Atoms spin backwards when they fly along a surface

Have you ever noticed that when a car is filmed, the wheels sometimes seem to turn back? For cars, turning the wheels in the opposite direction to the movement of the car is an artifact. But, for atoms, it can actually happen.

Imagine this

Let's set the stage. A sheet of flat metal, suspended in the void: the camera is panning to see a single atom moving flat a few nanometers from the surface. The electrons surrounding the nucleus of the atom move the electrons away from the metal surface of the metal, creating a kind of front wave load in front of the core and a load wake behind it. What we are looking at is the very image of a flat quantum salt runner.

The forces that generate the bow wave and the wake are carried by virtual photons exchanged between the metal surface and the atom. In the process of exchange, the atom will emit a constant stream of real photons in the direction of displacement. The momentum given by the launch of these photons slows the atom down. It is ultimately friction for a single atom.

The calculation of this scenario is old and only takes into account the translation movement. But the researchers wondered if the atom was spinning too. More precisely, are the forces between the surface and the atom such that they could produce a couple?

Rotation is prohibited

The simple answer to this question is no. Previous calculations have shown that photons emitted by the atom are polarized linearly, which means that they carry no spin pulse. This apparently excludes them as a source of angular momentum that would spin the atom. If the atom was to start spinning, then something else has to provide the kinetic momentum. In the quantum world, this can only happen if electrons or photons take away or deliver an angular momentum.

In this case, researchers have shown that photons with an angular momentum of spin are emitted, which means that the atom must start rotating to keep everything balanced.

But the equations also show that these photons can be emitted only in the opposite direction of the direction of movement of the atom, which will cause its acceleration. In other words, the atom does not only begin to turn, it also accelerates in the direction of its movement. In fact, on the lookout, all the frictions seem to have disappeared, which seemed unrealistic.

To obtain a realistic result, the researchers had to abandon the standard approach of assuming a local balance between the atom, the light fields and the plate. Instead, the researchers, in their calculations, demanded that the speed remain the same. For this to happen, an external force must be applied to overcome the friction forces.

The resulting calculation shows that the total friction force on the atom is reduced but does not completely disappear. This is somewhat analogous to the difference between slip friction and rolling friction. You can drag a tire down the road, but the friction forces that prevent slippage are much greater than rolling. It is somewhat remarkable that the friction experienced by an atom moving near the surface of a metal plate is also reduced by rotation.

However, this is nothing if we consider that the atom turns in the wrong direction, which is wrong compared to what we expect if it was a car tire. Imagine, if you please, that the tires of your car were turning back, but that the car was moving quite happily.

Rubidium runners

To make their calculations a little more concrete, the researchers identified the forces and accelerations for specific combinations of atoms and metal surfaces. They found that, for a Rubidium atom flying over a gold surface at 30 km / s, the friction forces were reflected by 30 nm / s.² slow-down. It's pretty close to the limits of our current ability to measure the acceleration of a single atom. In addition, a lithium atom flying over a sodium surface at 10 km / s will undergo a deceleration of 2.5 μm / s.², which should be measurable (even if creating a flat surface of sodium is difficult).

In the end, measuring the atomic rotation directly can be easier. The rotating atom must emit photons with a frequency corresponding to the rotation speed, which is about 25 MHz (for example, below the FM radio frequencies). Single photons at 25 MHz are hard to detect, but a constant flow of atoms can emit enough radiation to allow us to detect it.

Assuming that the calculations and subsequent experiments work, I still do not know what these results mean. The concept of rotating atom is very vague and ill defined. It's not as if the electrons were in planetary orbits and that a very small cat was upsetting a tiny solar system. Some electrons have spherical orbits, which means that the rotation does not change anything. The other electronic orbitals have a defined orientation but only one with respect to the other. Then, of course, there is the nucleus. Does it turn out too?

dumbbells

After reflection, I think that rotation makes sense only in the following way. Some electrons are stored in atomic orbitals that look a bit like perpendicular dumbbells. If all these orbitals are filled with electrons, the rotation then has no meaning because the atom is symmetrical and the rotation effectively exchanges the orbital tags for each electron – nothing really moves. Even if the orbitals are deformed by the surface and have different energies, the electrons also exchange energy between them.

However, if an atom has only one electron in one of the dumbbells orbitals, then the permutation trick of labels might not work and it does not work. There are no electrons with which to exchange energy. When the orbit rocks into space, the electron can gain energy as it approaches the surface and surrender energy as a photon when it jumps onto the next dumbbell orbital. This, at least conceptually, is very similar to rotation.

Physical Review Letters, 2019, DOI: 10.1103 / PhysRevLett.123.120401 (About DOIs)