Mailman 3 - Kwant-discuss

Conductance plot for continuum.discretize hamiltonian

by declanburke95＠gmail.com

I defined a QAHE hamiltonian: ham = ("alpha * (k_x * sigma_x - k_y * sigma_y)" "+ (m + beta * kk) * sigma_z" "+ (gamma * kk + U) * sigma_0") subs = {"kk": "k_x**2 + k_y**2"} Upon using the continuum.discretize I was able to plot bands etc but am struggling to plot the conductance. Using the method from the kwant documentation I am getting the following error: UserCodeError: Error occurred in user-supplied value function "onsite". See the upper part of the above backtrace for more information. I am new to kwant so any help would be appreciated!

2 days, 17 hours

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adding a scatterer with internal degree of freedom

by Ahmet Mert Bozkurt

Dear KWANT developers, I would like to add a localized magnetic impurity into a quantum spin hall system with spin-flip interaction (sigma_plus I_- + sigma_- I_+ where I is the magnetic impurity spin operator. The spin-flip interaction depends on the state of the magnetic impurity (whether it is spin-up or spin-down). If it's spin-up, it needs to interact only with the spin down electrons (term sigma_+) flip them upwards, while the magnetic impurity state needs to be updated to spin-down. I've written a code using two different lattices (lat_up and lat_down) and simply check transmission from one lattice lead to the other lattice lead. But this method becomes impractical if i want to add more magnetic impurities because i need to add more and more lattice (2^N). Is there any easier way to include a scatterer with internal degree of freedom (which can be updated perhaps using tkwant or any other way)? BTW this is the model I'm working on:https://arxiv.org/abs/1705.04985 Below is the relevant part of my code and thank you so much in advance! Mert # Scattering region syst[lat_up.shape(shape, (0, 0))] = onsite syst[lat_down.shape(shape, (0, 0))] = onsite syst[kwant.HoppingKind((1, 0), lat_up)] = hopx syst[kwant.HoppingKind((1, 0), lat_down)] = hopx syst[kwant.HoppingKind((0, 1), lat_up)] = hopy syst[kwant.HoppingKind((0, 1), lat_down)] = hopy syst[(lat_up(10, 2), lat_down(10, 2))] = 0.1*0.5*np.kron(pauli.sp, pauli.s0) syst[(lat_down(10, 2), lat_up(10, 2))] = 0.1*0.5*np.kron(pauli.sm, pauli.s0) # two lines above correspond to spin-flip of the magnetic impurity, jumping from one #lattice to the other. #### Define the leads. #### sym_left = kwant.TranslationalSymmetry((-1, 0)) lead_up = kwant.Builder(sym_left) lead_down = kwant.Builder(sym_left) lead_up[lat_up.shape(lead_shape, (0, 0))] = onsite_lead lead_down[lat_down.shape(lead_shape, (0, 0))] = onsite_lead lead_up[kwant.HoppingKind((1, 0), lat_up)] = hopx_lead lead_up[kwant.HoppingKind((0, 1), lat_up)] = hopy_lead lead_down[kwant.HoppingKind((1, 0), lat_down)] = hopx_lead lead_down[kwant.HoppingKind((0, 1), lat_down)] = hopy_lead # Attach leads syst.attach_lead(lead_up) syst.attach_lead(lead_down) syst.attach_lead(lead_up.reversed()) syst.attach_lead(lead_down.reversed()) def plot_conductance(sys, p, mu): # Compute conductance p.mu = mu data = [] smatrix = kwant.smatrix(sys, energy = 0.0, params = dict(p=p)) no_flip = smatrix.transmission(2, 0) flip = smatrix.transmission(1,0) return no_flip, flip

2 days, 20 hours

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Conductance plot for continuum.discretize hamiltonian

by declanburke95＠gmail.com

I defined a QAHE hamiltonian: ham = ("alpha * (k_x * sigma_x - k_y * sigma_y)" "+ (m + beta * kk) * sigma_z" "+ (gamma * kk + U) * sigma_0") subs = {"kk": "k_x**2 + k_y**2"} Upon using the continuum.discretize I was able to plot bands etc but am struggling to plot the conductance. Using the method from the kwant documentation I am getting the following error: UserCodeError: Error occurred in user-supplied value function "onsite". See the upper part of the above backtrace for more information. I am new to kwant so any help would be appreciated!

3 days

1
0
0 0

Conductance plot for continuum.discretize hamiltonian

by declanburke95＠gmail.com

I defined a QAHE hamiltonian: ham = ("alpha * (k_x * sigma_x - k_y * sigma_y)" "+ (m + beta * kk) * sigma_z" "+ (gamma * kk + U) * sigma_0") subs = {"kk": "k_x**2 + k_y**2"} Upon using the continuum.discretize I was able to plot bands etc but am struggling to plot the conductance. Using the method from the kwant documentation I am getting the following error: UserCodeError: Error occurred in user-supplied value function "onsite". See the upper part of the above backtrace for more information. I am new to kwant so any help would be appreciated!

