Finally, a Room-Temperature Superconductor. However, Enormous Pressure Required.
Edis Osmanbasic posted on October 23, 2020 |
A compound of hydrogen, carbon, and sulfur can conduct electricity without any resistance.
(Image courtesy of Source: Northwestern Engineering News.)
Room temperature superconductivity can be used to levitate objects. (Image courtesy of Northwestern Engineering News.)

Observing room-temperature superconductivity is an enduring challenge in the field of experimental physics. Recently, scientists claim to have developed a new material that can conduct electricity without any resistance at temperatures up to about 15 °C.  There is one catch, however. The new material survives only under enormous pressures, such as you would find at the Earth's center, so don't expect any practical applications soon, says Nature journal reporting on the breakthrough. 

Superconductors have numerous potential applications such as high-performance generators for wind turbines, the superconducting quantum interference devices, fast digital circuits, powerful superconducting electromagnets (maglev trains), MRIs, fusion reactors, and mobile phone towers.  

However, wider use of superconductors is still hindered by their need for massive cryogenics—the production and behavior of materials at very low temperatures. Scientifically, a gas is assumed to be cryogenic if it can be liquefied at or below −150 °C [1]. 

Today, conventional superconductors work at atmospheric pressures and only if kept very cold. Even the most advanced superconductors, such as the ones using copper oxide-based ceramic materials, can only work at temperatures below −140 °C. Thus, room-temperature working superconductors can impact this industry immensely. 

Recently, high-temperature conventional superconductivity in hydrogen-rich materials under high pressure has been reported in several instances [2], [3], [4].

In 2015, Mikhail Eremets at the Max Planck Institute for Chemistry in Mainz, Germany, reported the first high-pressure, high-temperature superconductor. It was a compound of hydrogen and sulfur that had zero resistance at temperatures up to −70 °C. This was an important development that eventually led to the discovery of a key component for room-temperature superconductivity−the pressure-driven disproportionation of hydrogen sulfide (H2S) to H3S, with a confirmed transition temperature of −70 °C at 155 GPa [2].

In 2018, researchers discovered that a high-pressure compound of hydrogen and lanthanum was superconductive at −13 °C [4]. Ashkan Salamat, a physicist at the University of Nevada, Las Vegas, stated that this was the first time this kind of superconductivity was produced by a compound of three elements rather than two. In this case, the material was made of carbon, sulfur and hydrogen. A third element expands the combinations for future experiments in the area of superconductors. Maddury Somayazulu, a high pressure materials scientist at Argonne National Laboratory in Lemont, Ill., stated that the study proved that you could bring down the superconductor operational pressure by judiciously selecting its third and fourth elements.

Evidence of high-temperature conductivity has been published in a study titled Room-temperature superconductivity in a carbonaceous sulfur hydride, which appeared in Nature (Vol. 586, 2020). Introducing methane at low pressures into the H2S + H2 precursor mixture for H3S produces a molecular exchange within a large assemblage of van der Waals solids that are hydrogen rich with H2 inclusions. These guest host structures are the foundation of superconducting compounds in extreme conditions. Superconductivity was reported in a photochemically transformed carbonaceous sulfur hydride system, starting from elemental precursors that had a maximum superconducting transition temperature of 287.7 K ± 1.2 K (about 15 °C) achieved at 267 ± 10 GPa.

Superconducting transition temperature of 276 K at 272 GPa. (Image courtesy of Nature.)
Superconducting transition temperature of 276 K at 272 GPa. (Image courtesy of Nature.)

Physicist Ranga Dias at the University of Rochester in New York, along with Salamat and other collaborators, placed a mixture of carbon, hydrogen and sulfur in a microscopic niche carved between the tips of two diamonds. The researchers then triggered chemical reactions in the sample using laser light, and watched as a crystal formed. As the experimental temperature was lowered, current resistance dropped to zero, demonstrating that the sample had become superconductive. When the pressure was increased, this transition occurred at higher and higher temperatures. The research team also proved that the crystal expelled its magnetic field at the transition temperature—a crucial test of superconductivity.

Superconductivity is established by the observation of zero resistance, magnetic susceptibility of up to 190 GPa, and reduction of the transition temperature under an external magnetic field of up to 9 T, with an upper critical magnetic field of about 62 T. 

Still, much about the material is remains unknown, such as the crystal’s exact structure and chemical formula. “As you go to higher pressures, the sample size gets smaller,” said Salamat. “That’s what makes these types of measurements challenging.”

In contrast, high-pressure superconductors made of hydrogen and one other element are well understood. Eva Zurek, a computational chemist at the State University of New York at Buffalo, stated that computer simulations of high-pressure mixtures of carbon, hydrogen and sulfur have been made. However, those studies cannot explain the remarkably high superconducting temperatures described by Ranga Dias’s research team. She concluded that many theoretical and experimental groups will address this issue once the research is published. 

References

[1] Elliot Snider, Nathan Dasenbrock-Gammon, Raymond McBride, Mathew Debessai, Hiranya Vindana, Kevin Vencatasamy, Keith V. Lawler, Ashkan Salamat & Ranga P. Dias. Room-temperature Superconductivity in a Carbonaceous Sulfur Hydride. Nature 586 (2020).

[2] Bilstein, Roger E. Stages to Saturn: A Technological History of the Apollo/Saturn Launch Vehicles (NASA SP-4206) (The NASA History Series). NASA History Office. 89–91. ISBN 978-0-7881-8186-3Cryogenics. (1996). 

[3] Drozdov, A. P., Eremets, M. I., Troyan, I. A., Ksenofontov, V. & Shylin, S. I. Conventional superconductivity at 203 kelvin at high pressures in the sulfur hydride system. Nature 525, 73–76 (2015).

[4] Drozdov, A. P. et al. Superconductivity at 250 K in lanthanum hydride under high pressures. Nature 569, 528–531 (2019). 

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