Mailman 3 April 2016 - Kwant-discuss

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' r' | but how the spin is included? Thanks in advance. Qingtiian Zhang The code is: import kwant from matplotlib import pyplot from numpy import matrix import tinyarray 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]]) sys=kwant.Builder() lat=kwant.lattice.square() a=1 t=1.0 alpha=0.5 e_z=0.08 W=2 L=2 sys[(lat(x, y) for x in range(L) for y in range(W))] = \ 4 * t * sigma_0 + e_z * sigma_z # hoppings in x-direction sys[kwant.builder.HoppingKind((1, 0), lat, lat)] = \ -t * sigma_0 - 1j * alpha * sigma_y # hoppings in y-directions sys[kwant.builder.HoppingKind((0, 1), lat, lat)] = \ -t * sigma_0 + 1j * alpha * sigma_x #### Define the left lead. #### lead = kwant.Builder(kwant.TranslationalSymmetry((-a, 0))) lead[(lat(0, j) for j in xrange(W))] = 4 * t * sigma_0 + e_z * sigma_z # hoppings in x-direction lead[kwant.builder.HoppingKind((1, 0), lat, lat)] = \ -t * sigma_0 - 1j * alpha * sigma_y # hoppings in y-directions lead[kwant.builder.HoppingKind((0, 1), lat, lat)] = \ -t * sigma_0 + 1j * alpha * sigma_x #### Attach the leads and return the finalized system. #### sys.attach_lead(lead) sys.attach_lead(lead.reversed()) kwant.plot(sys) sys=sys.finalized() print "Hamiltonian=" print sys.hamiltonian_submatrix() Smatrix=kwant.smatrix(sys, energy=3.) print "The scattering matrix=" print matrix.round((Smatrix.data),2) print "T=",(Smatrix.transmission(1, 0))

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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 terms that I don't know whether this is correct anymore. Any suggestions? 2. How can we set up the spin polarization for a particular site? For example, in some of the topological insulator, can we set up spin polarization on the edge as up and the other side of the edge as down, and then do the transport? Or this question is completely wrong? Thanks for advance. Sincerely, Liming

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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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Kwant for bulk system
by Weiyuan Tong June 8, 2016

June 8, 2016

Dear all, I want to calculate the conductance and the band structure of a bulk graphene system.(infinite in y direction) I have checked the source code for the calculation of band structure. https://github.com/kwantproject/kwant/blob/7c55b0cb2d0dec163e5483dea8ffdbc8… It seems that we can get the sys.cell_hamiltonian and hopping between the sys.cell_hamiltonian in kwant. In y direction, we can also have sys.cell_hamiltonian and hopping, then it can be extended to infinite in y direction: V^\dagger e^{iky} + H + Ve^{-iky}. I am not familiar with kwant coding. Can anyone help me to write some code lines to do these calculations? Or you can also change the Tutorial 2.2.3 as an example. Thanks in advance! Best wishes Weiyuan Tong

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Installing Kwant on Windows-8 system
by Joseph Kulik April 29, 2016

April 29, 2016

I am trying to install Kwant on my Windows-8 system. (I am new to this. I have never used Python.) I tried to follow the instructions on the web page http://kwant-project.org/install#microsoft-windows. The final step in the installation is the following command: python3 -c "import pip, glob; pip.main(['install', '--no-deps'] + glob.glob('*.whl'))" When I try to execute this, I get a message that says ‘python3’ is not recognizable as a command. Can anyone tell me what I've done wrong? Thanks. -- Joseph Kulik

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Some simple and general questions
by Gayles, Jacob Davis April 25, 2016

April 25, 2016

Dear All, I am really liking this Kwant program and I find it very intuitive. However my python is a bit rusty. 1) I would like to know what is the simplest way the number of sites, the positions, and the tuples of these sites. I would like to print them all out. 2) In this graphene flake example (http://kwant-project.org/doc/latest/tutorial/tutorial4 <http://kwant-project.org/doc/latest/tutorial/tutorial4>) if i wanted to make a hall bar setup, how would I exactly attach the leads purely in the x-axis and purely only the y-axis. (more of an aesthetic question). 3) in terms of the hopping, when one has sys[lat.neighbors(1)] =t sys[lat.neighbors(2)] =t is the hopping mediated by the distance i.e. t_1=t/a_1 and t_2=t/a_2. 4) I have calculated the energy DOS for a lead and this looks correct, what I’m more unsure of is this the same as the DOS of the propagating modes. And I see how to find the velocity and things from the documentation, but is there a way to find the energy. Sorry if this is a stupid question I could not find anything on the forum 5) similar to question 1) how do I find things like the number of propagating modes. 6) what is the best way to go about writing observables, sorry this is quite a general question and most likely up to the user, but for instance the pauli matrices are quite simple. However the position operator, or velocity operator changes with the way the Hamiltonian is described, is there something simple general method to calculate these? Thanks very much, and sorry if I did not see these answers in the mailing list Regards, Jacob JGIU INSPIRE GROUP - Prof. Sinova FB 08 - Institut für Physik Staudinger Weg 7 55128 Mainz ____________________________ room 01-523 (1. OG) phone +49 (0) 1517/ 39-27424 fax +49 (0) 6131/ 39-26267 email jgayles(a)uni-mainz.de <mailto:sinova-group@uni-mainz.de>

