Superconductivity: Theoretical procedures and experimental protocol

There has been a lot of progress and development in the superconductivity (zero electrical resistance) front owing to its wide range of applications in MRI machines, particle accelerators, and low-loss power cables. [31] University of Chicago scientists are part of an international research team that has discovered superconductivity-the ability to conduct electricity perfectly-at the highest temperatures ever recorded.

2022

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We calculate superconducting transition temperatures (Tc) in sulfur hydrides H2S and H3S from first principles using the density functional theory for superconductors. At pressures of 150 GPa, the high values of Tc (≥130 K) observed in the recent experiment [A. P. Drozdov, M. I. Eremets, and I. A. Troyan, arXiv:1412.0460] are accurately reproduced by assuming that H2S decomposes into R3m-H3S and S. For the higher pressures, the calculated Tcs for Im3m-H3S are systematically higher than those for R3m-H3S and the experimentally observed maximum value (190 K), which suggests the possibility of another higher-Tc phase. We also quantify the isotope effect from first principles and demonstrate that the isotope effect coefficient can be larger than the conventional value (0.5) when multiple structural phases energetically compete.

arXiv (Cornell University), 2023

The phenomenon of high temperature superconductivity, approaching room temperature, has been realized in a number of hydrogen-dominant alloy systems under high pressure conditions 1-12. A significant discovery in reaching room temperature superconductivity is the photo-induced reaction of sulfur, hydrogen, and carbon that initially forms as van der Waals solids at sub-megabar pressures. Carbonaceous sulfur hydride has been demonstrated to be tunable with respect to carbon content, leading to different superconducting final states with different structural symmetries. A modulated AC susceptibility technique adapted for a diamond anvil cell confirms a Tc of 260 kelvin at 133 GPa in carbonaceous sulfur hydride. Furthermore, direct synchrotron infrared reflectivity measurements on the same sample under the same conditions reveal a superconducting gap of ~85 meV at 100 K in close agreement with the expected value from Bardeen-Cooper-Schrieffer (BCS) theory 13-18. Additionally, x-ray diffraction in tandem with AC magnetic susceptibility measurements above and below the superconducting transition temperature, and as a function of pressure at 107-133 GPa, reveal the Pnma structure of the material is responsible for the close to room-temperature superconductivity at these pressures. Main The determination of the underlying interaction that governs the formation of Cooper pairs has become one of the major goals in the understanding of new superconducting materials 17-21. In particular, hydrogen-dominant alloys, such as the superhydrides, have been found to exhibit high superconducting critical temperatures (Tc) in excess of 200 K at megabar pressures (>100 GPa) 1-8,10-12. These materials, including both binary and an increasing number of ternary hydride systems, have been explored by simulation-with the theoretical calculations in some cases first predicting

Physical Review B

Frontiers in Electronic Materials, 2022

The search for room temperature superconductivity has accelerated in the last few years driven by experimentally accessible theoretical predictions that indicated alloying dense hydrogen with other elements could produce conventional superconductivity at high temperatures and pressures. These predictions helped inform the synthesis of simple binary hydrides that culminated in the discovery of the superhydride LaH10with a superconducting transition temperatureTcof 260 K at 180 GPa. We have now successfully synthesized a metallic La-based superhydride with an initialTcof 294 K. When subjected to subsequent thermal excursions that promoted a chemical reaction to a higher order system, theTconset was driven irreversibly to 556 K. X-ray characterization confirmed the formation of a distorted LaH10based backbone that suggests the formation of ternary or quaternary compounds with substitution at the La and/or H sites. The results provide evidence for hot superconductivity, aligning with re...

Materials Today, 2021

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 LaY 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)H 10 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 Im3m-YH 6 and Fm3m-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 Im3m-LaH 6 and Fm3m-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.

Hydrogen dense materials of the form AH3 (where A can be Al, Sc, Ga, S, Cr, Se, Y, La, P) are gaining interest with respect to study high temperature superconductivity at pressure with the reach of available techniques. In the present work, we have used first principle calculations to correlate the ionization energies and the superconducting critical temperatures for the metal hydrides. Using a linear regression, a straight line fit of the correlation implies a certain limit for sum of the ionization energy needed for superconductivity to occur. Alkali C60 superconductors shown similar nature with ionization energy.

Journal of Applied Physics, 2021

Recently, YH 6 was synthesized as a first compound from theoretically predicted stable compressed MH 6 hydrides with bcc Im-3m crystal structures. Superconductivity of pressurized YH 6 was confirmed with critical temperature (T c) that is considerably lower than the predicted value by Migdal-Eliashberg (ME) theory. Here, we present theoretical reinvestigation of the superconductivity for selected MH 6 hydrides. Our results confirm that YH 6 and ScH 6 with Im-3m structure at corresponding GPa pressures are superconductors but with an anti-adiabatic character of superconducting ground state and a multiple-gap structure in one-particle spectrum. Transition into superconducting state is driven by strong electron-phonon coupling with phonons of H atom vibrations. Based on anti-adiabatic theory, calculated critical temperature T c in YH 6 is ≈ 231 K, i.e. just by ≈7 K higher than the experimental value. For ScH 6 the calculated critical temperature is T c ≈ 196 K. This value is by 27 K higher than a former theoretical prediction. Unexpected results concern CaH 6 and MgH 6 in Im-3m structure at corresponding GPa pressures. Calculated band structures (BS) indicate that in CaH 6 and MgH 6 the couplings to H stretching vibrations do not induce transitions into superconducting anti-adiabatic state and these hydrides remain stable in adiabatic metal-like state, which contradicts to former predictions of ME theory. These discrepancies are discussed in association with BS structure and a possible role of dorbitals on the involved metals, while we stress that the anti-adiabatic theory uses BS topology and its stability as a key input.

Advanced Materials

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 LaH10, encounters a serious complication because of the large upper critical magnetic field HC2(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 LaH10: each atomic % of Nd causes decrease in TC by 10-11 K. Using strong pulsed magnetic fields up to 68 T, we constructed the magnetic phase diagram of the ternary (La,Nd)H10 superhydride, which appears to be surprisingly linear with HC2 ∝ |T-TC|. The pronounced suppression of superconductivity in LaH10 by magnetic Nd atoms and the robustness of TC with respect to nonmagnetic impurities (e.g., Y, Al, C) under Anderson's theorem indicate the isotropic (s-wave) character of conventional electron-phonon pairing in the synthesized superhydrides.