[Wien] Questions about Na2IrO3 (PBE+SOC+U calculation with magnetic order)
Peter Blaha
pblaha at theochem.tuwien.ac.at
Mon Aug 7 09:18:30 CEST 2017
> 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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--
P.Blaha
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