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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

8 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.

    [September 2022 update]

    As noted below, the study profiled in Wood’s article re superconductivity research was retracted by the journal Nature. As Neil deGrasse Tyson says, that’s par for the bleeding edge of scientific research.

    • > “‘Something is seriously wrong’: Room-temperature superconductivity study retracted” by Eris Hand (Sep 26, 2022) – After doubts grew, blockbuster Nature paper is withdrawn over objections of study team

    (quote) Eremets [Mikhail Eremets, an experimental physicist at the Max Planck Institute for Chemistry] is skeptical that Dias’s new superconductors will stand up to scrutiny. “How is this possible? Everything he touches turns to gold.” But he is confident that patient work of science, underpinned by painstaking replication, will sort the real promise of hydrides from the questionable claims. “Science is not afraid of these things,” he says. “The truth, sooner or later, will come.”

  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.”

  5. Continued research on magic graphene … using “a scanning tunneling microscope (STM) to view the infinitesimally small and complex world of electrons.”

    • > “Unmasking the magic of superconductivity in twisted graphene” by Princeton University (October 20, 2021)

    (quote) Princeton researchers [showed] an uncanny resemblance between the superconductivity of magic graphene and that of high temperature superconductors.

    When the team analyzed the data, they noticed two major characteristics, or “signatures,” that stood out, tipping them off that the magic bilayer graphene sample was exhibiting unconventional superconductivity. The first signature was that the paired electrons that superconduct have a finite angular momentum, a behavior analogous to that found in the high-temperature cuprates twenty years ago. When pairs form in a conventional superconductor, they do not have a net angular momentum, in a manner analogous to an electron bound to the hydrogen atom in the hydrogen’s s-orbital.

    The Princeton team also discovered how magic bilayer graphene behaves when the superconducting state is quenched by increasing the temperature or applying a magnetic field.

  6. Visualization

    A breakthrough discovery in understanding high-temperature superconductors models breaking a previously “hidden” particle-hole symmetry.

    • > “Physicists elucidate connection between symmetry and Mott physics” (the physics underlying high-temperature superconductors) by University of Illinois at Urbana-Champaign (March 22, 2022)

    The cuprates, a class of high-temperature superconductors, hold the record for the highest superconducting transition temperature at ambient pressure – these are the so-called Mott insulators.

    [Philip Anderson] “Mott insulators are often thought of as things that don’t break any symmetries. And because they don’t break any symmetries in this view, they’re difficult to characterize. What we found is that they do break a symmetry, namely, the hidden symmetry [one that is associated with interchanging particles and holes for just a single spin species] pointed out by Anderson and Haldane.”

    This observation proves to be a crucial step. The key insight the researchers made in the current work is that upon breaking this symmetry – for example, by adding or removing particles or holes via doping – one “destroys” a Fermi liquid.

    In their latest publication, the researchers demonstrated that the HK [Hatsugai-Kohmoto] model is the simplest model [vs. the Hubbard model] that breaks the particle-hole symmetry.


    Bardeen-Cooper-Schrieffer (BCS) theory

    BCS theory [1957] or Bardeen–Cooper–Schrieffer theory (named after John Bardeen, Leon Cooper, and John Robert Schrieffer) is the first microscopic theory of superconductivity since Heike Kamerlingh Onnes’s 1911 discovery.

    The theory describes superconductivity as a microscopic effect caused by a condensation of Cooper pairs.

    The theory is also used in nuclear physics to describe the pairing interaction between nucleons in an atomic nucleus.

    Fermi liquid theory

    Hubbard model

    The Hubbard model is an approximate model used, especially in solid-state physics, to describe the transition between conducting and insulating systems.

    The Hubbard model, named after John Hubbard, is a simple model of interacting particles in a lattice, with only two terms in the Hamiltonian (see example below): a kinetic term allowing for tunneling (“hopping”) of particles between sites of the lattice and a potential term consisting of an on-site interaction.

    The particles can either be fermions, as in Hubbard’s original work, or bosons, in which case the model is referred to as the “Bose–Hubbard model”.

    • Hatsugai-Kohmoto model

  7. More research on twisted graphene – understanding why not all twists achieve superconductivity.

    • > “In a sea of magic angles, ‘twistons’ keep electrons flowing through three layers of graphene” by Ellen Neff, Columbia University Quantum Initiative (April 8, 2022)

    Adding a third layer of graphene improves the odds of finding superconductivity, but the reason was unclear.

    Using a microscope capable of imaging down to the level of individual atoms, the team saw that groups of atoms in some areas were scrunching up into what Simon Turkel, a Ph.D. student in the Pasupathy Lab, dubbed “twistons.” These twistons [which depend on the angle mismatch between the top and bottom layers] appeared in an orderly fashion, allowing the device as a whole to better maintain the magic angles necessary for superconductivity to occur.

  8. Visualization of fermion atom pairing
    Caption: In order to model how electrons pair up in a superconducting material, MIT physicists captured 1000’s of images of particles pairing up in clouds of a very dilute gas of about 1,000 potassium-40 atoms. In this data visualization, the red and blue balls are spin-up and spin-down fermions, and some are paired together. The white sites are doubly occupied sites. Credit: Thomas Hartke, MIT.

    Here’s an interesting article on visualizing electron pairing with analogous systems of fermion atoms – to better understand superconductivity. After a years-long team effort by members of MIT’s Department of Physics, the MIT-Harvard Center for Ultracold Atoms, and the Research Laboratory of Electronics.

    Terms: Hubbard model, optical lattice

    • Phys.or > “Physicists generate first snapshots of fermion pairs, shed light on how electrons form superconducting pairs” by Jennifer Chu, MIT (July 6, 2023)

    To get a decent view of their [fermion pairs] behavior, Zwierlein [Martin Zwierlein, the Thomas A. Frank Professor of Physics at MIT] and his colleagues study the particles as a very dilute gas of about 1,000 atoms, that they place under ultracold, nanokelvin conditions that slow the atoms to a crawl. The researchers also contain the gas within an optical lattice, or a grid of laser light that the atoms can hop within, and that the researchers can use as a map to pinpoint the atoms’ precise locations.

    “In this dynamic soup, [as predicted by the Hubbard model,] the particles are constantly hopping on top of each other, moving away, but never dancing too far from each other,” Zwierlein notes.

    “If you normalize our gas of atoms to the density of electrons in a metal, we think this pairing behavior should occur far above room temperature,” Zwierlein offers. “That gives a lot of hope and confidence that such pairing phenomena can in principle occur at elevated temperatures, and there’s no a priori limit to why there shouldn’t be a room-temperature superconductor one day.”

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