Historical research guide · 1148 · 2015
Conventional superconductivity at 203 K at high pressures
A. P. Drozdov · M. I. Eremets · I. A. Troyan · V. Ksenofontov · S. I. Shylin
In one minute
This is the measurement the site's superconductor pages have been teaching around without being able to name. Alexander Drozdov and Mikhail Eremets at the Max Planck Institute for Chemistry in Mainz squeezed hydrogen sulfide — ordinary, cheap, easy to handle — in a diamond anvil cell to about ninety gigapascals, where it turns metallic, and cooled it. The resistance fell by three to four orders of magnitude. Pushed further, and annealed, the transition climbed to 203 kelvin: seventy degrees below freezing, and about forty kelvin above the best any copper-oxide superconductor had ever reached. The result is not a new kind of physics but an old one carried out: the compound they think they made, H₃S, superconducts by the ordinary phonon mechanism of Bardeen, Cooper and Schrieffer, which puts no ceiling on the transition temperature so long as the lattice vibrates fast enough. Hydrogen is the lightest atom there is.
Connection
Chapter 11 teaches the mechanism of superconductivity and then has to answer the reader's real question — how warm, and in what. This is the answer with a material, a pressure and a temperature attached — and it is settled the way an extraordinary claim has to be, by four independent signatures rather than one.
From the source
Hydrogen sulfide becomes metallic under a pressure of about 90 gigapascals; on cooling, the resistance drops by three to four orders of magnitude, marking the transition to a superconducting state. The transition temperature rises with further pressure.Main text, the paragraphs following the description of the diamond-anvil loading; Fig. 1a and 1b
The superconducting transition reaches an onset of 203(1) kelvin, confirmed in magnetic susceptibility as a sharp change from diamagnetic to paramagnetic on the zero-field-cooled curve, at a pressure of 155 gigapascals. Magnetization measurements at that pressure put the onset at 203.5 kelvin.Main text, numbered point (4); Fig. 4a and 4c captions
The isotope effect is the evidence that the mechanism is phonon-assisted and conventional: substituting sulfur deuteride shifts the transition to lower temperature, and above 170 gigapascals the measured isotope exponent is about 0.3, against the value of about 0.5 that Bardeen-Cooper-Schrieffer theory gives for a pure mass dependence.Main text, numbered point (2); Fig. 2b and 2c
The material is a type II superconductor: magnetization against field shows a pronounced hysteresis. The transition temperature falls with applied field only slowly over the 7 tesla available, and extrapolating that curve gives an estimated critical magnetic field as high as 70 tesla.Main text, numbered points (3) and (4); Fig. 3 and Fig. 4c
The authors attribute the high transition temperature not to the H₂S they loaded but to H₃S, formed when hydrogen sulfide partly decomposes under pressure — which is why the sample has to be annealed at 150 to 250 kelvin before the high transition appears.Abstract, final sentences; main text, the annealing paragraph
What to watch: the authors' own forward claim is that room-temperature superconductivity can be expected in other hydrogen-based materials, because hydrogen supplies both the high-frequency phonon modes and the strong electron-phonon coupling that the theory asks for. The pressure, roughly a million and a half atmospheres, is the part that is not yet engineering.Abstract, final sentence; main text, introduction citing Ashcroft
The way in
https://arxiv.org/abs/1506.08190How to cite it
A. P. Drozdov, M. I. Eremets, I. A. Troyan, V. Ksenofontov, S. I. Shylin (2015) Conventional superconductivity at 203 K at high pressures doi:10.1038/nature14964