Diffrence between Normal mode and phonons

[dmit11]

Hello all

What is the definition of Normal mode? And what is the difference between normal mode and phonon? Are they same?

Thank you!

 

[kanato]

Normal modes come from a network of masses on springs, where everything is classical. Phonons are the quantized "version" of normal modes. For many purposes in solid state physics, these terms are often used interchangeably.

 

[genneth]

A normal mode is defined to be a motion of the system where the whole system oscillates at a fixed frequency.

 

[stephenhky]

Normal modes are modes that a system of particles/oscillators move at the same frequency. Its quantized version is the quasiparticles.

Phonon is an example of quasiparticles. It is also a normal mode of vibrations of particles at the same frequency in solids. It is just like photons in vacuum, that a photon refers to an EM wave at a fixed frequency.

 

[mavipranav]

Phonons are more correctly known as collective-excitations rather than quasiparticles.

Collective excitations occur when the interaction between the particles of the unexcited system is strong, so that one cannot really identify a single-particle to describe the dynamics of the system, but rather must include the entire system.

Quasiparticles on the other hand, are a result of exciting a system from its ground state, whose particles are weakly interacting; they scatter off other unexcited-particles (hence they are "particles") but have large life-times on account of Pauli's exclusion principle (hence they are "quasi").

Both of them are elementary excitations.

 

[stephenhky]

thanks!