Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Optical phonons

  1. May 12, 2007 #1
    I thought optical phonons only existed in crystals with two or more types of atoms in a unit cell. But I keep reading references of optical phonons in single atom crystals, like silicon.
    What's the deal here?
  2. jcsd
  3. May 12, 2007 #2

    Dr Transport

    User Avatar
    Science Advisor
    Gold Member

    The crystal symmetry, [itex] O_h [/itex] doesn't eliminate the possibility of optical transitions in Si, but the deformation potentials and dipole mements in silicon are very small yeilding quite small transition intensities.
  4. May 9, 2011 #3
    <111> direction in silicon has two different spacings between atoms and thus two spring constants. The eigenvalue equation sets up the same as if there were two different masses.
  5. May 10, 2011 #4
    Remember that the total number of phonon modes is equal to the number of atoms. The number of distinct wave-vectors possible is equal to the number of lattice sites. There are therefore a number of different modes at each wavevector equal to the number of atoms per lattice site.

    Even in mono-atomic structures, you can have a non-trivial lattice (not a simple cubic) and so have more than one atom per lattice point.
  6. May 11, 2011 #5


    User Avatar
    Science Advisor

    To put it differently, silicium cristallizes in the diamond lattice. A primitive cell of the diamond lattice contains two atoms. Compare this to some simpler monoatomic lattice, e.g. I_2 (Iodine).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook