Paul M Sutter Gives Us The Superatom – a cooler name for a Rydberg Polaron!

In the late 1800’s Johann Rydberg studied atoms, electrons and energy levels before we really knew what these were. His observations showed him that certain atoms gave off certain frequencies of light and he noticed that when the atoms were slowly heated up they emitted light of progressively higher frequency, higher frequency meant higher energy light emissions BUT with diminishing energy gaps. The gap between the lower frequency levels were much bigger than between the higher frequency levels.

In the 1920’ Neils Bohr’s model showed electrons orbiting an atomic nucleus a bit like a solar system. The Bohr model is still taught in many schools and frequently used to denote science. It is however wrong - electrons do not orbit a nucleus in circular orbits. We know all models are wrong but the Bohr model was useful at the time because it helped expand our understanding of energy levels, electrons orbiting the nucleus but only at certain discrete energy levels. Electrons could move from one orbit to another but would then emit a discrete amount of energy helping explain Rydberg’s results.

Further investigation showed us that the electron is a quantum mechanical beasty with a wave function giving us it is fuzzy probability function rather than a discrete point particle.

Only in the specific case where an electron is almost, but not quite, kicked out of its atom so it is very far away from its atomic center can it be described as physically orbiting its nucleus. The atoms other electrons are (probabilistically) closer to the nucleus so the distant electron experiences the single net positive charge counterbalancing its own negative charge. In this state the outer electron loses its quantum mechanical nature and behaves as a classical negative point particle orbiting a positive classical atomic nucleus – a Rydberg atom! In any other situation you need a full quantum mechanical treatment to understand really what is going on.

However to get a Rydberg Polaron – a superatom - you need a Bose Einstein Condensate (BEC).

Let’s remember that fermions are the building blocks of matter - electrons, quarks etc and cannot share the same quantum state. Bosons are the force particles that can share the same quantum state and so overlap each other. If fermions are cooled enough they start to behave like bosons and reflect their quantum mechanical nature, their wave function – they can then overlap, sharing a quantum state. Fermions having the same number of positive and negative charges can then pile on top of one another and behave like bosons. Cooled enough they overlap more and more and they act as a single unified whole – a BEC.

If we find a random atom in this BEC and then zap it with a tiny amount of energy so that one of its electrons is almost kicked out of the atom to a super high energy level we have formed a Rydberg atom. But all the other atoms are crammed together in the BEC in the same quantum state so there are other atoms sitting between the electron and its home nucleus! This means that the wandering electron will influence its environment, attracting positive nuclei and repelling negative electrons by tiny but measurable amounts. The particles will then orient themselves in relation to this electron.

Physicists have found that it is easier to describe the whole situation rather than the individual bits. By zooming out a little bit and looking at the effect the electron is having on the entire system it is embedded within it becomes easier to describe that instead as a quasiparticle. Think about when you hit something and it makes a sound as it vibrates. Those vibrations quantum mechanically are called phonons and they are quasiparticles. This means you don’t have to describe every single particle in the vibrating sound system but instead describe the phonon as its own particle which has say its own certain frequency.

So, returning to our wandering electron in a bath of atoms the effect the electron has on the atoms are called polarons relating to the polar effect between the negative and positive poles aligning themselves.

Atoms stuffed inside other atoms that are described as a single unit rather than the sum of its parts giving us a Rydberg Polaron – like an atom but more so, a super atom!

Maybe able to leap over tall buildings in a single bound!

Paul M Sutter’s Ask A Spaceman podcast Link here and video here

Researchers Report the creation of Rydberg polarons In A Bose Gas 2018

Physicists Build Rydberg Polarons, Rice University

Observation Of Rydberg Polarons: One Atom to Rule them all here

A more thorough explanation of the theory of excitons in a Rydberg Polaron here