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Room Temperature Superconductivity

January 4, 2021

Although the most certain way of making a living is performance at a real job, many people are attracted to get-rich-quick schemes. One fictional proponent of these is the Ralph Kramden character in the mid-1950s television sitcom, The Honeymooners.[1] Ralph, as played by Jackie Gleason, was stuck in a low wage job, but he always had one idea, or another, to attain fortune. Those of us who bemoan such First World problems as having last year's smartphone should watch at least one episode as a reality check to see how some people lived in the 1950s.

Principal cast of The Honeymooners sitcom

Principal cast members of The Honeymooners television sitcom. From left to right, Jackie Gleason, as Ralph Kramden, Art Carney, as Ed Norton, and Audrey Meadows, as Alice Kramden. Not shown is Joyce Randolph, who played Ed Norton's wife, Trixie.

Audrey Meadows was the younger sister of Hollywood leading lady, Jayne Meadows. Audrey and Jayne's parents were Episcopal missionaries in the Wuchang District of Wuhan, China, now noted as the epicenter of the COVID-19 pandemic. (Wikimedia Commons image.)

Corporations are managed by people, so they demonstrate many human characteristics, including a tendency towards get-rich-quick schemes. This was evident after the 1986 discovery of the high superconducting transition temperature (Tc) of 35K in the ceramic material, lanthanum barium copper oxide (LaBaCuO, or LBCO) by Bednorz and Müller. Further research lead to much higher transition temperatures in the ceramic compounds, yttrium barium copper oxide (YBCO, Tc=92K) and bismuth strontium calcium copper oxide (BSCCO, Tc=107K). To put these results into perspective, superconductivity until that time was limited to predominantly metal alloys with transition temperatures below 30K where they had existed with very little improvement for many decades.[2]

The advantage of these high temperature superconductors (HTSCs) is that they are easier to cool to superconducting temperatures. While previous alloy superconductors needed liquid helium (4.2 K) as a refrigerant, HTSCs can use liquid nitrogen (77 K) instead, at a huge cost advantage. As a consequence, many corporations were blinded by dollar signs into filing many worthless, speculative patent applications about uses for HTSCs. These ceramic materials, however, proved far more difficult to process for the desired applications.

One obvious high volume application is electric power transmission cables for which the resistivity of conventional conductors causes I2R loss that wastes energy, which leads to estimated annual losses of $20 billion. Another application is wire for electromagnets used for magnetic resonance imaging. A superconducting cable can be 100% efficient, and also require less material. Ceramics, however, are brittle materials that cannot be drawn into wire. Early progress was made in such techniques as mixing particles of these materials into a matrix such as silver and extruding the result.[3]

One difficulty is that magnetism and superconductivity are incompatible, and such matrix superconductors can't withstand high magnetic fields that destroy the superconductivity. This is despite the fact that the HTSCs are type-II superconductors that are somewhat resistant to magnetic fields. This phenomenon is true not just for electromagnets, but for any current-carrying wire, such as a power transmission line. Since the magnetic field scales linearly with current, these self-generated magnetic fields can become high.

Mercury - Roman god and the first superconductor

Should Mercury be the god of superconductivity?

Superconductivity was discovered in 1911 by Heike Kamerlingh Onnes (1853-1926), who was awarded the 1913 Nobel Prize in Physics. Onnes saw an abrupt transition to zero resistance in mercury (the chemical element) at 4.2 K and immediately suspected a problem with his apparatus, but repeated checks proved his result. Mercury was a fortuitous choice for his experiments, since the obvious metals to try, such as gold, silver, or copper, are not superconducting.

Onnes published his results in 1911 in his research institute journal, "Communications of the Physical Laboratory at the University of Leiden."[4-6]

(Right image a woodcut from "Greek Mythology Systematized" by Sarah Amelia Scull, Porter & Coates, 1880, P.167; and left image, from a paper by Onnes; both Wikimedia Commons.)

