Quasiparticles

Quasiparticles are emergent excitations in many-body systems that behave like particles within a limited regime. By encoding complex interactions into effective degrees of freedom, they enable a simplified description in which excitations have a well-defined energy and momentum, a finite lifetime, and a characteristic dispersion.

Common examples include phonons (lattice vibrations), magnons (spin waves), plasmons (collective charge oscillations), excitons (bound electron–hole pairs), and polarons (electrons dressed by lattice distortions). In superconductors, Bogoliubov quasiparticles describe elementary excitations above the ground state.

Quasiparticles are described by dispersion relations, effective masses, and lifetimes. They are analyzed with many-body techniques such as Green's functions and self-energies, which yield spectral functions with peaks corresponding to quasiparticle states. Landau's Fermi liquid theory formalizes this picture for electrons near the Fermi surface.

They provide a practical language for understanding transport, optical properties, and collective phenomena in solids, including conduction, heat transport, and superconductivity, as well as interpretations of spectroscopic data like angle-resolved photoemission.

Quasiparticles are not fundamental; their usefulness declines when interactions are strong or energies are high, where lifetimes shorten and spectral features broaden, making the quasiparticle picture ill-defined.