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Re: [Kwant] wave function at each site
by Johnny Wu April 18, 2016

April 18, 2016

Dear all, I obtained the wavefunction of the system by using ham = sys.hamiltonian_submatrix() evecs = la.eigh(ham)[1] Is the wavefunction corresponding to site i given by evecs[i] ? Thanks! Best, Johnny On 18 April 2016 at 03:44, Abbout Adel <abbout.adel(a)gmail.com> wrote: > Dear Johnny, > > you need just to finalize your system before doing calculation. > so : > sys=sys.finalized() > m=len(sys.sites) > > and it will work . > Best regards, > Adel > > On Sun, Apr 17, 2016 at 7:23 PM, Johnny Wu <darknesswtc(a)gmail.com> wrote: > >> Dear Dr. Adel >> >> I could not compute len(sys.sites) since sys.sites is of the type >> "method". How should I change it? Thanks. >> >> Best, >> Johnny >> >> On 14 April 2016 at 18:44, Abbout Adel <abbout.adel(a)gmail.com> wrote: >> >>> Dear Johnny, >>> >>> if you want to get the index of a site in you system, you can do it as >>> follow: >>> >>> sys=make_system() >>> sys=sys.finalized() >>> >>> Positions=[sys.sites[i].pos for i in range(len(sys.sites))] # >>> list of all the positions of the sites >>> >>> >>> Since you know the form of you system, you can obtain the positions >>> of the sites at the edge. >>> for example if the site at (3,5) is at the edge you can find its >>> index straightforwardly: >>> >>> Index_i= Positions.index((3,5)) >>> >>> once you know the index of your site, you can do whatever you want. for >>> example delete that site: >>> >>> del sys[sys.sites[index_i]] >>> >>> >>> or you can pass it to a function which has site as an argument : >>> >>> function(sys.sites[index_i]) >>> >>> I hope that this helps >>> >>> Best regards, >>> Adel >>> www.abboutadelhomepage.com >>> >>> >>> >>> >> > > > -- > Abbout Adel >

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Position dependent effective mass by discretization interface.
by Emil Johansson April 18, 2016

April 18, 2016

Dear all, I am using Kwant to discretize an effective mass approximation scheme. Efforts to implement position dependent effective mass to model a metal-semiconductor hetrojunction have so far been futile. To avoid having to discretize the entire system with the high accuacy needed in the metal parts (the leads) i want to have different discretisation in different parts of the system. So my question is if it is possible to have such an interface between discretizations and if so, how could it be implemented in Kwant? Thank you in advance. Emil Johansson PS: Have tried the straight on approach with having a polyatomic lattice and in one part only having one of the monoatomic sublattices present. This however do no generate results in agreement with analytical results for a simple 2D wire test system. Attached a plot of the test system.

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Re: [Kwant] wave function at each site
by Johnny Wu April 17, 2016

April 17, 2016

Dear Dr. Adel I could not compute len(sys.sites) since sys.sites is of the type "method". How should I change it? Thanks. Best, Johnny On 14 April 2016 at 18:44, Abbout Adel <abbout.adel(a)gmail.com> wrote: > Dear Johnny, > > if you want to get the index of a site in you system, you can do it as > follow: > > sys=make_system() > sys=sys.finalized() > > Positions=[sys.sites[i].pos for i in range(len(sys.sites))] # > list of all the positions of the sites > > > Since you know the form of you system, you can obtain the positions of > the sites at the edge. > for example if the site at (3,5) is at the edge you can find its index > straightforwardly: > > Index_i= Positions.index((3,5)) > > once you know the index of your site, you can do whatever you want. for > example delete that site: > > del sys[sys.sites[index_i]] > > > or you can pass it to a function which has site as an argument : > > function(sys.sites[index_i]) > > I hope that this helps > > Best regards, > Adel > www.abboutadelhomepage.com > > > >

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Wavefunction at each site
by Johnny Wu April 14, 2016

April 14, 2016

Dear Kwant users and developers, I have a question about plotting wavefunction. This question is based on http://kwant-project.org/doc/1.0/tutorial/tutorial6. If I want to print the amplitude of wavefunction at the A site / B site only at the edge, I suppose I have to get the index of the A/B atom at the edge first. Is there anyway by which we can get the index of them? Thank you very much. Best, Johnny

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