Classifying superconductivity in compressed H3S

The discovery of high-temperature superconductivity in compressed H3S by Drozdov and co-workers (A. Drozdov, et. al., Nature 525, 73 (2015)) heralded a new era in superconductivity. To date, the record transition temperature of Tc = 260 K stands with another hydrogen-rich compound, LaH10 (M. Somayazulu, et. al., arXiv:1808.07695) which becomes superconducting at pressure of P = 190 GPa. Despite very intensive first-principle theoretical studies of hydrogen-rich compounds compressed to megabar level pressure, there is a very limited experimental dataset available for such materials. In this paper, we analyze the upper critical field, Bc2(T), data of highly compressed H3S reported by Mozaffari and co-workers (S. Mozaffari, et. al., LA-UR-18-30460, DOI: 10.2172/1481108) by utilizing four different models of Bc2(T). In result, we find that the ratio of superconducting energy gap, {\Delta}(0), to the Fermi energy, {\epsilon}F, in all considered scenarios is 0.03 < {\Delta}(0)/{\epsilon}F < 0.07, with respective ratio of Tc to the Fermi temperature, TF, 0.012 < Tc/TF < 0.039. These characterize H3S as unconventional superconductor and places it on the same trend line in Tc versus TF plot, where all unconventional superconductors located.Comment: 17 pages, 5 figure

Dc self-field critical current in superconductor dirac-cone material/superconductor junctions

Recently, several research groups have reported on anomalous enhancement of the self-field critical currents, Ic(sf,T), at low temperatures in superconductor/Dirac-cone material/superconductor (S/DCM/S) junctions. Some papers attributed the enhancement to the low-energy Andreev bound states arising from winding of the electronic wave function around DCM. In this paper, Ic(sf,T) in S/DCM/S junctions have been analyzed by two approaches: Modified Ambegaokar-Baratoff and ballistic Titov-Beenakker models. It is shown that the ballistic model is an inadequate tool to analyze experimental data from S/DCM/S junctions. The primary mechanism for limiting superconducting current in S/DCM/S junctions is different from the conventional view that the latter is the maximum value within the order parameter phase variation. Thus, there is a need to develop a new model for self-field critical currents in S/DCM/S systems. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.02.A03.21.0006AAAA-A18-118020190104-3Funding: This research was funded by the State Assignment of Minobrnauki of Russia, theme “Pressure” No. AAAA-A18-118020190104-3, and by Act 211 Government of the Russian Federation, contract No. 02.A03.21.0006

dd-wave superconductivity as a model for diborides apart MgB2_2

Recently, Pei et al. (arXiv2105.13250) reported that ambient pressure ${\beta}$-MoB$_2$ exhibits a phase transition to ${\alpha}$-MoB$_2$ (space group: $P6/mmm$) at pressure P~70 GPa and this high-pressure phase is a high-temperature superconductor exhibited $T_c=32 K$ at P~110 GPa. Despite ${\alpha}$-MoB$_2$ has the same crystalline structure as ambient pressure MgB$_2$2 and the $T_c$'s of ${\alpha}$-MoB$_2$ and MgB$_2$ are very close, the first principles calculations showed that in ${\alpha}$-MoB$_2$ the states near the Fermi level, ${\epsilon}_F$, are dominated by the $d$-electrons of Mo atoms, while in MgB$_2$ the $p$-orbitals of boron atomic sheets dominantly contribute to the states near the ${\epsilon}_F$. More recently, Hire et al. (arXiv2212.14869) reported that the $P6/mmm$-phase can be stabilized at ambient pressure in $Nb_{1-x}Mo_{x}B_{2}$ solid solutions, and these ternary alloys exhibit $T_c=8 K$. In addition, Pei et al. (Sci. China-Phys. Mech. Astron. 65, 287412 (2022)) showed that compressed WB$_2$ exhibits $T_c=15 K$ at P~121 GPa. Here, we analyzed experimental data reported for $P6/mmm$-phases of $Nb_{1-x}Mo_{x}B_{2}$ (x = 0.25; 1.0) and highly-compressed WB$_2$, and showed that these three phases exhibit $d$-wave superconductivity. We also deduced the gap-to-transition temperature ratio for these three phases. We found that $Nb_{0.75}Mo_{0.25}B_{2}$ exhibits high strength of nonadiabaticity, which is quantified by the ratio of $T_{\theta}/T_F=3.5$, which is by one order of magnitude exceeds the ratio in MgB$_2$, ${\alpha}$-MoB$_2$, WB$_2$, pnictides, cuprates, and highly-compressed hydrides.Comment: 23 pages, 9 figure

Quantifying the nonadiabaticity strength constant in recently discovered highly-compressed superconductors

Superconductivity in highly-pressurized hydrides became primary direction for the exploration of fundamental upper limit for the superconducting transition temperature, Tc, after Drozdov et al (Nature 2015, 525, 73) discovered superconducting state with $T_c=203 K$ in highly-compressed sulphur hydride. To date several dozens of high-temperature superconducting polyhydrides have been discovered. In addition, recently, it was reported that highly-compressed titanium and scandium exhibit record-high $T_c$ (up to 36 K), which is by manifold exceeded $T_c=9.2 K$ of niobium, which is the record high-$T_c$ ambient pressure metallic superconductor. Here we analysed experimental data on for recently discovered high-pressure superconductors (which exhibit high transition temperatures within their classes): elemental titanium (Zhang et al, Nature Communications 2022; Liu et al, Phys. Rev. B 2022), $TaH_3$ (He et al, Chinese Phys. Lett. 2023), $LaBeH_8$ (Song et al, Phys. Rev. Lett. 2023), and black (Li et al, Proc. Natl. Acad. Sci. 2018) and violet (Wu et al, arXiv 2023) phosphorous, to reveal the nonadiabaticity strength constant, $T_{\theta}/T_F$ (where $T_{\theta}$ is the Debye temperature, and $T_F$ the Fermi temperature) in these superconductors. The analysis showed that ${\delta}$-phase of titanium and black phosphorous exhibit the $T_{\theta}/T_F$ which are nearly identical to ones associated in A15 superconductors, while studied hydrides and violet phosphorous exhibit the constants in the same ballpark with $H_3S$ and $LaH_{10}$.Comment: 27 pages, 12 figure

Quantifying interaction mechanism in infinite layer nickelate superconductors

The relationship between the long-range antiferromagnetic order in cuprates and the high-temperature superconductivity in these compounds represents unresolved, nearly four-decades long scientific problem. Because recently discovered nickelate superconductors are crystallographical counterparts of cuprates, many properties and difficulties into describing these compounds are common to both families. Recently, Fowlie et al (2022 Nature Physics 18 1043) aimed to detect the antiferromagnetic order in $R_{1-x}Sr_{x}NiO_{2}$ (R = Nd, Pr, La, x ~ 0.2) films by using the muon spin rotation (muSR) technique. This research group reported on the existence of short-range antiferromagnetic order in all studied nickelates. Here, we aimed to reveal the existence of this interaction in the same nickelate films by analyzing the temperature dependent resistivity, $\rho(T)$, reported by the same research group. Global $\rho(T)$ data fits to the advanced Bloch-Gr\"uneisen model showed that each of R1-xSrxNiO2 compounds can be characterized by a unique power-law exponent, p (where p=2 for the electron-electron scattering, p=3 for the electron-magnon scattering, and p=5 for the electron-phonon scattering), and global characteristic temperature, $T_{\omega}$ (which has the meaning of the Debye temperature at p=5). We found that p=2.0 in Nd- and Pr-based compounds, and p=1.3 for La-based compound. The latter value does not have any interpretation within established theoretical models. We also analyzed $\rho(T)$ data for $Nd_{1-x}Sr_{x}NiO_{2}$ (0.125 < x < 0.325) reported by Lee et al (2022 arXiv2203.02580). Because our analysis showed that p-values in nickelates are remarkably different from p=3, we call for the developent of a new theoretical model to describe $\rho(T)$ in materials exhibiting a short-range antiferromagnetic order.Comment: 26 pagers, 11 figure

Debye temperature, electron-phonon coupling constant, and microcrystalline strain in highly-compressed La3_3Ni2_2O7−δ_{7-\delta}

Recently Sun et al (Nature 621, 493 (2023)) reported on the discovery of high-temperature superconductivity in highly-compressed La$_3$Ni$_2$O$_{7-\delta}$. In addition to ongoing studies of the phase structural transition, pairing mechanism, and other properties/parameters in this highly-pressurized nickelate, here explore a possibility for the electron-phonon pairing mechanism in the La$_3$Ni$_2$O$_{7-\delta}$. To do this, we analyzed experimental data on temperature dependent resistance, $R(T)$, and extracted pressure dependent Debye temperature, $\Theta_D$, for the $Fmmm$-phase (high-pressure phase). Derived ballpark value is $\Theta_D(P = 25 GPa) = 550$ $K$. We also estimated the electron-phonon coupling constant, $\lambda_{e-ph}(P=22.4 GPa) = 1.75$, for La$_3$Ni$_2$O$_{7-\delta}$ sample exhibited zero-resistance transition. Performed analysis of XRD data showed that the crystal lattice strain, $\epsilon(P)$, is higher in the $Fmmm$-phase in comparison with the $Amam$-phase (low-pressure phase). Based on performed $\epsilon(P)$ analysis, we proposed possible reason for the presence/absence of the zero-resistance state in La$_3$Ni$_2$O$_{7-\delta}$.Comment: 19 pages, 9 figures, 96 reference