International. In 1911, while studying the properties of matter at very low temperatures, Dutch physicist Heike Kamerlingh Onnes and his team discovered that the electrical resistance of solid mercury is zero below 4.2 K (−269 °C). This was the first observation of the phenomenon of superconductivity.
Below a certain "critical" temperature (Tc), most chemical elements undergo a transition to the superconducting state, characterized by two basic properties: first, they offer no resistance to the passage of electric current. When the resistance drops to zero, a current can circulate within the material without any energy dissipation. Secondly, as long as they are weak enough, external magnetic fields will not penetrate the superconductor. This phenomenon of field ejection is known as the Meissner effect, named after the physicist who first observed it in 1933. The technical superconductors in use today allow partial penetration of the magnetic field, making them suitable for electrotechnical applications.
Superconductivity has a number of technological applications including powerful superconducting electromagnets used, for example, in magnetic levitation trains, magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) machines, magnetic confinement fusion reactors (e.g., tokamaks), and focus and steering magnets used in particle accelerators.
Superconductivity at room temperature: the "holy grail" of physics
Currently, "ordinary" or metallic superconductors typically have critical temperatures below 30 K (-243.2 °C) and must be cooled with liquid helium to achieve superconductivity. High-temperature superconductors have a Tc as high as 138 K (-135°C) and can therefore be cooled using liquid nitrogen.
A room-temperature superconductor, if it existed, would allow for more efficient generation and use of electricity, improved energy transmission around the world, and more powerful computer systems, among many other possibilities.
The pursuit of superconductivity at room temperature is a long-standing challenge. Since Kamerlingh Onnes, countless scientists have been searching for a material, whose Tc exceeds room temperature.
The discovery of high-temperature superconductivity in copper oxides raised Tc above the temperature of liquid helium. Since 1994, one of the copper oxides has held the record for the highest Tc (133 K at atmospheric pressure and 164 K at high pressure). Despite intense research, it took another 20 years to break this record in an entirely new class of systems: in 2015, the compression of hydrogen sulfide at 150 GPa (1.5 million bar), or about 40% of the pressure found in the Earth's core, He gave a Tc of 203 K.
A new breakthrough in the field of superconductors
Notably, two independent groups, the first led by Russell Hemley at George Washington University in Washington, DC, USA, and the second by Mikhail Eremets at the Max Planck Institute of Chemistry, Germany, have just reported experiments indicating that a lanthanum hydride (LaH10) compressed to 170–185 GPa (1.7 to 1.85 million rods) has a Tc of 250–260 K (-23–13 °C).
According to Eva Zurek of the University at Buffalo, these findings strongly suggest superconductivity, but to prove it beyond any doubt, it would be necessary to also observe the Meissner effect. However, measuring this effect is a challenge: for the previous recording of high Tc, sulfur hydride, the Meissner effect could only be demonstrated several years after the initial superconductivity report. Since lanthanum hydride samples are significantly smaller than sulfur hydride samples, demonstrating the Meissner effect for lanthanum hydride will require substantial experimental efforts.
Additional experimental and theoretical work will also be needed to identify the multiple crystal lattices contained in the samples. The data suggest that one of these is LaH10, but the identity of the other structures remains unknown. This information will be essential to understand the relationship between crystal structure and superconductivity and possibly to reveal new superconducting phases that could have an even greater Tc. And the high Tc of LaH10 will certainly motivate experimenters to investigate similar systems, such as yttrium hydride, whose predicted Tc exceeds room temperature.
The new findings obtained by the two teams of researchers bode well for the search for room-temperature superconductors. As the German scientists underlined in their paper: "This jump, by 50 K, from the previous critical temperature record of 203 K indicates the real possibility of achieving superconductivity at room temperature in the near future at high pressures, and the prospect of conventional superconductivity at ambient pressure."
Russell Hemley concludes that "this is just the beginning of a new era of superconductivity."
Source: International Refrigeration Institute.
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