Non-Fermi-Liquid Behavior of Superconducting SnH4_4

We studied chemical interaction of Sn with H$_2$ by X-ray diffraction methods at pressures of 180-210 GPa. A previously unknown tetrahydride SnH$_4$ with a cubic structure (${fcc}$) exhibiting superconducting properties below ${T}$$_C$ = 72 K was obtained; the formation of a high molecular ${C2/m}$-SnH$_{14}$ superhydride and several lower hydrides, ${fcc}$ SnH$_2$ and ${C2}$-Sn$_{12}$H$_{18}$, was also detected. The temperature dependence of critical current density ${J}$$_C$(T) in SnH$_4$ yields the superconducting gap 2$\Delta$(0) = 20-22 meV at 180 GPa. The SnH$_4$ superconductor has unusual behavior in strong magnetic fields: linear temperature dependences of magnetoresistance and the upper critical magnetic field ${B}$$_{C2}$(T) $\propto$ (${T}$$_C$ - ${T}$). The latter contradicts the Wertheimer-Helfand-Hohenberg model developed for conventional superconductors. Along with this, the temperature dependence of electrical resistance of ${fcc}$ SnH$_4$ in normal resistivity state exhibits a deviation from what is expected for phonon-mediated scattering described by the Bloch-Gr\"uneisen model, and is beyond the framework of the Fermi liquid theory. Such anomalies occur for many superhydrides, making them much closer to cuprates than previously believed

Anomalous high-temperature superconductivity in YH6_6

Pressure-stabilized hydrides are a new rapidly growing class of high-temperature superconductors which is believed to be described within the conventional phonon-mediated mechanism of coupling. Here we report the synthesis of yttrium hexahydride Im3m-YH$_6$ that demonstrates the superconducting transition with T$_c$ = 224 K at 166 GPa, much lower than the theoretically predicted (>270 K). The measured upper critical magnetic field B$_c$$_2$(0) of YH$_6$ was found to be 116-158 T, which is 2-2.5 times larger than the calculated value. A pronounced shift of T$_c$ in yttrium deuteride YD$_6$ with the isotope coefficient 0.4 supports the phonon-assisted superconductivity. Current-voltage measurements showed that the critical current I$_c$ and its density J$_c$ may exceed 1.75 A and 3500 A/mm$^2$ at 0 K, respectively, which is comparable with the parameters of commercial superconductors, such as NbTi and YBCO. The superconducting density functional theory (SCDFT) and anharmonic calculations suggest unusually large impact of the Coulomb repulsion in this compound. The results indicate notable departures of the superconducting properties of the discovered YH$_6$ from the conventional Migdal-Eliashberg and Bardeen-Cooper-Schrieffer theories.Comment: arXiv admin note: text overlap with arXiv:1902.1020

Superconductivity in Cu Co-Doped SrxBi2Se3 Single Crystals

In this study, we grew Cu co-doped single crystals of a topological superconductor candidate Sr x Bi 2 Se 3 , and studied their structural and transport properties. We reveal that the addition of even as small an amount of Cu co-dopant as 0.6 atomic %, completely suppresses superconductivity in Sr x Bi 2 Se 3 . Critical temperature (∼2.7 K) is rather robust with respect to co-doping. We show that Cu systematically increases the electron density and lattice parameters a and c. Our results demonstrate that superconductivity in Sr x Bi 2 Se 3 -based materials is induced by significantly lower Sr doping level x < 0.02 than commonly accepted x ∼ 0.06 , and it strongly depends on the specific arrangement of Sr atoms in the host matrix. The critical temperature in superconductive Sr-doped Bi 2 Se 3 is shown to be insensitive to carrier density

