Question about electric dipole in heterostructure

[sophie_weber]

I have a question/sanity check about treatment of electrostatics in VASP. I think my confusion may just stem from not thinking properly about the physics, but nevertheless I want to ask about this.

I am performing calculations of a heterostructure consisting of an antiferromagnetic insulator (Cr2O3) and a heavy metal (Pt), terminated with vacuum on both sides. Due to the difference in electrostatic potential between the heavy metal and the insulator , I would expect an electric dipole to be formed at the heavy metal-insulating interface, and also separately, an electric dipole formed across the entire slab due to the difference in the delta E between vacuum and the insulating interface, and the delta E between vacuum and the heavy metal interface. I attach the INCAR and structure file here.

If we look at the LOCPOT plotted along the c-axis of the heterostructure (generated with LVHAR=.TRUE.), we do see a finite slope in the vacuum region, consistent with the second dipole source I described. Also, there is a clear "crossover region" between the insulator and heavy metal at their interface, before the potential on either side look like that of the isolated consitutents.

What I'm confused by is why the average electrostatic potential across the insulator (between about .5 and .7 of the fractional c-coordinate) appears completely flat, and does not have a slope. I guess maybe I can understand for the heavy-metal region (0.7-1) as the electric dipole might be screened here, but for the insulating compound I don't see how the dipole would be screened. Clearly, the slab has a finite electric dipole through the structure as manifested in the vacuum area. But I don't understand why this doesn't also seem to show up in the electrostatic potential profile within the slab region. As an aside, this calculation was done without dipole corrections but the same thing happens if I include them (i.e. slope in vacuum, no slope across structure ).

Does this simply have to do with the details of electrostatic boundary conditions in VASP? Or is this profile consistent with what you would expect based on physics (in which case if you could clarify that would be great.)

Thanks in advance!

[henrique_miranda]

Thank you for your detailed question.

I am not sure that I understood it completely.
In particular, I don't understand what is the red/blue lines in the plot.
Could you explain how you obtained them?

[sophie_weber]

Hello,

Sorry for not explaining better. the red lines are the "average" electrostatic potential (where the electrostatic potential is plotted in dark blue and is simply the output of the locpot for LVHAR=TRUE) where the average is separately taken in the different regions divided by the vertical blue lines. basically I take the average in the region of the insulator in the heterostructure, and in the matallic region of the heterostructure (and I separate out the interface region where the electrostatic potential clearly transitions).

The point I am struggling with is why there is not a linear slope superimposed on the periodic electrostatic potential variation due to atoms in the slab, throughout the who slab, rather than just in the vacuum region. Clearly their is an electric dipole generated at the interface so why doesn't this manifest throughout the entire slab?

Best,
Sophie

[henrique_miranda]

Ok, thank you for the clarification!
I think I understand your question now: because the Cr O compound is an insulator, you would expect to see a linear slope superimposed in the periodic potential, indicating a finite electric field through the slab similarly to what one sees in the vaccum region.
Unfortunately, I don't know how to explain it either.

The first explanation that comes to my mind would be that the layer structure of the Cr O compound is not an insulator in this heterostructure, but I don't know.
I guess you could try plotting the bandstructure projected onto the atomic sites of Cr O and see how they are located w.r.t the fermi level.
One way to do it would be using py4vasp reading the vaspout.h5 file:

Code: Select all

import py4vasp
c = py4vasp.Calculation.from_path('.')
graph = c.band.to_graph('Cr O')
graph.show()