On external electric or magnetic field

[BO PENG]

Dear Dr He,

I hope you are doing well! I was trying to play with the magnons under external electric or magnetic field based on DFT, but interestingly, the magnons under these fields from TB2J are exactly the same. I wonder whether it would be possible to run DFT with different fields and then provide TB2J such data. If not, would it be possible if we could just add external field to the exchange term directly, saying, an external magnetic field B to all the diagonal term along z? Thank you in advance!

Best wishes,

Bo

[matthieu verstraete]

Hi Bo,

Interesting idea! Indeed if you run the dft with finite fields applied, then extract wannier functions, the resulting J and tb2j runs should reflect the coupling. One issue is that you'll need many different parameters, but perhaps you can do some finite difference with fields strength. 

Best

Matthieu

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[BO PENG]

Hi Matthieu,

Thank you for the clarification! Long time no see ;)

I tried different parameters such as magnetic or electric field, as well as photoexcitation in DFT, but the changes in J were tiny, and the magnons looked exactly the same with the unperturbed ones. That's why I am trying to find whether we can just add the external magnetic field to the diagonal J terms in the Heisenberg Hamiltonian directly, and then use the new Hamiltonian to generate magnons. Thanks once again for your swift response!

Best wishes,

Bo

[matthieu verstraete]

This probably means that the magneto-electric coupling is very small - DFT predicts these coefficients relatively well in standard cases like BFO.

For the explicit B field, this is usually included in the ASD code itself, and is easy to add, once you have the gyromagnetic ratio and magnetic moment. 

From the Js you can directly calculate the magnon/spinwave spectrum. TB2J does this in a simple way by Fourrier transform. No need to add an explicit B field. NB: actual (LLG) atomistic spin dynamics will include more complex interactions, damping, and renormalized frequencies. An intermediate solution with FFT magnons under applied B field might be analytic as well.

ciao

Matthieu

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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Professor Matthieu J Verstraete

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[Xu He]

Hello, 

Adding external magnetic  field into the spin Hamiltonian is indeed possible in the linear spin wave theory.  This is not in the current implementation of TB2J. But it can be found in SpinW (see https://spinw.readthedocs.io/en/latest/intro.html#model). We're currently working on the magnon spectrum code in TB2J, and this is likely an interesting feature to include!

Cheers, 

HeXu   

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[BO PENG]

Dear Matthieu and Xu,

Thank you for the clarification! This is extremely helpful to know that most ASD codes such as SpinW can deal with the problem. Thanks once again!

Best wishes,

Bo