Mailman 3 May 2021 - Kwant-discuss

To to separate spins, electron and hole
by Khani Hosein Oct. 11, 2024

Oct. 11, 2024

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 … [View More]

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Realistic values of hopping parameters
by Yu Li Oct. 19, 2023

Oct. 19, 2023

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 … [View More]

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Gate-defined Nanowire Quantum Dot
by Jonathan Fields Sept. 15, 2022

Sept. 15, 2022

Dear Reader, I am new to KWANT and trying to figure out if it is suitable for calculating transport properties of gate-defined quantum dots made out of nanowires, along the lines of the structure used in https://arxiv.org/abs/cond-mat/0609463. Now, I understand that the mailing list is not a place to ask people to do my work for me, and my question is also not for someone to do this. What I am looking for is some insights into if this is possible (and not all that difficult) with the package. … [View More]At first sight (and going through the tutorials as well as the APS March Meeting material) this does not seem straightforward; what I struggle with is how to define this type of dot in KWANT. Now, one way would of course be to built the entire 3D geometry of the system, but this will be computationally expensive, and probably also rather tricky to do in terms of defining everything, from the hexagonal wire itself to all of the gates and oxide layers and such. Some abstraction would be preferred, perhaps even reducing the dimensionality and making a 2D system, such as often done in the tutorial. Is this a good idea? My problem is that if one does this, then I run into the problem of how to model the 'half wrap-around' type gates typically used in these devices. In the transport through barrier section of the APS March Meeting material there is a section on how to model realistic potentials, but this would not work all that well for these wrap around type gates. On the other hand, in a way they are perhaps not so different from the QPC in that section. One could model the three gates as something like https://i.imgur.com/WZC983c.png as they do essentially form channels in the wire. Apart from if this sacrifices too much of the nanowire nature in favor of a 2DEG system, I do suppose such a system should in principle be able to be treated as a quantum dot. I haven't been able to confirm this as I am not yet sure how one can apply a source-drain voltage in KWANT; I first thought that this would probably be related to the energy of the modes, but then again I have not been able to produce Coulomb diamonds in this way. The question is getting rather lenghty, and also a bit unclear at this point. Perhaps I should finish up by stating it in a concise form; would you think that it is possible to simulate the transport of such a gate defined nanowire quantum dot device with KWANT, and if so, is the approach I am suggesting above a viable one, or would you go about it very differently? Kind regards Jonathan <http://aka.ms/weboutlook> [View Less]

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current-voltage diagram
by loojoo026＠gmail.com April 8, 2022

April 8, 2022

dear all I have done my project using Kwant and I have drawn a conductance-energy diagram for the system. The question I have is how can I get the current-voltage diagram? thanks Leo

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How the spin is positioned in the Smatrix?
by Qingtian Zhang March 6, 2022

March 6, 2022

Dear all, I am trying square lattice with spin-orbit coupling and Zeeman splititng just like the example of Tutorial 2.3.1. Now I want to get the spin-dependent conductance：Guu,Gud,Gdu,and Gdd, where "u" indicates up spin, d indicates down spin. We can have the scattering matrix through Smatrix=kwant.smatrix(sys, energy), but how the spin is positioned in the scattering matrix? I have checked some simple cases, it seems the Smatrix is organized like this: | r t | S= | t' … [View More]

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Spin polarized transport calculation
by L.M J March 6, 2022

March 6, 2022

Dear Kwant users and developers I have two questions related to the spin calculation using kwant. 1. I'm wondering if we can do spin-polarized transport in kwant? For example, can we can get the separate Conductance Vs Energy diagram for spin up and spin down terms. I'm thinking one way of doing this is to set up the Rashba Hamiltonian as a whole. And then manually extract the spin up and spin down hamiltonian and do the transport calculation separately. But this will eliminate some extra … [View More]

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Graphene Conductor-Superconductor Conductance above Gap
by Henry Axt Feb. 21, 2022

Feb. 21, 2022

Hello, I set up a graphene lattice with periodic boundary conditions that is connected to a superconductor (simulated with an electron and hole lattice). When observing the transmission between the electron and hole lattice, I see a small amount conductance above the superconducting gap. I do not see this with something like a square lattice. What could explain this?