3 days

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Kpoint-resolved transmission spectrum

by Zhou Jiaqi

Dear all, We all know that it is easy to realize the energy-resolved transmission spectrum using Kwant. I am looking for the method to realize kpoint-resolved transmission spectrum using Kwant, like the output of non-equilibrium Greens function - density functional theory (NEGF-DFT) in the following link : https://www.researchgate.net/figure/shows-that-the-features-of-k-resolved-t… I note that we can use “params” to use the selected kpoint in 'kwant_model.hamiltonian_submatrix'. Take Gamma (0, 0, 0) as an example : Input : kwant_model.hamiltonian_submatrix(params={'k_x': 0, 'k_y': 0, 'k_z': 0}) Output : (the hamiltonian at Gamma kpoint) array([[-0.91848142+0.j , -3.13863545+0.05174286j, 0.01464058-0.12995645j, 5.49753359+1.2518773j ], [-3.13863545-0.05174286j, -0.91858637+0.j, -5.63827375-0.00282617j, 0.01464063-0.1299569j ], [ 0.01464058+0.12995645j, -5.63827375+0.00282617j, -0.91851442+0.j , -3.13862945+0.05174286j], [ 5.49753359-1.2518773j , 0.01464063+0.1299569j , -3.13862945-0.05174286j, -0.91857308+0.j ]]) Then after diagonalization we can get eigenvalues at Gamma point and k-resolved band. I wonder if we can use this method to get k-resolved transmission spectrum. I tried the following instruction : kwant.smatrix(kwant_model, params={'k_x': 0, 'k_y': 0, 'k_z': 0}) However, it does not work, i.e., the output does not vary with different kpoint (I am sure it should vary in physics). So here is the question: how to (maybe use ‘params’ in SMatrix) study transport at different kpoint using Kwant? Any suggestions will be greatly appreciated, thanks a lot! Sincerely, Jiaqi

1 week, 1 day

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Kwant parallelization

by Zhou Jiaqi

Hi everyone, I am writing to discuss the parallelization of Kwant. I installed Kwant with MUMPS from Ubuntu(https://launchpad.net/ubuntu/+source/mumps). When I run a Kwant script (which will be shown in the end) in the normal way on a 4-core laptop (2 threads per core), the screenshot (the output of htop, see https://drive.google.com/open?id=1TwEuc21DMjRnVQ9yG3XpkVilhFiAKaeb) shows that all the 8 CPU threads are involved, but 21 python threads. However, according to the tutorial: " Kwant uses only the sequential, single core version of MUMPS. The advantages due to MUMPS as used by Kwant are thus independent of the number of CPU cores of the machine on which Kwant runs.” So it confuses me whether Kwant 1.4.1 has already employed parallelization or not, and why there are 21 python threads? Moreover, I find the usage of concurrent package slows down the calculation. Thanks a lot for your time, and it would be greatly appreciated if anyone could share some news about the Kwant parallelization development. Best regards, Jiaqi UCLouvain ______________________ import kwant import tbmodels import numpy as np import matplotlib.pyplot as plt model = tbmodels.Model.from_wannier_files( hr_file='graphene_hr.dat', wsvec_file='graphene_wsvec.dat', xyz_file='graphene_centres.xyz', win_file='graphene.win', h_cutoff=0.01) lattice = model.to_kwant_lattice() wire = kwant.Builder() def shape(p): x, y, z = p return -5 < x < 5 and -5 < y < 5 and -1 < z < 1 wire[lattice.shape(shape, (0, 0, 0))] = 0 model.add_hoppings_kwant(wire) sym_lead_x = kwant.TranslationalSymmetry(lattice.vec((-2, 0, 0))) lead_x = kwant.Builder(sym_lead_x) def lead_shape(p): x, y, z = p return -5 <= x <= 5 and -5 < y < 5 and -1 < z < 1 lead_x[lattice.shape(lead_shape, (0, 0, 0))] = 0 model.add_hoppings_kwant(lead_x) wire.attach_lead(lead_x) wire.attach_lead(lead_x.reversed()) syst = wire.finalized() def trans(energy): smatrix = kwant.smatrix(syst, energy) data = smatrix.transmission(1, 0) return data def main(): energies = np.linspace(0, 1, 100) tc = map(trans, energies) # transmission coefficient te = zip(energies, tc) lte = list(te) print(lte) if __name__ == '__main__': main()

