Unveiling the origin of the record superconductivity

Achieving room temperature superconductivity is among the most pursued but elusive goals of scientists. A paper uploaded to the arXiv ¹ in December 2014 claims to have observed superconductivity as high as 190 K in hydrogen sulfide at high pressure, breaking all the records thus far. If this observation is confirmed, cuprates will be knocked from their position as the highest temperature superconductors. The result represents a huge step towards a room-temperature superconductor and remarks that very high transition temperatures can be achieved with simple compounds.

Hydrogen sulfide (H₂S) at ambient conditions is a gas recognizable for its rotten-egg odor. Despite its chemical similarity to water (H₂O), it is poisonous, flammable, and explosive. H₂S is present in nature as it might result from the bacterial breakdown of organic matter when oxygen lacks, and occurs in volcanic as well as natural gases. However, under pressure its properties completely change. It first becomes a molecular insulating solid, then the molecules dissociate forming an atomic solid, and, finally, it becomes a metal around 96 GPa ². Now, the experiments performed using a diamond anvil cell (see Figure 1) by Drozdov et al. indicate that compressing hydrogen sulfide further, up to 200 GPa, it can superconduct at 190 K, the highest superconducting critical temperature (Tc) ever observed [1].

The observation is revolutionary because it confirms experimentally for the first time the theoretical intuition by Ashcroft that high-temperature superconductivity could be attained in metallic hydrogen or hydrogen-rich materials ³,⁴. The expected extraordinary values are not originated from an exotic coupling mechanism as in the pnictides or cuprates, but from the well-understood interaction between the electrons and the vibrations of the atoms in the solid, the phonons. As it was already described in a previous post in MappingIgnorance, Tc in an electron-phonon superconductor is enhanced when the average phonon frequency and the electron-phonon coupling constant are large. According to Ashcroft, this may be the case in metallic hydrogen and hydrogen-rich compounds.

As soon as the results of the record Tc were communicated to the scientific community, many theoretical researchers devoted themselves to understand what had happened in the experiment. Some authors ⁵ have published theoretical indications that H₂S might have dissociated into H₃S and S, and have argued that superconductivity, driven by the electron-phonon mechanism, might have been observed in the H₃S compound. Interestingly, before the experiments were communicated, H₃S had been predicted to be a high-temperature superconductor ⁶. Other authors have instead claimed that the mechanism behind superconductivity is not the electron-phonon mechanism ⁷.

A group of scientists from the Donostia International Physics Center (DIPC) in collaboration with researchers from France, UK, Canada, and China have now shown making use of theoretical first-principles calculations that, indeed, the electron-phonon coupling mechanism explains the observed superconducting Tc, but only if anharmonic effects are considered in the hydrogen motion. The authors have published their conclusions in Physical Review Letters ⁸.

In the first place, the researchers of the latter work have studied the stability and possible decomposition of hydrogen sulfide when it is pressurized to 200 GPa. Their theoretical results indicate that H₂S is not a stable stoichiometry at the pressures where the extraordinary value of Tc was observed. In fact, it can dissociate following two different mechanisms: 3H₂S → 2H₃S + S, which had already been described before [5], or 5H₂S → 3H₃S + HS₂, not identified earlier. Thus, superconductivity could have occurred both in HS₂ or H₃S. When studying the possibility of electron-phonon superconductivity in the HS₂ compound, the authors estimated that it stops superconducting approximately at 30 K, and thus cannot explain the experiment. On the contrary, the H₃S phase that crystallizes in the cubic structure shown in Figure 2, has a very large electron-phonon coupling that can potentially explain the measured extraordinary Tc.

The calculated Tc however does not match the experimental value when the phonons, which drive the superconducting mechanism here, are calculated in the standard harmonic approximation. It is considerably overestimated. In the harmonic approximation it is assumed that the force felt by the atoms is linear with the displacement from the equilibrium position, as in the well-known Hooke’s law. The harmonic approximation is generally valid if the atoms vibrate very close from the equilibrium position. Although high compression of H₃S should limit the range of atomic displacement, the hydrogen atoms are significantly displaced from their equilibrium positions due to their very light mass, yielding to important effects beyond the harmonic approximation. Including the so-called anharmonic effects in the calculations, strong corrections were predicted for the phonons involving hydrogen atoms. Combining the anharmonic phonons with the electron-phonon interaction, the experimental Tc was reproduced. Thus, the researchers conclude that the record high-temperature superconductivity observed is due to the large interaction between electrons and anharmonic phonons.

The experimental result by Drozdov et al. and the theoretical explanation that followed [8] clearly address that very large superconducting Tc’s can be obtained even with the conventional electron-phonon pairing mechanism. Considering that this measurement is the first ever to have observed high Tc in hydrides, it is expected that many new experiments will follow. Will room-temperature superconductivity be observed under high-pressure in a different hydrogen-rich compound soon? Time will tell.