When your laptop or smartphone heats up, it’s because of energy lost in translation. The same with the power lines that transmit electricity between cities. In fact, about 10 percent of the energy produced is lost in transmission. That’s because the electrons that carry the electric charge do so as free agents. and interact with other electronics as they travel together through wires and cables. All this jostling creates friction, and, in the end, heat.
But when the electrons pair up, they can rise above the diameter and slide through the material without friction. This “superconducting” behavior occurs in various materials, even at cold temperatures. If these materials can be made to conduct electricity close to room temperature, they could pave the way for zero-loss devices, such as heat-free laptops and phones, and high-performance cables. But first, scientists must understand how electrons pair up in the first place.
Now, new images of paired particles in clouds of atoms can provide clues to how electrons pair up in materials that conduct high voltages. The snapshot was taken by MIT physicists and is the first image to directly match a fermion—an important type of particle that includes electrons, as well as protons, neutrons, and some 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 some superconducting materials. They developed a technique to image supercooled clouds of potassium-40 atoms, which allowed them to observe paired particles, even when separated by a small distance. They can also pick up interesting patterns and behaviors, such as couples making checkerboards, which are interrupted by singles passing by.
Observations, reported today in Science, can be a visual diagram for how electrons may pair up in superconducting materials. The results may also help explain how neutrons pair up to form a dense superfluid and churning superfluid inside a neutron star.
“Fermion pairing is at the basis of superconductivity and many other phenomena in nuclear physics,” said Martin Zwierlein, the Thomas A. Frank Professor of Physics at MIT. “But no one has seen this matchup on location. So it’s pretty amazing to see these images on screen, to be honest.”
Co-authors of the study include Thomas Hartke, Botond Oreg, Carter Turnbaugh, and Ningyuan Jia, all members of MIT’s Physics Department, the MIT-Harvard Center for Ultracold Atoms, and the Electronics Research Laboratory.
A suitable perspective
To observe electron pairing directly is an impossible task. They are too small and too fast to photograph with existing photographic techniques. To understand their behavior, physicists like Zwierlein looked at the analog system of atoms. Both electrons and atoms, despite the difference in their size, are similar in that they are fermions – particles that exhibit a property called “half spin”. When fermions of opposite spin interact, they can pair up, as electrons do in superconductors, and as some atoms do in clouds of gas.
Zwierlein’s group studied the behavior of potassium-40 atoms, known as 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 a pair of electrons. But under normal conditions, at room temperature, atoms react in a blur that is difficult to grasp.
To get a better view of their behavior, Zwierlein and his colleagues studied the particles as a very dilute gas of about 1,000 atoms, which they placed under extremely cold nanokelvin conditions that slowed down the atoms. Researchers also contain the gas inside an optical lattice, or a grid of laser light that atoms can hop inside, and researchers can use as a map to determine the precise location of atoms.
In their new study, the team has improved their existing technique for imaging fermions that allows them to freeze atoms in an instant, then take separate snapshots of potassium-40 atoms with one particular spin or the other. Researchers can then superimpose images of one atom on top of another, and look to see if the two types match up, and how.
“It was hard to get to the point where we could take these pictures,” Zwierlein said. “You can imagine at first getting a big fat hole in your image, your atoms running away, nothing working. We have had very complex problems to solve in the lab over the years, and the students are very patient, and finally, to be able to see these images is really satisfying.”
What the team saw was a match between the behavior of atoms predicted by the Hubbard model – a widely held theory that is believed to be important for the behavior of electrons in high-temperature superconductors, materials that exhibit superconductivity at relatively high (although still very cold) temperatures. Predictions of how the electrons pair up in these materials have been tested through this model, but never directly observed until now.
The team created and imaged thousands of different clouds of atoms and translated each image into a digital grid-like model. Each grid shows the location of atoms of both types (shown in red and blue in their paper). From these maps, they can see grid squares with red or blue atoms, and squares where both red and blue atoms pair up locally (shown in white), as well as empty squares without red. or blue (black) atoms.
Individual pictures show many local pairs, and red and blue atoms in close proximity. By analyzing a series of hundreds of images, the team was able to show that the atoms appear in definite pairs, sometimes connected in tight pairs within a square, and at other times forming looser pairs, separated by one or more grid gaps. This physical separation, or “non-local pairing,” was predicted by the Hubbard model but never directly observed.
The researchers also noticed that the collection of pairs seems to be a wider pattern, a checkerboard, and this pattern wobbled in and out of formation as a partner of one pair vented outside its square and immediately distorted the checkerboard of the other pair. This phenomenon, called “polarity”, has also been predicted but never seen directly.
“In this dynamic soup, the particles are constantly on top of each other, moving, but never jumping away from each other,” Zwierlein observed.
The pairing behavior between these atoms must also occur in superconducting electrons, and Zwierlein says the team’s new imaging will help inform scientists’ understanding of high-temperature superconductors, and perhaps provide insight into how these devices may be tuned to higher, more practical temperatures. .
“If you normalize our atomic gas to the electron density in the metal, we think this pairing behavior should occur above room temperature,” Zwierlein proposed. “That gives a lot of hope and confidence that such a pairing phenomenon can in principle happen at high temperatures, and there’s no such limit that there shouldn’t be a room temperature controller one day.”
This research was supported, in part, by the US National Science Foundation, the US Air Force Office of Scientific Research, and a Vannevar Bush Faculty Fellowship.
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