On the road to room-temperature superconductivity

25 Jan 2019 Belle Dumé

Lanthanum superhydride

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 (Tc). 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 Tc of 4.2K 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 Tc 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 Tc of 260 K 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 Tc of 250 K for lanthanum hydride synthesized at pressures around 170 GPa in independent work posted on the arXiv in December 2018.

Hemley and colleagues use a special modulated heating method to synthesize their superhydride material at 180-200 GPa and temperatures of between 1000 to 2000 K while the material is in a diamond anvil cell. In their electrical conductivity studies, they do this with carefully mounted micro-electrodes on the tips of the diamond anvil to measure the superconducting properties of the material as it is cooled. They observe a dramatic resistance drop at 260 K. Additional experiments indicate that the Tc could reach 280 K depending on the synthesis conditions.

Quantum-mechanics-based calculations for “materials by design”The Washington team has also studied the effect of applied current on the Tc of their sample. “This measurement gives us a critical current estimate that is remarkably high,” explains Hemley. “We have performed low-temperature X-ray diffraction on LaH10 too to determine if the transition as a function of temperature is associated with a major structural change – which we find it is not.”

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.

The fact that researchers were able to predict high-Tc superconductivity in this material using quantum-mechanics-based calculations before actually synthesizing it meant that they were able to guide the experiments in the right direction and identify other potentially interesting compounds. This “materials by design” approach will be important for motivating experimentalists to investigate similar systems, such as yttrium hydride, whose predicted Tc exceeds room temperature, says Hemley. “Other promising new superconductors include carbides and other low atomic number compounds with similar structures that in fact may be stable at ambient pressures.”

The researchers are now indeed busy exploring a broader range of compositions based on their own and other groups’ calculations. “We believe that LaH10is just one of many superhydrides with likely high Tcs, some that could be superconductors even above room temperature” Hemley tells Physics World.

25/1/2019 from phhsicsworld.com