Superconductor world record at 15 °C
US researchers have succeeded in transporting electricity without loss at plus degrees in a laboratory experiment. However, important questions about the superconductivity experiment remain unanswered.
A team of researchers led by Ranga Dias from the University of Rochester in the US state of New York has achieved this feat. It is a result that will go around the world - and which experts consider to be an important symbolic milestone. "It's difficult to overestimate the significance," says Alexander Goncharov from the Carnegie Institution for Science, for example, who was not involved in the experiments. His colleague Lilia Boeri from La Sapienza University in Rome is also very impressed: "This is great."
Perfect power cables, floating trains
Superconductivity has been a promise for more than 100 years. It holds out the prospect of cables without electrical resistance. They could be used to transmit electricity over long distances without loss, and more economical microchips and smaller MRIs are also conceivable. And as superconductors displace magnetic fields from their interior, some trams would probably make way for a Transrapid.
Unfortunately, nature has so far thwarted such dreams. The vast majority of materials only lose their electrical resistance near absolute zero at minus 273 degrees Celsius. And even "high-temperature" superconductors such as copper-containing cuprates still require liquid nitrogen at around minus 200 degrees as a coolant - and are usually very brittle.
However, there is a trick that makes even simpler compounds superconducting at high temperatures: if a large amount of pressure is applied to their surface, their atomic lattice changes. This allows electrons to communicate via targeted lattice vibrations. In this way, they come together to form "Cooper pairs" that move through the solid without losing energy - "conventional" superconductivity, which for a long time was only known from ultracold materials.
A Mainz team led by Mikhail Eremets from the Max Planck Institute for Chemistry popularised the research field of high-pressure superconductivity five years ago. The scientists packed at that time a tiny sample of sulphur and hydrogen (H3S) between the tips of a diamond press. At 100 gigapascals, a million times the air pressure on Earth, the compound became a superconductor, despite a temperature of minus 70 degrees.
Building plan for a superconductor
"That was the real milestone," says Lilia Boeri. "Everything that has happened since then is a logical consequence of that." In 2018, the Mainz-based researchers set the next record. According to this, the metal compound lanthanum decahydride (LaH10) is also a perfect conductor if it is compressed extremely strongly. And that at a comparatively warm 13 degrees below zero.
The remarkable thing about the Mainz results: Theorists had calculated in advance that the compounds would have to become superconducting under high pressure. With the chemically complex high-temperature superconductors from the cuprate family or the iron-based pnictides, such a prediction was practically never successful.
The secret of superconductivity
At room temperature, atoms are constantly wriggling around. Electrons moving through a solid are therefore constantly slowed down and lose energy. The more momentum they lose, the greater the electrical resistance of a material.
Atomic lattices only come to rest near absolute zero (minus 273.15 degrees Celsius). Conduction electrons can therefore easily deform their environment in the solid. This causes vibrations to propagate in the lattice, which pave the way for other electrons. According to the BCS theory formulated in 1957, the charge carriers join together to form "Cooper pairs", which can whizz through the solid without any electrical resistance.
At the lowest level of the temperature scale, this happens automatically for many elements in the periodic table, for lithium as well as lead. In 1986, however, physicists realised that superconductivity in some chemical compounds also persists at significantly higher temperatures. These cuprates consist of a handful of different types of atoms whose quantum physical influences add up just enough to guide electrons elegantly through the lattice.
However, the exact mechanisms of this "unconventional" superconductivity are still unknown; researchers assume a complex interplay of charge, spin, orbital motion and lattice vibrations. They are therefore looking for simpler materials that remain superconducting at high temperatures. Since 2015, experts have been increasingly relying on diamond presses, which compress hydrogen-containing material samples enormously. This changes the properties of the atomic lattices - and in some cases leads to classic BCS superconductivity, which is otherwise only known from extremely cold bodies.
This is why the diamond pressing experiments immediately made many researchers dream of the ultimate goal of their discipline: a superconductor that still works at room temperature around 20 degrees Celsius. In fact, theorists with computer support have identified a whole series of element duos whose "transition temperature" should be close to this symbolic limit. Since then, several laboratories around the globe have been busy turning the predictions into reality.
