Mailman 3 February 2023 - Kwant-discuss

To to separate spins, electron and hole
by Khani Hosein 11 Oct '24

11 Oct '24

Dear all, I am using Kwant to study a system with both spins and electron and hole, so the hopping matrix is 4X4. Now I want to know how to separate the spins, electron and hole. I study this through the "2.6. Superconductors: orbital degrees of freedom, conservation laws and symmetries" smatrix = kwant.smatrix(syst, energy) data.append(smatrix.submatrix((0, 0), (0, 0)).shape[0] -smatrix.transmission((0, 0), (0, 0)) +smatrix.transmission((0, 1), (0, 0))) I do not find more information about this for kwant. For my system, if I have 3 leads, and the hopping and onsite energy is set in this order: (e↑,0,0,0)- spin up electron,first row of the 4X4matrix (0,e↓,0,0)- spin down electron,second row of the 4X4matrix (0,0, h↑ ,0)- spin up hole,third row of the 4X4matrix (0,0, 0, h ↓ )- spin down hole,fourth row of the 4X4matrix I want to obtain the transmissions from 0 →2. smatrix = kwant.smatrix(syst, energy) The transmission from h↑ to e↑ is: smatrix.transmission((2, 1), (0, 2)) The transmission from h ↓ to e↑ is: smatrix.transmission((2, 1), (0, 3)) Is my understanding correct? Thanks very much in advance! Hosein Khani

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Realistic values of hopping parameters
by Yu Li 19 Oct '23

19 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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How to add magnetic field (Peierls substitution) for the continuum model
by dielectric_hjzhao＠126.com 16 Mar '23

16 Mar '23

Dear kwanters, I am using kwant for handle a k.p Hamiltonian, for the conductance calculations. I start from a k.p model, and transform it to a tight-binding model using kwant.continuum tools [procedures listed here https://kwant-project.org/doc/1/tutorial/discretize]. Now I want to study the conductance of the system under magnetic field. I try to do Peierls substitution for hoppings between site i and site j (i-j being parallel to the lead). However, I find that using the kwant.continuum scheme, there are no information regarding the lattice information such as "lat = kwant.lattice.honeycomb() ". This prevents the application of Peierls substitution by the following way: _____________________________ def hopx(site1, site2, B): y = site1.pos[1] return - np.exp(-1j * B * y) syst[kwant.builder.HoppingKind((1, 0), lat.a, lat.b)] = hopx ____________________________ My question is how to add magnetic field during the calculations for the systems generated from a continuum model by kwant.continuum? Thank you very much!

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Questions about kwant nanoribbon transmission
by leegunaun＠inu.ac.kr 20 Feb '23

20 Feb '23

While studying 2.3.transport_through_barrier.ipynb, a tutorial in gitlab, I tried to find the transimission by changing the lattice to a graphene nanoribbon rather than a square. Here is the code I tried. import numpy as np import kwant from matplotlib import pyplot lat = kwant.lattice.general([[1, 0], [1/2, np.sqrt(3)/2]], # lattice vectors [[0, 0], [0, 1/np.sqrt(3)]]) # Coordinates of the sites a, b = lat.sublattices def make_lead_x(W=10, t=1): syst = kwant.Builder(kwant.TranslationalSymmetry([-1, 0])) syst[(lat(0, y) for y in range(W))] = 4 * t syst[lat.neighbors()] = -t return syst def make_wire_with_flat_potential(W=10, L=2, t=1): def onsite(s, V): return (4 - V) * t # Construct the scattering region. sr = kwant.Builder() sr[(lat(x, y) for x in range(L) for y in range(W))] = onsite sr[lat.neighbors()] = -t # Build and attach lead from both sides. lead = make_lead_x(W, t) sr.attach_lead(lead) sr.attach_lead(lead.reversed()) return sr def plot_transmission(syst, energy, params): # Compute conductance trans = [] for param in params: smatrix = kwant.smatrix(syst, energy, args=[param]) trans.append(smatrix.transmission(1, 0)) pyplot.plot(params, trans) _syst = make_wire_with_flat_potential() kwant.plot(_syst) _syst = _syst.finalized() kwant.plotter.bands(_syst.leads[0]) plot_transmission(_syst, 1, np.linspace(-2, 0, 51)) Error says 'Polyatomic' object is not callable. I did a search lat = kwant.lattice.general([[1, 0], [1/2, np.sqrt(3)/2]], # lattice vectors [[0, 0], [0, 1/np.sqrt(3)]]) # Coordinates of the sites a, b = lat. sublattices I understand that this code is polyatomic, and it is not correct to apply it to (lat(x, y). But i can't find a solution. Can you help me?

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