Optical phonons

1. May 12, 2007

BeauGeste

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. May 12, 2007

Dr Transport

The crystal symmetry, $O_h$ 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.

3. May 9, 2011

Joe-b716

<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.

4. May 10, 2011

genneth

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.

5. May 11, 2011

DrDu

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).