From cuprates to Room Temperature Superconductors

Dear Colleagues,

Room Temperature Superconductivity  (RTS)  has been realized in highly pressurized LaH_y, with y≈10, which is capable of exhibiting superconductivity at temperatures below 260 K (-13 °C) by  a collaboration of the groups of Russell J. Hemley at George Washington University, of Viktor V. Struzhkin, at Geophysical Laboratory of Carnegie Institution and of Yue Meng at Argonne National Laboratory (arXiv:1808.07695), increasing the critical temperature found few days before by Eremets group at some lower temperatures (arXiv:1808.07039) in  a similar system.

 

Superconductivity at winter-time temperatures near the temperature at which ice forms is an epochal scientific achievement. The highest T_c known previously was in highly pressurized hydrogen sulfide, H₃S, where the transition temperature is 203 K (−70 °C).  The record for superconductivity at atmospheric pressure is held by cuprates, which superconduct at temperatures as high as 138 K (−135 °C) .

Since quantum phenomena are known to manifest at atomic or subatomic scales, or at very high energies, the majority of researchers have previously had doubts about whether room-temperature superconductivity was actually achievable. In fact, superconductivity is a manifestation of quantum coherence at a macroscopic level, which was considered by the majority of scientists to be possible only near zero temperature. This was supported by the well-established BCS theory formulated in 1957, which predicted that the dream of technologists, a room-temperature superconductor, could not exist; the maximum temperature for superconductivity according to BCS theory was just 30 K. However, BCS considers superconductivity based on cooper pairing mediated by phonons in a weak coupling regime appearing in a homogeneous 3D metal with high electron-density, with an isotropic Fermi surface and a phonon spectrum with much less energy than the Fermi surface’s energy.

The experimental realization of room temperature superconductivity has changed this perception, showing that quantum coherence on a large, macroscopic scale is possible nearly at room temperature, supporting the unpopular ideas of some scientists who proposed reaching RTS in systems that are beyond the standard BCS approximations.

Little (1964) proposed RTS driven by an electron-mediated mechanism in one-dimensional organic conductors. Ginsburg (1968) proposed RTS driven by an excitonic mechanism. Ashcroft (1968) proposed RTS in metallic hydrogen at very high pressure. Eagles (1969) proposed strong paring without superconductivity near the limit for BEC condensation at very low electron density. Müller (1986) proposed approaching RTS driven by very large electron-phonon coupling driven by the Jahn-Teller effect, and discovered high-temperature superconductors in doped perovskites. Bianconi (1993) proposed RTS driven by a Fano resonance between a BCS condensate and a BCS-BEC condensate in the appearing Fermi surface by tuning the chemical potential around the Lifshitz transition to produce the appearance of a new Fermi surface spot in quasi 2D or quasi 1D electronic matter. Perali et al (1996) have provided a theory for the numerical solution of Bogoliubov equations for multi gap systems that are able to predict RTS in superlattices of “1D” or “2D” structures.

The present experimental result supports the Neil Ashcroft theory developed in these last 50 years as for example in

• Hemley, R. J. & Ashcroft, N. W. The revealing role of pressure in the condensed matter sciences. Physics Today 51, 26-32 (1998). doi:10.1063/1.882374.
• Ashcroft, N. W. Symmetry and Higher Superconductivity in the Lower Elements, In Symmetry and Heterogeneity in High Temperature Superconductors, NATO Science Series II: Mathematics, Physics and Chemistry (Bianconi A. ed.) 214 (2006), pp. 3-20, (Springer Netherlands, 2006 doi:10.1007/1-4020-3989-1_1 
• Liu, H., Naumov, I. I., Hoffmann, R., Ashcroft, N. W. & Hemley, R. J. Potential high-Tc superconducting lanthanum and yttrium hydrides at high pressure. Proceedings of the National Academy of Sciences 114, 6990-6995 (2017). doi:10.1073/pnas.1704505114

The data accumulated in the successful search for RTS should be used as the key ingredients for other RTS in hydrides which is one of key topics of this Special Issue. Moreover today the researchers look for how to realize superconducting devices at atmospheric pressure operating at high temperatures for quantum computers. New atomic scale systems like  “2D”, graphene-like, and “1D” structures,  as thick as a single atom or molecule are synthesized opening new venues for room temperature superconductors.

Regards,

Prof. Antonio Bianconi
Prof. Andrea Perali
Guest Editors

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