Historical research guide · 1149 · 2019
Superconductivity at 250 K in lanthanum hydride under high pressures
A. P. Drozdov · P. P. Kong · V. S. Minkov · S. P. Besedin · M. A. Kuzovnikov · S. Mozaffari · L. Balicas · F. Balakirev · D. Graf · V. B. Prakapenka · E. Greenberg · D. A. Knyazev · M. Tkacz · M. I. Eremets
In one minute
Four years after the sulfur hydride result, the same Mainz group came back with fifty kelvin more. The prediction they were testing came from density-functional calculations: put a heavy atom at the centre of a cage of hydrogen — a clathrate — and the hydrogen lattice vibrates fast enough to superconduct near room temperature. Lanthanum hydride compressed to about 170 gigapascals does it at 250 kelvin, which is twenty-three degrees below the freezing point of water and warmer than a winter day in much of the world. They did not rest the claim on one measurement. Resistance goes to zero; swapping hydrogen for deuterium moves the transition down by the amount the phonon mechanism predicts; a magnetic field pushes it down, and extrapolating that gives an upper critical field of 120 tesla; and synchrotron X-ray diffraction identifies the phase that is doing it.
Connection
Chapter 11 can now say what the ceiling actually is, and who is pushing it. This is also the clearest live example on the site of theory naming a material before anyone made it — the calculation asked for a hydrogen cage, and the cage was built and measured.
From the source
Superconductivity is reported with a transition temperature of about 250 kelvin in the face-centred-cubic Fm-3m structure of lanthanum hydride at a pressure of about 170 gigapascals — a leap of roughly 50 kelvin above the previous record of 203 kelvin in H₃S.Abstract, final third; main text, the pressure-dependence paragraph
The transition temperature against pressure has a maximum: it rises with pressure to 250 to 252 kelvin at about 170 gigapascals and then falls abruptly at higher pressures.Main text, the paragraph describing Tc(P); Fig. 1
Three independent confirmations are offered for the transition: zero resistance, an isotope effect on substituting deuterium, and suppression of the transition by an external magnetic field.Abstract, sentence beginning 'We proved the existence of superconductivity'
Extrapolating the measured upper critical fields to zero temperature, by fitting the Ginzburg-Landau expression to data taken under the 90 per cent and 50 per cent resistance criteria, gives an upper critical field of about 120 tesla.Abstract; main text, the paragraph on the extrapolation of Hc2(T); Fig. 2b caption
The phase responsible was identified rather than assumed: synchrotron X-ray powder diffraction on a sample showing a 249 kelvin transition at 150 gigapascals resolves a dominant face-centred cubic Fm-3m lattice with refined lattice constant 5.1019(5) ångström, and the refined volume gives the stoichiometry LaH₉·₆ rather than the ideal LaH₁₀.Main text, the crystallography paragraph; Extended Data Fig. ED2 and the X-ray diffraction figure caption
What to watch: the authors name their own target in the abstract — room-temperature superconductivity, meaning 273 kelvin, in the near future at high pressures, and then conventional superconductivity at ambient pressure. The pressure is the open problem, not the temperature.Abstract, final sentence
The way in
https://arxiv.org/abs/1812.01561How to cite it
A. P. Drozdov, P. P. Kong, V. S. Minkov, S. P. Besedin, M. A. Kuzovnikov, S. Mozaffari, L. Balicas, F. Balakirev, D. Graf, V. B. Prakapenka, E. Greenberg, D. A. Knyazev, M. Tkacz, M. I. Eremets (2019) Superconductivity at 250 K in lanthanum hydride under high pressures doi:10.1038/s41586-019-1201-8