[Wien] Questions about Na2IrO3 (PBE+SOC+U calculation with magnetic order)
Hung Yu Yang
yanghw at bc.edu
Wed Aug 9 00:05:43 CEST 2017
Thanks a lot for the replies from both of you! And in this case the orbital
moment is indeed not negligible when I checked :ORB (~0.4).
Yours sincerely,
Hung-Yu
On Mon, Aug 7, 2017 at 4:27 AM, pieper <pieper at ifp.tuwien.ac.at> wrote:
> Of course! The local moments in an AF insulator can be anything. I just
> saw that there is only one type of magnetic atom in the unit cell (Ir) and
> did not think properly. Sorry for that one.
>
> ---
> Dr. Martin Pieper
> Karl-Franzens University
> Institute of Physics
> Universitätsplatz 5
> A-8010 Graz
> Austria
> Tel.: +43-(0)316-380-8564
>
>
> Am 07.08.2017 09:18, schrieb Peter Blaha:
>
>> The Ir moments are read from case.scf file, but why should it be near
>>> an integer, and what is the unit of the reported moments in case.scf
>>> file? If the unit is bohr magneton, then for a spin 1/2, should it be
>>> ~1.7?
>>>
>>
>> For a FERROmagnetic insulator, the TOTAL spin moment/cell must be
>> integer. The moments of individual atoms can have any value.
>>
>> For an ANTIferromagnet, the total spin moment/cell must be zero, and
>> of course the atomic moments can again have any value.
>>
>> What you get by :MMT or MMIxxx are spin moments only.
>>
>> The moments are given in Bohr magnetons (mu_B), but are the spin
>> moments, not the effective moments. Thus if you have one unpaired
>> electron the spin moment is one.
>>
>> PS: For your compound, the orbital moments are probably as important
>> as the spin moments and you must add them to get the total moment.
>>
>>
>>
>>
>>> The gap can be roughly seen along the k-path I plotted and is checked by
>>> DOS plot, which shows it's an insulator.
>>>
>>> Yours sincerely,
>>>
>>> Hung-Yu
>>>
>>> On Sun, Aug 6, 2017 at 11:03 AM, pieper <pieper at ifp.tuwien.ac.at
>>> <mailto:pieper at ifp.tuwien.ac.at>> wrote:
>>>
>>> A few thoughts on this:
>>>
>>> ad 1)
>>> Presumably Ir is the magnetic ion, so what you describe seems to be
>>> ok with me. I did not check your .struct file, however.
>>>
>>> ad 2)
>>> See chapters 4.5.6 and 7.4 of the UG (Wien2k 17.1) on ther
>>> orb-program, and the references therein. Personally I found the .pdf
>>> file of his talk on LDA+U very usefull that E. Assmann posted on the
>>> Wien2k-site.
>>>
>>> Note the recommendation in the UG for the SIC-mode of orb for of
>>> strongly correlated systems: set J=0 and use only U_eff=U-J. The
>>> value of U_eff is something you will have to decide, perhaps based
>>> on the approach described by Madsen and Novak cited in the UG. You
>>> also might want to take a look at eece as an alternative (UG chapter
>>> 4.5.7)
>>>
>>> >From many comments here in the mailing list and from the UG (again
>>> e.g. chapter 5.5.7, 4.5.8) I take it that PBE is what you should do
>>> if you want to calculate spacial charge and spin distributions, but
>>> to calculate gaps you may have to switch to numerically much more
>>> costly hybrid methods.
>>>
>>> You probably can (mis)use U_eff as a free parameter to adjust the
>>> gap in your PBE calculation to your favorite value. However, the
>>> physical meaning of the value would be dubious (imho), and there is
>>> no guarantee that the Ir-moments simultaneously come near your
>>> favorit 'theoretical moments' (whatever the actual value and origin
>>> of those is).
>>>
>>> ad 3) Don't bother with the starting values of local moments for
>>> atomic configurations, and for an antiferromagnet the interstitial
>>> moment obviously should stay close to zero.
>>>
>>> But the moments you give in your table are very far from integer.
>>> Are this Ir-spin moments from case.scf or did you add orbital
>>> moemnts calculated by lapwdm? The spin moments should be somewhere
>>> near integer for an insulator.
>>>
>>> So, how did you determine the gap in the table? Did you plot a DOS?
>>> Is this really an insulator, or are there in fact bands crossing
>>> E_F? You might severly misjudge the (direct?) gap depending on where
>>> in k-space it is and the points in your k-list.
>>>
>>> Good luck
>>>
>>> ---
>>> Dr. Martin Pieper
>>> Karl-Franzens University
>>> Institute of Physics
>>> Universitätsplatz 5
>>> A-8010 Graz
>>> Austria
>>> Tel.: +43-(0)316-380-8564 <tel:%2B43-%280%29316-380-8564>
>>>
>>>
>>>
>>> Am 04.08.2017 19:22, schrieb Hung Yu Yang:
>>>
>>> Dear WIEN2k developers and users,
>>>
>>> I am trying to do a calculation on Na2IrO3, which has a band gap
>>> ~340meV and a zigzag antiferromagnetic order in its ground
>>> state, and
>>> I have some questions as follow:
>>>
>>> 1. To assign the zigzag antiferromagnetic order, what I did is
>>> that I
>>> first made a cif file that has two inequivalent Ir atoms, and let
>>> WIEN2k decide the symmetry for me. I adopted the generated
>>> structural
>>> file (attached in this mail), checked the cif file in some
>>> visulization software and made sure they were structurally
>>> equivalent
>>> except that there were 2 inequivalent Ir atoms instead of 1. Is
>>> this
>>> the proper way to generate structural files for magnetically
>>> ordered
>>> systems?
>>>
>>> 2. After generating the desired structural file, I put up on Ir1
>>> atom
>>> and down on the other (Ir2), used several different combinations
>>> of U
>>> and J, and I got the following results:
>>>
>>> U(eV) J(eV) Ueff=U-J (eV) Moment(Ir1, Ir2)
>>> Gap(meV)
>>> 2.1 0.6 1.5
>>> 0.22532,-0.22439 ~700meV
>>> 2.4 0 2.4
>>> 0.30105,-0.30109 ~750meV
>>> 3 0.6 2.4
>>> 0.23225,-0.23235 ~900meV
>>> 3 1.5 1.5
>>> 0.17203,-0.17210 ~900meV
>>>
>>> First, in this test, it seems that the gap is affected by U
>>> only, not
>>> Ueff=U-J. Second, the moments of Ir seemed to be closely related
>>> to J.
>>> In this situation, what is the proper way to assign U and J? I
>>> am not
>>> sure how much it means if I just try to tune U and J until they
>>> match
>>> the experimental gap and/or theoretical moments.
>>>
>>> 3. I also have a question about the unit of moment given here; I
>>> understand that the moments depend on the RMT sizes and I
>>> assumed the
>>> unit is in bohr magneton, but the first value shown for Ir atom
>>> is
>>> 0.76697 and I am not sure in what unit is this value and how it
>>> assigns the initial moment for a certain element (say, Ir in my
>>> case).
>>> Also, the interstitial moments are nearly 0 among all the cycles.
>>>
>>> For this calculation, kpoints=450, RKmax=8 and I had to use the
>>> TEMP
>>> scheme for it to converge. I appreciate any reply from you.
>>>
>>> Yours sincerely,
>>>
>>> Hung-Yu
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