The levitating power of quantum materials

The Redpath Museum hosted Tami Pereg-Barnea, an associate professor in McGill’s Department of Physics, on March 12 as part of the Cutting Edge Lecture in Science series. An expert in nano-scale materials, Pereg-Barnea discussed her current research, in which she applies quantum mechanical properties to understand the behaviour of superconductors.

Pereg-Barnea started by giving a crash course on quantum mechanics, highlighting the revolutionary concept of wave-particle duality, the idea that materials can be described as either a particle or a wave.

“Quantum mechanics is a theory that we use for microscopic scales,” Pereg-Barnea said.

Interactions between subatomic particles such as electrons are better described by quantum physics than classical physics: While larger-scale objects do not usually act as waves, physicists have found that small particles at the quantum level do. However, unlike ocean waves, quantum-level waves are not made of any material constituent; rather, they are mathematical functions, called wave functions, that describe the probability of finding a particle at different positions. 

Just as normal waves can be superimposed to cancel or amplify one other, quantum materials, such as electrons, can interact to produce wave functions with peaks and plateaus. The probability of finding an electron in these peaks is high, and almost nonexistent in the plateaus. This wave-particle duality is what distinguishes quantum mechanics from Newtonian physics, which considers objects only as particles.

Quantum mechanics helps explain the behaviour of superconductors, which are materials that can transport electrons between atoms with no resistance. Usually, the current of electrons through a material is slowed by their collisions with each other. However, by being cooled down to extremely low temperatures, a superconductor minimizes these collisions. According to Pereg-Barnea, lowering electron collisions can be used to avoid economic costs.

“Hydro-Quebec is losing about six per cent of the energy that it is sending us even before it arrives at our homes,” Pereg-Barnea said. “This is because of these collisions.”

Superconductors play on the force of magnetism, too. 

“A superconductor is not only a perfect conductor, [it] also repels magnetic fields from its interior,” Pereg-Barnea said. “This is the Meissner effect.”

The Meissner effect refers to the ability of a superconductor to repel a magnetic field. This effect is possible because of the coordinated nature of electric currents inside superconductors.

To show superconductors in action, Pereg-Barnea prepared a track made of magnetic material and demonstrated how a superconductor playing the role of a train would levitate above this track. Since the superconductor levitates, there is no friction. Therefore, given an initial push, the levitating train would theoretically continue forever.

“This looks like magic,” Pereg-Barnea said. “But it’s actually just physics.”

The mechanism behind this macro-scale event is quantum mechanics. When exposed to a magnetic field, the material creates a well-coordinated flow of electrons that bring a magnetic field opposing that of the track. Based on the wave-particle duality proposed by quantum mechanics, electrons do not have to actually hit each other in a collision but can rather ‘go through’ each other. 

“You should not think of them as particles,” Pereg-Barnea said. “You should think of them as waves.” 

When a system is cooled, the electrons in the system want to be at the lowest energy level, or ground state. In ground state, all electrons move in a coordinated way. Therefore, the cooling effect combined with the quantum mechanical collision of electrons allows the superconductor to produce a magnetic field exactly opposite to what it experiences from the track. 

While quantum mechanics requires a great deal of abstraction, it has many real-world applications, such as electrical conductance, lasers, and MRI scanners. Quantum materials also hold great potential in the realm of sustainability, though Pereg-Barnea made clear that scientific advance cannot be impactful without real-world insight. 

“I think we have enough materials nowadays to tackle sustainability issues,” Pereg-Barnea said. “However, this is still an engineering problem of making it more affordable and more efficient.”

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