Mailman 3 May 2016 - Kwant-discuss

How the spin is positioned in the Smatrix?
by Qingtian Zhang 07 Mar '22

07 Mar '22

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 07 Mar '22

07 Mar '22

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 12 Sep '21

12 Sep '21

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 08 Jun '16

08 Jun '16

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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About excitation energy for LDOS calculation
by Anant vijay 30 May '16

30 May '16

Hello Everyone! There is given a parameter Excitation energy when we calculate scattering matrix or LDOS. Can anyone please give more details about this parameter.All I can understand is that it is the energy with which electron travel in scattering region. when we calculate LDOS we get the probability with which electron of that excitation energy can move in crystal. What is confusing that I have already applied voltage across the device. How still conductance changes when I change the value of Excitation energy. Is it calculating conductance for that energy level only or what? Information about how this value get processed would be helpful. Thank you

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slow code to setup hoppings
by Sergey Slizovskiy 26 May '16

26 May '16

Dear Colleagues, Imagine, I have two different 2D lattices in a heterostructure and want to set up interlayer hoppings between the close-by atoms. I solve this problem in the way presented below. This works, but to my surprise it works so slowly that it became a bottle-neck of my whole computation. Is there a way to improve the code below? (I suspect, it may be the tag comparison that slows down the thing, but I am not sure) def connect(sys,crit): for site1, site2 in it.product(sys.sites(), repeat=2): if crit(site1, site2): yield (site1, site2) def critInterlayer(site1,site2): ## gamma1 hopping integral diff = site1.pos-site2.pos [x,y] = diff if (abs(x)+abs(y)<0.1) and (site1.tag!=site2.tag): return True else: return False sys[connect(sys,critInterlayer)]=hopping Best wishes, Sergey

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regarding band structure of closed system (QD)
by anant vijay 23 May '16

23 May '16

Dear users, I am trying to calculate band structure of Quantum dot (simple sqaure lattice implementation). Since KWANT's bands() function needs infinite system to calculate band structure. Please share if anyone has a trick to calculate BS of quantum dot.

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scatter without a center area
by Chong Wang 17 May '16

17 May '16

Hi, I want to calculate conductance between two different leads. This is what I have tried: 1. build an empty center area. This does not work because kwant doesn't know how to attach leads to an empty area. 2. use lead_1.attach_lead(lead_2). Kwant.plot does not correctly plot two leads, so I suppose it doesn't work. I don't want to build a center area that is the same with one of the leads, because my system is very large and I suppose this will be time-consuming. Best! Chong Wang Ph. D. candidate Institute of Advanced Study, Tsinghua University, Beijing 100084

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Jupyter notebook
by Joseph Kulik 13 May '16

13 May '16

My question is not really about Kwant, but about how to install Jupyter. I have a 64-bit operating system with Windows 8. I have successfully installed Python 3.4. I want to install Jupyter notebook, but I am not sure what files I need from Cristoph Gohlke's website ( http://www.lfd.uci.edu/~gohlke/pythonlibs/). When I eventually get the required files, what is the installation procedure? Can any one advise? Thank you. -Joe -- Joseph Kulik

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PLEASE UNSUBSCRIBE FROM THIS FORUM
by siddheshwar chopra 11 May '16

11 May '16

Dear Developers, It is a humble request to unsubscribe me from this mailing list. I could not find a suitable way of doing it on the website. Regards,

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Kwant-discuss May 2016