Ranga Dias and his team now seem to have pulled off a coup in two respects. Not only does their 15-degree record fall quite clearly within the rather roughly defined range of room temperature among physicists. With their measurement, the US researchers have also entered the realm of three-element compounds. Specifically, they mixed methane (CH4) into their sample container filled with hydrogen sulphide - and thus sprinkled some carbon into the mix. When compressed in the diamond press, the gas mixture then turned into a superconducting metal.
"At first I didn't believe in the result myself, but now we're sure," says Dias. In the end, the young assistant professor carried out the experiment more than 30 times, measuring the electrical resistance and magnetic sense of the sample each time. In doing so, he probably also wanted to prevent a repeat of an experience from 2017, when he and his older colleague Isaac Silvera had somewhat hastily published evidence of a metallic phase of pure hydrogen that would be something like the ultimate superconductor. To date, however, the measurements from back then have not been reproduced, which has earned Dias and Silvera much criticism.
In the express through peer review
This time, things look better at first glance, confirm all the experts contacted by "Spektrum.de". However, the current publication also had to be done quickly: because of the huge competition in the field, he requested an extra fast peer review from the specialist journal "Nature", says Dias. He submitted the manuscript at the end of August. It then took just six weeks for the paper to be published - an unusually short period of time.
"It's hard to imagine that a thorough peer review took place during this time," criticises Graeme Ackland from the University of Edinburgh. At first glance, the study appears to be quite solid, but some important questions remain unanswered. Bernhard Keimer, Director at the Max Planck Institute for Solid State Research in Stuttgart, takes a similar view. "We don't actually know what kind of material it is," he says.
So far, it is completely unclear what lattice structure the sulphur, hydrogen and carbon atoms form in the tiny sample container used by Dias and his team. The previous record holder LaH10 was different: calculations showed that the hydrogen atoms form a kind of cage around the heavier foreign atom. This creates a symmetrical lattice that resembles that of metallic hydrogen.
Three is better than two
In the complex of C, S and H atoms from Dias' experiment, on the other hand, something else is more likely to favour superconductivity: It is possible that the three elements form extremely stable "covalent" bonds under pressure, which make the atomic lattice very rigid. As a result, vibrations could easily propagate through the material, bringing electrons together to form Cooper pairs. This is certainly the case with H3S, which was on the podium in 2015.
It remains to be seen whether this also explains the superconductivity of the new record holder. Measurements in which X-rays are scattered by the sample could have shed light on this, says Mainz-based competitor Mikhail Eremets. "It is a mystery why the team has not published such data."
Ranga Dias justifies this by saying that such measurements are not meaningful for C-S-H compounds and are generally overestimated in diamond pressing experiments. According to him, he and his team are working on a different X-ray method that should allow conclusions to be drawn about the atomic structure of the sample.
One way or another, theorists should now get to work. Over the next few months, they will be running various atomic lattice configurations through their computers to see which of them can reproduce the results of the experiment from Rochester. "That's the next race," says Lilia Boeri.
On the way to the next record
She believes that more and more records in room-temperature superconductivity will follow in the coming years. This is because researchers have only just started working on hydride compounds made from three elements. The periodic table offers a total of 1770 possible combinations.
One of them, for example, NH3BH3, which is made up of boron, hydrogen and nitrogen, could still be a superconductor at an incredible 280 degrees Celsius. At least that was the result of a measurement that another US group claimed to have succeeded in taking in summer 2020. However, as the team had to interrupt the measurements due to the Covid-19 lockdown, the result is considered preliminary. However, experts believe it shows the potential of the research field.
In general, the experts hope that some of the three-element compounds will turn out to be easier-to-clean room-temperature superconductors, meaning they will retain their special properties even at low pressures. "In their current form, clamped in a diamond press, the materials are certainly not suitable for any application," says Bernhard Keimer.
Ranga Dias also recognises this. Nevertheless, he recently founded a company to earn money with his discovery. There is no business plan yet, he says. But that could still change if he and his team continue to experiment.
However, the history of superconductivity is a history of disappointed expectations. Even when the cuprate was discovered at the end of the 1980s, some researchers were convinced that the age of levitating trams was just around the corner. In the end, things turned out to be much more complicated than expected.
And so many researchers are rather sceptical when it comes to a superconductor that works its magic even at room temperature and without additional external pressure. "I think we will eventually find a material that can be used well at normal pressure and minus 100 degrees," says Lilia Boeri. That would be enough for some special applications - but probably not enough for the big material revolution.
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, via Wikimedia Commons
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