I am experimentally investigating the effect of Joule heating in different ultrathin materials by looking at the relationship between velocity saturation and optical phonon emission. I am trying to understand using this logic why gold (and similar metals) do not have a saturating velocity compared with semiconductors in high fields.
As gold has 4 atoms in its unit cell, I understand it should have optical phonon modes. However, the phonon dispersions only show acoustic modes. Are the optical phonon energies in gold very high? Or do they not have optical modes and I have misunderstood? I am struggling to find information on the topic.
Dear Anneke Smits
you ca check this work:
Article Phonon frequencies of gold
Your question is interesting. Gold is a metal, therefore, you may find interesting the following thread:
https://www.researchgate.net/post/why_pure_metals_cannot_show_raman_bands
Accordingly to the previous thread, using symmetry arguments, they show that fcc metals such as Gold do not show first order Raman bands.
Thank you so much Pedro L. Contreras E. ! The link to the Raman bands question may have answered this for me. I had not read before that pure metals of fcc structure could not have OPs. I will read the articles you have suggested.
You are welcome Dear Anneke Smits
Your question is interesting. Usually for optical phonons, we use a model with 2 different masses, so try m1 = m2 in the equation for omega in the diatomic 1D lattice solution & you will see. Thank you so much for the thread.
Pedro L. Contreras E., sorry to continue, but having refreshed my memory on the 1D diatomic chain with m1=m2, as you suggested, I actually calculate an optical branch would in fact still arise, giving at the centre of the first Brillouin zone (k=0) the solutions, omega=0 or 2*sqrt(C/M), where C is the spring constant and M the mass of the atom. This method is often used to show that centred cells do still have optical modes if I remeber correctly.
Have I misunderstood your suggestion?
I appreciate the help.
Yes Dear Anneke Smits
That is what I mean, there is a solution.
I asked the question because all elements have only one type of atoms (masses are identical) but if the primitive unit cell contains at least two identical atoms, optical phonons will also exist .
Gold's unit cell contain 4 lattice points, the crystal structure of gold is FCC & it contains four gold atoms which touch only along the face diagonal.
I am no specialist in Ramman, but I found the following thread that says: "...silver and gold themselves are not Raman-active..."
https://www.researchgate.net/post/Is_silver_Raman_active
Is it the same as saying that Ag & Au cannot show Ramman bands?
Following a group theoretical calculation which is more elegant, probably an answer could be also obtained. I am only guessing since group theory is a very powerful tool in Physics. This classical paper for example, says that some elements can be Ramman innactive or active, & it has all the fundamental theorems. Keep it for future reference (it looks that is is very instructive to study group theory and Ramman spectra):
https://www.ias.ac.in/article/fulltext/seca/009/03/0224-0258
Will follow.
Dear Anneke Smits
Please look at the following threads, I found them yesterday, but in vzla internet only comes a few hours every day so only now I have some conectivity. I think all threads are related. The first & second talk about the role of centre optical phonons & give further ideas about the implications of crystalline structures. The third one talks about the group theoretical notation.
https://www.researchgate.net/post/I_could_not_understand_the_idea_of_Phonons_full_dispersion_relation_and_Phonons_near_the_center_of_the_Brillouin_zone_Can_someone_please_explain
https://www.researchgate.net/post/Why_Raman_signal_is_related_to_zone-centre_optical_phonons
https://www.researchgate.net/post/What_is_criteria_to_decide_whether_Raman_modes_are_Ag_Bg_or_Eg
Yes Dear Prof. M. Markov
I think you wrote the answer in another thread. I will replay in your thread again :)
Dear Anneke Smits
The following work does explain it, the appendix to the article is free & we can look at the figs.
https://onlinelibrary.wiley.com/doi/abs/10.1002/0471266965.com060.pub2
Raman Spectroscopy of Solids by Thomas M. Devine & Fran Adar,
Raman Spectroscopy of Solids, 2012.
Appendix A: Group Theory and Vibrational Spectroscopy
I quote:..."The phonon dispersion curves for a simple, one‐dimensional (1D), monoatomic lattice and a 1D diatomic lattice are presented in Figures 21 and 22. Since there are no available states at the center of the BZ in a monatomic lattice [for a three‐dimensional (3D) lattice as well as a 1D], such structures are not Raman active. Consequently, all metals with one atom per unit cell are Raman inactive. Diatomic lattices, whether homonuclear or heteronuclear, do possess phonon modes at the center of the BZ (see Fig. 18). Those crystals in which atoms are sitting on centers of symmetry will have only IR active phonons and will not be Raman active. But crystals such as diamond, silicon, and germanium have their centers of symmetry between the atoms, so they will be Raman active. Crystals in the zincblende structure, and its hexagonal analog (wurtzite), such as GaN, do not have centers of symmetry and are all Raman active..."
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