New snapshots reveal how electrons pair up in superconducting materials

When your laptop or smartphone heats up, it’s because some of the energy is lost during transmission. The same goes for power lines that carry electricity between cities. Approximately 10 percent of the generated energy is lost during this process. This loss occurs because electrons, which carry electric charge, move through power cords and transmission lines as free agents, colliding with other electrons along the way. These collisions create friction and generate heat.

However, when electrons pair up, they can move through a material without experiencing friction. This behavior, known as superconductivity, occurs in certain materials but requires extremely low temperatures. If scientists can achieve superconductivity at higher temperatures, it could lead to the development of devices like heat-free laptops, phones, and highly efficient power lines. However, before that can happen, researchers need to understand the mechanism behind electron pairing.

Recently, physicists from MIT captured snapshots of particles pairing up in a cloud of atoms, offering insights into how electrons might pair up in superconducting materials. The team focused on fermions, a class of particles that includes electrons, protons, neutrons, and specific types of atoms. They developed a technique to image a supercooled cloud of potassium-40 atoms, simulating the behavior of electrons in certain superconductors. This allowed them to observe the pairing of particles, even when they were close together. They also observed intriguing patterns and behaviors, such as pairs forming checkerboard-like arrangements that were disrupted by solitary particles passing by.

The results of this study, published in Science, provide a visual framework for understanding how electrons could pair up in superconducting materials. They could also contribute to our understanding of how neutrons pair up to form dense superfluids within neutron stars.

“Fermion pairing is fundamental to superconductivity and many phenomena in nuclear physics,” explains Martin Zwierlein, a study author and professor of physics at MIT. “But no one had witnessed this pairing directly before. So seeing these images onscreen was truly awe-inspiring.”

The study’s co-authors include Thomas Hartke, Botond Oreg, Carter Turnbaugh, and Ningyuan Jia, all affiliated with MIT’s Department of Physics, the MIT-Harvard Center for Ultracold Atoms, and the Research Laboratory of Electronics.

A decent view

Directly observing the pairing of electrons is an extremely challenging task due to their small size and high speed, making them difficult to capture using existing imaging techniques. To gain insights into their behavior, physicists like Zwierlein have turned to analogous systems involving atoms. Although atoms are much larger than electrons, they share similarities as fermions, particles with “half-integer spin.” When fermions of opposite spins interact, they can form pairs, similar to how electrons pair up in superconductors or certain atoms in a gas cloud.

Zwierlein’s research group focuses on studying potassium-40 atoms, which are fermions that can exist in two different spin states. When atoms of different spins interact, they can form pairs analogous to superconducting electron pairs. However, under normal room-temperature conditions, the atoms’ interactions occur rapidly, making them challenging to observe.

To overcome this hurdle, Zwierlein and his colleagues manipulate a dilute gas containing around 1,000 atoms and cool it to ultracold temperatures, close to absolute zero, effectively slowing down the atoms’ movements. They confine the gas within an optical lattice—a grid of laser light that allows the atoms to hop around—and use this lattice as a map to precisely locate the atoms.

In their recent study, the team improved their imaging technique for fermions. They were able to momentarily freeze the atoms in place and capture separate snapshots of potassium-40 atoms with each spin state. By overlaying these images, they could identify where the two types of atoms paired up and analyze their behavior.

“Reaching the point where we could capture these images was a highly challenging process,” Zwierlein explains. “At first, there were significant obstacles—imaging issues, atoms escaping, and numerous complex problems to solve in the lab over the years. The students demonstrated great perseverance, and finally being able to visualize these images was absolutely exhilarating.”

Pair dance

The observations made by the team align with the predictions of the Hubbard model, which is a widely accepted theory that sheds light on the behavior of electrons in high-temperature superconductors. These materials exhibit superconductivity at relatively higher temperatures (although still extremely cold). While the predictions regarding electron pairing in these materials have been tested using the Hubbard model, they had never been directly observed until now.

To conduct their experiments, the team created multiple clouds of atoms and captured images of them, converting each image into a digital grid representation. Each grid displayed the positions of both types of atoms, represented as red and blue in the research paper. By analyzing these grids, the researchers were able to identify squares containing single red or blue atoms, squares where a red and blue atom were paired locally (depicted as white), and empty squares devoid of either atom (black).

Individual images already revealed numerous local pairs of atoms, with red and blue atoms in close proximity. By analyzing hundreds of images, the team demonstrated that atoms indeed formed pairs, sometimes tightly within a single square, and at other times with a looser pairing across one or several grid spacings. This separation, referred to as “nonlocal pairing,” was predicted by the Hubbard model but had never been observed directly.

The researchers also noticed that pairs of atoms tended to form a larger checkerboard pattern, which periodically distorted as one member of a pair moved outside its square, momentarily disrupting the checkerboard pattern of other pairings. This phenomenon, known as a “polaron,” had been predicted but had never been directly observed.

“In this dynamic environment, the particles are constantly hopping on top of each other, moving away but never straying too far apart,” explains Zwierlein.

The observed pairing behavior between the atoms is expected to also occur among superconducting electrons. The team’s novel snapshots will enhance scientists’ understanding of high-temperature superconductors and could offer insights into methods for achieving higher and more practical superconducting temperatures.

“If we scale the density of our atom gas to match that of electrons in a metal, we believe this pairing behavior should occur well above room temperature,” Zwierlein suggests. “This gives us hope and confidence that such pairing phenomena can potentially occur at elevated temperatures. There is no inherent reason why we couldn’t achieve room-temperature superconductivity in the future.”

Source: Massachusetts Institute of Technology

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