# How to model Topological Insulators from first-principles?

Topological insulators and quantum materials are gaining increasing interest across the physical, chemical and materials communities.

Today, one can go to the Topological Materials Database and see whether a given bulk system is a Topological Insulators or Weyl Semimetal, but it remains unclear (at least to a moral like me) what steps could be taken to determine this from scratch.

If a DFT practitioner wants to model a a material and determine whether it has topological properties from scratch, with zero prior experience in these materials (but experience in semiconductors, surfaces, adsorption), what would be the key elements and steps required to carry out such a study?

Note: Assume that the researcher has already modeled the electronic band structure of a selected material with and without spin-orbit coupling.

There is a whole zoo of topological phases, and hopefully someone will provide a more complete answer, but here are some thoughts.

Symmetry and dimension. The topological classification of a material with a gap (topological insulator or topological superconductor) depends on (i) symmetry and (ii) dimension. These relations are summarized by the so-called tenfold way shown in this table (from this paper):

The "Symmetry" columns correspond to time reversal symmetry (T), particle-hole symmetyry (C), and chiral symmetry (S), with "0" meaning no symmetry, "$$\pm1$$" is the square of the antiunitary operator of the symmetry. The "Dimension" columns correspond to the spatial dimension (1, 2, and 3 are most relevant for real systems, but some higher-dimensional synthetic systems have also been explored). A well-known example are time-reversal invariant topological insulators (what are typically called topological insulators), which obey time reversal symmetry (and for electrons T$$^2=-1$$) and fall into class AII. You will see this means that there is no topological classification for 1D, and both 2D and 3D have a $$\mathbb{Z}_2$$ classification, as is well-known for these materials. Another example are Chern insulators, which fall into class A and admit a $$\mathbb{Z}$$ classification in even dimensions only.

So how do you figure what are the topological properties of your material? First you need to figure out what the relevant symmetries are and what the dimension is, so that you know where it falls in the tenfold way. For example for a "topological insulator" in 3D, the relevant symmetry is time-reversal symmetry and the dimension is obviously 3. Then, you need to figure out how to calculate the corresponding topological invariant, in this case $$\mathbb{Z}_2$$. As Shahid Sattar described in their answer, in this case it can be done using a number of standard packages like Z2Pack or WannierTools.

Topological quantum chemistry. Beyond the three symmetries described above, crystalline symmetries further constrain topological order. Additionally, semimetallic systems also admit a topological classification. The database that you refer to in your question uses the formalism known as topological quantum chemistry to classify these phases. In short, it uses the symmetry of the various high-symmetry $$\mathbf{k}$$ points in the Brillouin zone to determine the degeneracies of the bands at those points, and then uses the symmetries along the paths connecting these points to determine the allowed connectivities of the bands. Each possible connectivity corresponds to a possible topological phase. Then depending on where the Fermi level is, one gets an insulating or a semimetallic phase.

The Topological Quantum Chemistry Database covers almost any material you may ever encounter, so they essentially have done the job for you. However, all their calculations are based on semilocal DFT, and this can be problematic (see for example this paper). Therefore, I would use the database as a good starting point, but then do your own calculations to figure out what is really going on (e.g. using hybrid functionals or the $$GW$$ approximation to get better estimates fo the bands).

• A nice answer again.
– Jack
Apr 6, 2021 at 13:35

Finding topological invariant number(called Z$$_2$$ number) can give information about topological invariance. Different codes are available for such calculations such as Z2pack or wanniertools and others.

• An alternative tool and database: SymTop and Materiae
– Thomas
Oct 26, 2020 at 17:32

As a first step, you can use the WannierTools.

There are five typical examples in there.

• Bi2Se3 (3D strong TI)
• MoS2 (2D QSHE)
• WTe2 (Type II Weyl semimetal)
• IrF4 (Nodal Chain metals)
• FeSi (Weyl point in Phonon system)