Excitón
en Un excitón es una cuasipartícula que describe el estado excitado de un sólido o más en general, de un sistema de la materia condensada. En un dieléctrico o semiconductor , puede ser visto como un estado legado de un electrón y de un hueco, interactuando mediante la Ley de Coulomb . Una imagen vívida de la formación de un excitón es como sigue: Un fotón entra en un semiconductor , excitando un electrón de la banda de valencia en la banda de conducción . El electrón perdido en la banda de valencia deja un hueco detrás, de carga eléctrica opuesta, el cual es atraído por la fuerza de Coulomb. El excitón resulta del vínculo del electrón y el hueco. ar:أكسيتون de:Exziton fr:Exciton it:Eccitone nl:Exciton ja:励起子 pl:Ekscyton ru:Экситон uk:Екситон física Categoría: Física del estado sólido photon enters a semiconductor, exciting an electron from the valence band into the conduction band . The missing electron in the valence band leaves a hole behind, of opposite electric charge, to which it is attracted by the Coulomb force . The exciton results from the binding of the electron with its hole; as a result, the exciton has slightly less energy than the unbound electron and hole. The wavefunction of the bound state is hydrogenic (an " exotic atom " state akin to that of a hydrogen atom ). However, the binding energy is much smaller and the size much bigger than a hydrogen atom because of the effects of screening and the effective mass of the constituents in the material. In a hydrogen atom the core and the electron can have parallel or antiparallel spin, the same is true for the exciton, and also for positronium , but not for the two electrons in the He-atom. Often excitons were given names which look like hydrogen orbital names, but have the wrong numbering for angular momentum , or other quantum number s. Subtypes Excitons can be treated in two limiting cases, which depend on the properties of the material in question. In semiconductors, the dielectric constant is generally large, and as a result, screening tends to reduce the Coulomb interaction between electrons and holes. The result is a Mott-Wannier exciton , which has a radius much larger than the lattice spacing. As a result, the effect of the lattice potential can be incorporated into the effective masses of the electron and hole, and because of the lower masses and the screened Coulomb interaction, the binding energy is usually much less than a hydrogen atom, typically on the order of 0.1 eV. This type of exciton was named for Sir Nevill Francis Mott and Gregory Wannier . When a material's dielectric constant is very small, the Coulomb interaction between electron and hole become very strong and the excitons tend to be much smaller, of the same order as the unit cell (or on the same molecule as with fullerenes), so the electron and hole sit on the same cell. This Frenkel exciton , named after Yakov Frenkel , has typical binding energy on the order of 1.0 eV. Alternatively, an exciton may be thought of as an excited state of an atom or ion , the excitation wandering from one cell of the lattice to another. Often there is more than one band to choose from for the electron and the hole leading to different types of excitons in the same material. Even high lying bands can be used as is seen in femtosecond two-photon experiments. At surfaces so called image states may occur, where the hole is inside the solid and the electron is in the vacuum. These electron hole pairs can only move along the surface. Dynamics The probability of the hole disappearing (the electron occupying the hole) is limited by the difficulty of losing the excess energy, and as a result excitons can have a relatively long lifetime. (Lifetimes of up to several milliseconds have been observed in copper (I) oxide ) Another limiting factor in the recombination probability is the spatial overlap of the electron and hole wavefunctions (roughly the probability for the electron to run into the hole). This overlap is smaller for lighter electrons and holes and for highly excited hydrogenic states. The whole exciton can move through the solid. With this additional kinetic energy the exciton may lie above the band-gap. The exciton propagating through molecular crystal is one that is of greatest concern. Several mechanisms have been proposed in the literature. Two are important. The first one is exciton energy dissipated due to interaction with phonon bath. The other one is energy carried away by radiation. Combination of the two has also been studied. Features
- Since an exciton is a bound state of an electron and a hole, the overall charge for this quasiparticle is zero. Hence it carries no electric current .