[Pw_forum] Hyperfine interaction in Li-Mn compounds?

[Davide Ceresoli]

Gregor Mali wrote:
> Dear PWscf/GIPAW users.
> 
> I am studying lithium-manganese oxides and lithium-manganese silicates.
> Isotropic shifts of 6Li MAS NMR signals in these compounds are
> predominantly determined by the contact hyperfine interaction (Fermi
> shifts), i.e. by the interaction between 6Li nuclei and unpaired
> electronic spins.
> 
> Is it possible to calculate these hyperfine shifts or the magnitude of
> hyperfine interaction by the PWsfc/GIPAW module using job = 'hyperfine'.
> I also wonder if the corresponding GIPAW input file requires any
> additional input parameters. The option 'hyperfine' is, namely, not
> documented in the INPUT_GIPAW.html file.
> 

Dear Gregor,
     the extra information to enter are the nuclear g-factors
for the isotopes. Here is an example for MnO (55Mn and 18O).

&inputgipaw
         job = 'hyperfine'
         prefix = 'mno'
         tmp_dir = './scratch/'
         spline_ps = .true.
         hfi_output_unit = 'MHz'
         hfi_nuclear_g_factor(1) =  1.387487      ! 55Mn
         hfi_nuclear_g_factor(2) = -0.757520      ! 18O
/

I've got the numbers from webelements.com, under "NMR properties".
There, the gyromangnetic ratio \gamma is listed in 10^7 rad T^{-1} s^{-1}.
To obtain the g-factor, multiply \hbar and divide by the nuclear
magneton (\mu_N):

E.g. using the command 'units':
You have: 6.6452546e7 hbar / nuclearmagneton T s
You want:
         Definition: 1.3874872

If the nuclear spin is N/2, N>1, you need to divide the output
quantities by N.

For Li, you should get good results for the Fermi contact, because
the unpaired electron are in the 2s orbital. But if the magnetization
comes form p and d orbitals, I've found that the core electrons (those
not included in the pseudopotential!) polarize opposite to the
valence, giving rise to large cancellations in the Fermi contact.

I've implemented the core relaxation in an experimental version of
the code. I haven't put it in the CVS, because it's not perfect, and
one should carefully check if the results make sense. If you are
interested, send me an e-mail.

> Best regards.
> 
> Gregor Mali
Best,
     Davide