As tiny as they are, there's a relatively large amount of empty space inside an atom. Now, scientists from Austria and the US have filled in some of those gaps, creating a new state of matter in the form of "giant atoms" filled with other atoms.

Normally, there's a bit of space between the nucleus of an atom and the electrons orbiting it. The distance of that orbit depends on the type of atom, and at several hundred nanometers wide, the Rydberg atom earns its nickname as the "giant atom." That's over a thousand times the radius of a hydrogen atom, which got the researchers wondering whether giant atoms could be stuffed with these smaller atoms.

To test the idea, researchers from TU Wien, Rice University and Harvard started with a Bose-Einstein condensate. This exotic state of matter is formed when atoms are cooled to just slightly above absolute zero, which causes them to slow right down and begin clumping together, exhibiting unusual behavior. Bose-Einstein condensates have recently helped scientists create strange new states of matter like supersolids, excitonium, and fluids with negative mass.

In this case, the starting point was a cloud of strontium atoms. After cooling them into a Bose-Einstein condensate, the team used a laser to energize one of the atoms, which lifts a single electron in that atom into a highly-excited state. The excited electron begins orbiting the nucleus at a much larger distance than usual, creating a Rydberg atom.

This electron's orbit becomes so large that other strontium atoms can easily fit inside it. The team observed up to 170 atoms crammed inside one Rydberg atom, but that number can depend on the Rydberg atom's radius and the density of the Bose-Einstein condensate.

The atoms do interact with each other, but very weakly. The Rydberg atom's electron is scattered very slightly by the neutral atoms in its path, but because the electron is so slow it isn't transferred into another state. The team ran computer simulations of the interaction and found that this weak interaction decreases the total energy of the system, forming a bond between the giant atom and the smaller ones inside it.

The atoms do not carry any electric charge, therefore they only exert a minimal force on the electron," says Shuhei Yoshida, co-author of the study. "It is a highly unusual situation. Normally, we are dealing with charged nuclei, binding electrons around them. Here, we have an electron, binding neutral atoms."

Weak as it may be, the bond means that the atoms form a new state of matter, which the team calls Rydberg polarons.

"For us, this new, weakly bound state of matter is an exciting new possibility of investigating the physics of ultracold atoms," says Joachim Burgdörfer, co-author of the study. "That way one can probe the properties of a Bose-Einstein condensate on very small scales with very high precision."

The research was published in the journal Physical Review Letters.

Source: TU Wien