Superconductivity is useful in other devices, such as SQUID magnetometers, fast digital circuitry based on Josephson junctions, and qubit devices for superconducting quantum computing. A room temperature superconductor (RTSC) would allow such devices to be more easily realized. One caveat, however, is that many such devices perform well, not just because they're superconducting, but because they're at low temperature.

A room temperature superconducting device will not always be as good as a low temperature superconducting device, and this is the case in superconducting quantum computing devices. Quantum computers are being developed to solve certain computing problems much faster than conventional computers. However, superconducting quantum computers need to have thermal energy (E=kBT) that is much lower than the energy specified by the qubit frequency (E=hν), and this is only true at very low absolute temperatures.

A Josephson junction device consists of superconducting materials separated by a thin insulating gap. electrons can tunnel through the insulator to allow a current flow. Regions of constant voltage will appear in the current-voltage curve of the device when when it's frequency-excited, so a Josephson junction can be used as an extremely precise voltage reference that's accurate at the parts per billion level. These voltages, Vn are given as

AC Josephson equation

where h is Planck's constant, and e is the elementary charge. Since the voltages depend only on fundamental constants, this is a very nice standard. The voltage standard chip shown in the photograph, below, produces a volt-level signal by connecting many junctions in series. It's driven at microwave frequency.

NIST Josephson array voltage reference

An array of 3020 superconducting Josephson junctions that act as an extremely precise voltage reference at liquid helium temperature.

(NIST image, via Wikimedia Commons.)

The discovery of a material that's superconducting without refrigeration has been a century's goal, and the discovery of the HTSCs in the 1980s gave renewed hope that this goal was within reach. While there have been published papers that give evidence that one unknown phase, or another, of a ceramic mixture superconducts at room temperature and atmospheric pressure, a uniquely specified room temperature superconductor has never been synthesized. Now, another major advance towards room temperature superconductivity has been attained in a carbonaceous sulfur hydride, albeit at extremely high pressures.[8-14] This research was done by scientists at the University of Rochester (Rochester, New York), Intel Corporation (Hillsboro, Oregon), and the University of Nevada (Las Vegas, Nevada).[8]

Scientists don't haphazardly select their study materials. They're always guided by past experiments and theory. In the case of the hydrides, Cornell University theorist, Neil Ashcroft, proposed in 1968 that hydrogen under great pressure would become metallic and a superconductor.[10] Attaining such a state in pure hydrogen proved to be difficult, and Ashcroft later proposed in 2004 that hydrogen embedded in the crystal lattice of another element would superconduct at lower pressure.[10-11] I published research on on a superconducting hydride with colleagues in 1976.[16] In 2015, H3S was found to be superconducting at 203 K (−94 degrees Fahrenheit) when compressed to 155 gigapascals (GPa); and, in 2018, lanthanum hydride was shown to be superconducting at about -13 °C while under larger pressure.[10-12]

The H3S in these experiments was formed under high pressure from dissociation of hydrogen sulfide (H2S).[8] Since H2S and methane (CH4) are miscible and of comparable size at high pressure, the Rochester/Oregon/Nevada research team decided to create mixtures of these and test them for superconductivity in a diamond anvil cell that applied the requisite pressure (see photo).[8-12] They placed solid particles of sulfur and carbon in the diamond anvil cell, exposed the solids to hydrogen, hydrogen sulfide, and methane gas, and heated them with a laser through the transparent diamond to create superconducting crystals.[10]

Dimaond anvil cell

The diamond anvil cell used in the experiments.

The anvil cell was invented by physicist, Percy W. Bridgman (1882-1961), who was awarded the 1946 Nobel Prize in Physics for his high pressure research.

Bridgeman's original anvil used tungsten carbide (WC), so the device was limited to generating pressures up to a few gigapascals.

Later, the tungsten carbide was replaced by the harder diamond.

(University of Rochester photo by J. Adam Fenster.)

