[Wien] Questions about Na2IrO3 (PBE+SOC+U calculation with magnetic order)

Mon Aug 7 00:45:17 CEST 2017

Hi,

For your doubts with regard to (3), my responses are as follow:

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?

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> 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
>
>
>
> 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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