The New Playground of an Unexpected Bose-Einstein Condensate

Most playgrounds feature slides, swings, and other structures that encourage visitors to explore cause-and-effect, test their physical limits, and try new things. Scientists like to engage in these activities too, although their playgrounds don’t look quite the same…

Photo by Olivia Bauso on Unsplash
In new research published in the American Physical Society’s journal Physical Review Letters, a team of researchers from the US and Canada have discovered what looks like a Bose-Einstein Condensate (BEC) in an unexpected place—a magnet based on the rare-earth element ytterbium.
 “This discovery provides a new playground for understanding Bose-Einstein condensation in magnetic systems, as well as how ytterbium breaks our expectations at low-temperatures in condensed matter systems,” says Gavin Hester, a graduate student working with Kate Ross at Colorado State University and one of the project leaders. 
 BECs are often called the fifth state of matter. It’s a state that can only be reached when a very dilute gas is cooled to nearly absolute zero. At that temperature, the atoms clump together and start to act collectively, as if they are one big atom rather than a bunch of individual atoms with their own agendas. Scientists have been fascinated by BECs since they were predicted by Albert Einstein in the 1920s, but their experimental realization took another 70 years.

This graphic shows 3D successive snapshots in time during which a BEC appears in a gas of rubidium atoms. The data, taken at JILA in 1995, cemented the discovery of the new phase of matter. L-R: Just before the appearance of a BEC, just after the appearance, a sample of nearly pure condensate. Credit: NIST/JILA/CU-Boulder, public domain. 
 In this new research, the scientists weren’t initially thinking about BECs. They were studying the material Yb2Si2O7—made of ytterbium (Yb), silicon (Si), and oxygen (O) atoms arranged in a crystalline structure—and whether it can take the exotic form of a spin liquid. Like a BEC, a spin liquid is a collective state of atoms that yields unusual properties. At least that’s what theorists predict; spin liquids haven’t been fully confirmed in the lab yet. 
 To test whether Yb2Si2O7 is a good candidate for a spin liquid, the researchers grew single crystal samples. Then they exposed the samples to a range of magnetic field strengths and very low temperatures and analyzed how the physical properties changed. One measurement raised eyebrows.
“We initially had inclinations that [Yb2Si2O7] might exhibit a BEC a few years into this six-year project, when measuring specific heat in a magnetic field,” says Hester. “This then led to us utilizing the incredibly powerful technique of neutron scattering to characterize this state and its excitations,” he says. 
 In neutron scattering, a sample is placed in the path of a neutron beam. A detector on the opposite side of the sample records the properties of the incoming neutrons. The result is a pattern that reflects the ways in which neutrons are influenced by the sample. From this pattern, a computer program reconstructs the atomic and magnetic structure of the sample. 
 The neutron scattering results confirmed the team’s suspicion, at temperatures close to absolute zero, the sample displayed a magnetic structure characteristic of a BEC.
Hester with one of the instruments used in the experiment, a triple-axis spectrometer. Photo courtesy of Gavin Hester.
BECs have been discovered in a wide variety of systems, from photons to ultracold atoms and magnets. However, the team was surprised that Yb2Si2O7 was one of those systems. “For a Bose-Einstein condensate to exist in a magnetic system it must interact with its neighbors in a very symmetric fashion. This requirement makes Bose-Einstein condensation quite unexpected in Yb2Si2O7,” says Hester. In a magnetic solid, the way a magnetic ion interacts with its neighbors is influenced by its mass. Put simply, ytterbium—a heavy element—shouldn’t interact with all of its neighbors in a symmetric way.
 Furthermore, the researchers discovered that at high magnetic fields, the BEC signal has unusual characteristics. Understanding this “mystery phase” is one of the focal points of their ongoing research.

These results raise fundamental questions about the nature of interactions in the system, what new phases related to BEC are possible, and more generally how quantum materials based on ytterbium behave at low temperatures. The questions and surprises are a large part of what makes this discovery a playground, providing a new context in which researchers can explore cause-and-effect, test physical limits, and try new things.

“The highest form of research is essentially play.”
– N. V. Scarfe, written in the 1962 article “Play is Education” published in Childhood Education
 -Kendra Redmond 
This is Kendra’s 200th article for the Physics Buzz Blog. We really appreciate her contributions and efforts to science writing. We look forward to her 201st article!

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