Vortex Phase Dynamics in Yttrium Superhydride YH₆ at Megabar Pressures

A comprehensive study of vortex phases and vortex dynamics is presented for a recently discovered high-temperature superconductor YH6 with Tc(onset) of 215 K under a pressure of 200 GPa. The thermal activation energy (U0) is derived within the framework of the thermally activated flux flow (TAFF) theory. The activation energy yields a power law dependence U0 ∝ Hα on magnetic field with a possible crossover at a field around 8–10 T. Furthermore, we have depicted the vortex phase transition from the vortex-glass to vortex-liquid state according to the vortex-glass theory. Finally, vortex phase diagram is constructed for the first time for superhydrides. Very high estimated values of flux flow barriers U0(H) = (1.5–7) × 104 K together with high crossover fields make YH6 a rather outstanding superconductor as compared to most cuprates and iron-based systems. The Ginzburg number for YH6 Gi = (3–7) × 10–3 indicates that thermal fluctuations are not so strong and cannot broaden superconducting transitions in weak magnetic fields

Vortex Phase Dynamics in Yttrium Superhydride YH₆ at Megabar Pressures

A comprehensive study of vortex phases and vortex dynamics is presented for a recently discovered high-temperature superconductor YH6 with Tc(onset) of 215 K under a pressure of 200 GPa. The thermal activation energy (U0) is derived within the framework of the thermally activated flux flow (TAFF) theory. The activation energy yields a power law dependence U0 ∝ Hα on magnetic field with a possible crossover at a field around 8–10 T. Furthermore, we have depicted the vortex phase transition from the vortex-glass to vortex-liquid state according to the vortex-glass theory. Finally, vortex phase diagram is constructed for the first time for superhydrides. Very high estimated values of flux flow barriers U0(H) = (1.5–7) × 104 K together with high crossover fields make YH6 a rather outstanding superconductor as compared to most cuprates and iron-based systems. The Ginzburg number for YH6 Gi = (3–7) × 10–3 indicates that thermal fluctuations are not so strong and cannot broaden superconducting transitions in weak magnetic fields

Superconductivity at 253 K in lanthanum–yttrium ternary hydrides

Here we report the high-pressure synthesis of a series of lanthanum–yttrium ternary hydrides obtained at pressures of 170–196 GPa via the laser heating of P6 3/mmc La–Y alloys with ammonia borane. As a result, we discovered several novel compounds: cubic hexahydride (La,Y)H$_6$ and decahydrides (La,Y) H$_{10}$ with a maximum critical temperature T$_C$ ~ 253 K and an extrapolated upper critical magnetic field B$_{C2}$ (0) of up to 135 T at 183 GPa. The current–voltage measurements show that the critical current density J$_C$ in (La,Y)H10 is 12–27.7 kA/mm$^2$ at 4.2 K, which is comparable with that of commercial superconducting wires such as NbTi and Nb$_3$ Sn. (La,Y)H$_6$ and (La,Y)H$_{10}$ are among the first examples of ternary high-T$_C$ superconducting hydrides. Our experiments show that part of metal atoms in the structures of recently discovered Im$\bar{3}$m-YH$_6$ and Fm$\bar{3}$m-LaH$_{10}$ can be replaced with lanthanum (~ 70%) and yttrium (~ 25%), respectively, with the formation of unique ternary superhydrides containing metal encapsulated cages La@H$_{24}$ and Y@H$_{32}$, which are specific for Im$\bar{3}$m-LaH$_6$ and Fm$\bar{3}$m-YH$_{10}$. This work demonstrates that hydrides, unstable in pure form such as LaH$_6$ and YH$_{10}$ , may nevertheless be stabilized at relatively low pressures in solid solutions with superhydrides having the desired structure

