MIT physicists generate first snapshots of fermion pairs

When your laptop or smartphone gets hot, it’s due to energy being lost in translation. The same goes for power lines that transmit electricity between cities. In fact, around 10 percent of the energy generated is lost in the transmission of electricity. This is because the electrons that carry electrical charge do so as free agents, bumping into and rubbing against other electrons as they collectively move through power cables and transmission lines. All of these pushes create friction and ultimately heat.

But when the electrons pair up, they can rise above the fray and glide through a frictionless material. This “superconducting” behavior occurs in a variety of materials, albeit at ultracold temperatures. If these materials can become superconducting closer to room temperature, they could pave the way for zero-loss devices such as heat-free laptops and phones, and ultra-efficient power lines. But first, scientists will have to understand how electrons pair up in the first place.

Now new snapshots of particles pairing up in a cloud of atoms may provide clues about how electrons pair up in a superconducting material. The snapshots were taken by MIT physicists and are the first images to directly capture the pairing of fermions, an important class of particles that includes electrons, as well as protons, neutrons, and certain types of atoms.

In this case, the MIT team worked with fermions in the form of potassium-40 atoms, and under conditions that simulate the behavior of electrons in certain superconducting materials. They developed a technique to image a supercooled cloud of potassium-40 atoms, allowing them to observe how the particles paired up, even when separated by a small distance. They were also able to identify interesting patterns and behaviors, such as the way couples formed checkerboards, that were disturbed by singles passing by alone.

the observations, reported today in Science, can serve as a visual model for how electrons can pair up in superconducting materials. The results may also help describe how neutrons pair up to form an intensely dense and churning superfluid inside neutron stars.

“Fermion pairing is at the basis of superconductivity and many phenomena in nuclear physics,” says study author Martin Zwierlein, the Thomas A. Frank Professor of Physics at MIT. “But no one had seen this couple in situ. So it was awesome to finally see these images on screen, faithfully.”

Study co-authors include Thomas Hartke, Botond Oreg, Carter Turnbaugh, and Ningyuan Jia, all members of the MIT Department of Physics, the MIT-Harvard Center for Ultracold Atoms, and the Electronics Research Laboratory.

a decent view

Directly observing pairs of electrons is an impossible task. They are simply too small and too fast to be captured with existing imaging techniques. To understand their behavior, physicists like Zwierlein have searched for analogous systems of atoms. Both electrons and certain atoms, despite their difference in size, are similar in that they are fermions, particles that exhibit a property known as “half-integer spin.” When fermions of opposite spin interact, they can pair up, as do electrons in superconductors and as certain atoms in a cloud of gas.

Zwierlein’s group has been studying the behavior of potassium-40 atoms, which are known fermions, which can be prepared in one of two spin states. When a potassium atom of one spin interacts with an atom of another spin, they can form a pair, similar to superconducting electrons. But under normal room temperature conditions, the atoms interact in a blur that’s hard to capture.

To get a decent insight into their behavior, Zwierlein and his colleagues study the particles as a very dilute gas of about 1,000 atoms, which they place in nanokelvin ultracold conditions that slow the atoms down. The researchers also contain the gas within an optical lattice, or lattice of laser light, into which the atoms can jump, and which the researchers can use as a map to pinpoint the precise locations of the atoms.

In their new study, the team made improvements to their existing technique to image fermions that allowed them to momentarily freeze the atoms in place, then take separate snapshots of the potassium-40 atoms with one particular spin or another. The researchers could then superimpose an image of one type of atom on top of the other and see where and how the two types matched up.

“It was very difficult to get to a point where we could take these images,” says Zwierlein. “You can imagine at first getting big holes in your image, your atoms leaking out, nothing works. We had terribly complicated problems to solve in the lab over the years, and the students had great stamina, and finally being able to see these images was absolutely exciting.”

Dance partner

What the team saw was the pairing behavior between atoms that was predicted by the Hubbard model, a widely accepted theory believed to hold the key to the behavior of electrons in high-temperature superconductors, materials that exhibit superconductivity at relatively low temperatures. high (although still very cold). ) temperatures. Predictions of how electrons in these materials pair up have been tested through this model, but have never been directly observed until now.

The team created and imagined different clouds of atoms thousands of times and translated each image into a digitized version that resembles a grid. Each grid showed the location of atoms of both types (represented as red versus blue on your paper). From these maps, they were able to see grid squares with a single red or blue atom, and squares where a red and blue atom paired locally (represented as white), as well as empty squares that did not contain a red or blue atom. or blue (black) atom.

The individual images already show many local pairs and red and blue atoms in close proximity. By analyzing sets of hundreds of images, the team was able to show that atoms are shown in pairs, sometimes coming together in a tight pair within a square, and other times forming looser pairs, separated by one or more grid spaces. . This physical separation, or “nonlocal pairing,” was predicted by Hubbard’s model but never directly observed.

The researchers also observed that the collections of pairs appeared to form a larger checkerboard pattern, and that this pattern moved in and out of the formation when one member of a pair ventured outside of their square and momentarily distorted the checkerboard. chess of other pairs. This phenomenon, known as “polaron”, was also predicted but never directly seen.

“In this dynamic soup, the particles are constantly jumping on top of each other, moving away, but never dancing too far from each other,” Zwierlein notes.

The pairing behavior between these atoms should also occur in superconducting electrons, and Zwierlein says the team’s new snapshots will help inform scientists’ understanding of high-temperature superconductors, and perhaps provide insight into how these materials might fit together. at higher temperatures and more practical. .

“If you normalize our gas of atoms to the electron density in a metal, we think this pairing behavior should occur well above room temperature,” Zwierlein offers. “That gives a lot of hope and confidence that such pairing phenomena can occur, in principle, at elevated temperatures, and there’s no a priori limit to why there shouldn’t be a room-temperature superconductor one day.”

This research was supported, in part, by the US National Science Foundation, the US Air Force Office of Scientific Research, and the Vannevar Bush College Grant.