QuantumATK Forum

QuantumATK => General Questions and Answers => Topic started by: jeong9090 on August 8, 2017, 21:38

Title: graphene phonon dispersion
Post by: jeong9090 on August 8, 2017, 21:38

Hi,

I have used VNL for academic user.

I'm currently trying to calculate graphene phonon dispersion.

I used two different geometries. One is only one graphene unit cell which is added from database and another is added from plug in nanosheet(10,2).

After running Job manager, there are different phonon dispersions.

I wonder why there is different phonon dipsersion.

I'd appreciate it if you let me know that.

Thanks,

JY

Title: Re: graphene phonon dispersion
Post by: Jess Wellendorff on August 9, 2017, 09:13

What you are seeing is mainly the effect of Brillouin Zone folding of the bands: The nanosheet has many more atoms (it's not a primitive graphene unit cell), so there are many more phonon modes in the large system than in the small one. That's equivalent do zone folding of electronic bands when doing electronic bandstructure of repeated cells; the bands are folded into the irreducible zone.

Title: Re: graphene phonon dispersion
Post by: jeong9090 on August 9, 2017, 16:40

Hi,

When I add graphene from database and I repeat A and B to make a large size, is it the same result like Brillouin Zone folding?

What I want to do is the phonon dispersion graphene on a substrate as function of different interface bonding.

If I make an interface between graphene and a substrate, I can't repeat its structure to obtain phonon dispersion.

Am I right?

Can I confirm right phonon dispersion with only unit cell instead of large size of graphene?

How can I have phonon dispersion without BZ folding with many atoms?


Thanks,

JY

Title: Re: graphene phonon dispersion
Post by: Daniele Stradi on August 10, 2017, 08:42

Hi,

When I add graphene from database and I repeat A and B to make a large size, is it the same result like Brillouin Zone folding?

Yes, repeating the cell along A and B will result in a folding of the phonon bands in the Brillouin zone.

What I want to do is the phonon dispersion graphene on a substrate as function of different interface bonding.
If I make an interface between graphene and a substrate, I can't repeat its structure to obtain phonon dispersion.
Am I right?

For a repeated structure, you will still obtain a phonon dispersion, but the bands will be folded, hence the analysis might be complicated.
For a 1x1 structure, the analysis of the phonon band structure of graphene on a substrate is simpler, see Nano Lett. 10, 4335 (2010)
http://pubs.acs.org/doi/abs/10.1021/nl101657v

How can I have phonon dispersion without BZ folding with many atoms?

In a repeated structure, you will still get the correct band dispersion, but the phonon bands will be folded and difficult to analyze.
If you want to unfold the phonon bands of the supercell and map them into those of a 1x1 unit cell, unfortunately this is not possible at the moment in VNL. 

Regards,
Daniele.

Title: Re: graphene phonon dispersion
Post by: jeong9090 on August 10, 2017, 16:37

Hi, Daniels

Thank you for your reply.

It is VNL limitation.

Thanks

JY