# Do acoustic phonons disappear in charge density wave states?

• A
• sfman
In summary, the mean-field theory of charge density wave (CDW) introduces phase and amplitude fluctuations as collective excitations, while also mentioning the Kohn anomaly in the acoustic phonon dispersion. As temperature increases towards the CDW transition temperature, phonons condense at momentum Q=2kF. However, in an incommensurate CDW phase, the location of phonons is unclear. It is unclear if there are still gapless acoustic phonons in the CDW phase, in addition to phasons, or if acoustic/optical phonons merge into phasons and amplitudons. The phason φ in cos(Q⋅x+φ) can be seen as a generalized acoustic phonon, with Q=
sfman
TL;DR Summary
Do acoustic phonons disappear in charge density wave states?
In most standard exposition of (the mean-field theory of) charge density wave (CDW), phase and amplitude fluctuations are introduced as the collective excitations. Kohn anomaly in the acoustic phonon dispersion is also mentioned as temperature goes from the above till the CDW transition temperature, at which phonons condense at momentum Q=2kF.

But where are the phonons deep in, say, an incommensurate CDW phase?
Despite the condensation, are there still gapless acoustic phonons in the CDW phase, in addition to the phasons?
Or acoustic/optical phonons disappear and merge into the phasons and amplitudons?

Intuitively, I tend to think phason φ in cos(Q⋅x+φ) as a generalized acoutic phonon since phonon seems to be just setting Q=0 as a special case for normal solids.

Please provide more background to your question. Not everyone has worked in your subfield. Of the many confusing points, I don't know what you means when you say phonons condense. Could you post a phonon spectrum to illustrate what you are thinking because I also don't know what you mean by gapless acoustic phonons.

One thing I can say is that acoustic phonons are so named because the low Q phonon spectrum is linear and the slope is a measure of the speed of sound of the material. If these didn't exist you would have an unphysical sample that wouldn't transmit sound.

## 1. What are acoustic phonons?

Acoustic phonons are collective vibrations in a crystal lattice that involve the displacement of atoms from their equilibrium positions.

## 2. What is a charge density wave state?

A charge density wave state is a type of electronic order that occurs in certain materials, where the electrons arrange themselves in a periodic pattern with a lower density of charge compared to the surrounding areas.

## 3. Do acoustic phonons disappear in charge density wave states?

In some cases, yes. Acoustic phonons can be affected by the presence of a charge density wave state, leading to a decrease in their intensity or disappearance altogether.

## 4. How do charge density wave states affect acoustic phonons?

The presence of a charge density wave state can cause a distortion in the crystal lattice, leading to a change in the phonon frequencies and potentially causing the disappearance of certain phonon modes.

## 5. Can acoustic phonons reappear in charge density wave states?

It is possible for acoustic phonons to reappear in charge density wave states under certain conditions, such as changes in temperature or pressure. However, this is not always the case and depends on the specific material and its properties.

• Atomic and Condensed Matter
Replies
5
Views
9K
• Beyond the Standard Models
Replies
11
Views
3K
• Other Physics Topics
Replies
10
Views
2K
• Science and Math Textbooks
Replies
19
Views
17K