I was looking at the data on this page for a NiWO4 calculation.

enter image description here

I can see unoccupied bands near the fermi level that appear to be below the band gap. What does this actually mean physically or is it an artifact of DFT? I suspect this may be related to it being a topological insulator.

  • $\begingroup$ I think you interpret the band coloring as an indicator of whether the respective band is occupied or not. But this coloring is only related to the band index. The occupation at each k-point may differ from this coloring scheme. $\endgroup$ – Gregor Michalicek Aug 27 '20 at 11:03

There are two different questions in your post:

  1. The band structure that you are reproducing is not that of a semiconductor. There are bands crossing the Fermi level, so the material is metallic. Looking at the link that you include in the post, there is a plot of the density of states (DOS) right next to the plot you reproduce here of the band structure, and in the DOS it is also clear that the Fermi level is within a group of bands, so the material is a metal.
  2. Whether the material really is a metal or this is an artifact of DFT is a different question. Semilocal DFT does underestimate band gaps, so if we were to assume that the calculations to generate this band structure are semilocal DFT, it could be that repeating them with a hybrid functional or with many-body perturbation theory techniques a gap opened and the system became a semiconductor. Then again, perhaps the system would stay metallic.

These points are not related to the material being a topological insulator: a topological insulator must have an insulating bulk, and the material you are looking at is a metal.

  • $\begingroup$ I find this really interesting, NiWO4 is a well known semiconductor material with a quite large band gap (2+ eV). For some reason based on this information, this source is classifying it as a topological insulator which is why I suspected this is the source of weirdness in band structure. $\endgroup$ – Tristan Maxson Aug 26 '20 at 20:21
  • $\begingroup$ Should a metallic material be able to exhibit this sort of "band gap" seen in the band structure though? $\endgroup$ – Tristan Maxson Aug 26 '20 at 20:22
  • 4
    $\begingroup$ Looking at NiWO4 in the Materials Project database, the material does have a reasonable band gap: materialsproject.org/materials/mp-25094 However, if you look at the details of the calculations there, they used GGA+U, so my guess is that the calculations that you reported in the post are using an inadequate level of theory. $\endgroup$ – ProfM Aug 26 '20 at 20:25
  • $\begingroup$ +1 for another rapid answer from ProfM! First HNQ in a while :) Keep them coming as it's great for our site! $\endgroup$ – Nike Dattani Aug 26 '20 at 20:29
  • $\begingroup$ That is a good observation. I may update this question someday with an answer if it is that since I will be doing some work on this material potentially. Thanks $\endgroup$ – Tristan Maxson Aug 26 '20 at 21:05

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