[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] The experimental discovery of near-room-temperature (NRT) superconductivity in highlycompressed H₃S, LaH₁₀ and YH₆ has restored fundamental interest in the electron–phonon pairing mechanism in superconductors. One of the prerequisites of phonon-mediated NRT superconductivity in highly compressed hydrides is strong electron–phonon interactions, which can be quantified by dimensionless ratios of the Bardeen–Cooper–Schrieffer (BCS) theory vs $\left(k_B{T}_c\right)/\left(\mathrm{\hslash <!--\; ???\; -->}{\mathrm{\omega <!--\; ??\; -->}}_{ln}\right)$ variable, where T_c is the critical temperature and ${\mathrm{\omega <!--\; ??\; -->}}_{ln}$ is the logarithmic phonon frequency (Mitrovic et al 1984 Phys. Rev. B 29 184). However, all known strong-coupling correction functions for the BCS ratios are applicable for $\left(k_B{T}_c\right)/\left(\mathrm{\hslash <!--\; ???\; -->}{\mathrm{\omega <!--\; ??\; -->}}_{ln}\right)$ < 0.20, which is not a high enough $\left(k_B{T}_c\right)/\left(\mathrm{\hslash <!--\; ???\; -->}{\mathrm{\omega <!--\; ??\; -->}}_{ln}\right)$ range for NRT superconductors, because the latter exhibit variable values of 0.13 < $\left(k_B{T}_c\right)/\left(\mathrm{\hslash <!--\; ???\; -->}{\mathrm{\omega <!--\; ??\; -->}}_{ln}\right)$ < 0.32. In this paper, we reanalyze the full experimental dataset (including data for highly compressed H₃S) and find that the strong-coupling correction functions for the gap-to-critical-temperature ratio and for the specific-heat-jump ratio are double-valued nearly linear functions of $\left(k_B{T}_c\right)/\left(\mathrm{\hslash <!--\; ???\; -->}{\mathrm{\omega <!--\; ??\; -->}}_{ln}\right)$. (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] Since the milestone experimental discovery by Drozdov et al( 2015 Nature 525 73–6) who reported the observation of near-room-temperature (NRT) superconductivity in highly-compressed sulphur hydride, the quest for room-temperature superconductivity is primarily focused on highly-compressed materials. Extreme conditions and space confinement inside a diamond anvil cell (DAC) dramatically limits the number of experimental techniques which can be applied to study highly-compressed superconductors. For this reason, the development of new approaches to characterize materials at extreme conditions is one of the central topics in the field of NRT superconductivity. In this paper, we describe an approach to categorize highly-compressed superconductors, including NRT superconductors, as unconventional superconductors. The primary idea for the classification is based on the empirical finding of Uemura (1997 Physica C 282–7 197) who showed that all unconventional superconductors have the ratio of the superconducting transition temperature, T _c, to the Fermi temperature, T _F, within a range of 0.01 ≤ T _c/T _F ≤ 0.05. To deduce the Fermi temperature in highly-compressed superconductors, we utilize temperature dependence of the upper critical field and the resistance data (which both can be more or less routinely measured for highly-compressed superconductors) and reported results by first principles calculations for these materials. We demonstrate the application of the approach for highly-compressed oxygen, sulphur, lithium, and recently discovered yttrium superhydride polymorphs, YH_n( n = 4,6,7,9) (Troyan et al( 2019 arXiv:1908.01534) and Kong et al( 2019 arXiv:1909.10482)). We also show the application of the approach for the newly discovered uncompressed Nd_2-xSr_xNiO₂ nickelate superconductor. (paper)

