Mailman 3 March 2020 - 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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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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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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Stability of the retarded Green's function calculation
by Joseph Weston (Aquent LLC - Canada) March 26, 2020

March 26, 2020

Hello Kwantoptians, I have a question regarding the speed/stability of computing the retarded Green's function of a transport setup using Kwant. In the Kwant source-code [1] it is noted that using `kwant.greens_function` is "often slower and less stable than the scattering matrix calculation". I was wondering if you could provide me with some references for this assertion. I have perused the Kwant paper [2] and (part I of) the thesis of Michael Wimmer [3] and cannot find any mention of a … [View More]

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3D Current Density
by Ousmane LY March 25, 2020

March 25, 2020

Dear Naveen, if you want to use a 2d system to represent local properties of a 3d system, the former shouldn’t be considered as a physical system. Therefore, its onsite and hopping values are not relevant and shouldn’t alter your calculations in any sens. Even attaching leads for the 2d system may not be necessary. However, you have to be sure that the 2d system represents well the 2d plane of the underlying 3d geometry. This would concern mainly the number of sites and links. You may have a … [View More]

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adding spin degrees of freedom to BdG form in the superconductor.py in Kwant
by Sayandip Dhara March 24, 2020

March 24, 2020

Hello, I want to do a transport calculation for a 1D/2D nanowire supporting Majorana fermions. So for that, I need to add spin and changing magnetic field and spin-orbit coupling. In the section for superconductors (example 2.6 )the BdG hamiltonian is expressed as a 2x2 system with tau matrices. Now if I want to add spin and spin-orbit coupling H_0 I have to write the onsite terms as 2x2 matrices (pauli matrices). So my question is how can I do that in this BdG formalism in Kwant. Also, I want … [View More]

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3D Current Density
by Naveen Yadav March 23, 2020

March 23, 2020

Dear KWANT developers, I want to plot the current density for a 3D model with finite width in x and y direction. Below is the energy dispersion of the model Hamiltonian. To calculate the current density profile, I have assumed scattering region with a width of 60 unit cells in z-direction and leads with different onsite and same hoppings (as that of scattering region) are attached. For better understanding the code is attached below. For plotting current density, I have used 2D slices of y-z … [View More]plane. Now my question is that, while building the second system, what onsite and Hopping terms should I have to use? Because I have already build a 3D system with particular onsite and hopping terms. Can I use onsite =0 for the second system? and what should be the hoppings for the second system? can I use simply lat2.neighbors() for the second system? I tried it in different ways, it gives me different results. I don't know which one correct. please suggest me the correct way. [image: image.png] *import kwantfrom numpy import exp,sin,linspace,sqrt, cos, piimport matplotlib as mplfrom matplotlib import pyplot as pltimport tinyarrayimport pandas as pd* *sigma_0 = tinyarray.array([[1, 0], [0, 1]])sigma_x = tinyarray.array([[0, 1], [1, 0]])sigma_y = tinyarray.array([[0, -1j], [1j, 0]])sigma_z = tinyarray.array([[1, 0], [0, -1]])lat=kwant.lattice.cubic(norbs=2)L=10W=80#H=60def onsite(site): return (1.0 * cos(1.05) + 2 * 1.0) * sigma_z* *def onsite_s(site): return (1.0 * cos(1.05) + 2 * 1.0) * sigma_z - 0.75 * sigma_0def hoppingx(site0, site1): return (-0.5 * 1.0 * sigma_z - 0.5 * 1j * 1.0 * sigma_x)def hoppingy(site0, site1): return -0.5 * 1.0 * sigma_z - 0.5 * 1j * 1.0 * sigma_ydef hoppingz(site0, site1): y = site1.pos[1] return (-0.5 * 1.0 * sigma_z - 0.5 * 1j * 0.25 * sigma_0) * exp(-2 * pi * 1j * 0.001 * 1.0 * (y-40))sys=kwant.Builder()sys[(lat(z,y,x) for z in range(0,10) for y in range(W) for x in range(L))]=onsite_ssys[kwant.builder.HoppingKind((0, 0, 1), lat, lat)] = hoppingxsys[kwant.builder.HoppingKind((0, 1, 0), lat, lat)] = hoppingysys[kwant.builder.HoppingKind((1, 0, 0), lat, lat)] = hoppingzsys[(lat(z,y,x) for z in range(10,70) for y in range(W) for x in range(L))]=onsitesys[kwant.builder.HoppingKind((0, 0, 1), lat, lat)] = hoppingxsys[kwant.builder.HoppingKind((0, 1, 0), lat, lat)] = hoppingysys[kwant.builder.HoppingKind((1, 0, 0), lat, lat)] = hoppingzsys[(lat(z,y,x) for z in range(70,80) for y in range(W) for x in range(L))]=onsite_ssys[kwant.builder.HoppingKind((0, 0, 1), lat, lat)] = hoppingxsys[kwant.builder.HoppingKind((0, 1, 0), lat, lat)] = hoppingysys[kwant.builder.HoppingKind((1, 0, 0), lat, lat)] = hoppingz* *# The scattering region 0 < W_z < 10 and 60 < W_z > 70 have onsite energy of leads.* *lead=kwant.Builder(kwant.TranslationalSymmetry((-1,0,0)))lead[(lat(0,y,x) for y in range(W) for x in range(L))] = onsite_slead[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)sys.attach_lead(lead.reversed())kwant.plot(sys)sysf=sys.finalized()Sites=list(sysf.sites)#print(Sites)wave=kwant.wave_function(sysf, energy=0.1)psi=wave(0)[0]#df = pd.DataFrame(Sites)#df.to_csv('data2.csv',index=False, header=False)def Plot(x,psi): def cut(site0,site1): return site0.pos[2]==x and site1.pos[2]==x J_0 = kwant.operator.Current(sysf, where=cut) current = J_0(psi) lat2=kwant.lattice.square(norbs=2) sys2=kwant.Builder() sys2[(lat2(site.pos[0],site.pos[1]) for site in Sites if site.pos[2]==x)]=onsite* * #sys2[(lat2(site.pos[0],site.pos[1]) for site in Sites if site.pos[2]==x)]=0* * sys2[kwant.builder.HoppingKind((1, 0), lat2, lat2)] = hoppingz sys2[kwant.builder.HoppingKind((0, 1), lat2, lat2)] = hoppingy* * #sys2[lat2.neighbors()]=-sigma_0* * #lead2=kwant.Builder(kwant.TranslationalSymmetry((-1, 0))) #lead2[(lat2(site.pos[0], site.pos[1]) for site in Sites if site.pos[2]==x)]=onsite_s #lead2[kwant.builder.HoppingKind((1, 0), lat2, lat2)] = hoppingz #lead2[kwant.builder.HoppingKind((0, 1), lat2, lat2)] = hoppingy #sys2.attach_lead(lead2) #sys2.attach_lead(lead2.reversed()) sys2f=sys2.finalized() #kwant.plot(sys2f) kwant.plotter.current(sys2f, current)for i in range(10): Plot(i,psi)* -- With Best Regards Naveen Yadav Ph.D Research Scholar Deptt. Of Physics & Astrophysics University Of Delhi. [View Less]

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