The craze caused by LK-99 at the end of July gradually subsided in mid-August. After many authoritative institutions successively falsified the superconductivity of LK-99, Nature officially issued a document on August 16 denying that LK-99 is a room temperature superconductor. But one question remains: Would a true room-temperature superconductor be revolutionary?


The answer depends on the area of ​​application and whether the hypothesized material possesses other key qualities. But in at least some fields of science, especially those that use strong magnetic fields, better superconductors could have a huge impact.

A superconductor is a material that can carry electrical current without resistance at a certain temperature and therefore does not produce waste heat.

But all confirmed superconductors exhibit this property only under low temperatures or extreme pressure conditions, or both.


Behavior of heat capacity (c(v), blue) and resistivity (ρ, green) during superconducting phase transition

Such materials are already widely available in the laboratory because researchers are able to use a range of techniques to reduce their temperatures, although this increases the cost and complexity of experiments.

But in daily applications, the low-temperature requirements of superconductors are a difficult threshold to overcome.

An extreme example is the Large Hadron Collider (LHC), an accelerator at CERN.


To move protons in a 27-kilometer circle, the LHC uses superconducting coils with a temperature of just 1.9 Kelvin (-271.25ºC) to generate a strong magnetic field.

To do this, a cryogenic system containing 96 tons of liquid helium is first required. It is the largest system of its kind in the world.

Luca Bottura, a magnet researcher and nuclear engineer at CERN, once said, "If we don't need extreme temperatures, engineering design will be greatly simplified."

Therefore, superconductors that can operate at or near room temperature will quickly revolutionize many fields of science.

But science is not there yet.

Quantum problem

Take quantum computers as an example. This emerging technology is expected to solve certain tasks that classical computers cannot complete.

One of the main ways to build a quantum computer is to store information in rings of superconducting materials.


quantum computer

The superconducting materials are cooled to near absolute zero (-273.15ºC) and then housed in expensive, Russian-doll-like devices called dilution refrigerators.


dilution refrigerator

In quantum computers based on superconductors, performance will rapidly degrade if the temperature increases by even a few tenths of a degree, for reasons that have nothing to do with superconductivity.

Yasunobu Nakamura, co-inventor of superconducting quantum computing, believes that quantum computing is extremely sensitive to any kind of noise, and thermal vibrations are a major enemy, which can produce spurious "quasiparticles."

He mentioned that the antagonistic effect of thermally excited quasiparticles can be seen at around 100-150 millikelvin.

In other cases, the experiment itself may not require extremely low temperatures, but the superconductor still needs to be maintained at a much lower temperature than when it transitions to superconducting (i.e., Tc).


Superconductors vary in their physical properties. But in many applications, especially in high-field magnets, two properties are critical: critical current and critical magnetic field.

That's because superconductivity is lost not only when the temperature rises, but also when the material is pushed to carry more than a certain amount of electric current or is exposed to a high enough magnetic field.


MIT's cryogenic system is packed with superconductors with high transition temperatures. Credit: David L. Ryan/The Boston Globe via Getty

Most importantly, both critical magnetic field and critical current are temperature-dependent: the lower the temperature, the greater the current and magnetic field the material can withstand.

So, although a superconductor has a high Tc, it does not mean that it can be used at any temperature below Tc.

In many applications, the properties of superconductors improve as the system temperature decreases.

Fortunately, the best superconductors discovered so far, including a class of superconductors called cuprate (or cuprate) superconductors, can also withstand very high magnetic fields as long as the temperature is kept low enough.

on site

Four years ago, the National High Magnetic Field Laboratory (NHMFL) in Tallahassee, Florida, used a copper oxide to obtain records of stable (non-pulsing) magnetic field strengths.

NHMFL's superconducting coils can generate a magnetic field of 45.5 Tesla, but only if they are kept in liquid helium, which is below 4.2 Kelvin.

"We use high-Tc superconductors not because of their high Tc value, but because of their high critical magnetic field," said physicist Laura Greene, NHMFL's chief scientist.

"If you want a high-field magnet, run it as low a temperature as possible, because that's where you get the real power of superconductivity," said Yuhu Zhai, a mechanical and electrical engineer at another U.S. national laboratory, the Princeton Plasma Physics Laboratory (PPPL) in New Jersey.

CERN is exploring options for a future particle collider that could eventually smash protons at seven times more energy than the Large Hadron Collider, a range in which physicists hope to discover new elementary particles.


Map of the Large Hadron Collider and Super Synchrotron at CERN

To reach these higher energies, particles must be accelerated using higher fields or along longer accelerator loops, or both.

To build such a machine, physicists dream of digging a 100-kilometer-long ring tunnel next to the Large Hadron Collider.

But even with such a large ring tunnel, a superconducting magnet like the one at the Large Hadron Collider, an 8-Tesla monster with a niobium-titanium coil, wouldn't be able to produce the required magnetic field, which is estimated to be at least 16 to 18 Tesla.

"At this point, we obviously have to move to other materials," Bottura said.

Current high-Tc superconductors can achieve this, but they may need to be kept at liquid helium temperatures.

A similar accelerator proposed by China, the Ring Electron-Positron Collider, will also use high-Tc superconducting magnets.

Wang Yifang, director of the Beijing Institute of High Energy Physics, said they have been considering high-temperature superconducting materials for some time, mainly cuprates and iron-based materials.

critical current

However, cuprate superconductors also have other drawbacks: They are brittle ceramic materials that are expensive to produce and difficult to make into cables.

In addition, Wang Yifang also mentioned that the critical current of this material is too low. Another type of iron-based superconductor has better performance in principle and is only half the cost of copper oxide.

Bottura and others are investigating the feasibility of a new kind of accelerator.

By replacing protons with muons, particles similar to electrons but 207 times more massive, the collider can study the same type of physics as the 100-kilometer-long proton-proton collider.

But the research collider's ring is much smaller and can even be placed in the existing Large Hadron Collider tunnel, allowing muons to circle around without involving a particularly high-strength magnetic field.

But the problem is that generating a beam of muons with the right characteristics may require magnets as high as 40 Tesla.

At this intensity, the problem is no longer the superconductor, but how to keep the coil in place, since the current within the electromagnetic coil tends to push the magnets apart.

At 40 Tesla, even the strongest steel cannot withstand mechanical stress.

Instead, magnets may require the use of stronger materials such as carbon fiber. (NHMFL magnets have less stringent strength requirements because they need to produce a high magnetic field in a space only a few centimeters wide).

So superconductors will play a huge role in proton and muon colliders, but other engineering challenges may also arise.

Integration journey

However, in another class of machines designed to harness nuclear fusion energy, structural strength has become a serious constraint.

An established fusion method has long been to use magnets arranged in a donut shape, also known as a tokamak, to confine a plasma, heating the plasma to millions of degrees and crashing various isotopes of hydrogen together.


The world's largest experimental tokamak, called ITER, is under construction in southern France and will use large liquid helium to cool magnets and generate magnetic fields close to 12 Tesla.


But according to Zhai, industry and publicly funded laboratories are working hard to design tokamak magnets based on high-Tc superconductors.

There are many reasons. Higher magnetic fields could significantly increase the rate at which a fusion reactor burns fuel, thereby in principle increasing the energy that can be produced, but many of the key steps in extracting energy from fusion have yet to be proven.


One positive result of industry efforts to increase production of high-Tc magnetic materials has been to make them less expensive, but they are still much more expensive than niobium-titanium materials.

In addition, Zhai also said that the tokamak should eventually abandon liquid helium cooling. On the one hand, this is because the cooling system is complex and difficult to build, and on the other hand, as helium is a scarce resource, it is difficult to build hundreds of ITER-sized reactors using liquid helium.

Greene believes that finding better superconducting materials is a high-risk undertaking, and there have been few success stories so far.

Still, she says, “It’s hard work, but it’s also exciting, world-changing work.”

References:

https://www.nature.com/articles/d41586-023-02681-8