New Frontiers in Superconductivity: Superhydrides at High Pressures

Superconductivityhas been one of the profound quantum phases in condensed matter physics.Efforts to identify and develop room temperature superconducting materials arean intensive area of research, motivated by both fundamental science and theprospects for applications. More than a century of rigorous research has ledphysicists to believe that the highest Tc that can beachieved is 40K for conventional superconductors. However, the recent discoveryof superconductivity in hydrogen sulfide at 203K [1] changed the notionof what might be possible for phonon–mediated superconductors. In this talk, Iwill discuss recent developments on high pressure room temperature superconductivity. An importantdiscovery leading to room-temperature superconductivity is thepressure-driven disproportionation of hydrogen sulfide (H2S) to H3S,with a confirmed transition temperature of 203 kelvin at 155 gigapascals [1]. Both H2S and CH4 readily mix with hydrogento form guest–host structures at lower pressures, and are of comparablesize at 4 gigapascals. By introducing methane at low pressures intothe H2S + H2 precursor mixture for H3S, molecularexchange is allowed within a large assemblage of van der Waals solids thatare hydrogen-rich with H2 inclusions; these guest–host structuresbecome the building blocks of superconducting compounds at extreme conditions. I shall present our most recentresults on superconductivity in a photochemically transformed carbonaceoussulfur hydride system, starting from elemental precursors, with a maximumsuperconducting transition temperature of 287.7 ± 1.2 kelvin (about15 degrees Celsius) achieved at 267 ± 10 gigapascals [2]. Superconductivity is established by the observation of zero resistance, amagnetic susceptibility, and reduction of the transition temperature under anexternal magnetic field of up to 9 tesla, with an upper critical magnetic fieldof about 62 tesla according to the Ginzburg–Landau model at zero temperature. TheRaman spectroscopy is used to probe the chemical and structural transformationsbefore metallization. The discovery achieves the more than a century long questto find room temperature superconductivity, a phenomenon that was firstobserved by Kamerlingh Onnes in 1911. Finally, I shall discuss future researchdirections in probing room temperature superconductivity by introduction ofchemical tuning within our ternary system at much lower pressures [3,4]. 1. Drozdov,A. P., Eremets, M. I., Troyan, I. A., Ksenofontov, V. & Shylin, S. I.Conventional superconductivity at 203 kelvin at high pressures in the sulfurhydride system. Nature 525, 73–76 (2015) 2. ElliotSnider, Nathan Dasenbrock-Gammon, Raymond McBride, Mathew Debessai, HiranyaVindana, Kevin Vencatasamy, Keith Lawler, Ashkan Salamat, Ranga P. Dias“Room TemperatureSuperconductivity in a Carbonaceous Sulfur Hydride” Nature 586, 373-377 (2020) 3. ElliotSnider, Nathan Dasenbrock-Gammon, Raymond McBride, Noah Meyers, Keith Lawler,Ashkan Salamat, Ranga P. Dias “Superconductivity to 262 kelvin via catalyzed hydrogenationof yttrium at high pressures” (In press PRL) 4. Suxing X. Hu, Reetam Paul, ValentinV. Karasiev, Ranga P. Dias, “Carbon-Doped Sulfur Hydrides as Room-TemperatureSuperconductors at 270 GPa" Under review (2021)