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April 2024

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[kwant] How to make a finite lead?
by 김봉수 12 Jun '24

12 Jun '24

Dear kwant users, I am trying to reproduce the paper following. https://arxiv.org/abs/1110.4280 (in this paper, fig5, 6 will help to understand my questions.) What I’m struggling with now is to build a finite(not infinite, and not having any translational symmetry) lead as below. I wonder 1. I think "kwant.builder.SelfEnergyLead()” function might be proper thing to build a finite lead without any translational symmetry. Is it correct? 2. If it's right, when I run this code using SelfEnergyLead(), the following error occurs. How can I solve it? ------------------------------------------------------------------------------------------------ lat = kwant.lattice.chain(norbs=1) syst = kwant.Builder() syst[(lat(i) for i in range(4))] = 4 syst[lat.neighbors()] = -1 def self_energy(energy, args=(), *, params=None): return -1j left_lead = kwant.builder.SelfEnergyLead(self_energy, interface=[lat(0)], parameters = []) right_lead = kwant.builder.SelfEnergyLead(self_energy, interface=[lat(3)], parameters = []) syst.leads.append(left_lead) syst.leads.append(right_lead) syst = syst.finalized() H = syst.leads[0].kwant.hamiltonian_submatrix kwant.plot(syst) ------------------------------------------------------------------------------------------------ AttributeError: 'SelfEnergyLead' object has no attribute ‘kwant' Any help is appreciated! Best, Bongsu Kim

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About the Warning "Hamiltonian breaks Conservation law, ignoring the symmetry in the computation.."
by GUIDO MENICHETTI 04 May '24

04 May '24

Dear developers, dear all, I am Guido Menichetti, a Post-Doc at the University of Pisa. I am currently working on computing the Andreev reflection in a 3D hexagonal lattice system, incorporating the spin degree of freedom and utilizing wraparound techniques. To facilitate this, I have been following the Kwant tutorial available at https://kwant-project.org/doc/1.0/tutorial/tutorial5. In my approach, I define a "conservation law" to decompose the transmission function into four blocks: two for the spin degree of freedom and two for the electron-hole degree of freedom. Initially, I successfully ran calculations in a 2D system with periodic boundary conditions, perpendicular to the transport direction, without encountering any warning messages. However, when attempting the same calculations in a 3D system with periodic boundary conditions in the perpendicular directions, I encountered a warning message: "[...] |envs\Kwant-env\lib\site-packages\kwant\system.py:316: UserWarning: Hamiltonian breaks Conservation law, ignoring the symmetry in the computation warnings.warn('Hamiltonian breaks ' + name +". This warning disappears when I set the superconductor gap Delta (as defined in https://kwant-project.org/doc/1.0/tutorial/tutorial5) to 0 (Delta = 0). The superconducting part is located in the right lead. I believe the issue does not stem from the Hamiltonian of the scattering region. However, I am puzzled by the origin of this warning and its potential impact on the results. Could this problem be related to the "wraparound" technique in the 3D system? The only difference between the 2D and 3D systems is the lattice: the 2D system lives on a square lattice, while the full 3D system is a hexagonal lattice. Your insights and guidance on resolving this issue would be greatly appreciated. Best regards, Guido Menichetti University of Pisa

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