Superconductors are a well studied phenomenon that has led to the development
of important technology such as MRI machines and maglev trains.
Superconductors are materials that are able to conduct electricity with no resistance and are impervious to magnetic fields, allowing for the incredibly strong magnets in these machines to function without melting the rest of the equipment. Although they are so well-studied and commonly used, the entire theory behind how superconductors work is now being challenged by the fact that bismuth can, against all odds, become superconductive.
Superconductors are materials that allow electricity to pass through them with absolutely no resistance, while becoming invulnerable to magnetic fields. Bismuth aside, superconductivity can usually only be achieved at very cool temperatures, close to absolute zero. The most widely accepted theory behind how superconductors work is called the Bardeen–Cooper–Schrieffer theory, or BCS theory. The BCS theory suggests that when a metal reaches a low enough temperature, the electrons slow down and align themselves in pairs called Cooper pairs. When the electrons are lined up like this, they are able to flow easily through the metal as electricity, almost like a fluid. In non-superconductive materials, it is the electrons bouncing around and being repulsed by each other that prevents this easy flow and creates resistance.
The usual aim of superconductor research is to find material that can become superconductive at higher temperatures, making them easier to produce. However, the discovery of bismuth superconductors is forcing scientists to reexamine the entire theory behind how superconductors work.
When bismuth was first tested as a possible superconductor, two things stood in the way. The first was that in initial testing, the bismuth was only cooled to a temperature of 0.01 kelvin. Years later, it was discovered that the threshold for bismuth becoming superconductive is actually 0.00053 kelvin, and scientists had given up just short of the mark. The second and more confusing obstacle to bismuth being a superconductor, is that bismuth only has 1 mobile electron per 100,000 electrons. In contrast to bismuth, most superconductors that we’ve discovered have roughly 1 free flowing electron per atom. This low “carrier density” of bismuth is so small that the BCS theory is not able to explain superconductivity in bismuth, as there are not enough electrons to line up and form Cooper pairs. Bismuth is actually the lowest carrier density superconductor ever found. If the BCS theory were to be applied to bismuth, it would predict that superconductivity could only be achieved at temperatures thousands of times lower than 0.00053 k.
While bismuth superconductors may not seem to help scientists reach their goal of finding a room temperature superconductor, they have made it clear that there are more forces at work in superconductors than they realized. Researchers must now reevaluate what they thought they knew about superconductors, taking the properties of bismuth into account. With further research into how bismuth can become superconductive, it is possible that we may discover other factors that affect superconductivity, and advance the entire field of superconducting technology.