Elementary suggestion

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The elementary excitation is a concept of many-body physics of solids , comprising the absorption or release of energy by a system composed of many particles describes.

If energy (e.g. in the form of heat ) is supplied to a solid , the energy is not absorbed by a single particle ( atom or electron ), but is quickly distributed over several particles due to the strong interactions between the particles. Instead of considering the energy ratios of the individual particles, this is described as a stimulus.

In a first approximation, elementary excitations are free of interaction, i. i.e., they do not influence each other. In the next approximation, however, interactions between elementary excitations must be considered. In contrast to the initial problem of the many-body system, these interactions are often "small" and can be treated by perturbation theory .

The classical mechanics knows a similar problem for the vibrational state of a system of mass points . Here, for example, the vibration of a string is described by normal modes ( fundamental and overtones ) instead of looking at the movement of individual atoms of the string.

Similarly, a solid can be treated as a system of interacting particles (lattice ions and electrons). Let us consider the oscillation of the lattice ions around their equilibrium position as an example : Fundamental oscillation and harmonics can be separated by introducing normal coordinates . (In general, if a system has n degrees of freedom , the introduction of new normal coordinates can decouple the equations of motion .) The normal vibrations can be quantized , the associated quanta of the lattice vibrations are called phonons . They are one of the most well-known elementary stimuli of the solid.

The elementary excitations of a solid, in which photons excite band transitions from charge carriers or are emitted during spontaneous band transitions , typically have a higher order of magnitude than phonons.

An elementary excitation of a solid, which can be described with effective field quantities , is called a quasiparticle .