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Register a new userWhat superconducts in sulfur hydrides under pressure, and why
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The recent discovery of superconductivity at 190~K in highly compressed H$_{2}$S is spectacular not only because it sets a record high critical temperature, but because it does so in a material that appears to be, and we argue here that it is, a conventional strong-coupling BCS superconductor. Intriguingly, superconductivity in the observed pressure and temperature range was predicted theoretically in a similar compound H$_{3}$S. Several important questions about this remarkable result, however, are left unanswered: (1) Does the stoichiometry of the superconducting compound differ from the nominal composition, and could it be the predicted H$_{3}$S compound? (2) Is the physical origin of the anomalously high critical temperature related only to the high H phonon frequencies, or does strong electron-ion coupling play a role? We show that at experimentally relevant pressures H$_2$S is unstable, decomposing into H$_3$S and S, and that H$_3$S has a record high $T_c$ due to its covalent bonds driven metallic. The main reason for this extraordinarily high $T_c$ in H$_3$S as compared with MgB$_2$, another compound with a similar superconductivity mechanism, is the high vibrational frequency of the much lighter H atoms.
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Due to its low atomic mass hydrogen is the most promising element to search for high-temperature phononic superconductors. However, metallic phases of hydrogen are only expected at extreme pressures (400 GPa or higher). The measurement of a record superconducting critical temperature of 190 K in a hydrogen-sulfur compound at 200 GPa of pressure[1], shows that metallization of hydrogen can be reached at significantly lower pressure by inserting it in the matrix of other elements. In this work we re-investigate the phase diagram and the superconducting properties of the H-S system by means of minima hopping method for structure prediction and Density Functional theory for superconductors. We also show that Se-H has a similar phase diagram as its sulfur counterpart as well as high superconducting critical temperature. We predict SeH3 to exceed 120 K superconductivity at 100 GPa. We show that both SeH3 and SH3, due to the critical temperature and peculiar electronic structure, present rather unusual superconducting properties.
The discovery of high-temperature conventional superconductivity in H3S with a critical temperature of Tc=203 K was followed by the recent record of Tc ~250 K in the face-centered cubic (fcc) lanthanum hydride LaH10 compound. It was realized in a new class of hydrogen-dominated compounds having a clathrate-like crystal structure in which hydrogen atoms form a 3D framework and surround a host atom of rare earth elements. Yttrium hydrides are predicted to have even higher Tc exceeding room temperature. In this paper, we synthesized and refined the crystal structure of new hydrides: YH4, YH6, and YH9 at pressures up to 237 GPa finding that YH4 crystalizes in the I4/mmm lattice, YH6 in Im-3m lattice and YH9 in P63/mmc lattice in excellent agreement with the calculations. The observed very high-temperature superconductivity is comparable to that found in fcc-LaH10: the pressure dependence of Tc for YH9 also displays a dome like shape with the highest Tc of 243 K at 201 GPa. We also observed a Tc of 227 K at 237 GPa for the YH6 phase. However, the measured Tcs are notably lower by ~30 K than predicted. Evidence for superconductivity includes the observation of zero electrical resistance, a decrease of Tc under an external magnetic field and an isotope effect. The theoretically predicted fcc YH10 with the promising highest Tc>300 K was not stabilized in our experiments under pressures up to 237 GPa.
The long-sought goal of room-temperature superconductivity has reportedly recently been realized in a carbonaceous sulfur hydride compound under high pressure, as reported by Snider et al. [1]. The evidence presented in that paper is stronger than in other similar recent reports of high temperature superconductivity in hydrides under high pressure [2-7], and has been received with universal acclaim [8-10]. Here we point out that features of the experimental data shown in Ref. [1] indicate that the phenomenon observed in that material is not superconductivity. This observation calls into question earlier similar claims of high temperature conventional superconductivity in hydrides under high pressure based on similar or weaker evidence [2-7].
With the motivation of discovering high-temperature superconductors, evolutionary algorithm is employed to search for all stable compounds in the Sn-H system. In addition to the traditional SnH$_4$, new hydrides SnH$_8$, SnH$_{12}$ and SnH$_{14}$ are found to be thermodynamically stable at high pressure. Dynamical stability and superconductivity of tin-hydrides are systematically investigated. I$bar{4}$m2-SnH$_8$, C2/m-SnH$_{12}$ and C2/m-SnH$_{14}$ exhibit higher superconducting transition temperatures of 81, 93 and 97 K compared to the traditional compound SnH$_4$ with T$_c$ of 52 K at 200 GPa. An interesting bent H$_3^-$ in I$bar{4}$m2-SnH$_8$ and novel liner H$_4^-$ in C2/m-SnH$_{12}$ are observed. All the new tin-hydrides remain metallic over their predicted range of stability. The intermediate-frequency wagging and bending vibrations have more contribution to electron-phonon coupling parameter than high-frequency stretching vibrations of H$_2$ and H$_3$.
Linear response methods are applied to identify the increase in electron-phonon coupling in elemental yttrium that is responsible for its high superconducting critical temperature Tc, which reaches nearly 20 K at 115 GPa. While the evolution of the band structure and density of states is smooth and seemingly modest, there is strong increase in the 4d content of the occupied conduction states under pressure. We find that the transverse mode near the L point of the fcc Brillouin zone, already soft at ambient pressure, becomes unstable (in harmonic approximation) at a relative volume V/Vo=0.60 (P ~ 42 GPa). The coupling to transverse branches is relatively strong at all high symmetry zone boundary points X, K, and L. Coupling to the longitudinal branches is not as strong, but extends over more regions of the Brillouin zone and involves higher frequencies. Evaluation of the electron-phonon spectral function $alpha^2F(omega)$ shows a very strong increase with pressure of coupling in the 2-7 meV range, with a steady increase also in the 7-20 meV range. These results demonstrates strong electron-phonon coupling in this system that can account for the observed range of Tc.
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