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How to caculate hall resistance
by ZHANG Bing Sept. 12, 2021

Sept. 12, 2021

Dear Sir, I am a PhD student of Hong Kong University of Science and Technology. I want to use KWANT to caculate Hall resistance of a Hall bar structure.We can get the conductance between 6 electrodes, but how to get hall resistance? Can you give me some help? Thank you very much. Best Regards, Zhang Bing

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PyPlot.plot will give the incorrect number
by 张金龙 May 16, 2021

May 16, 2021

Hello I felt confusing, when I use pyplot.plt. The data in figure is different from the printed. There are the codes: # ================================================================ # Physics background # ------------------ # Kwant features highlighted # -------------------------- # - Numpy matrices as values in Builder import kwant import numpy as np # For plotting import matplotlib.pyplot as plt # For matrix support import tinyarray import kwant.qsymm # … [View More]define Pauli-matrices for convenience s_0 = tinyarray.array([[1, 0], [0, 1]]) s_x = tinyarray.array([[0, 1], [1, 0]]) s_y = tinyarray.array([[0, -1j], [1j, 0]]) s_z = tinyarray.array([[1, 0], [0, -1]]) mz=0 # AFM mu=0.0 # stargged chemical potential n_theta=12 def make_system(constant=1, W=30, L=30, mz=0, m_in=0, mu=0): lat = kwant.lattice.honeycomb(a=constant,norbs=2, name=['a', 'b']) # a, b = lat.sublattices sym = kwant.TranslationalSymmetry(lat.vec((1, 0))) syst = kwant.Builder() def onsite_a(site,theta): return mz*s_z+m_in*(np.cos(theta)*s_x+np.sin(theta)*s_y)+mu*s_0 def onsite_b(site,theta): return -mz*s_z+m_in*(np.cos(theta)*s_x+np.sin(theta)*s_y)-mu*s_0 syst[(lat.a(x, y) for x in range(L) for y in range(W))] = onsite_a syst[(lat.b(x, y) for x in range(L) for y in range(W))] = onsite_b syst[lat.neighbors()] = s_0 # define lead left_sym=kwant.TranslationalSymmetry(lat.vec((1, 0))) left_lead=kwant.Builder(left_sym) left_lead[(lat.a(0, y) for y in range(W))] = onsite_a left_lead[(lat.b(0, y) for y in range(W))] = onsite_b left_lead[lat.neighbors()] = s_0 syst.attach_lead(left_lead) syst.attach_lead(left_lead.reversed()) return syst def plot_conductance(syst, energy, theta_list): # Compute conductance data = np.zeros(n_theta) num_i=-1 for theta_i in theta_list: num_i=num_i+1 smatrix = kwant.smatrix(syst, energy, params=dict(theta=theta_i)) data[num_i]=smatrix.transmission(1, 0) plt.figure() # x=np.linspace(-1,1,n_theta) # data_i=np.linspace(0,0,n_theta) # print(x,data_i) # plt.plot(x,data_i,'ow') print(data) plt.plot(theta_list, data) plt.xlabel(r'$\theta/\pi$') plt.ylabel("conductance [e^2/h]") plt.savefig('tran_PHE.jpg') #plt.show() def main(): syst = make_system(m_in=0.0001) # Check that the system looks as intended. kwant.plot(syst,file='transport_lattice.pdf') syst = syst.finalized() theta_num=np.linspace(-np.pi,np.pi,num=n_theta) plot_conductance(syst, energy=0.0001, theta_list=theta_num) if __name__ == '__main__': main() The result would be printed in terminal: [1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1.]. However, the figure plotted is wrong. -- Best Wishes Jinlong Zhang jlzhang(a)163.com [View Less]

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Creating a three-wire junction
by Sayandip Dhara May 12, 2021

May 12, 2021

Hello, I want to create a three-wire "(1D each wire) junction for calculating conductance. Can I use kwant.lattice.chain() to use three separate chains and then join them somehow? I also need to change the angle between these 3 wires. It would be really helpful if anyone can give me the idea to make such a graph (lattice ) in kwant. Thanks, Sayandip Dhara

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