The story of superconductivity began in 1911, when Dutch physicist Heike Kamerlingh Onnes was experimenting with the effects of liquid helium cooled to 0.9 Kelvin, the closest any scientist had come to absolute zero at the time. He placed liquid mercury into a series of tubes, cooled the metal with liquid helium, and measured its resistance, or how the material reduced electrical flow.
Kamerlingh Onnes was awarded the 1913 Nobel Prize in Physics for what happened to that mercury, and it laid the groundwork for a century of further experimentation that today still promises to revolutionize everything from communications to transportation if it can be figured out.
“The experiment left no doubt that, as far as accuracy of measurement went, the resistance disappeared…Thus the mercury at 4.2K has entered a new state, which, owing to its particular electrical properties, can be called the state of superconductivity,” he said in his Nobel Lecture.
In other words, Onnes had discovered an amazing property in some materials: electric current flowed through them with perfect efficiency.
“In 10, 15 years, we probably will be seeing a different world”
He noted that two other metals, tin and lead, also exhibited superconductive properties at extremely low temperatures, and he offered a prediction about the importance of the newly discovered materials: “Now that we are able to use these metals, which are easy to work, all types of electrical experiments with resistance-free apparatus have become possible,” he said.
Over 100 years later, superconductors are important to some medical technologies, but they are expensive to create and require specific conditions in order to be implemented successfully. Recent discoveries of new types of superconductors and the first evidence for a room-temperature superconductor, however, leave experts with hope that superconductivity will yet live up to its incredible potential.
What is superconductivity?
“Superconductivity is magic,” said David Larbalestier, chief materials scientist at the National High Magnetic Field Laboratory and a mechanical engineering professor at the FAMU-FSU College of Engineering. “You're used to wasting electricity every time you send it from here to there, and superconductivity is electricity without friction.”
Fundamentally, electricity is the flow of electrons from one place to another. Materials that facilitate this flow are called conductors, and they’re ubiquitous. From the copper wiring in power cables to the silicon semiconductor embedded in a mobile phone, conducting is vital to modern technology, and by extension, society.
There’s just one problem: conductors lose energy everytime they transmit electricity because of resistance. This in turn places a limit on efficiency and causes electronics to waste energy in the form of released heat. This is where superconductors come in. They are materials that, in theory, don’t lose energy from resistance. Most are compounds based on metals like mercury, copper, and iron.
Superconductors are in use today to power MRI machines and a city in Germany even installed a one kilometer-long superconducting energy cable in 2014. But current applications are a far cry from physicists’ predictions of world-changing potential.
Larbalestier said that if superconductors could replace copper wiring, electrical utilities would become much more efficient. Superconductors can also be used to power quantum computers or as electromagnets to suspend maglev trains. Several magnetically levitating train systems exist around the world, and they apply various techniques to keep trains slightly off the ground to avoid friction caused by a track. Japan’s SCMaglev even uses superconducting magnets, but the magnets must be constantly cooled to near absolute zero with liquid nitrogen and helium.
The limiting factor for widespread application, thus far, has been temperature. Kamerlingh Onnes needed to cool mercury to 4.2 Kelvin (about -452 Fahrenheit) for it to exhibit superconducting properties. Since then, scientists have realized that they can increase temperature if they also increase the pressure applied to the superconductors.
“We will go from a semiconducting society to a superconducting society”
Paul Chu, the founding director and chief scientist of the Texas Center for Superconductivity at the University of Houston, helped set a longstanding record for the highest-temperature superconductor in 1994. Without applying pressure, the material—a mercury-based cuprate—demonstrated superconductive properties at 134 Kelvin (-218 Fahrenheit) without pressure, and 164 Kelvin (-165 Fahrenheit) with pressure.
To achieve this kind of pressure, which is measured in gigapascals, scientists squeeze their superconducting material in between two diamonds that have been cut off at the tip, Chu said. Since pressure is inversely proportional to area, this superconductor sandwich must be very small.