The crystal were superconducting at 147 K and 148 GPa, and 287 K (about 15 °C) at 267 GPa.[10] Superconductivity was verified by magnetic field expulsion as well as by conductivity measurement.[8,11] The material was a type-II superconductor with an upper critical field of about 62 tesla.[8] Says Ashkan Salamat, a principal investigator of this study at the University of Nevada,
"The discovery is new, and the technology is in its infancy and a vision of tomorrow, but the possibilities are endless. This could revolutionize the energy grid, and change every device that's electronically driven."[9]

Since it isn't possible to probe for hydrogen using diffraction techniques, the crystal structure of this superconducting material is not yet known.[12] Once it's known, there's the possibility of chemically tuning the compound to produce superconductors at lower pressures.[8] The principal investigators of this research have founded a company, Unearthly Materials, to search for a possible room temperature superconductors that functions at ambient pressure.[9]


  1. The Honeymooners (1955=1956), Frank Satenstein, Director, on the Internet Movie Database.
  2. One example of a non-metal conventional superconductor is niobium nitride, which is superconducting below 16 K.
  3. Charles N. Wilson, "Superconductive metal matrix composites and method for making same," US Patent No. 5,041,416, August 20, 1991, via Google Patents.
  4. H. Kamerlingh Onnes, "Further experiments with liquid helium. C. On the change of electric resistance of pure metals at very low temperatures, etc. IV. The resistance of pure mercury at helium temperatures." Comm. Phys. Lab. Univ. Leiden; No. 120b, 1911.
  5. H. Kamerlingh Onnes, "Further experiments with liquid helium. D. On the change of electric resistance of pure metals at very low temperatures, etc. V. The disappearance of the resistance of mercury." Comm. Phys. Lab. Univ. Leiden; No. 122b, 1911.
  6. H. Kamerlingh Onnes, "Further experiments with liquid helium. G. On the electrical resistance of pure metals, etc. VI. On the sudden change in the rate at which the resistance of mercury disappears." Comm. Phys. Lab. Univ. Leiden; No. 124c, 1911.
  7. Celebrating the Centennial of the Discovery of Superconductivity, NIST Online Museum of Quantum Voltage Standards.
  8. Elliot Snider, Nathan Dasenbrock-Gammon, Raymond McBride, Mathew Debessai, Hiranya Vindana, Kevin Vencatasamy, Keith V. Lawler, Ashkan Salamat, and Ranga P. Dias, "Room-temperature superconductivity in a carbonaceous sulfur hydride," Nature, vol. 586, October 14, 2020, pp. 373-377, https://doi.org/10.1038/s41586-020-2801-z.
  9. UNLV and University of Rochester Physicists Observe Room-Temperature Superconductivity, University of Nevada, Las Vegas, Press Release, October 14, 2020 .
  10. Robert F. Service, "After decades, room temperature superconductivity achieved," Science, October 14, 2020, doi:10.1126/science.abf2621.
  11. Davide Castelvecchi , "First room-temperature superconductor excites — and baffles — scientists," Nature, vol. 586, October 14, 2020, p.349, doi: https://doi.org/10.1038/d41586-020-02895-0.
  12. Charlie Wood, "Room-Temperature Superconductivity Achieved for the First Time," Quanta Magazine, October 14, 2020.
  13. Paul Rincon, "Superconductors: Material raises hope of energy revolution," BBC News, October 15, 2020.
  14. The World's First Room Temperature Superconductor, YouTube Video by the University of Rochester, October 14, 2020.
  15. E.F. Talantsev, "The electron-phonon coupling constant, the Fermi temperature and unconventional superconductivity in a room-temperature superconductor carbonaceous sulfur hydride," arXiv, October 20, 2020.
  16. P. Duffer, D.M. Gualtieri, and V.U.S. Rao, Pronounced Isotope Effect in the Superconductivity of HfV2 Containing Hydrogen (Deuterium), Phys. Rev. Lett., vol 37, no. 21 (November 22, 1976) pp. 1410-1413, DOI:https://doi.org/10.1103/PhysRevLett.37.1410.

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