Mailman 3 May 2023 - Kwant-discuss

Realistic values of hopping parameters
by Yu Li 18 Oct '23

18 Oct '23

Dear all, Now I’m studying some transport property based on multilayered system, with nonmagnetic layer/ferromagnetic layer (NM/FM) stack. May I ask when creating a tight binding system, and setting hoppings by the following function: syst[(lat(1, 0), lat(1, 1))] = t, how to associate the hopping with a realistic system? Such as hoppings between FM and FM atoms, or between FM and NM atoms? I found in many literatures people just simply put t=1, but I assume the value should depend on the type of atoms. My another question is, is it possible to define more than one type of hopping between two sites? For the tight-binding approximation of p electrons, there are two types of hopping integrals: Vppσ, and Vppπ, so I’m wondering if it’s possible to simulate the effect of both hoppings? Thanks a lot for reading my questions! Cheers, Yu Li

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Methods for calculating Josephson Current ?
by guilhermewells＠gmail.com 06 Jun '23

06 Jun '23

Hi, I'm new to Kwant and I've been looking for calculating Josephson Current with it. The past three weeks I've been reading a lot the tutorials, the mailing list and some online courses. The problem is that I can't get a straight answer on where it is possible to calculate Josephson current reallistically. The most complete answer was given by J. Weston here [1], the problem is that the methods suggested are also not so clear, since I coundn't find tutorials using properly Self-Energy and Green's function with Kwant. Most of the topics are questions, so they usually are of people with code not working, so it doesn't point out the proper way to use. So, I'd like to know if I'm missing some tutorials with these subjects, or some online courses to understand this. The documentation itself is not enough for the applications (at least for me). I'm learning kwant alone, there is no one around in my department that even have heard of it. Right now I'm got a discontinuous current (only at the junction interface) through my junction using directly "kwant.operator.Current" and "plotter", which I think is to be expected due to the issue of the Andreev bound states. Thanks, I'm looking foward to use kwant at higher level [1] https://mail.python.org/archives/list/kwant-discuss@python.org/thread/6IP2Q… -- Guilherme Vieira Phd Student at Centro Brasileiro de Pesquisas Físicas Brazil, RJ

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Beyond nearest interaction in continuous Hamiltonian.
by Adel Belayadi 31 May '23

31 May '23

Dear Kwant core team, Dirac or Majorana Fermions might be used in kwant with Discretizing continuous Hamiltonian on a square lattice. However the way that kwant descritizes the Dirac Hamiltonian gives rise to what is called doubling Fermions (even the desicritization preserves the hermicity ’symmetric derivative') . In this context, several approaches have been used to get rid of the extra Dirac cones in the Brillouin zone. Without going in deeper details, each of those proposed methods has some issue related to preserving either chiral or symplectic symmetry.To avoid this issue recent methods goes beyond nearest neigbour interaction and then my question come to raise. *How can we include second or third nearest interaction in kwant.continuum.discretize *. Best regards Adel

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Different hopping parameters
by Gabriel Garcia 25 May '23

25 May '23

Hi develops, I'm trying to build a diatomic chain 1D in kwant, with two differents hopping parameters (intracell and extracell). But when i try to attach the lead, it doesn't work. Can anyone help me? -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- L = 5 C = 5 t1 = 4 # hopping intracell t2 = 5 # hopping extracell lat = kwant.lattice.general([(C,0),(0,1)],[(0,0),(2,0)], norbs=1) a, b = lat.sublattices syst = kwant.Builder() for i in range(L): syst[a(i, 0)] = 0 syst[b(i, 0)] = 0 syst[kwant.builder.HoppingKind((1, 0), a, b)] = -t1 syst[kwant.builder.HoppingKind((1, 0), b, a)] = -t2 kwant.plot(syst) plt.show() sym_lead1 = kwant.TranslationalSymmetry(lat.vec((-1, 0))) lead1 = kwant.Builder(sym_lead1) lead1[lat(0,0)] = 0 lead1[lat(1,0)] = 0 lead1[kwant.builder.HoppingKind((1, 0), a, b)] = -t1 lead1[kwant.builder.HoppingKind((1, 0), b, a)] = -t2 syst.attach_lead(lead1) kwant.plot(syst); --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- TypeError Traceback (most recent call last) <ipython-input-38-06cd19c77c10> in <module> 3 4 lead1 = kwant.Builder(sym_lead1) ----> 5 lead1[lat(0,0)] = 0 6 lead1[lat(1,0)] = 0 7 lead1[kwant.builder.HoppingKind((1, 0), a, b)] = -t1 TypeError: 'Polyatomic' object is not callable

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Confusion regarding conductance unit
by sahu.ajit92＠gmail.com 18 May '23

18 May '23

Hello Kwant Developers, 1. I am new to Kwant. In tutorial 2.5, it is mentioned that the conductance unit is [e^2/h] whereas I find some papers using Kwant mentioned as [2e^2/h] for e.g. https://doi.org/10.1016/j.physleta.2020.126990. Please clarify this. 2. I calculate the energy (unit in eV) from the KPM method and then calculate the conductance in Kwant. What's the unit for the conductance in my case? Please find below the code that I used to calculate the conductance. def dos_eng(model1): rho = kwant.kpm.SpectralDensity(model1) eng1, densities = rho.energies, rho.densities return eng1, densities def calc_conductance(sys, energies): # Compute transmission as a function of energy data = [] for energy in energies: smatrix = kwant.smatrix(sys, energy) data.append(smatrix.transmission(1, 0)) return data Thanking You, Ajit

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