A possible major step in the quest of room-temperature superconductivity

Two researcher teams have just reported experiments indicating that a hydride of lanthanum compressed to 1.7 to 1.85 million bar has a critical temperature of 250–260 K (-23 – -13°C).

In 1911, while studying the properties of matter at very low temperature, the Dutch physicist Heike Kamerlingh Onnes and his team discovered that the electrical resistance of solid mercury goes to zero below 4.2 K (-269°C). This was the very first observation of the phenomenon of superconductivity.1

Below a certain “critical” temperature (Tc), the majority of chemical elements undergo transition into the superconducting state, characterized by two basic properties: firstly, they offer no resistance to the passage of electrical current. When resistance falls to zero, a current can circulate inside the material without any dissipation of energy. Secondly, provided they are sufficiently weak, external magnetic fields will not penetrate the superconductor. This field expulsion phenomenon is known as the Meissner effect, name of the physicist who first observed it in 1933.1 The technical superconductors in use today allow partial penetration of the magnetic field, which renders them suitable for electrotechnical applications.

Superconductivity has a number of technological applications including powerful superconducting electromagnets used for example in maglev trains, magnetic resonance imaging (MRI) and Nuclear magnetic resonance (NMR) machines, magnetic confinement fusion reactors (e.g. tokamaks), and the beam-steering and focusing magnets used in particle accelerators.

Room-temperature superconductivity: the “holy grail” of physics

Currently, "ordinary" or metallic superconductors usually have critical temperatures below 30 K (-243.2°C) and must be cooled using liquid helium in order to achieve superconductivity. High-temperature superconductors have a Tc as high as 138 K (-135°C) and can thus be cooled using liquid nitrogen.

A room-temperature superconductor, if it existed, would allow for more efficient generation and use of electricity, enhanced energy transmission around the world and more powerful computing systems, among many other possibilities.

The quest for room-temperature superconductivity is a longstanding challenge. Ever since Kamerlingh Onnes, countless scientists have been searching for a material, the Tc of which exceeds room temperature.

The discovery of high-temperature superconductivity in copper oxides raised Tc above liquid helium temperature. Since 1994, one of the copper oxides has held the record for the highest Tc (133 K at atmospheric pressure and 164 K under high pressure). Despite intense research, it took another 20 years to beat this record in a completely new class of systems: in 2015, the compression of hydrogen sulfide to 150 GPa (1.5 million bar), or about 40% of the pressure found in Earth’s core, yielded a Tc of 203 K.

A new breakthrough in superconductors field

Remarkably, two independent groups, the first led by Russell Hemley at the George Washington University in Washington, DC, USA2, and the second by Mikhail Eremets at the Max Planck Institute for Chemistry, Germany 3, have just reported experiments indicating that a hydride of lanthanum (LaH10) compressed to 170–185 GPa (1.7 to 1.85 million bar) has a Tc of 250–260 K (-23–-13°C).

According to Eva Zurek from the University at Buffalo, SUNY, Buffalo NY, USA, these findings are strongly suggestive of superconductivity, but to prove it beyond any shadow of doubt, it would be necessary to also observe the Meissner effect. Measuring this effect is, however, challenging: for the previous high-Tc record holder, sulfur hydride, the Meissner effect could only be demonstrated several years after the initial report of superconductivity. Since the lanthanum hydride samples are significantly smaller than the sulfur hydride samples, demonstrating the Meissner effect for lanthanum hydride will require substantial experimental efforts.4

Further theoretical and experimental work will also be needed to identify the multiple crystalline lattices contained in the samples. The data strongly 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 unveil new superconducting phases that might have an even higher Tc. And the high Tc of LaH10 will certainly motivate experimentalists to investigate similar systems, such as yttrium hydride, whose predicted Tc exceeds room temperature.4

The new findings obtained by the two teams of researchers bode well for the search for room-temperature superconductors. As stressed by the German scientists in their paper: “This leap, by 50 K, from the previous critical temperature record of 203 K indicates the real possibility of achieving room-temperature superconductivity in the near future at high pressures, and the perspective of conventional superconductivity at ambient pressure.”3

For his part, Russell Hemley concludes that “This is just the beginning of a new era of superconductivity”.

1 https://home.cern/science/engineering/superconductivity

2 M. Somayazulu et al. “Evidence for superconductivity above 260 K in lanthanum superhydride at megabar pressures,” Phys. Rev. Lett. 122, 027001 (2019). https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.122.027001

3 A. P. Drozdov et al., “Superconductivity at 250 K in lanthanum hydride under high pressures,” arXiv:1812.01561. https://arxiv.org/ftp/arxiv/papers/1812/1812.01561.pdf

4 E. Zurek, Viewpoint: Pushing Towards Room-Temperature Superconductivity, https://physics.aps.org/articles/v12/1