[en] Any theory of electron-phonon mediated superconductivity requires knowledge of the full phonon spectrum ${\mathrm{\alpha <!--\; ??\; -->}}^2\left(\mathrm{\omega <!--\; ??\; -->}\right)\cdot <!--\; ???\; -->F\left(\mathrm{\omega <!--\; ??\; -->}\right)$ in order to calculate superconducting transition temperature, T _c. However, there is currently no experimental technique for measuring ${\mathrm{\alpha <!--\; ??\; -->}}^2\left(\mathrm{\omega <!--\; ??\; -->}\right)\cdot <!--\; ???\; -->F\left(\mathrm{\omega <!--\; ??\; -->}\right)$ in highly-compressed near-room-temperature (NRT) superconductors. In this paper, we propose to advance McMillan’s approach (1968 Phys Rev 167 331), which utilises the Debye temperature $T_{\mathrm{\theta <!--\; ??\; -->}}$ (an integrated parameter for the full phonon spectrum), deduced via the fit of experimentally measured temperature-dependent resistance data R(T) to the Bloch-Grüneisen equation for highly-compressed black phosphorous, boron, GeAs, SiH₄, H_xS, D_xS, LaH_x, and LaD_y. By utilizing the relations between T _c, T_θ, and the electron-phonon coupling strength constant λ _e-ph (which can be computed from first-principles calculations), it is possible to affirm/disprove the electron-phonon coupling mechanism in given superconductors. We show that computed λ _e-ph for highly-compressed black phosphorous, boron, GeAs, SiH₄, and for one sample of LaH₁₀ are in a good agreement with λ _e-ph values deduced from experimental data. A remarkable constancy of $T_{\mathrm{\theta <!--\; ??\; -->}}=1531\pm <!--\; ??\; -->70\mathrm{K}$ for H₃S at different ageing stages is also found. We also show that if the phonon spectra of two isotopic counterparts share an identical shape (or, in the case of highly-compressed superconductors, the same material at different pressures), then within electron-phonon phenomenology, these materials should obey the relation of T_θ,₁/T_{θ ,2} = T _c,1/T _c,2 = ω _ln,1/ω _ln,2 (where subscripts 1 and 2 designate two isotopic counterparts). We further report that H₃S-D₃S pair ratios of T _{c,H3 S}/T _{c,D3 S} = ω _{ln,H3 S}/ω _{ln,D3 S} = 1.27 are largely different from deduced T_{θ ,H3 S}/T_{θ ,D3 S} = 1.65. This implies that NRT superconductivity in H₃S-D₃S systems originates from more than one mechanism, where the electron-phonon coupling lifts T _c in H₃S vs D₃S, but the primary origin for the NRT background of T _c ∼ 150 K in both H₃S and D₃S remains to be discovered. (paper)

[en] Recently, Snider et al (2020 Nature 586 373) reported on the observation of superconductivity in highly compressed carbonaceous sulfur hydride, H_x(S,C)_y. The highest critical temperature in H_x(S,C)_y exceeds the previous record of T _c = 280 K by 5 K, as reported by Somayazulu et al (2019 Phys. Rev. Lett. 122 027001) for highly compressed LaH₁₀. In this paper, we analyze experimental temperature-dependent magnetoresistance data, R(T,B), reported by Snider et al. The analysis shows that H_x(S,C)_y compound exhibited T _c = 190 K (P = 210 GPa), has the electron–phonon coupling constant λ _e−ph = 2.0 and the ratio of critical temperature, T _c, to the Fermi temperature, T _F, in the range of 0.011 ⩽ T _c/T _F ⩽ 0.018. These deduced values are very close to the ones reported for H₃S at P = 155–165 GPa (Drozdov et al 2015 Nature 525 73). This means that in all considered scenarios the carbonaceous sulfur hydride 190 K superconductor falls into the unconventional superconductor band in the Uemura plot, where all other highly compressed super-hydride/deuterides are located. It should be noted that our analysis shows that all raw R(T,B) data sets for H_x(S,C)_y samples, for which Snider et al (2020 Nature 586 373) reported T _c > 200 K, cannot be characterized as reliable data sources. Thus, independent experimental confirmation/disproof for high-T _c values in the carbonaceous sulfur hydride are required. (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)