quasiparticle

Quasiparticle is an emergent entity in quantum many-body systems that behaves like a particle despite arising from interactions among many underlying constituents. In solids, the complex interactions between electrons and lattice vibrations, impurities, and other excitations produce dressed excitations that carry energy and momentum with properties different from a bare electron. Quasiparticles are useful because they allow a complicated many-body problem to be described in terms of weakly interacting, particle-like excitations.

In Landau's Fermi-liquid theory, low-energy excitations near the Fermi surface act as quasiparticles with a one-to-one correspondence to noninteracting electrons but with renormalized parameters, such as an effective mass and a renormalization factor Z, which is the weight of the quasiparticle pole in the electron's Green's function. Their lifetimes scale inversely with the square of the energy measured from the Fermi surface, so they are long-lived close to the Fermi level but decay at higher energies.

Quasiparticles come in many guises. Electron quasiparticles (the dressed electrons) and hole quasiparticles (missing electrons in a filled band) are common. Other examples include phonons (quantized lattice vibrations), plasmons (collective charge oscillations), magnons (spin waves), excitons (bound electron-hole pairs), polarons (electrons dressed by lattice distortions), and Cooper pairs in superconductors, which behave as bosonic quasiparticles.

Quasiparticles are valid as long as the system permits well-defined, long-lived excitations; in strongly correlated materials or near quantum critical points, quasiparticle descriptions can fail (non-Fermi liquids).

Experimentally, quasiparticles are inferred from dispersions observed in angle-resolved photoemission spectroscopy, tunneling spectra, and from features in transport and optical measurements; their properties are encoded in the poles and residues of the many-body Green's function via the self-energy.