Quasiparticles: A useful perspective

Recently, an international team of researchers claimed that it had discovered particles called Majorana fermions in a topological superconductor. The international press went gaga but, unfortunately, there wasn’t much discussion on whether what had been found was a particle or a quasiparticle. Many news outlets assumed that they were one and the same – but they’re really not. One physicist even thinks that the quasiparticle that was found in the superconductor could’ve been of synthetic origin, not natural. I asked condensed-matter physicist Vijay Shenoy, whose lab at IISc has been doing some great work on topological materials, about this. His replies are presented in full below.

Can all quasiparticles be treated as particles?

VS: The idea of a ‘quasiparticle’ is a very subtle one. At the risk of being technical, let me try this:

An excitation is called a particle if, for a given momentum of the excitation, there is a well-defined energy. Quite remarkably, this definition of a particle embodies what we conventionally think of as a particle: small hard things that move about.

Now, to an example. Consider a system made of atoms at a very low density. It will be in a gaseous state. Due to their kinetic energy, the atoms will be freely moving about. Such a system has particle-like excitations. These particle-like excitations correspond to the behaviour of individual atoms.

Now consider the system at a higher density. The atoms will be strongly interacting with each other and, therefore, make up a solid. You will never “see” these atoms as low-energy excitations. There will now be new type of excitations that are made of the collective motion of atoms and which will be particle-like (since there is a well-defined energy for a given momentum). These particle-like excitations are called phonons. Note that the phonon excitation is very different from the atom that makes up the solid. For example, phonons carry sound within a solid – but when the sound propagates, you don’t have atoms being carried from place to place!

A ‘quasiparticle’ excitation is one that is very nearly a particle-like excitation: for the given momentum, it is a small spread of energy about some average value. The manifestation is such that, for practical purposes, if you watch this excitation over longer durations, it will behave like a particle in an experiment…

Other examples: neutrons and protons are particle-like excitations of the quark-gluon system!

If I find a quasiparticle with certain unique properties, can I also be sure that I will also be able to find a particle with the same properties?

VS: I assume by “find a particle” you mean like a ‘fundamental particle’. I know examples where this is (as far as we know) not true. For example, the 1/3-filled Landau level has Laughlin quasiparticles that have a charge 1/3rd that of an electron! Other filling factors will have excitations with other fractional charges. We do not know any fundamental particle that could have these properties.

Actually, the idea of a ‘fundamental particle’ is itself not a very useful concept in physics. Something may look fundamental to us at scales of energies that are accessible to us – but if we probe at higher energy scales, we may see that it is also made up of other even more fundamental things (neutrons/protons are really quarks held together by gluons). We will then say that the original ‘fundamental particle’ is a quasiparticle excitation of the system of ‘even more fundamental things’! You could actually ask where this will end, at what energy scales… We really do not know the answer to this question. This is why the concept of a ‘fundamental particle’ is not a very useful concept in physics.