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Quest for room temperature superconductivity

[“Quest” series]

This is a topic which I’ve followed for decades. A holy grail of physics: room temperature superconductivity. The physics of Cooper pairs: what makes two electrons pair up when their charge actually makes them repel each other?

SciTechDaily > “Breakthrough in Understanding the Physics of High-Temperature Superconductivity” by Helmholtz-Zentrum Dresden-Rossendorf (May 12, 2020).

How do electrons form pairs in high-temperature superconductors such as cuprates?

In superconductivity, electrons combine to create “Cooper pairs,” which enables them to move through the material in pairs without any interaction with their environment. But what makes two electrons pair up when their charge actually makes them repel each other?

For conventional superconductors, there is a physical explanation: … One electron distorts the crystal lattice, which then attracts the second electron. For cuprates, however, it has so far been unclear which mechanism acts in the place of lattice vibrations.

At this point “Higgs oscillations” enter the stage: In high-energy physics, they explain why elementary particles have mass. But they also occur in superconductors, where they can be excited by strong laser pulses. They represent the oscillations of the order parameter – the measure of a material’s superconductive state, in other words, the density of the Cooper pairs.

So, “Higgs oscillations” = oscillations in the density of the Cooper pairs? Such oscillation can be induced via cyclic high-energy laser pulses. Like using your legs to continuously maintain a swing, eh.

[Image caption] By applying a strong terahertz pulse (frequency ω), they stimulated and continuously maintained Higgs oscillations in the material (2ω). Driving the system resonant to the Eigenfrequency of the Higgs oscillations in turn leads to the generation of characteristic terahertz light with tripled frequency (3ω). Credit: HZDR / Juniks

4 thoughts on “Quest for room temperature superconductivity

  1. A landmark in condensed matter physics …

    • Quanta Magazine > “Room-Temperature Superconductivity Achieved for the First Time” by Charlie Wood (October 14, 2020) – Physicists have reached a long-sought goal. The catch is that their room-temperature superconductor requires crushing pressures to keep from falling apart [maintain a metallic lattice].

    [Image] A novel metallic compound of hydrogen, carbon and sulfur [“the group doesn’t know how the atoms are arranged, or even the substance’s exact chemical formula”] exhibited superconductivity at a balmy 59 degrees Fahrenheit when pressurized between a pair of diamond anvils. (J. Adam Fenster / University of Rochester)

    … the substance superconducts at room temperature only while being crushed between a pair of diamonds to pressures roughly 75% as extreme as those found in the Earth’s core.

    Cooper pairs … stream together in a coherent swarm that passes through the metal’s lattice unimpeded, experiencing no resistance whatsoever.

    Researchers have spent decades searching for a superconductor whose Cooper pairs tango tightly enough to withstand the heat of everyday environments.

  2. More research on terahertz quantum cascade lasers …

    • > “Researchers develop a high-power, portable terahertz laser” by Michaela Jarvis, Massachusetts Institute of Technology (Nov 2, 2020)

    In a paper published in Nature Photonics, MIT Distinguished Professor of Electrical Engineering and Computer Sciences Qing Hu and his colleagues report that their terahertz quantum cascade laser can function at temperatures of up to 250 K (-10 degrees Fahrenheit), meaning that only a compact portable cooler is required.

    “These are very complex structures with close to 15,000 interfaces between quantum wells and barriers, half of which are not even seven atomic layers thick,” says co-author Zbig Wasilewski, professor of electrical and computer engineering and University of Waterloo Endowed Chair in Nanotechnology. “The quality and reproducibility of these interfaces are of critical importance to the performance of terahertz lasers. It took the best in molecular beam epitaxial growth capabilities – our research team’s key contribution—together with our MIT collaborators’ expertise in quantum device modeling and fabrication, to make such important progress in this challenging sector of THz photonics.”



    Quantum well structure

  3. The quest continues for room temperature superconductivity, with yttrium hydride (YH6) requiring less extreme pressure than the reigning leader. [See prior Oct 18, 2020 comment.] A new puzzle as to why “the critical magnetic field observed in the experiment is 2 to 2.5 times greater as compared to theoretical predictions.” A goal to attain superconductivity at lower extreme pressures.

    • EurekAlert (American Association for the Advancement of Science) > “Scientists have synthesized a new high-temperature superconductor” by SKolkovo Institute of Science and Technology (Skoltech) (March 10, 2021)

    Yttrium hydrides rank among the three highest-temperature superconductors known to date.

    The leader among the three is a material with an unknown S-C-H [sulfur-carbon-hydrogen] composition and superconductivity at 288 K [~15 °C or ~60 °F … at 267 GPa or ~2,635,085 atmospheres],

    which is followed by lanthanum hydride, LaH10 (superconducting at temperatures up to 259 K),

    and, finally, yttrium hydrides, YH6 and YH9, with maximum superconductivity temperatures of 224 K and 243 K, respectively. The superconductivity of YH6 was predicted by Chinese scientists in 2015.

    All of these hydrides reach their maximum superconductivity temperatures at very high pressures: 2.7 million atmospheres for S-C-H and about 1.4 – 1.7 million atmospheres for LaH10 and YH6.

    The high pressure requirement remains a major roadblock for quantity production.

  4. Research on superconductivity in twisted bilayer graphene continues. All hail, Cooper pairs!

    • > “Research finds surprising electron interaction in ‘magic-angle’ graphene” by Kevin Stacey, Brown University (March 19, 2021)

    Twisted bilayer graphene model
    [Caption] Researchers have discovered a way to manipulate the repulsive force between electrons in “magic-angle” graphene, which provides new insight into how this material is able to conduct electricity with zero resistance. Credit: Li lab / Brown University

    Since the initial discovery [in 2018], researchers have been working to understand this exotic state of matter [twisted one-atom-thick sheets of carbon]. … In a study published in the journal Science, the researchers show that magic-angle superconductivity grows more robust when Coulomb interaction is reduced … .

    Cooper pairing locks electrons together at a specific distance from each other. That pairing competes with the Coulomb interaction, which is trying to push the electrons apart.

    To manipulate the Coulomb interaction for this study, the researchers built a device that brings a sheet of magic-angle graphene in very close proximity to another type of graphene sheet called a Bernal bilayer. Because the two layers are so thin and so close together, electrons in the magic-angle sample become ever so slightly attracted to positively charged regions in the Bernal layer. That attraction between layers effectively weakens the Coulomb interaction felt between electrons within the magic-angle sample, a phenomenon the researchers call Coulomb screening.

    “Nobody has ever built anything like this before,” Li said. “Everything had to be incredibly precise down to the nanometer scale, from the twist angle of the graphene to the spacing between layers. Xiaoxue really did an amazing job. We also benefitted from the theoretical guidance of Oskar Vafek, a theoretical physicist from Florida State University.”

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