# Concept of phonons?

1. May 2, 2007

### pivoxa15

How should I think about the concept of phonons?

As I understand it, it is a wave which propagates with a speed similar to sound and models the movement of atoms in crystals. However how many atoms per phonon? And isn't phonons meant to be analogous to photons which are single units hence one particle per phonon?

Or does the phonon is a wave travelling at a certain speed and frequency hence energy and all particles with that speed is represented by the phonon? Hence the solid is represented by phonons only. Which is usually less than the number of particles.

In this article they showed 6 waves with distinct k values. They represent 6 different phonons? Hence each phonon can be made up of many particles.
http://en.wikipedia.org/wiki/Phonons

Last edited: May 2, 2007
2. May 2, 2007

### ZapperZ

Staff Emeritus
You may want to read this paper:

"Visualizing the phonon wave function", S.C. Johnson and T.D. Gutierrez, Am. J. Phys. v.70, p. 227 (2002).

Zz.

3. May 3, 2007

### pivoxa15

How about thinking phonons as sound waves? In sound, the particles oscillate forwards and backwards about some point. And a travelling wave represents the movement of the oscillations reaching further away from the initial point where the sound was generated. Although the particles are not necessary moving or migrating but are oscillating about a point. In other words, the moving wave reprsents the oscillations moving forwards and reaching more particles away from the initial point. To model it in a solid, one must install boundary conditions at the ends so that the wave dissapear at the ends. Because we don't want the particles to 'leak' out of the solid. Each wave with a certain frequency or wavenumber is a certain phonon.

4. May 3, 2007

### ZapperZ

Staff Emeritus
But this is not accurate either.

With phonons, you have BOTH acoustical (sound) and optical (light) branches. While phonon modes are sometime called "first sound" in older solid state texts, to think that these are only acoustic-active vibration would not be accurate.

Zz.

5. May 3, 2007

### pivoxa15

What do you mean by the optical (light branches)?

Do you mean thinking about the particles tied together via a string and the oscillations of the string like a transverse string tied together at two ends so more like an EM wave tied at two ends then a sound wave that disappear at the boundaries? Except the wave in this case is made up of particles.

Last edited: May 3, 2007
6. May 4, 2007

### Sojourner01

Sort of got it...

Optic phonon vibrations are those modes of vibration which cause the positive ions to be some degree out of phase with negative ones. This causes an oscillating electric dipole - which as you'd expect, couples strongly to EM radiation. Thus optical branch phonons can both absorb and emit radiation.