An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and some molecular crystals. Excitons are created when light is absorbed by a material and they play an important role in optical properties of these materials.
The term exciton was introduced by Arthur Schawlow and Charles Townes in 1951. They proposed that absorption of light in semiconductors creates excited states called excitons. These excitons are variously referred to as Frenkel excitons or Wannier-Mott excitons. Frenkel excitons occur when the electron and hole are both localized on the same atom, while Wannier-Mott excitons occur when the electron and hole are localized on different atoms but still within the same unit cell of the lattice.
The binding energy of anexciton is typically tens or hundreds of meV. The size of anexciton (also known as its Bohr radius) is generally larger than the sizeof an atom, often quoted as several angstroms (Å). This is because themost prevalent type of interaction between electrons and holes involvesthe creation of phonons, which requires a certain amount of space forthe process to take place effectively. The effective mass offree carriersis also reduced inside anexciton relative to their isolatedatom values, due largely to screening effects from other carriers inthe material; this effect can be seen clearly in semiconductor bandstructures where there is a “light-hole” branch belowthe main conduction and valence bands which becomes more pronouncedwith decreasing carrier concentration (and thus increasing dielectricconstant). As such, it should not be surprising that many bulkpropertiesare dramatically altered in the presenceof strong Coulombic interactions between fermionic quasiparticles(such as those found insideanexciton).
One particularly interesting consequenceof strong Coulombic interactions between fermions is their abilityto give rise to new phasesof matter, such as Bose-Einstein condensatesand superconductivity. In general terms, when fermions interact withone another via short-range forces they tend to occupy differentenergy levels (or orbitals), whereas long-range forces allow themto share energy levels leading potentially to macroscopic quantumcoherence effects such as those mentioned above. It has been suggestedthat Frenkel excitonic insulators could providea route towards realising novel electronic phasesbased on this principle; if successful, these would represent amaterials class beyond currently known Mott insulatorswhich show similar kinds of behaviour resulting from very differentmicroscopic mechanisms (namely correlations between localisedelectrons on atomic sites).