c. What are Quasi Particles?
The termQuasi particles describes a physical concept, which treats elementary excitations in solids, like spin waves, as particles. As the particles do not consist of matter, they are called quasi particles.
At first it must sound curious to treat excitations as particles. But let us think about what matter is. From Einstein’s relativistic theory we know that matter and energy are essentially the same. Matter can be converted into energy, like in fission and fusion reactors. On the other hand energy can be converted into matter. It happens naturally; when high energy particles or radiation hit the earth’s atmosphere, a whole cascade of secondary particles is created. This phenomenon is called “air shower”. Partially the created particles originate from kinetic energy of incoming high energy particles or they are entirely produced through the energy of a high energy photon. In large particle accelerators, new particles are produced from the impact energy that occurs when two particles with nearly the speed of light collide with each other.
So it seems that what we call matter also consist of elementary excitations of a medium called “space”. Some of these excitations have a very short lifetime like muons or pions for example. Others seem to have infinite lifetimes like protons and electrons. Although it might come out that their lifetime is just incredibly long, so that no decay could be observed till the up come of modern science.
From this point of view it seems not to be weird when we treat elementary excitations in solids as particles. These particles have in most cases finite lifetimes, like the magnon or exciton. But there are also some quasi-particles with infinite lifetime like the cooper-pair in superconductors. We see that there is a striking similarity between matter-particles and quasi-particles and to some extent they can be treated in the same way.
The next section will treat the quasi particle of spin waves and the question: