Atom Jam-Packed With Atoms Is New State of Matter

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The electron (blue) orbits the nucleus (red). Its orbit encloses many other atoms of the Bose-Einstein-condensate (green). TU Wien

Updated | Scientists have packed an atom full of other atoms to create a new, exotic state of matter called "Rydberg polarons."

This giant greedy atom can help scientists probe the reaches of ultracold atom physics.

The space between an atom's nucleus and electron is usually barren. But, when an electron's orbit sits relatively far away from its proton and neutron-packed centre, could you fill it with other atoms?

That was the question on the lips of an international group of researchers, who have effectively stretched out the orbit of an electron and stuffed the space with other atoms.

Their research was published in Physical Review Letters.

"The average distance between the electron and its nucleus can be as large as several hundred nanometres—that is more than a thousand times the radius of a hydrogen atom," Joachim Burgdörfer, one of the study's authors, said in a statement.

Highly excited electron

The team first created a state of matter called a Bose-Einstein condensate using ultracold strontium atoms. This condensate is an extremely low-density gas that has been cooled to a very low temperature. The scientists used a laser to transfer energy to one of the strontium atoms and excite a single electron, making a "Rydberg atom."

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This highly excited electron orbited its nucleus at a much greater distance than before. The orbit became so wide it encompassed other atoms in the condensate, which form a weak bond and become a new state of matter: Rydberg polarons.

Depending on the radius of an excited electron's path, a Rydberg atom could enclose up to 170 strontium atoms.

Minimal force from the neutral atoms

The Rydberg atom's electron is somewhat influenced by the neutral atoms along its orbit, scattering the particle slightly as it spins around its nucleus. Quantum physics allows this minimal scattering, which does not shift the electron from its orbit.

"The atoms do not carry any electric charge, therefore they only exert a minimal force on the electron," says Shuhei Yoshida, another study author based at Technische Universität Wien in Austria.

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Computer simulations show this weak interaction reduces the system's total energy, while a bond is created between the Rydberg atom and the other atoms.

"It is a highly unusual situation," says Yoshida. "Normally, we are dealing with charged nuclei binding electrons around them. Here, we have an electron binding neutral atoms."

Ultracold atoms

Atoms vibrate with heat, which can break the very weak bond between these atoms. The Rydberg polarons, therefore, require very low temperatures for detection.

The work opens new avenues for research into ultracold atom physics.

"For us, this new, weakly bound state of matter is an exciting new possibility of investigating the physics of ultracold atoms," says Burgdörfer, who is also based at Technische Universität Wien. "That way one can probe the properties of a Bose-Einstein condensate on very small scales with very high precision."

This article has been updated to include more information about Bose-Einstein condensate.