Room Tc...

The cage-like crystal structure (LaH₁₀) thought to be responsible for the high-temperature superconductivity observed in this study. Courtesy: R Hemley

Topics: Green Energy, Materials Science, Quantum Mechanics, Superconductors

Note: Room temperature is 300 K, which is 26.85 Celsius, 80.33 Fahrenheit.

A team of researchers from George Washington University in the US is saying that a hydride of lanthanum compressed to 200 GPa (2 Mbars) could be superconducting at temperatures near room temperature – a result that has been backed up with findings from another group in Germany. The results could be a major step towards realizing the long-sought goal of room-temperature superconductivity for energy applications.

Superconductivity is the ability of a material to conduct electricity without any resistance. It is observed in many materials when they are cooled to below their superconducting transition temperature (T_c). In the Bardeen-Cooper-Schrieffer (BCS) theory of (“conventional”) superconductivity, this occurs when electrons overcome their mutual electrical repulsion and form “Cooper pairs” that then travel unheeded through the material as a supercurrent.

Superconductivity was first observed in 1911 in solid mercury below a T_c of 4.2K (--268.95 Celsius, --452.11 Fahrenheit) and the search for room-temperature superconductors has been on ever since. Room-temperature superconductivity would help considerably improve the efficiency of electrical generators and transmission lines, as well simplify current applications of superconductivity, such as superconducting magnets in particle accelerators.

Researchers came a step closer to this holy grail with the high-temperature superconducting copper oxides, which were discovered in the 1990s and which have a Tc above liquid helium temperatures. It was only in 2015, however, that they discovered that hydrogen sulphide has a Tc of 203 K when compressed to pressures of 150 GPa. This result spurred a flurry of interest in the compressed hydrides – that is, solid materials containing hydrogen atoms bonded to other elements.

Dramatic resistance drop at 260 K

“We believe that a T_c at – or very near – room temperature has finally been realized,” says Russell Hemley, who led this latest research effort.

Thanks to quantum-mechanics-based calculations, Hemley’s group first predicted that lanthanum hydride (LaH₁₀) could be superconducting in July 2017. The researchers then synthesized the material, and reported direct measurements of its conductivity that indicated a T_c of 260 K (-13.15 Celsius, 8.33 Fahrenheit) at 180-200 GPa in May 2018, posting a paper on the arXiv in August 2018 that has now been published in Physical Review Letters. A team led by Mikhail Eremets at the Max Planck Institute for Chemistry in Germany reported on a T_c of 250 K (-23.15 Celsius, -9.67 Fahrenheit) for lanthanum hydride synthesized at pressures of around 170 GPa in independent work posted on the arXiv in December 2018.

Quantum-mechanics-based calculations for “materials by design”

The researchers say they have reproduced their result many times and also have preliminary magnetic susceptibility data that point to room-temperature superconductivity. To unequivocally prove, however, that this is indeed the case will require them to observe the Meissner effect (the expulsion of magnetic field from a material when it becomes superconducting) in LaH₁₀. This is challenging, they admit, but preliminary results from experiments on their samples at the Argonne National Laboratory in Illinoisare encouraging. Further work is also needed to characterize the superconducting properties of structures other than LaH₁₀ in their samples that they have predicted and observed using X-ray diffraction.

On the road to room-temperature superconductivity, Belle Dumé, Physics World