What Do Surfers and Electrons Have in Common

In this blogpost I will try to explain the project in which I am participating. The project is called Visualisation of Electron – Phonon Coupling in Organic Semiconducting Materials. I am assuming that the main target audience for this blogpost is not highly constituted by physicists, so this blogpost is supposed to be an easy to read post. For physicist fellows, I hope they enjoy my way of explaining physical notions to the non-physicist people. (By the way, all of the phenomena explained in this blogpost are quantum mechanical objects and quantum mechanical objects do not behave according to the common sense. In order to make it easy to understand, I will be using objects from daily life. But never mind and keep reading.)

As we all know, electrical current is the movement of electrons. All of the atoms have electrons around their nuclei. But they are all bounded to the nuclei, some of them strongly and some of them loosely bounded to the nuclei of atom. When they form up a material, some of their electrons become free. By free we mean that they can move freely through the material and that movement is called electric current. But the electrons cannot really go through all the way along the material freely; there are some imperfections in the material. Electrons are scattered by this imperfections or dislocations and this is an adverse effect for the electric current.

We also have phonons in the materials. Phonons can be considered as propagating waves through the material, like water waves moving through the sea. Sea can be considered as the material and the waves as phonons. Of course there are surfers who like to ride the waves and for our material the surfers are electrons. But you know surfers are not always successful to climb up to the top of the wave and ride the wave; they sometimes fail. That is the situation for electrons. Sometimes phonons can carry the electrons through the material, in this situation the electrical current is enhanced. Sometimes electrons are scattered by the phonons and this is a contribution to the electrical resistance.

Let’s consider this one dimensional array of atoms. It is ok to think that they are connected by springs. If I move one of the atoms a little bit along the line and release it; we will have a propagating wave. That propagating wave is a close definition for phonons.

That is what we try to understand in the project: Do the phonons cooperate with the electrons or do they make it hard to move for the electrons? Or under which circumstances they cooperate?