SOC-induced exciton weight change

[Guo_BIT]

Dear Developers,

I used YPP to calculate the contribution of excitons. When symmetry is turned off and SOC is not considered, the lowest energy state and the second-lowest energy state are independently contributed by (0.5, 0, 0) and (0, 0.5, 0), respectively.

Code: Select all

#    Band_V             Band_C             Kv-q ibz           Symm_kv            Kc q ibz           Symm_kc            Weight  
]#
         36                 37                 649               1                   649               1               0.876284     

However, when taking SOC into account, the exciton of the ground state is contributed by both of these k-points together

Code: Select all

#                       K-point [iku]                         Weight
# :    0.00000000       -0.500000000         0.00000000        0.554620981   
# :  -0.500000000         0.00000000         0.00000000         1.00000000

Considering the distribution of these k-points in reciprocal space, I am wondering whether this result is physical significance or is simply a computational artifact.

This is important for our current research, and I look forward to your response.

Best wishes
Jingda Guo

[Daniele Varsano]

Dear Jingda,

are you sure that you are not considering degenerate excitons all together? To avoid this, you should set Degen_Step=0.0 eV in your ypp input file.

Can you please post the output file of your calculations?

Best,

Daniele

[Guo_BIT]

Dear Daniele:

To avoid degeneracy, I set the following in the ypp.in file:

Code: Select all

excitons                         # [R] Excitonic properties
amplitude                        # [R] Amplitude
States= "1 - 8"                  # Index of the BS state(s)
BSQindex= 1                    # Q-Index of the BS state(s)
Degen_Step= 0.00000       eV    # Maximum energy separation of two degenerate states

then I calculated the weight of the first exciton

Code: Select all

#    Maximum Residual Value =  0.35041E+00
#   
#    E [ev]             Strength           Index
#
    0.338274464E-1     0.352137751E-3      1.00000000
    0.338313021E-1     0.886224792E-3      2.00000000
    0.338357650E-1     0.186500802E-4      3.00000000
    0.338525586E-1     0.333322123E-5      4.00000000
    0.338968784E-1     0.917177204E-5      5.00000000
    0.340177938E-1      1.00000000         6.00000000
    0.340196043E-1     0.281616807         7.00000000
    0.344031453E-1     0.252245002E-6      8.00000000

The specific weights are as follows, where 72 and 73 represent the valence and conduction bands

Code: Select all

# Electron-Hole pairs that contribute to Excitonic State 1 for iq=1 more than  5.000000%
#                       K-point [iku]                         Weight
# :    0.00000000       -0.500000000         0.00000000        0.554620981   
# :  -0.500000000         0.00000000         0.00000000         1.00000000   
#    
#
#    Band_V             Band_C             Kv-q ibz           Symm_kv            Kc q ibz           Symm_kc            Weight             Energy [eV]
#
         71                 74                289                1                  289                1                  0.298324           0.079534
         71                 73                289                1                  289                1                  0.250963           0.079534
         72                 74                13                 1                  13                 1                  0.174108           0.079536
         71                 73                13                 1                  13                 1                  0.095660           0.079536
         72                 73                289                1                  289                1                  0.075195           0.079534
         71                 74                13                 1                  13                 1                  0.050809           0.079536

[Daniele Varsano]

Dear Jingda,

please note that the weight reported at the beginning of the files on the relevant k points are normalized to the largest weight, and this can be misleading when comparing two different calculations.
I suggest you to plot weight to the whole BZ, or along a band structure, to inspect differences in the two calculations.
This can be done using the yambo-py utility, as explained in this tutorial:
https://wiki.yambo-code.eu/wiki/index.p ... _databases

In any case, from the sort file you posted, it is evident that you have many degenerate excitons. As they are degenerate, you can have different separate contributions or a linear combination of them. In order to recover the entire picture, probably you need to consider a linear combination of them, or analyze them one by one at the same foot, i.e. not considering the tiny energy difference they display.

Best,

Daniele