It is said that science is moving faster than ever before. In fact, the pace is usually limited by the flow of information – I can’t respond to your results until I know about them, and magazines are notoriously slow. The arXiv, which contains manuscripts that can be submitted for peer review, is not. Deploy your preprint today, watch it go public tomorrow… and watch it precipitate three weeks later.
This has all been unfolding before our very eyes with an extraordinary claim for room temperature superconductivity, marred by some suspicious data.
Why is superconductivity important?
Superconductivity is a hidden workhorse of today’s medical and scientific establishment. The way to get high magnetic fields is to use a very large current loop. In ordinary conductors, the imperfect conduction of electrons heats the metal, increasing the resistance of the metal. The whole feedback loop ends when your finely designed metal coil turns into a small glowing metal puddle — often considered a bad thing.
This can be avoided because some metals become superconductors at low temperatures and have no resistance. But superconductivity has limits: resistance returns if the current is too high or if the magnetic field is too high.
Although there are several high-temperature superconductors (with high meaning anywhere from -196 to -90°C), their use has proved difficult. All magnets used in large devices use superconductors operating at liquid helium temperatures (-270°C). Liquid helium is very expensive and it is a limited resource that can be subject to some very strange market forces.
This combination of applications and conditions makes any claim of superconductivity at room temperature very exciting – and makes anyone researching superconductivity very suspicious.
Superconductors operate at low temperatures because they rely on pretty delicate physics. Resistance is due to an electron not really knowing what is going on. It is surrounded by an apparently regular array of metal atoms. Based on that information, he leaves, like a confident drunk driver. But atoms vibrate and crystals aren’t perfect, so our drunken electron bounces back atom after atom. Each time it changes direction and loses energy.
Below a certain temperature, electrons can pair to form something that behaves like a single expanded particle. In a sort of physical irony, the vibrations that previously helped create resistance help the electron pairs together. The pairs spread out in a giant wave that contains information about the atomic positions. The electrons are guided by this wave so that they can flow without resistance. The electrons will pass even through thin non-conductive barriers and still lose no energy.
That pairing process is very delicate. If the temperature is too high, the magnetic field too large or the number of electrons too high, the pairs are broken and the superconductivity disappears.
Pairing while the temperature is high
The researchers, a pair of scientists from the Indian Institute of Science, wanted something different. They wondered if the structure of a material could cause electrons to be scattered in such a way that they pair up, using a material’s properties to promote superconductivity.
To test this, the researchers used silver nanoparticles embedded in gold. The choice for silver and gold makes sense because electrons in gold and silver do not notice much vibration in either metal. This means that the coupling mechanism of standard superconductivity is inefficient for their materials.
Unfortunately, I don’t see how randomly structuring a material can cause electrons to pair and fall into a superconducting state. I can’t even really discuss how they made their choices for nanoparticle sizes or gold to silver ratios because there isn’t much information to go on.
However, according to their data, they see a transition to superconductivity at temperatures between -123°C and 77°C – yes, that’s +77°C – depending on the ratio of gold to silver. For the higher transition temperature, they have never actually observed the transition: they claim that the critical temperature, if it exists, is above 77°C and could not be reached in their equipment.
The data is also noteworthy for another reason: The critical magnetic field appears to be extremely high. The researchers still observe superconductivity at -123°C for applied fields of three to five Tesla. This is big: I think niobium is the best pure metal superconductor with a critical field of about 0.2T (some alloys are better).
That’s not noise
However, the biggest problem seems to be the data itself. A keen-eyed researcher noticed that two of the tracks had identical noise, just slightly offset from each other. I can’t think of any reasonable explanation for this. Noise is random and should never be repeated. The graph can be the result of a series of errors: the same data set processed twice with an error in the process in one run. Then a dataset is mislabeled and used in the newspaper. But I seriously doubt that, which raises the possibility of data misrepresentation.
I also found it strange that no one else has investigated the electrical properties of silver nanoparticles in gold. After a quick search, it seems I’m right and wrong. The optical properties – the bit I’m usually interested in – are more than lethal. Nothing was clear about the electrical properties, although that doesn’t mean there are no papers on the conductivity of silver nanoparticles in gold.
It’s possible that the researchers were lucky and that what they observe is largely reality (except for the dodgy sound). But I think I need to see this done in another lab before I accept the results presented. Fortunately, I think that can be done fairly quickly.
arXiv.org2018, ID: 1807.08572, 1808.02929, (About the arXiv).