Quasiparticle

In physics, quasiparticles and collective excitations (which are closely related) are emergent phenomena that occur when a microscopically complicated system such as a solid behaves as if it contained different weakly interacting particles in free space. For example, as an electron travels through a semiconductor, its motion is disturbed in a complex way by its interactions with all of the other electrons and nuclei; however it approximately behaves like an electron with a different mass (effective mass) traveling unperturbed through free space. This 'electron with a different mass' is called an 'electron quasiparticle'. In another example, the aggregate motion of electrons in the valence band of a semiconductor or a hole band in a metal is the same as if the material instead contained positively charged quasiparticles called electron holes. Other quasiparticles or collective excitations include phonons (particles derived from the vibrations of atoms in a solid), plasmons (particles derived from plasma oscillations), and many others. In physics, quasiparticles and collective excitations (which are closely related) are emergent phenomena that occur when a microscopically complicated system such as a solid behaves as if it contained different weakly interacting particles in free space. For example, as an electron travels through a semiconductor, its motion is disturbed in a complex way by its interactions with all of the other electrons and nuclei; however it approximately behaves like an electron with a different mass (effective mass) traveling unperturbed through free space. This 'electron with a different mass' is called an 'electron quasiparticle'. In another example, the aggregate motion of electrons in the valence band of a semiconductor or a hole band in a metal is the same as if the material instead contained positively charged quasiparticles called electron holes. Other quasiparticles or collective excitations include phonons (particles derived from the vibrations of atoms in a solid), plasmons (particles derived from plasma oscillations), and many others. These particles are typically called 'quasiparticles' if they are related to fermions, and called 'collective excitations' if they are related to bosons, although the precise distinction is not universally agreed upon. Thus, electrons and electron holes are typically called 'quasiparticles', while phonons and plasmons are typically called 'collective excitations'. The quasiparticle concept is most important in condensed matter physics since it is one of the few known ways of simplifying the quantum mechanical many-body problem. Solids are made of only three kinds of particles: electrons, protons, and neutrons. Quasiparticles are none of these; instead, each of them is an emergent phenomenon that occurs inside the solid. Therefore, while it is quite possible to have a single particle (electron or proton or neutron) floating in space, a quasiparticle can only exist inside interacting many-particle systems (primarily solids). Motion in a solid is extremely complicated: Each electron and proton is pushed and pulled (by Coulomb's law) by all the other electrons and protons in the solid (which may themselves be in motion). It is these strong interactions that make it very difficult to predict and understand the behavior of solids (see many-body problem). On the other hand, the motion of a non-interacting classical particle is relatively simple; it would move in a straight line at constant velocity. This is the motivation for the concept of quasiparticles: The complicated motion of the real particles in a solid can be mathematically transformed into the much simpler motion of imagined quasiparticles, which behave more like non-interacting particles.