Anomalous High‐Temperature Superconductivity in YH6

Pressure‐stabilized hydrides are a new rapidly growing class of high‐temperature superconductors, which is believed to be described within the conventional phonon‐mediated mechanism of coupling. Here, the synthesis of one of the best‐known high‐TC superconductors—yttrium hexahydride 3-YH6 is reported, which displays a superconducting transition at ~ 224 K at 166 GPa. The extrapolated upper critical magnetic field Bc2(0) of YH6 is surprisingly high: 116–158 T, which is 2–2.5 times larger than the calculated value. A pronounced shift of TC in yttrium deuteride YD6 with the isotope coefficient 0.4 supports the phonon‐assisted superconductivity. Current–voltage measurements show that the critical current IC and its density JC may exceed 1.75 A and 3500 A mm−2 at 4 K, respectively, which is higher than that of the commercial superconductors, such as NbTi and YBCO. The results of superconducting density functional theory (SCDFT) and anharmonic calculations, together with anomalously high critical magnetic field, suggest notable departures of the superconducting properties from the conventional Migdal–Eliashberg and Bardeen–Cooper–Schrieffer theories, and presence of an additional mechanism of superconductivity.The work on the high‐pressure experiments was supported by the Ministry of Science and Higher Education of the Russian Federation within the state assignment of the FSRC “Crystallography and Photonics” of RAS and by the Russian Science Foundation (project no. 19‐12‐00414). A.G.G. acknowledges the use of the facilities of the Center for Collective Use “Accelerator Center for Neutron Research of the Structure of Substance and Nuclear Medicine” of the INR RAS. A.G.K. thanks the Russian Foundation for Basic Research (project no. 19‐03‐00100) for the financial support of this work. A.R.O., D.V.S., and A.G.K. thank the Russian Science Foundation (grant 19‐72‐30043). The reported study was funded by the RFBR, project 20‐32‐90099. A.R.O and D.V.S. thank the Ministry of Science and Higher Education agreement No. 075‐15‐2020‐808. Portions of this work were performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS was supported by the National Science Foundation—Earth Sciences (EAR‐1634415) and Department of Energy—GeoSciences (DE‐FG02‐94ER14466). Use of the GSECARS Raman Lab System was supported by the NSF MRI Proposal (EAR‐1531583). This research used the resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE‐AC02‐06CH11357 and R.B. acknowledges the support from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement no. 802533). R.B. thankfully acknowledges the computer resources at Altamira and the technical support provided by Physics Institute of Cantabria (IFCA) (RES‐FI‐2020‐3‐0028). The research used resources of the LPI Shared Facility Center. A.V.S., O.A.S. and V.M.P. acknowledge support of the state assignment of the Ministry of Science and Higher Education of the Russian Federation (Project No. 0023‐2019‐0005). A.B. acknowledges financial support from the Spanish Ministry of Science and Innovation (PID2019‐105488GB‐I00). R.A. performed the calculations at the Supercomputer Center at the Institute for Solid State Physics in the University of Tokyo. The authors thank Igor Grishin (Skoltech) for proofreading of the manuscript

Effect of paramagnetic impurities on superconductivity in polyhydrides: s\textit{s}-wave order parameter in Nd-doped LaH10_{10}

Polyhydrides are a novel class of superconducting materials with extremely high critical parameters, which is very promising for applications. On the other hand, complete experimental study of the magnetic phase diagram for the best so far known superconductor, lanthanum decahydride LaH$_{10}$, encounters a serious complication because of the large upper critical magnetic field $\textit{H}$$_{C2}$(0), exceeding 120-160 T. Partial replacement of La atoms by magnetic Nd atoms results in a decrease of the upper critical field, which makes it attainable for existing pulse magnets. We found that addition of neodymium leads to significant suppression of superconductivity in LaH$_{10}$: each atomic % of Nd causes decrease in $\textit{T}$$_{C}$ by 10-11 K. Using strong pulsed magnetic fields up to 68 T, we constructed the magnetic phase diagram of the ternary (La,Nd)H$_{10}$ superhydride, which appears to be surprisingly linear with $\textit{H}$$_{C2}$ $\propto$ |$\textit{T}$ - $\textit{T}$$_C$|. The pronounced suppression of superconductivity in LaH$_{10}$ by magnetic Nd atoms and the robustness of $\textit{T}$$_C$ with respect to nonmagnetic impurities (e.g., Y, Al, C) under Anderson's theorem indicate the isotropic ($\textit{s}$-wave) character of conventional electron-phonon pairing in the synthesized superhydrides.Comment: Supporting Information is include