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1. High-Temperature Superconductivity in Hydrides: Experimental Evidence and Details

   https://doi.org/10.1007/s10948-022-06148-1  

   Eremets, M. I. ; Minkov, V. S. ; Drozdov, A. P. ; Kong, P. P. ; Ksenofontov, V. ; Shylin, S. I. ; Bud’ko, S. L. ; Prozorov, R. ; Balakirev, F. F. ; Sun, Dan ; et al ( March 2022 , Journal of Superconductivity and Novel Magnetism)

   Since the discovery of superconductivity at ~ 200 K in H₃S [1], similar or higher transition temperatures,T_cs, have been reported for various hydrogen-rich compounds under ultra-high pressures [2]. Superconductivity was experimentally proved by different methods, including electrical resistance, magnetic susceptibility, optical infrared, and nuclear resonant scattering measurements. The crystal structures of superconducting phases were determined by X-ray diffraction. Numerous electrical transport measurements demonstrate the typical behavior of a conventional phonon-mediated superconductor: zero resistance belowT_c, shift ofT_cto lower temperatures under external magnetic fields, and pronounced isotope effect. Remarkably, the results are in good agreement with the theoretical predictions, which describe superconductivity in hydrides within the framework of the conventional BCS theory. However, despite this acknowledgement, experimental evidences for the superconducting state in these compounds have recently been treated with criticism [3–7], which apparently stems from misunderstanding and misinterpretation of complicated experiments performed under very high pressures. Here, we describe in greater detail the experiments revealing high-temperature superconductivity in hydrides under high pressures. We show that the arguments against superconductivity [3–7] can be either refuted or explained. The experiments on the high-temperature superconductivity in hydrides clearly contradict the theory of hole superconductivity [8] and eliminate it [3].

    

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2. High-temperature superconductivity on the verge of a structural instability in lanthanum superhydride

   https://doi.org/10.1038/s41467-021-26706-w  

   Sun, Dan ; Minkov, Vasily S. ; Mozaffari, Shirin ; Sun, Ying ; Ma, Yanming ; Chariton, Stella ; Prakapenka, Vitali B. ; Eremets, Mikhail I. ; Balicas, Luis ; Balakirev, Fedor F. ( November 2021 , Nature Communications)

   The possibility of high, room-temperature superconductivity was predicted for metallic hydrogen in the 1960s. However, metallization and superconductivity of hydrogen are yet to be unambiguously demonstrated and may require pressures as high as 5 million atmospheres. Rare earth based “superhydrides”, such as LaH₁₀, can be considered as a close approximation of metallic hydrogen even though they form at moderately lower pressures. In superhydrides the predominance of H-H metallic bonds and high superconducting transition temperatures bear the hallmarks of metallic hydrogen. Still, experimental studies revealing the key factors controlling their superconductivity are scarce. Here, we report the pressure and magnetic field dependence of the superconducting order observed in LaH₁₀. We determine that the high-symmetry high-temperature superconductingFm-3mphase of LaH₁₀can be stabilized at substantially lower pressures than previously thought. We find a remarkable correlation between superconductivity and a structural instability indicating that lattice vibrations, responsible for the monoclinic structural distortions in LaH₁₀, strongly affect the superconducting coupling.

    

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3. Superconducting phase diagram of H3S under high magnetic fields

   https://doi.org/10.1038/s41467-019-10552-y  

   Mozaffari, Shirin ; Sun, Dan ; Minkov, Vasily S. ; Drozdov, Alexander P. ; Knyazev, Dmitry ; Betts, Jonathan B. ; Einaga, Mari ; Shimizu, Katsuya ; Eremets, Mikhail I. ; Balicas, Luis ; et al ( June 2019 , Nature Communications)

   The discovery of superconductivity at 260 K in hydrogen-rich compounds like LaH₁₀re-invigorated the quest for room temperature superconductivity. Here, we report the temperature dependence of the upper critical fieldsμ₀H_c2(T) of superconducting H₃S under a record-high combination of applied pressures up to 160 GPa and fields up to 65 T. We find thatH_c2(T) displays a linear dependence on temperature over an extended range as found in multigap or in strongly-coupled superconductors, thus deviating from conventional Werthamer, Helfand, and Hohenberg (WHH) formalism. The best fit ofH_c2(T) to the WHH formalism yields negligible values for the Maki parameterαand the spin–orbit scattering constantλ_SO. However,H_c2(T) is well-described by a model based on strong coupling superconductivity with a coupling constantλ~ 2. We conclude that H₃S behaves as a strong-coupled orbital-limited superconductor over the entire range of temperatures and fields used for our measurements.

    

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4. Superconductivity in La2Ni2In

   https://doi.org/10.1103/PhysRevB.102.165125  

   Maiwald, J ; Mazin, I I ; Gurevich, A ; Aronson, M C ( October 2020 , Physical review)

   null (Ed.)

   We report here the properties of single crystals of La 2 Ni 2 In . Electrical resistivity and specific heat measurements concur with the results of density functional theory calculations, finding that La 2 Ni 2 In is a weakly correlated metal, where the Ni magnetism is almost completely quenched, leaving only a weak Stoner enhancement of the density of states. Superconductivity is observed at temperatures below 0.9 K. A detailed analysis of the field and temperature dependencies of the resistivity, magnetic susceptibility, and specific heat at the lowest temperatures reveals that La 2 Ni 2 In is a dirty type-II superconductor with likely s -wave gap symmetry. Nanoclusters of ferromagnetic inclusions significantly affect the subgap states resulting in a nonexponential temperature dependence of the specific heat C ( T ) at T ≪ T c . 

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5. Structural transitions, octahedral rotations, and electronic properties of A3Ni2O7 rare-earth nickelates under high pressure

   https://doi.org/10.1038/s41535-024-00648-0  

   Geisler, Benjamin ; Hamlin, James_J ; Stewart, Gregory_R ; Hennig, Richard_G ; Hirschfeld, P_J ( April 2024 , npj Quantum Materials)

   Motivated by the recent observation of superconductivity withT_c ~ 80 K in pressurized La₃Ni₂O₇¹, we explore the structural and electronic properties ofA₃Ni₂O₇bilayer nickelates (A = La-Lu, Y, Sc) as a function of pressure (0–150 GPa) from first principles including a Coulomb repulsion term. At ~ 20 GPa, we observe an orthorhombic-to-tetragonal transition in La₃Ni₂O₇at variance with x-ray diffraction data, which points to so-far unresolved complexities at the onset of superconductivity, e.g., charge doping by variations in the oxygen stoichiometry. We compile a structural phase diagram that establishes chemical and external pressure as distinct and counteracting control parameters. We find unexpected correlations betweenT_cand thein-planeNi-O-Ni bond angles for La₃Ni₂O₇. Moreover, two structural phases with significantc⁺octahedral rotations and in-plane bond disproportionations are uncovered forA = Nd-Lu, Y, Sc that exhibit a pressure-driven electronic reconstruction in the Nie_gmanifold. By disentangling the involvement of basal versus apical oxygen states at the Fermi surface, we identify Tb₃Ni₂O₇as an interesting candidate for superconductivity at ambient pressure. These results suggest a profound tunability of the structural and electronic phases in this novel materials class and are key for a fundamental understanding of the superconductivity mechanism.

    

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