1 week, 2 days

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discontinuity between lead and scattering region

by Naveen Yadav

Dear KWANT Developers, I am trying to plot the current density. The procedure is straightforward. I have attached leads to the scattering region (leads have same onsite and hopping as of the scattering region) as *sys[(lat(z,y,x) for z in range(H) for y in range(W)for x in range(L))]=onsitesys[kwant.builder.HoppingKind((1, 0, 0), lat, lat)] = hoppingzsys[kwant.builder.HoppingKind((0, 1, 0), lat, lat)] = hoppingysys[kwant.builder.HoppingKind((0, 0, 1), lat, lat)] = hoppingxlead=kwant.Builder(kwant.TranslationalSymmetry((1,0,0)))lead[(lat(z,y,x) for z in range(H) for y in range(W)for x in range(L))]=onsitelead[kwant.builder.HoppingKind((1, 0, 0), lat, lat)] = hoppingzlead[kwant.builder.HoppingKind((0, 1, 0), lat, lat)] = hoppingylead[kwant.builder.HoppingKind((0, 0, 1), lat, lat)] = hoppingxsys.attach_lead(lead, add_cells=80)sys.attach_lead(lead.reversed())* [image: image.png] As you can see from the plot there is a discontinuity in the plot at (W=20). Why it is so even if the scattering region and leads have same onsite and hopping and there is translation symmetry throughout ? -- Best Regards, Naveen Yadav Research Scholar Department of Physics & Astrophysics University Of Delhi New Delhi-110007

2 weeks

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Applicability of Kwant to time-dependent gating of a TMDC quantum dot

by tiessen2＠uic.edu

To whom it may concern, I am a graduate student at the University of Illinois at Chicago (UIC) and I am interested in studying qubit manipulation in single and double quantum dots made from TMDCs. Specifically, I am interested in simulating a type of qubit/quantum dot configuration which was proposed in the following paper: https://iopscience.iop.org/article/10.1088/1367-2630/ab5ac9 I noticed that Kwant does seem to allow for time-dependent electric fields in its calculations but I wasn't sure if it could handle this specific type of problem. What I would like to know is the following: 1. Is Kwant capable of simulating a quantum dot similar to the one proposed in the linked paper 2. If not, what kind of modifications would I need to make to the code to make it possible for it to accurately simulate the proposed structure Thank you for your time, John Tiessen

2 weeks, 2 days

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[Kwant]Mailing list migration

by Christoph Groth

Dear subscribers of kwant-discuss, the kwant-discuss list has been migrated to a new server. If you receive this message then the operation must have been successful. The new server, running Mailman 3 and kindly provided by the nice people at python.org, offers a greatly improved web interface with the look and feel of a well-made web forum. Check it out for yourself: https://mail.python.org/archives/list/kwant-discuss@python.org/ At the same time, the list remains a classical mailing list, just like before. We are confident that the new server provides the best of both worlds. All subscriptions to kwant-discuss (and also kwant-announce that was also migrated) were taken over to the new list. This is also true for the subscription options with the notable exception of the old passwords. If you would like to log in (for example to modify delivery options), you will have to create a new account with the email address that is used for delivery. The new posting address is kwant-discuss(a)python.org, but the old address should continue to work as a redirection towards the new one. Please share your observations and let us know about any problems by posting to the list, or if this is not possible, do not hesitate to contact me directly. Regards Christoph

2 weeks, 5 days

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An issue with plotting two systems with different dimension in one system

by tavakkolidjawad

dear all I want to plot two different structures in one system. Although it is easy to plot separate lattices with the same structure, I will not be able to solve the problem when the structures are different. For example, I plotted two lattices with the same diamond structure, but when I replaced one of them with a honeycomb structure, I got an error after plotting the system. Apparently and logically, I think there is no bug in my code. Is there a way to solve this problem? I ask you to take a look at my code and report any possible bugs to me. Thanks ################################################################### import kwant from math import sqrt lat = kwant.lattice.general([(0, 0.5, 0.5), (0.5, 0, 0.5), (0.5, 0.5, 0)], [(15, 15, 15), (15.25, 15.25, 15.25)], name=['a1', 'a2']) a1, a2 = lat.sublattices honeycomb = kwant.lattice.general([(1, 0), (1 / 2, sqrt(3) / 2)], [(0, 0), (0, 1 / sqrt(3))]) subA , subB = honeycomb.sublattices def make_system(a=25, b=25, c=30): syst = kwant.Builder() def cuboid_shape(pos): x, y, z = pos return 15 <= x < a and 15 <= y < b and 15 <= z < c syst[a1.shape(cuboid_shape, (15, 15, 15))] = 1 syst[a2.shape(cuboid_shape, (15, 15, 15))] = 1 syst[lat.neighbors()] = 1 def graphene_shape(pos): x,y = pos return 0 <= x < 10 and 0 <= y < 10 syst[honeycomb.shape(graphene_shape,(0,0))] = 1 syst[honeycomb.neighbors()] = 1 return syst def main(): syst= make_system() kwant.plot(syst) syst = make_system(a=1.1, b=1.1, c=1.1) if __name__ == '__main__': main() ###################################################################

1 month

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