[Pw_forum] Monolayer MoS2 band Structure

[Thomas Brumme]

Hey Cameron,

I think that the main problem is your lattice parameter.
You use 5.8364 bohr which is 5.8364*0.529177 = 3.088 Angstrom.
Thus, your MoS2 is under compressive strain. That's why you
(correctly) get an indirect band gap between K and Q.
See also Fig. 3(b) of Phys. Rev. B 85, 033305:

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.85.033305

Cheerio

Thomas

On 09/24/2015 08:20 AM, Cameron Foss wrote:
> Hello,
>
> I am attempting to obtain the band structure of monolayer MoS2, 
> however my results so far do not agree well with what has been 
> documented for monolayer (ML) MoS2. I consistently get an indirect 
> band gap for ML MoS2 caused by the 'Q-valley' minimum seen in bulk MoS2.
>
> I have tested the code with interplanar distances of 15 and 30 
> angstroms to eliminate any interplanar interactions. There have been 
> several papers that report the band structure for ML MoS2 from first 
> principles, they have used functionals from LDA, GGA, and an HSE 
> hybrid functional. My best success has been with GGA-PW91(see input 
> file below). Note in all my simulations I have measured a band gap of 
> ~1.9eV which is in agreement with reported values.
>
> I am uncertain as to what the issue could be? I have tested 3 
> different PPs.
>
> %%%%%%%%%%%%%%%%%%%%
> Details on simulations:
>
>  Specifically, my simulations continue to show that the 'Q-valley' is 
> slightly lower (16meV~30meV) than the minimum at the K point, 
> suggesting an indirect band gap. However ML mos2 has been widely shown 
> to have a direct band gap at the K point where this 'Q-valley' along 
> the conduction band is suppressed above the K-point minimum.
>
> I run a scf (pw.x) calculation first, followed by an nscf (pw.x) run, 
> and a bands calculation with pw.x (and of course bands.x to retrieve 
> the energy values). I specify 16 bands for nbnd, kpoints along 
> symmetry paths, and a MP grid of 8x8x1 with a 1 1 1 offset.
>
> Input file:
>
> &control
>     calculation='scf'                      !nscf, then bands
>     restart_mode='from_scratch',
>     !pseudo_dir='directory where pseudopotentials are stored/',
>     !outdir='directory where large files are written/'
>     pseudo_dir='/home/cameron/QE/espresso-5.1/pseudo/',
>     outdir='/home/cameron/QE/espresso-5.1/2dout'
>     prefix='mos2-gga',
>  /
>  &system
>     ibrav=4, celldm(1)=5.8364, celldm(3)=10,
>     nat=3, ntyp=2, ecutwfc =70, ecutrho=300         ! specified nbnd=16
>  /
>  &electrons
>     conv_thr =  1.0d-15                   ! reduced to 1.0d--12 for 
> nscf, bands
>     mixing_beta = 0.5                     ! increased to 0.7 for nscf, 
> bands
>  /
> ATOMIC_SPECIES
>  Mo  95.94    Mo.pw91-n-van.UPF
>  S   32.065   S.pw91-n-mt.UPF
> ATOMIC_POSITIONS bohr
> S        0.00002  -0.00001   0.000000000
> Mo       2.91822   1.68481   2.929886569
> S        0.00002  -0.00001   5.859870374
> K_POINTS automatic
>  8 8 1 1 1 1
>
> I ran into some convergence issues with all the bands with this 
> conv_thr in the nscf and bands calculations, so i had to reduce it to 
> 1.0d-12 for those calculations. I also used a mixing beta of 0.7 for 
> both the nscf and bands calculations. I am uncertain as to whether 
> changing mixing_beta's between calculations is of importance or not as 
> well, but I do not think this is the case.
>
> Best regards,
> Cameron
>
>
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://pwscf.org/mailman/listinfo/pw_forum

-- 
Dr. rer. nat. Thomas Brumme
Max Planck Institute for the Structure and Dynamics of Matter
Luruper Chaussee 149
22761 Hamburg

Tel:  +49 (0)40 8998 6557

email: Thomas.Brumme at mpsd.mpg.de

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20150924/7fc93bbf/attachment.html>