[Wien] EELS of L3 Ti edge

Fri Jan 14 23:22:07 CET 2005

There is probably no explanation as to why LDA works.

As for the multiplet mentioned in this message and one of the follow-ups, it has indeed remarkable advantages.

Wien looks at the full, complicated electronic structure of the crystal - but only in a 'simple' way (ground state LDA or GGA, say).  Multiplet programs like the code of Frank de Groot look at a very basic structure (basically an atom, and one or two crystal field parameters, I believe) - but in a more complex way, taking the multiplets into account (the multiplets get the branching ratio right, and that makes the Ti L edge of TiO wonderful since there is not much to say about it except for the branching ratio ;-)).  Yet this seemingly simple approach has proven to yield a lot of useful results.

tddft/bse applications may help us in the next years to remedy the shortcomings of current dft+lda elnes calculations (eg., inclusion of tdlda in feff has enabled them to get branching ratios quite close to experiment).

Kevin Jorissen

EMAT - Electron Microscopy for Materials Science   (http://webhost.ua.ac.be/emat/)
Dept. of Physics

UA - Universiteit Antwerpen
Groenenborgerlaan 171
B-2020 Antwerpen
Belgium

tel  +32 3 2653249
fax + 32 3 2653257
e-mail kevin.jorissen at ua.ac.be

________________________________

Van: wien-admin at zeus.theochem.tuwien.ac.at namens Maurits Haverkort
Verzonden: do 13-1-2005 17:11
Aan: wien at zeus.theochem.tuwien.ac.at
Onderwerp: Re: [Wien] EELS of L3 Ti edge

Dear all

I do not understand why LDA is able to calculate EELS and XAS L2,3 spectra
in the first place.

For L2,3 XAS and EELS the core hole created interacts with the 3d valence
electrons, creating bound states, or excitons. This has been very nicely
shown for d0 compounds, where the XAS and EELS spectra always consist of 6
sharp peaks (in Oh symmetry), independent of the material. Take CaO or CaF2
for example (PRB. 43 6899 (1991)). These spectra should be expected when the
3d electrond bonds with the 2p core hole, making an exciton. This has been
verified using cluster calculations. Using the concept of having a final
state that is excitonic XAS spectra of a large variety of TM oxides have
been understood with the use of cluster calculations.

Back to my question. If LDA can calculate XAS spectra than what is the
reason why it works. When including a super cell calculation with a
core-hole included do you start to calculate excitons? In other words is the
valence charge density around the core hole increased? Do you find 2p 3d
multiplet effects in LDA? If I have a 2pz corehole my 3dz^2 orbitals will be
lower in energy than my 3dx^2-y^2. Is this correctly reproduced in LDA, due
to the effective potential taken into account?

Many questions on the basics of LDA I guess, but probably important.

If anybody knows some answers or publications I would be very happy.

Best regards,

Maurits Haverkort

----- Original Message -----
From: "Peter Blaha" <pblaha at zeus.theochem.tuwien.ac.at>
To: <wien at zeus.theochem.tuwien.ac.at>
Sent: Thursday, January 13, 2005 3:42 PM
Subject: Re: [Wien] EELS of L3 Ti edge

> I'd just like to raise a few points you need to consider when you
> calculate spectra:
>
> TM oxides are often fairly correlated materials, so you might need LDA+U
> in some cases.
>
> EELS (and X-ray absorption) spectra have a core-hole as final state. In
> most cases this should NOT be neclected!
> You can simulate it with WIEN by creation of a small supercell and put
> a core hole on one of your atoms, promoting this electron into valence.
> /edit case.inc and reduce the core oppupation; edit case.in2 and add this
> electron.
> run_lapw
> edit case.in2 and set the number of electrons back to "normal"
> run you spectra.
>
> L2,L3: You may take the splitting from the core eigenvalues (case.scf) and
> overlay the spectra yourself (eg. in Excel) with the proper shift and
> with desired intensity ratio. I know this is not perfect, but not so bad
> either.
>
> Anyway, several people have shown that WIEN can be used quite successfully
> to calculate EELS spectra.
>
> Regards
>
> > I'm doing experimental and theoretical research on EELS of Ti (oxides
mainly).
> > I found big differences in the experimental and computed spectra (for L3
edge)
> > - the computed spectra look very, very far from the experimental ones.
We
> > tried to include other orbitals in the computation (changing the Ar core
with
> > Ne and even He) but without any positive result. There are some articles
in
> > which is shown that the expected ratio of the L3:L2 intensities should
be 2:1
> > (on the basis of 2j+1 degerancy). It looks like that this can be the
reason
> > for these differences with the experimental results. In the same
articles is
> > shown that the ratio L3:L2 has to be 0.8 and in this case the spectral
> > features in the computed and experimental spectra are quite similar one
to
> > each other. My question is it is possible to change somehow these ratios
or
> > how we can improve the quality of the spectra? I would like just to
notice,
> > that in the case of Cr we don't have such differences - the computed
spectra
> > look quite good.
> >
> > Thanks in advance for your help
> > Best regards,
> > Emil Stoyanov
> >
> > _______________________________________________
> > Wien mailing list
> > Wien at zeus.theochem.tuwien.ac.at
> > http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
> >
>
>
>                                       P.Blaha
> --------------------------------------------------------------------------
> Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
> Phone: +43-1-58801-15671             FAX: +43-1-58801-15698
> Email: blaha at theochem.tuwien.ac.at    WWW:
http://info.tuwien.ac.at/theochem/
> --------------------------------------------------------------------------
>
> _______________________________________________
> Wien mailing list
> Wien at zeus.theochem.tuwien.ac.at
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