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Ред 53:
Кристална решетка при температура равна на [[абсолютна нула|абсолютната нула]] се намира в [[основно състояние]] и не съдържа фонони. Когато решетката е с ненулева [[температура]] нейната енергия не е постоянна, а флуктуира във времето около някаква средна стойност. Тези енергийни флуктуации се причиняват от вибрации в кристалната решетка, които се разглеждат като "фононен газ". Произволното осцилиращо движение на атомите в решетката се отнася до [[количество топлина|количеството топлина]].
<!--A crystal lattice at [[absolute zero|zero temperature]] lies in its [[ground state]], and contains no phonons. According to [[thermodynamics]], when the lattice is held at a non-zero [[temperature]] its energy is not constant, but fluctuates [[random|randomly]] about some [[Arithmetic mean|mean value]]. These energy fluctuations are caused by random lattice vibrations, which can be viewed as a ''gas of phonons''. (Note: the random motion of the atoms in the lattice is what we usually think of as [[heat]].) Because these phonons are generated by the temperature of the lattice, they are sometimes referred to as '''thermal phonons'''.
Unlike the atoms which make up an ordinary gas, thermal phonons can be created or destroyed by random energy fluctuations. In the language of statistical mechanics this means that the chemical potential for adding a phonon is zero. It is very important to note that this behaviour takes us away from the harmonic potential mentioned earlier, and into the anharmonic regime. The behaviour of thermal phonons is similar to the ''photon'' gas produced by an [[electromagnetic cavity]], wherein photons may be emitted or absorbed by the cavity walls. This similarity is not coincidental, for it turns out that the electromagnetic field behaves like a set of harmonic oscillators; see [[Black-body radiation]]. Both gases obey the [[Bose-Einstein statistics]]: in thermal equilibrium and within the harmonic regime, the average number of phonons (or photons) in a given state is
Ред 62:
where ''ω<sub>k,s</sub>'' is the frequency of the phonons (or photons) in the state, ''k<sub>B</sub>'' is [[Boltzmann's constant]], and ''T'' is the temperature.
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[[Категория:Квантова механика]]
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