[Wien] Questions about Na2IrO3 (PBE+SOC+U calculation with magnetic order)
pieper
pieper at ifp.tuwien.ac.at
Mon Aug 7 10:27:20 CEST 2017
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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