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Condensed matter physics is one of the largest and most important subdisciplines of physics today, and one of the "holy grails" of condensed matter physics is high-temperature superconductivity.
On Tuesday afternoon, at the APS March Meeting's annual March meeting in Las Vegas, Ranga Dias, a physicist at the University of Rochester, gave a sold-out speech in which he announced that he and his team had fulfilled a century-old dream in the field: a superconductor that could operate at room temperature and near-normal pressure.
If anyone can achieve superconductivity at room temperature, it can be said that a new technological revolution has been started: interest in the speech is so strong that security personnel start preventing more people from entering overcrowded rooms 15 minutes before the speech starts. Shortly before Dias begins speaking, they can be heard driving away curious onlookers.
The study by Dias et al., published today in the journal Nature, seems to show that a conventional conductor – a solid made of hydrogen, nitrogen and rare earth metal lutetium – is transformed into a flawless material capable of conducting electricity with perfect efficiency.
Previously, superconductivity could only be observed at extremely cold temperatures or ultra-high pressures – conditions that prevented experimental materials from being used for long-term, routine applications such as non-destructive power transmission, hovering high-speed trains and affordable medical imaging equipment.
In the new study, the forged compound successfully conducted electrical current resistively at 21 degrees Celsius (69.8 degrees Fahrenheit, 294 K) and a pressure of about 1 gigapascal. That's still a lot of pressure — about 10 times the deepest pressure in the Mariana Trench — but it's more than 100 times lower than previously required for experiments using similar materials.
In the past hundred years, scientists have explored superconducting materials.
Here is the full video of the author's speech, which readers who understand can savor:
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James Hamlin, a physicist at the University of Florida, commented: "If this is true, then their research is completely revolutionary. Room temperature superconductivity has been a dream for a century. Existing superconductors require expensive and bulky cooling systems to conduct electricity frictionlessly. The creation of room-temperature superconductors will make power grids, computer chips, and superconductors needed for maglev trains and nuclear fusion power generation more efficient."
21 °C, achieve room temperature superconductivity
The superconducting forms involved in this study require electrons to couple to each other to form so-called Cooper pairs (Cooper pairs were first proposed by American physicist Leon Cooper in 1956 to describe a pair of electrons or other fermions bound together in some way at low temperatures). One factor contributing to the formation of Cooper pairs is the high-frequency vibrations (called phonons) between the nuclei associated with these electrons. This is easier to arrange in light nuclei, while hydrogen is the lightest around. Therefore, finding ways to stuff more hydrogen into chemicals is considered a viable way to produce superconductors at higher temperatures.
However, the surest way to do this is to be under extreme stress. These pressures can prompt hydrogen to enter the crystalline structure of the metal or form hydrogen-rich chemicals that are unstable at lower pressures. Both methods produce chemicals with very high critical temperatures, which is the highest point at which they support superconductivity. However, while these are close to room temperature, the pressure required is more than multiple gigapascals – each gigapaska is nearly 10,000 times the atmospheric pressure at sea level.
Essentially, this is equivalent to trading unrealistic temperatures for unrealistic pressure.
What we want, however, is room temperature and pressure, where we can use these chemicals to determine the general principles that produce this hydrogen-rich superconductivity, and then use those principles to identify other chemicals that exhibit similar behavior under conditions that are easier to maintain.
The team focused their attention on lutetium (Lutetium, atomic number 71) because the occupation of its electron orbitals should provide more electrons that may be involved in forming Cooper pairs, making superconductivity more achievable. The study also added trace amounts of nitrogen to make the structure more stable by doping the material, making it possible to reduce the required pressure.
It is clear that the mixture of lutetium/nitrogen/hydrogen underwent some changes before the measurements were taken. Under ambient conditions, the addition of these two gases turns lutetium blue, possibly due to hydrogen seeping into the metal. But as the pressure increased to thousands of atmospheres, the mixture turned a dramatic pink, which turned out to be related to the mixture's turning into metal. Continuing to increase the pressure to more than 30000 times the atmospheric pressure, it loses its metallic properties and turns a deeper red.
Superconducting pressure of lutetium-nitrogen-hydrogen in near environments
Superconductivity is possible from 3000 to 30000 times atmospheric pressure. Therefore, the researchers tested in this pressure range to help find the pressure that supports the highest critical temperature. The study found that at a temperature of 294 K, about 21°C, about the same as room temperature, the material appeared to lose resistance to electric current, provided that the peak air pressure was about 10,000 times the atmospheric pressure.
In addition, superconductivity also changes the magnetic properties of materials, and much of the paper discusses measuring the magnetic properties of samples. Studying sample magnetism is not an easy task, considering that samples can be very small, and they are often sandwiched between all the hardware that needs to crush the sample under extreme pressure.
To figure out what this material is, scientists have done a lot of work. It almost certainly contains some hydrogen and nitrogen incorporated into the metal, but it's unclear how much there is because any extra two gases can simply be excluded from the sample. The researchers tried to crystallize it, but the results were somewhat ambiguous. The signal of hydrogen (atomic weight 1) is drowned out by the signal of lutetium (atomic weight 175), and it is possible for hydrogen to move through the material.
So while they identified possible locations for hydrogen in the material, it wasn't clear how many of those locations were actually occupied. And this will make it a challenge to extract larger principles from the behavior of this material.
Retraction doubts
Ranga Dias。 图源：J. Adam Fenster/University of Rochester
In light of the recent retraction by Ranga Dias' research group, many physicists say they will no longer believe it easily. "I think they have to do some real work and be really open for people to believe in it," Hamlin said.
Jorge Hirsch, a physicist at the University of California, San Diego (who also inventored H-index) and a critic of the team's early work, put it even more bluntly: "I'm skeptical of the new results because I don't trust the authors."
It is worth mentioning that Jorge Hirsch's APS March Meeting report and Ranga Dias were arranged in the same venue, and the front and back feet went on stage, which can be said to add to the atmosphere of confrontation.
Jorge Hirsch (stander) and Ranga Dias. Image from Zhihu @芝了, https://www.zhihu.com/question/588302961
In 2020, a team led by physicist Ranga Dias caused a stir by reporting superconductivity in tiny spots of carbon, sulfur, and hydrogen (CSH), which the team achieved by squeezing the material between two diamond tips to millions of times atmospheric pressure. Scientists have previously created other hydrogen-rich superconductors, known as hydrides, but they must be cooled to 250K (-23°C) or lower. CSH is superconducting at a temperature of 287K, the temperature of a wine refrigerator.
At the time, Science magazine reported on the study with the headline "Finally, room-temperature superconductivity achieved."
But other researchers were unable to replicate the CSH results and complained that the study's formulation was vague and incomplete. Other researchers found problems with the way the team measured the material's magnetic behavior, a key marker of superconductivity.
A year later, Dias and Ashkan Salamat, a physicist at the University of Nevada, Las Vegas, with whom he often collaborates, published raw data in the form of a 149-page document detailing an unusual and complex method of removing background magnetic interference. This approach is inconsistent with the procedure they described in the original paper.
Eventually, with opposition from all the authors, Nature retracted the paper in September 2022.
So this time, will other labs be able to replicate the material and confirm its superconductivity?
Hamlin says that while only a handful of groups in the world have been able to work under incredible diamond high-pressure anvils to see superconductivity in CSH, there are probably dozens of labs that have been able to work in pressure regimes for lower lutetium-based materials. Dias says his lab has been working on a way to completely remove the diamond anvil chamber from the process over the past few months, which could further accelerate the confirmation of the discovery.
In order for other labs to replicate these results exactly, the group must be willing to share the entire raw data set along with detailed sample preparation methods, or send samples of their material to other labs for testing. However, external access may fall short of the community's expectations. Dias and Salamat founded a startup, Unearthly Materials, which has raised more than $20 million in funding from investors including the CEOs of Spotify and OpenAI. They also recently filed a patent on lutetium hydride, which would prevent them from mailing samples.
"We have clear, detailed instructions on how to make our samples," says Dias. "Given the exclusivity of our process and the intellectual property that exists, we do not intend to distribute this material."
What do netizens think?
After a conference report, the news has already caused a global sensation. A major scientific breakthrough that receives a lot of attention around the world must also withstand skepticism, especially for the Ranga Dias team, which has experienced the retraction storm.
Some netizens said that in view of the author's previous behavior, this work needs to be treated with caution.
Source: Zhihu netizens @Tycho
After watching the live broadcast last night, some netizens raised questions about the experimental data and details in the speech. The results of this study would be more convincing if Ranga Dias' team could shed light on the intent of some of the settings in the experiment and explain the doubts in the data.
Source: Zhihu netizens @洗芝溪
Some netizens said that if the results do not stand up to scrutiny, the credibility of scientists will be greatly damaged.
It seems that whether Dias's impact on the holy grail of condensed matter physics is successful this time needs further verification by more scientists.
Reference content:
https://www.zhihu.com/question/588302961
https://meetings.aps.org/Meeting/MAR23/Session/K20.2
https://www.quantamagazine.org/room-temperature-superconductor-discovery-meets-with-resistance-20230308/
https://arstechnica.com/science/2023/03/room-temperature-superconductor-works-at-lower-pressures/