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Diffrence between Normal mode and phonons

  1. Apr 25, 2010 #1
    Hello all

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

    Thank you!
  2. jcsd
  3. Apr 26, 2010 #2
    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.
  4. Apr 27, 2010 #3
    A normal mode is defined to be a motion of the system where the whole system oscillates at a fixed frequency.
  5. Apr 28, 2010 #4
    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.
  6. May 1, 2010 #5
    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.
  7. May 3, 2010 #6
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