superconductors based on I _c,surfB can be extended to in-field conditions for 1G HTS wire. We also studied effects of the flux trap and magnetic hysteresis upon increasing/cycling the transport current, I, in this conduit superconducting wire. (paper)

[en] In this paper we present the experimental results on the temperature dependences of c-axis resistivity, ${\mathrm{\rho <!--\; ??\; -->}}_c$(T), for epitaxial films of n-type superconductor Nd_2–xCe_xCuO_4+δ deposited on (110) SrTiO₃ single crystal substrate in a wide range of Ce content from lightly to heavily substitution regions ($x=0.12-<!--\; ???\; -->0.20$) under an optimal annealing. Alternative empirical models were used for a description of the observed semiconductor–like ${\mathrm{\rho <!--\; ??\; -->}}_c\left(T\right)$ dependence. An excellent quantitative depiction of the experimental ${\mathrm{\rho <!--\; ??\; -->}}_c\left(T\right)$ dependences turned out to be possible for all investigated samples in a model of natural superlattice with temperature - dependent barrier height. (paper)

[en] We present our recent results in introducing artificial flux-pinning centers in metal-organic deposited YBa₂Cu₃O₇ coated conductors. In particular, we describe methods for creating point-like and planar defects through precursor and process modifications, and linear defects using post-process heavy-ion irradiation. We observe these defects through transmission-electron microscopy. Each type of defect contributes a particular critical-current signature and combinations of defects can be used to tailor the superconductor for specific applications.