Dear Anton,
Thank you for your reply. I’m probably more interested in 3D and will scan 
magnetic fields. Could you please help clarify whether the following 
understandings are correct?

1. Both the PRR method and the built-in kwant.kpm.correlator can calculate the 
Chern marker. The former is more efficient in 3D. 

2. Although the code in my first post for a 2D QHE is probably not yet enough 
tuned, conceptually it is correct: define x,y as kwant density operators and 
then feed to kwant.kpm.correlator. (I'm not sure since there is no example in 
the documentation other than the special case of kpm.conductivity.)

3. The main difference between the two methods lies in how they use kpm 
expansion. kwant.kpm.correlator uses the so-called two-dimensional kpm 
expansion. The PRR method uses the conventional kpm expansion to approximate 
the projection below Fermi energy. 

4. Actually, I wonder what makes the PRR method special as I want to code it 
myself. Is it only 2.? Or anything else?

Thank you very much!

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