When I started my master’s degree, I kept hearing about quasiparticles but never got a clear definition. So I was a bit confused—what is a quasiparticle, really?
At its core, a quasiparticle is something that behaves like a particle. It can have a mass, a momentum, a position… and it obeys the same kinds of equations as real particles (like the Schrödinger equation). But what truly sets it apart is the context in which it exists.
If you break a symmetry in the vacuum, you can get excitations of the spacetime field itself—those are what we call particles. But if you break a symmetry in a medium—like a metal, a semiconductor, or even a fluid—different kinds of collective excitations appear. These are quasiparticles.
Quasiparticles are deeply tied to the material they emerge from. You won't find an exciton floating around in a vacuum, for example. Their behavior arises from the collective motion of many particles.
One simple and illustrative example is the hole. Imagine you excite an electron in an atom so it jumps to the conduction band. Now that atom is missing one electron, and we call that absence a hole. Electrons from neighboring atoms might move in to fill the gap, leaving behind new holes as they do. What we observe is that this hole seems to move through the material, as if it were a real particle—with a positive charge and an effective mass. And yes, just like particles and antiparticles, a hole and an electron can annihilate each other.
Examples
Just a few examples of quasiparticles I’ve come across:
- In a metal, a plasmon is the quantum of plasma oscillations—collective vibrations of the free electron gas.
- In a semiconductor, an exciton is a bound state of an electron and a hole, held together by the Coulomb force.
- Polaritons emerge when light (an electromagnetic wave) strongly couples with a material excitation—like a plasmon or an exciton. This gives rise to hybrid quasiparticles like plasmon-polaritons or exciton-polaritons. I’ll explain exciton-polaritons in more detail in another article!
- In a fluid, a vortex can behave like a particle, although it’s usually not stable enough to qualify as a proper quasiparticle.
- But in a quantum fluid, a vortex becomes a topological defect that can’t just disappear—it’s a real quasiparticle!
- And finally, in a Bose-Einstein condensate, a bogolon is a small excitation of the condensate, behaving like a particle.
Quasiparticles are everywhere once you know where to look!