According to Chu and Larbalestier, other factors that limit the accessibility of superconductors include the costs of diamonds, purified raw materials, and helium, of which there is a global shortage.
Because of all these constraints, high-temperature superconductors lost some of their allure, Chu said.
“High-temperature superconductivity was discovered in 1986—at that time, the whole field was excited, not just scientists,” he said. “But then later on, we could not overcome that threshold [of 164 Kelvin], and people lost interest in it.”
Recent experiments, however, have reinvigorated interest and buoyed hopes that superconductors may be able to function without extreme temperature regulation.
A room-temperature superconductor
In October 2020, scientists announced an unexpected breakthrough: the creation and experimental validation of a room-temperature superconductor composed of carbon, sulfur, and hydrogen. Unlike previous experiments, their material superconducted at a relatively balmy 59 degrees Fahrenheit.
The research represents “an amazing combination of theoretical and experimental science,” Larbalestier said. (Neil Ashcroft, a theoretical physicist, predicted in 1968 that hydrogen could become a superconducting material under intense pressure.)
Ranga Dias, a mechanical engineer at the University of Rochester who co-led the research, called their paper a “game-changer" in an interview.
"We will go from a semiconducting society to a superconducting society. This is a very exciting time as a scientific community, if we can really pull this off,” Dias said.
For their sulfur hydride to superconduct at such a high relative temperature, Dias’ team had to both raise the pressure and dope the hydrogen with carbon.
Dias compared doping to trying to increase the density of people in a room. On the one hand, you could bring the four walls of the room closer and closer together—that’s the traditional method of increasing mechanical pressure. On the other hand, you could add more people into the room, ratcheting up density without closing the walls in. This method is analogous to chemically doping a material to increase conductivity without an unfeasible increase in pressure or decrease in temperature.
Carbon, the material chosen to dope the hydrogen, mattered, Dias said, just like the people you choose to place in the room.
“If you add 10 people there, you'll feel the room getting crowded," he explained. "But what if you replace these 10 people with 10 defensive linemen? Now all of a sudden, even though the number of people stay the same, you feel definitely squeezed.”
Still, the pressure required to sustain superconductivity was about three-quarters of that found at Earth’s core, so Dias said that there is work to be done to move room-temperature superconducting out of the lab.
His group and others are now looking into metastability, or the ability of a material to remain stable even in energetically disadvantageous scenarios. An example of this in nature is diamond, which is stable in its configuration at high pressure and remains in that form at ambient pressure even though it would be more stable as graphite.
“I think if you can stabilize this high pressure state, you can change the world,” Chu said.
The group published their research in the journal Nature and established a company to work towards mass-producing their superconducting material at room temperature and ambient pressure.
The future of superconductivity
Even though a sulfur hydride broke the record for the highest-temperature superconductor, Dias said that there should and will still be research into copper-, iron-, and lead-based superconductors, as each may be best for a given application.
There are both practical and theoretical questions left to answer, Larbalestier said, noting that copper oxide superconductors in particular are not well understood.
“There's a sense that there's at least one Nobel Prize still left for really explaining why copper oxide superconductors superconduct up to 100 Kelvin,” he said.
Additionally, high-powered computers have aided researchers in discovering new complex crystal structures that could be good candidates for superconducting. While Ashford initially used a pencil and paper to think about a hydrogen-based superconductor, physicists today have been able to compute several hundred such metallic hydrogen compounds, Larbalestier said.
Scientists at CERN are looking to extend the research done by the Large Hadron Collider with an even more massive Future Circular Collider (FCC). Larbalestier said that he is part of a group tasked with developing a superconducting material to coat the ring of the FCC and increase the collider’s efficiency.
With the fine-tuning and commercialization of superconductors, Dias said that some technology from science fiction—like the hoverboards in Back to the Future—are now possible, even if they aren’t practical.
“This can really flip the whole world in terms of the technology we're using right now—that's why I think so many researchers are putting their full effort to make this a reality,” Dias said. “In 10, 15 years, we probably will be seeing a different world.”