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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 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 (T
c) of 35
K 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.
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=k
BT) 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
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.
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, H
3S 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 H
3S in these experiments was formed under high pressure from
dissociation of
hydrogen sulfide (H2S).[8] Since H
2S 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]
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]
References:
- The Honeymooners (1955=1956), Frank Satenstein, Director, on the Internet Movie Database.
- One example of a non-metal conventional superconductor is niobium nitride, which is superconducting below 16 K.
- Charles N. Wilson, "Superconductive metal matrix composites and method for making same," US Patent No. 5,041,416, August 20, 1991, via Google Patents.
- 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.
- 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.
- 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.
- Celebrating the Centennial of the Discovery of Superconductivity, NIST Online Museum of Quantum Voltage Standards.
- 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.
- UNLV and University of Rochester Physicists Observe Room-Temperature Superconductivity, University of Nevada, Las Vegas, Press Release, October 14, 2020 .
- Robert F. Service, "After decades, room temperature superconductivity achieved," Science, October 14, 2020, doi:10.1126/science.abf2621.
- 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.
- Charlie Wood, "Room-Temperature Superconductivity Achieved for the First Time," Quanta Magazine, October 14, 2020.
- Paul Rincon, "Superconductors: Material raises hope of energy revolution," BBC News, October 15, 2020.
- The World's First Room Temperature Superconductor, YouTube Video by the University of Rochester, October 14, 2020.
- 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.
- 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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