Physicists generate the first snapshots of fermion pairs
The images shed light on how electrons form superconducting pairs that
glide through materials without friction.

Date:
July 6, 2023
Source:
Massachusetts Institute of Technology
Summary:
Physicists captured the first images that directly show the
pairing of fermions. The snapshots of particles pairing up in a
cloud of atoms can provide clues to how electrons pair up in a
superconducting material.


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FULL STORY ==========================================================================
When your laptop or smartphone heats up, it's due to energy that's lost
in translation. The same goes for power lines that transmit electricity
between cities. In fact, around 10 percent of the generated energy is
lost in the transmission of electricity. That's because the electrons
that carry electric charge do so as free agents, bumping and grazing
against other electrons as they move collectively through power cords
and transmission lines. All this jostling generates friction, and,
ultimately, heat.

But when electrons pair up, they can rise above the fray and glide
through a material without friction. This "superconducting" behavior
occurs in a range of materials, though at ultracold temperatures. If
these materials can be made to superconduct closer to room temperature,
they could pave the way for zero-loss devices, such as heat-free laptops
and phones, and ultraefficient 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 can
provide clues to how electrons pair up in a superconducting material. The snapshots were taken by MIT physicists and are the first images that
directly capture the pairing of fermions -- a major 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, which allowed them
to observe the particles pairing up, even when separated by a small
distance. They could also pick out interesting patterns and behaviors,
such as a the way pairs formed checkerboards, which were disturbed by
lonely singles passing by.

The observations, reported today in Science, can serve as a visual
blueprint for how electrons may pair up in superconducting materials. The results may also help to describe how neutrons pair up to form an
intensely dense and churning superfluid within 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 pairing
in situ. So it was just breathtaking to then finally see these images
onscreen, faithfully." The study's co-authors include Thomas Hartke,
Botond Oreg, Carter Turnbaugh, and Ningyuan Jia, all members of MIT's Department of Physics, the MIT-Harvard Center for Ultracold Atoms,
and the Research Laboratory of Electronics.

A decent view To directly observe electrons pair up is an impossible
task. They are simply too small and too fast to capture with existing
imaging techniques. To understand their behavior, physicists like
Zwierlein have looked to 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 electrons do in superconductors, and as certain atoms do in
a cloud of gas.

Zwierlein's group has been studying the behavior of potassium-40
atoms, which are known fermions, that 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 is difficult to capture.

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

In their new study, the team made enhancements to their existing technique
for imaging fermions that enabled them to momentarily freeze the atoms
in place, then take snapshots separately of potassium-40 atoms with
one particular spin or the other. The researchers could then overlay
an image of one atom type over the other, and look to see where the two
types paired up, and how.

"It was bloody difficult to get to a point where we could actually
take these images," Zwierlein says. "You can imagine at first getting
big fat holes in your imaging, your atoms running away, nothing is
working. We've had terribly complicated problems to solve in the lab
through the years, and the students had great stamina, and finally, to
be able to see these images was absolutely elating." Pair dance What
the team saw was pairing behavior among the atoms that was predicted by
the Hubbard model -- a widely held theory believed to hold they key to
the behavior of electrons in high-temperature superconductors, materials
that exhibit superconductivity at relatively high (though still very cold) temperatures. Predictions of how electrons pair up in these materials have
been tested through this model, but never directly observed until now.

The team created and imaged different clouds of atoms thousands of
times and translated each image into a digitized version resembling a
grid. Each grid showed the location of atoms of both types (depicted
as red versus blue in their paper). From these maps, they were able to
see squares in the grid with either a lone red or blue atom, and squares
where both a red and blue atom paired up locally (depicted as white), as
well as empty squares that contained neither a red or blue atom (black).

Already individual images show many local pairs, and red and blue atoms
in close proximity. By analyzing sets of hundred of images, the team
could show that atoms indeed show up in pairs, at times linking up in a
tight pair within one square, and at other times forming looser pairs, separated by one or several grid spacings. This physical separation,
or "nonlocal pairing," was predicted by the Hubbard model but never
directly observed.

The researchers also observed that collections of pairs seemed to form
a broader, checkerboard pattern, and that this pattern wobbled in and
out of formation as one partner of a pair ventured outside its square and momentarily distorted the checkerboard of other pairings. This phenomenon, known as a "polaron," was also predicted but never seen directly.

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

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

"If you normalize our gas of atoms to the density of electrons in a metal,
we think this pairing behavior should occur far above room temperature," Zwierlein offers. "That gives a lot of hope and confidence that such
pairing phenomena can in principle occur 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
U.S. National Science Foundation, the U.S. Air Force Office of Scientific Research, and the Vannevar Bush Faculty Fellowship.

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========================================================================== Related Multimedia:
* Data-figure_of_particles_pairing_up_in_a_cloud_of_atoms ========================================================================== Journal Reference:
1. Thomas Hartke, Botond Oreg, Carter Turnbaugh, Ningyuan Jia, Martin
Zwierlein. Direct observation of nonlocal fermion pairing in an
attractive Fermi-Hubbard gas. Science, 2023; 381 (6653): 82 DOI:
10.1126/ science.ade4245 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/07/230706152721.htm

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