About dark exciton

[andrea marini]

Well, of course to be completely dark the oscillator strength should be zero. But when it is already 1/100 of the main peak you can hardly observe it in the absorption spectrum. So I would say that both 10^-7 and 10^-4 correspond to pretty dark excitons. Of course in the case where explicit selection rules prevent an exciton to absorb the oscillator should be numerically zero.

Andrea

[sdwang]

Tanks!
In the o.exc_E_sorted , I get
# E [ev] Strength Index
#
4.948 0.9463E-6 1.000
5.545732 1.000000 2.000000
5.611 0.7413E-5 3.000
5.925 0.2374E-6 4.000
5.971 0.8421E-4 5.000
5.982299 0.829999 6.000000
6.004 0.4682E-9 7.000
......
So, as you mentioned above, the fisrt one is dark exciton.But strictly, it is absolutely zero. In yambo, can we get the absolute zero exciton energy level? Or can we have another way to sort the dark one ?

[sdwang]

Well, of course to be completely dark the oscillator strength should be zero. But when it is already 1/100 of the main peak you can hardly observe it in the absorption spectrum. So I would say that both 10^-7 and 10^-4 correspond to pretty dark excitons. Of course in the case where explicit selection rules prevent an exciton to absorb the oscillator should be numerically zero.

Andrea

Tanks!
In the o.exc_E_sorted , I get
# E [ev] Strength Index
#
4.948 0.9463E-6 1.000
5.545732 1.000000 2.000000
5.611 0.7413E-5 3.000
5.925 0.2374E-6 4.000
5.971 0.8421E-4 5.000
5.982299 0.829999 6.000000
6.004 0.4682E-9 7.000
......
So, as you mentioned above, the fisrt one is dark exciton.But strictly, it is absolutely zero. In yambo, can we get the absolute zero exciton energy level? Or can we have another way to sort the dark one ?

[andrea marini]

So, as you mentioned above, the fisrt one is dark exciton.But strictly, it is absolutely zero. In yambo, can we get the absolute zero exciton energy level? Or can we have another way to sort the dark one ?

Dear sdwang, first of all remember to include your complete affiliation in the signature.

Then, what do you mean for "absolute zero exciton energy level" ? If you are wondering if Yambo can give an exactly zero oscillator strength my asnwer is: I dunno. The numbers you get at the end are the result of complex calculations where a certain numerical noise cannot be eliminated. To my understanding 0.9463E-6 is, for a code written in single precision, is zero if compared to 1.

Andrea

[sdwang]

Thanks!
Now I have some results with yambo about dark exciton.I give a picture to demonstrate it as following,
picture1:
______ bright2
.........dark2

_______bright1
..........dark1

picture2:
.........dark2
______bright2

........dark1
______bright1

In picture1, the dark excitons below the bright ones, we could say the dark ones may give the nonradiative channel which make the luminescence efficiency low.
But in picture2, the dark one are above the bright ones, can we make the same conclusion as the picture1? I thinch maybe the bright2 in picture2 would relax to the dark1 level, so it also give poor luminescence efficiency .
Are my conclusions right above? (the dark ones and the bright ones are seperated by tens meV)
Thanks in advance!!!

[sdwang]

So, as you mentioned above, the fisrt one is dark exciton.But strictly, it is absolutely zero. In yambo, can we get the absolute zero exciton energy level? Or can we have another way to sort the dark one ?

Dear sdwang, first of all remember to include your complete affiliation in the signature.

Then, what do you mean for "absolute zero exciton energy level" ? If you are wondering if Yambo can give an exactly zero oscillator strength my asnwer is: I dunno. The numbers you get at the end are the result of complex calculations where a certain numerical noise cannot be eliminated. To my understanding 0.9463E-6 is, for a code written in single precision, is zero if compared to 1.

Andrea

Thanks!
Now I have some results with yambo about dark exciton.I give a picture to demonstrate it as following,
picture1:
______ bright2
.........dark2

_______bright1
..........dark1

picture2:
.........dark2
______bright2

........dark1
______bright1

In picture1, the dark excitons below the bright ones, we could say the dark ones may give the nonradiative channel which make the luminescence efficiency low.
But in picture2, the dark one are above the bright ones, can we make the same conclusion as the picture1? I thinch maybe the bright2 in picture2 would relax to the dark1 level, so it also give poor luminescence efficiency .
Are my conclusions right above? (the dark ones and the bright ones are seperated by tens meV)
Thanks in advance!!!

[andrea marini]

This post has been answered here.