What are quasiparticles?


Quasiparticles emerge when a vast number of fundamental particles interact with each other at close distances. When solids or liquids are subject to extreme temperatures and pressures, the resulting complex system of particles behaves as if it contains a few extra particles. These quasiparticles are long-lived excitations occurring in a solid, liquid or plasma that effectively behave as weakly interacting particles.

While they are not elementary particles, and can’t exist outside of a system of many interacting particles, quasiparticles can be quite stable with well-defined properties like mass and charge. Since they were first introduced by Lev Landau in the 1930s, the list of types of quasiparticles is still increasing, with physicists creating various types of quasiparticles with similar properties to real fundamental particles, such as charge or spin.


Science: Standard Model Of Physics, Explained | Boomers Daily

Types of Quasiparticles


One of the most well known types of quasiparticles in matter are holes. Holes are just the absence of an electron in an atom or atomic lattice where one should exist, and can move through a lattice, hopping from one atom to another, acting as a positively-charged particle.

Phonons are a type of quasiparticle arising from vibrations of atoms in a solid. While vibrations in a lattice can occur at any frequency, on a quantum scale vibrational energy occurs in discrete packets, resulting in a particlelike movement of energy around a lattice, called phonons. 

A polaron arises from an electron in a lattice interacting with the polarization of its surrounding ions. This was coined by Landau in 1933 to describe an electron moving in a dielectric crystal where the atoms are displaced from their equilibrium positions. This displacement effectively dampens the charge and mobility of an electron and increases its effective mass, resulting in a particlelike polaron which behaves as a negatively charged particle with a mass greater than an electron. They are mostly observed in ionic solids where because the forces between the electron and ions are very strong.

There are many other quasiparticles with applications in fields including condensed matter and semiconducting physics, such as a plasmon (a collective excitation of plasma oscillations) or an exciton (consisting of an electron and hole bound together). 


Observing Quasiparticles 


In 2021, a team of physicists in Denmark cooled a gas of atoms to a temperature as low as a billionth of a degree above absolute zero creating a Bose Einstein condensate where the atoms coalesce into a single quantum mechanical state. Under such conditions the system produced a short-lived Bose polaron, a type of quasiparticle that is made of atoms immersed in a Bose-Einstein condensate.

In 2022, reserachers in the City College of New York announced the emergence of a previously unobserved class of polaritonic quasiparticles, a type of light and matter quasiparticle arising from the strong coupling of the spin-correlated excitons and photons.

The list of possible quasiparticles is still growing, with matter and materials being investigated under new and varying extreme conditions.


Images from https://www.quantamagazine.org/a-video-tour-of-the-standard-model-20210716/

Life and Death of a Bose Polaron (2022) https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.4.043093

The Near Magic Mystery Of Quasi Particles (2021) https://www.quantamagazine.org/the-near-magical-mystery-of-quasiparticles-20210324/

Quasiparticles, flat bands and the melting of hydrodynamic matter. Nature Physics, 2023; DOI: 10.1038/s41567-022-01893-5

Dirnberger, F., Bushati, R., Datta, B. et al. Spin-correlated exciton–polaritons in a van der Waals magnet. Nat. Nanotechnol, 1060–1064 (2022). https://doi.org/10.1038/s41565-022-01204-2

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