X-ray absorption by means of the BSE: for shallow core levels or all the core levels??

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X-ray absorption by means of the BSE

X-ray absorption by means of the BSE: for shallow core levels or all the core levels??

Hajar (guest) 06 Dec 2015 17:15

Dear exciting developers,

I have a question, please:
Is the 'x-ray absorption spectra ' valid for shallow core levels such as Li, or it is valid for the deep core levels such as Zn K or L2,3 edges?

Re: X-ray absorption by means of the BSE: for shallow core levels or all the core levels??

Caterina Cocchi 07 Dec 2015 09:16

Dear Hajar,

the calculation of x-ray absorption spectra, as described in the related tutorial, is generally valid, independently of the energy of the core level. One should be however be aware that spin-orbit coupling, which can impact the accuracy of the results for L and M edges, is not yet implemented.

Please let us know if you encounter specific problems when performing such calculations.

Best regards,
Caterina Cocchi
(exciting team)

Hajar (guest) 07 Dec 2015 11:23

Dear Caterina
Thank you so much for your reply.

According to your awareness, due to the lake of spin-orbit coupling in this type of calculation of exciting-code, the x-ray calculations for L2,3 edges may be redounded to the wrong results. yes?

Caterina Cocchi 07 Dec 2015 15:52

Dear Hajar,

the accuracy of the results of X-ray absorption spectra from the L edges is pretty much dependent of the system and of the relevance of spin-orbit coupling effects in that specific case. This should not prevent you from performing such calculations. My hint is to consider this as a possible source of discrepancy, if any, in comparison with the experimental data.

Best regards,
Caterina Cocchi
(exciting team)

Hajar (guest) 07 Dec 2015 19:38

Thank you so much dear Caterina,
Your explanations were useful. I will run x-ray absorption calculations for my cases and will use your help.
All the best for you.
Hajar

Hajar (guest) 09 Dec 2015 08:36

Dear Caterina and exciting developers,
I have some questions concerning the x-ray absorption spectra (XANES) in BSE implemented in the exciting-code:

1. We know that in the XANES, by a x-ray photon, a core electron of a single atom, such as 1s electron of a single N atom in BN, is excited to the conduction states. It seems that in x-ray absorption calculation in the exciting, the 1s electrons of all the N atoms in BN crystal absorb the x-ray (not a single N atom)!

2. By attention to above point: Can one compare this type of calculation with the corresponding energy loss near edge structure (ELNES) spectrum (N 1s edge ELNES)?? As you know, ELNES and XANES spectra nearly have the same dispersion, but in ELNES, the incident beam is electron. In the ELNES, the incident electron excites a core electron (such as 1s electron of N) to the conduction states and a hole is left by the electron transition. Again as you know, in the wien2k, one removes 1s electron of a single atom (not all N atoms) and adds that to the background, and with the corehole approximation, one makes supercell in order to reducing the mutual interaction of coreholes (a result of the periodic boundary). Now, I think that the XANES in exciting does not consider this point: I mean that the final absorption spectrum in exciting-code is due to the absorption by all the N atoms. Because one introduces a single species file for the excited N atom and the other N atoms.

By considering above points, I think that I cannot compare the XANES of exciting-code with the corresponding ELNES.
Please clarify these point for me.

All the best for you

Hajar

Caterina Cocchi 09 Dec 2015 09:18

Dear Hajar,

I reply to your questions below.

1. When performing x-ray absorption calculations with exciting, as described in our tutorial, full flexibility is left to the user to include only transitions from specific atoms. In fact, in principle, each atom can have its own species file. Moreover, in the BSE part of the input file, the user can explicitly define the initial and final states of the transitions.

2. It is well established in the condensed-matter theory community that our BSE approach for core-level spectroscopy is valid for both XANES and ELNES [see for review: I. Tanaka and T. Mizoguchi, J. Phys.: Condens. Matter 21, 104201 (2009)]. This given, as explained in point 1., the user has the possibility to choose explicitly which transitions to include in the BSE.

I hope this solves your doubts.

Best regards,
Caterina Cocchi
(exciting team)

Hajar (guest) 09 Dec 2015 09:49

Thank you for your reply, dear Caterina

(a)If each atom can have its own species file, why you do not consider this in the tutorial? (b) Does this result in the changes in the absorption spectrum? (c) Moreover, is the N (or Li)K edge spectrum in the tutorial the x-ray absorption spectrum for all the N (or Li) atoms not only in the unitcell but also in whole of the crystal. yes?

(d) Can you guide me for making an input.xml that introduce a species file for a distinct N atom (that absorb the x-ray) and a species for all the other atoms?

Thank you again

Caterina Cocchi 09 Dec 2015 10:07

Dear Hajar,

below the answers to your questions.

(a) The tutorial presents the general guidelines to perform BSE calculations for x-ray absorption spectra. Specific modifications, which are problem- or system-dependent, are obviously left to the users.

(b) The computed absorption spectrum depends explicitly on the number of transitions included in the BSE. If only transitions from one specific atoms are included, the spectrum may change with respect to the case when transitions from all atoms are considered.

(c) The results presented in the tutorial are valid for the considered (infinitely extended) bulk materials.

(d) The explanation given in the tutorial on x-ray absorption from BSE about how to modify species files are generally valid, no matter if applied to one or more atoms in the unit cell. If you want to replace a core electron with a local orbital only for a specific atom of a given species in the unit cell, you may follow the procedure described in the tutorial, assigning the new species file only to that atom, leaving all the other atoms of the given species with the default species file.

Best regards,
Caterina Cocchi
(exciting team)

Hajar (guest) 10 Dec 2015 05:35

Dear Caterina,
I am really grateful of you for your replyes.

I am a little confused. Concerning the question 1:
As you know, in the experimental XANES, the absorption occurs for a single atom in the crystal, i.e., a single nitrogen atom that is distinct from the other nitrogen atoms in the crystal. Or as another example, I want to calculate x-ray absorption of a single phosphor atom in the phosphorus bulk. The phosphorus unitcell contains two types of phosphor atom. In addition to introducing two species files for these two types of phosphor atom in unitcell, "I have to separate this single phosphor from the other infinite phosphor atoms through the crystal, due to the periodic conditions". Yet, I don't understand how does exciting consider this separation?

Thank you,
Hajar

Caterina Cocchi 10 Dec 2015 16:11

Dear Hajar,

it is important to clarify that in exciting you can specify species files and define transitions for each atom inside the unit cell. If you want to consider absorption from one individual atom in a larger portion of the infinite crystal, you have to build a supercell and proceed as specified in the previous posts for one single atom in such supercell.

Best regards,
Caterina Cocchi
(exciting team)

Hajar (guest) 10 Dec 2015 19:03

Thanks a lot dear Caterina,
Now, I got what I wanted.

All the best

Yours sincerely,
Hajar