Could high-pressure hydrides be high-temperature superconductors?
28 Dec 2018 Belle Dumé
Researchers at the Max Planck Institute for Chemistry in Mainz, Germany say that lanthanum hydride (LaH10) could be superconducting at the remarkably high temperature of 250 K (–23 °C), albeit at extreme pressures of around 170 GPa. Meanwhile, another team from George Washington University in the US says that it has found evidence of superconductivity in the same material at even higher temperatures of 280 K (7 °C) under 202 GPa pressures. If confirmed, the findings could be a major step towards finding room-temperature superconductors.
Superconductivity is the complete absence of electrical resistance and 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.
Long sought-after goal
The best superconductors available today unfortunately need to be cooled down to liquid helium or liquid nitrogen temperatures. “Room-temperature superconductivity (generally accepted as being 273 K or 0 °C) is thus a long sought-after goal for scientists,” says Mikhail Eremets, whose research group discovered high-temperature superconductivity in hydrogen sulphide under 150 GPa pressures in 2014. Room-temperature superconductivity would help considerably improve the efficiency of electricity generators and transmitters, as well simplify current applications of superconductivity, such as superconducting magnets in particle accelerators.
Before 2014, the highest Tc for conventional superconductors was 39 K (–230 °C) for magnesium diboride and many believed that it would be impossible to reach any higher. Although superconductivity has been achieved at 164 K at high pressure in copper-oxide systems, these are known as unconventional superconductors, for which an accepted theory is still lacking.
New family of hydrides
“The discovery of superconductivity at 203 K in H2S, brought attention back to conventional superconductors that can be described by the BCS and Migdal-Eliashberg theories,” says Eremets. “These theories predict that high and even room-temperature superconductivity is possible in metals with certain favourable parameters such as lattice vibrations at high frequencies. We can now predict superconducting materials with the aid of first principles calculations based on density functional theory and these suggest a new family of hydrides with a clathrate structure in which the host atom (Ca, Y or La) is at the centre of a cage formed by hydrogen atoms.
“Indeed, for LaH10 and YH10, superconductivity with Tcs ranging from 240 to 320 K is predicted at megabar pressures.”
Three different kinds of measurements
The researchers say they have now found evidence for this thanks to three different kinds of measurements on LaH10. The first is the characteristic decrease in resistance as the temperature decreases. The second is the so-called isotope effect, in which the hydrogen atoms in the sample are replaced with heavier deuterium. The corresponding shift in Tc (to 168 K as expected) is direct evidence of the pairing of Cooper electrons in conventional superconductivity. Finally, the researchers confirmed the superconducting nature of the transition at 250 K thanks to how it changes depending on an external applied magnetic field. “The magnetic field reduces the Tc in conventional superconductors through the orbital effect or by breaking the spin-singlet state of the Cooper pair, explains Eremets.
“Our experiments are seemingly simple but they are in fact quite challenging to set up,” he tells Physics World. “We place a piece of lanthanum metal in a diamond anvil cell in a hydrogen (or deuterium) atmosphere and then heat it to make a hydride (or deuteride). The pressure can be as high as 200 GPa (2 megabars), so the samples thus needs to very small – just 10 microns across. They also need to be connected to four electrodes for us to be able to obtain direct evidence of superconductivity. Finally, we need to analyse the material’s crystal structure at an advanced synchrotron (APS GeSCARS in our case).”
The Meissner effect
The small size of the samples means that the researchers have not yet been able to measure the Meissner effect (another crucial indicator of superconductivity) in their material. “We detected this effect in the case of H2S, but these samples had a much bigger diameter of 100 microns. Our LaH10 samples are 10 times smaller, which means that their magnetization signal is below the sensitivity of a SQUID magnetometer. We will thus need to develop other experimental techniques to detect this signal.”
Meanwhile, in similar experiments, researchers led by Russell Hemley of George Washington University in the US say that they have observed a sudden drop of electrical resistance at 280 K (7 °C) in LaH10 at pressure of up to 202 GPa. What is more, they have even detected the Meissner effect in this material in follow-up work (not yet published). They did their experiments at the Argonne National Lab in Illinois
Both Eremet’s group’s result and Hemley and colleagues’ still need to be independently confirmed, however, says Jorge Hirsch of the University of California at San Diego, who was not involved in either study. “We have to be careful not to place too high a confidence in experimental results obtained under very difficult experimental conditions when they appear to agree with predictions of BCS theory, and perhaps consider the possibility of experimental artifacts,” he comments.
Eremets and colleagues have published their findings on arXiv and Hemley and colleagues will publish their work in Physical Review Letters.
4/1/2019 FROM PHYSICSWORLD.COM
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