The surprising past and present of superconductivity

Heike Kamerlingh Onnes was cool. Well, he was really good at making things cool. Born in 1853, he was the first person to cool helium to its liquid form at -271 °C, then the coldest temperature ever achieved on Earth. After creating liquid helium, he wondered about cooling liquid mercury into a solid state and using it as a wire to conduct electricity. Scientists at the time predicted that supercooled metals would be terrible conductors, stopping an electrical current in its tracks. 

When Kamerlingh Onnes tried it out, the opposite happened: all electrical resistance disappeared. 

Kamerlingh Onnes called this phenomenon “superconductivity.” In the 115 years since, researchers have discovered why superconductivity happens, found new superconducting materials, and invented myriad ways to use its curious properties.

So what exactly is superconductivity? How does it work on the quantum level? And how close are we to having a viable superconductor to use in everyday life? 

What is superconductivity? 

While Kamerlingh Onnes discovered superconductivity in mercury in 1911, it took the development of quantum mechanics and other insights to truly understand how superconductivity operates at the subatomic level.

Superconductivity occurs when materials conduct electricity with zero resistance below a certain temperature.

Electrical currents form when charged particles move energy through a conductive material. Usually, these charged particles are electrons. Electrical resistance happens when the electrons flowing through a material bump into the conductive material’s atoms. Those collisions remove energy from the circuit. That, at a subatomic level, is resistance: the collisions of electrons into atoms. 

Metals are often good conductors because their atoms are held together in a unique lattice structure by strong electrostatic forces, allowing a “sea” of free-flowing electrons.

In superconductors, however, the electrons never bump into the atoms. Normally, electrons repel one another because they are negatively charged, and in chemistry, like repels like. But at a certain low temperature, electrons will overcome their repulsion and instead join up into something called a Cooper pair.

Cooper pairs have a unique property in that their components start to act like a different type of fundamental particle altogether. Electrons are a type of fundamental particle called a fermion and, as stated by the Pauli exclusion principle, no two identical fermions can occupy the same quantum state within a system. But while in a Cooper pair, electrons start to behave like bosons. Bosons are special because they can occupy the same quantum state in a system. So instead of acting like fermions, as electrons usually do, the Cooper pair acts like a boson. 

The result: all the Cooper pairs in a superconductor can now occupy the same state. The practical effect of this is that Cooper pairs do not run into atoms, since running into an atom would cause an electron to move into a different energy level, and thus a different quantum state. Therefore, the electrons flow without facing any resistance.

The future of superconductors

Superconductors have the chance to completely transform how we move energy. Imagine power lines that could move electricity over long distances without ever losing power. Computer chips that could connect faster with zero-resistance wiring. Or even high-speed trains that use superconductivity’s magnetic effects to move faster, using less energy.

The challenge, of course, is maintaining the extreme cold temperatures required. Research into superconductors today often focuses on finding more viable options for regular use. Since superconductivity is a phase change, some combination of cooling or pressure is needed, but effort are underway to find superconductors that work at warmer temperatures and lower pressures. Today, the best high temperature superconductors at standard atmospheric pressure, known as cuprates, operate at around -135°C.

Superconductors at these temperatures have already found their way into technologies, including medical sensors and particle detectors, electric motors, and more. But a room temperature superconductor could do even more.

Here at Perimeter, researchers have contributed to breakthroughs in superconducting materials. In 2014, for example, Perimeter faculty member Roger Melko and graduate student Lauren Hayward (who is now PI’s Associate Director of Training Programs) co-authored a paper about the transition phase of one superconducting material. Research into quantum matter and condensed matter can also provide insights into tomorrow’s superconductors.

Researchers have come a long way from the days of freezing mercury - but we have much further to go in unlocking superconductivity’s full potential.