Together with collaborators in Spain (Prof. Errea and his team), Italy (Prof. Mauri and his team), France (Prof. Calandra), Germany (Dr Sanna), and Japan (Prof. Arita, Prof. Koretsune and Prof. Tadano) we have shown, in our most recent publication that the crystal structure of the record superconducting LaH10 compound is stabilised by nuclear quantum fluctuations. This result suggests that superconductivity approaching room temperature may be possible in hydrogen-rich compounds at much lower pressures than previously expected with classical calculations!
The prestigious journal of Nature publishes the results on Feb. 5, 2020. You can see the article here: https://rdcu.be/b1fCK.
The possibility of high-temperature superconductivity in LaH10, a super-hydride formed by lanthanum and hydrogen, was anticipated by crystal structure predictions back in 2017. These calculations suggested that above 230 gigapascals a highly symmetric LaH10 compound (Fm-3m space group), with a hydrogen cage enclosing the lanthanum atoms (see figure), would be formed. It was calculated that this structure would distort at lower pressures, breaking the highly symmetric pattern. However, experiments performed in 2019 were able to synthesise the highly symmetric compound at much lower pressures, from 130 to 220 gigapascals, and measure superconductivity around 250 kelvin. The crystal structure of the record superconductor, and thus its superconductivity, remained not entirely clear. Now, thanks to our results, we know that atomic quantum fluctuations “hold” the symmetric structure of LaH10 in the experimental pressure range (in which superconductivity has been observed).
More in detail, the calculations show that if atoms are treated as classical particles, that is, as simple points in space, many distortions of the structure tend to lower the energy of the system. This means that the classical energy landscape is very complex, with many local minima, like a highly deformed mattress because many people are standing on it. However, when atoms (nuclei and electrons) are treated like quantum objects, which are described with a delocalized wave function, the energy landscape is completely reshaped: only one minimum is evident, which corresponds to the highly symmetric Fm-3m structure. Somehow, quantum effects get rid of everybody in the mattress, but only one-foot person’s remain deforming the mattress.
Furthermore, the estimations of the critical temperature using the quantum energy landscape agree satisfactorily with the experimental evidence. This additional supported the Fm-3m high-symmetry structure as responsible for the superconducting record. The results are especially relevant because they also demonstrate how nuclear quantum fluctuations can stabilise crystal structures even at more than 100 gigapascals below their classical instability pressure. Our work shows that the “classical” instabilities are due to the enormous electron-crystal lattice interaction that makes this compound a record superconductor. In other words, quantum effects stabilise crystal structures with substantial superconducting temperatures that would otherwise be unstable. Consequently, new hopes are opened to discover high-temperature superconducting hydrogen compounds at much lower pressures than expected classically, maybe even at ambient pressure.
Reference: Ion Errea, Francesco Belli, Lorenzo Monacelli, Antonio Sanna, Takashi Koretsune, Terumasa Tadano, Raffaello Bianco, Matteo Calandra, Ryotaro Arita, Francesco Mauri & José A. Flores-Livas. “Quantum crystal structure in the 250-kelvin superconducting lanthanum hydride” Nature 578, pages 66–69 (2020) See the article here: https://rdcu.be/b1fCK.