Mailman 3 January 2017 - 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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Using Kwant for finding tunneling probability from metal to vacuum
by Kristjan Eimre Feb. 14, 2017

Feb. 14, 2017

Hello Kwant experts, I have studied Kwant a bit, but before I delve deeper into it, I want to know if it can be applied to my problem. I have a 3d electron potential landscape for a metal-vacuum surface calculated with DFT. (More detailed info and pictures in [1]). Can I use Kwant to calculate the transmission probability of electrons incident on the surface? And if kwant is not suitable for this problem, perhaps you know of any other numerical software or method that would allow this calculation? [1] - http://physics.stackexchange.com/questions/295329/quantum-tunneling-probabi… Best regards, Kristjan

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Re: [Kwant] How to attach the leads manually?
by Татьяна Григоренко Jan. 31, 2017

Jan. 31, 2017

TranslationalSymmetry does not work with my SimpleSite Family. Therefore, I attach the leads manually. I Use class kwant.builder.BuilderLead(builder, interface). I define a builder in the following way: #The tight-binding system of a left lead. left_lead = kwant.Builder() #Add sites lead left_lead[lat(x1, y1)] = 1.5 left_lead[lat(x2, y2)] = 1.5 etc #Add hoppings left_lead[lat(x2, y2), lat(x1, y1)] = 2j etc The same way I define the interface(wich called left_sequence) but without the hoppings. l_lead = kwant.builder.BuilderLead(left_lead, left_sequence). But it does not work. Where I'm wrong? As the lead I want to use the left and right boundary of my graphene atoms. С уважением, Татьяна Григоренко

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Regarding smatrix and spin
by Camilla Espedal Jan. 24, 2017

Jan. 24, 2017

Hi again, This question is basically the same as this: https://www.mail-archive.com/kwant-discuss@kwant-project.org/msg00076.html I want to calculate some things using the scattering matrix. I started out with a very simple system, most basic two-terminal system. For some energy there is one propagating mode. I now add matrix structure to the mix (just multiply by s_0 everywhere) and there are now 2 propagating modes (which makes sense). Now, if I look at the reflection coefficients for lead 0 by using submatrix(0,0), it is now a 2x2 matrix after I introduced the matrices. How are the elements ordered? Is it [[r_upup, r_updown],[r_downup, r_downdown]] I know that I could make two lattices, but since I do not plan to use the other functions such as transmission. I just want the smatrix. Hope you can help me, and thanks in advance. Best regards, Camilla

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Regarding tranlational invariance along 2 directions
by ANANT VIJAY Jan. 24, 2017

Jan. 24, 2017

Hello everyone, I am writing this to ask about how can I finalize the sytem with translational invariance along two directions. Code that I am using is the following as I want to make a slab system for band structure. sym = kwant.TranslationalSymmetry([1, 0, 0], [0, 1, 0]) Error: ValueError: Currently, only builders without or with a 1D translational symmetry can be finalized. Is there any solution to this? I also happen to get one code from notes on internet by edx course where thwe following code has been used using wraparound function. I did not get much out of it. Also, I checked I don't have this function in my kwant module. how can I get this functions module? Please anyone can explian what this function is doing in the code: def make_scatter_sys(): from functions import wraparound syst = wraparound(bhz(Z=1, system_type='slab')) syst.attach_lead(make_lead()) syst.attach_lead(wraparound(bhz(), keep=0)) syst = syst.finalized() syst.leads[0] = TRIInfiniteSystem(syst.leads[0], trs) return syst ANANT VIJAY VARMA Research Scholar IISER-KOLKATA WEST BENGAL.

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Installation instructions for Kwant on WSL
by Anton Akhmerov Jan. 23, 2017

Jan. 23, 2017

Hi everyone, Andrey Antipov kindly shared his installation instructions for Kwant using the Windows subsystem for Linux. If you are interested, please check it out over here: https://gist.github.com/aeantipov/3839d09529cb5dd2cedc00b76ccd2eb8 Best, Anton

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Re: [Kwant] Units of the wavefunction and velocity
by Harshad Sahasrabudhe Jan. 23, 2017

Jan. 23, 2017

Great, thanks a lot! On Mon, Jan 23, 2017 at 10:15 AM, Anton Akhmerov < anton.akhmerov+kd(a)gmail.com> wrote: > > Thank you. Then to calculate the velocity, should I just divide the > > probability current by the integral of |𝜓|2 over the unit cell? > > You could do that, but this is already done in Kwant, and you can read > the velocities off from the modes object [1]. > > Best, > Anton > > [1]: https://kwant-project.org/doc/1/reference/generated/kwant. > physics.PropagatingModes#kwant.physics.PropagatingModes >

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Units of the wavefunction and velocity
by Harshad Sahasrabudhe Jan. 23, 2017

Jan. 23, 2017

Hi, I was wondering what units would the wavefunction obtained from Kwant have? I was thinking they would have the units 1/sqrt(nm.eV) (since my energies are in eV and lengths are in nm) as the modes are normalized according to [image: Inline image 1] How would I calculate the velocity of the mode, if the modes are normalized to carry unit current? Thanks, Harshad

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the path in the Brillouin zone
by Weiyuan Tong Jan. 18, 2017

Jan. 18, 2017

Dear all, I used kwant to calculate the band structure of zigzag and armchair graphene nanoribbons. For zigzag graphene nanoribbons, we have sym= kwant.TranslationalSymmetry(lat.a.vec((-1, 0))). For armchair graphene nanoribbons, we have sym= kwant.TranslationalSymmetry(lat.a.vec((-1, 2))). My question is: which is the path in the Brillouin zone that we use and label the x-axis accordingly? My figure is attached. Thanks in advance! Weiyuan

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Re: [Kwant] Fwd: Regarding smatrix and spin
by Andrey Antipov Jan. 18, 2017

Jan. 18, 2017

Не вопрос, напишу мануал. On Tuesday, January 17, 2017 at 02:51 Anton Akhmerov <anton.akhmerov+kd(a)gmail.com> wrote: Привет, можешь девушке помочь? (В смысле поделиться на нашем списке рассылки как ты квант на WSL установл.) ---------- Forwarded message ---------- From: Anton Akhmerov Date: Tue, Jan 17, 2017 at 10:48 AM Subject: Re: [Kwant] Regarding smatrix and spin To: Camilla Espedal Cc: "kwant-discuss(a)kwant-project.org" Dear Camilla, It seems that you are trying to install Kwant on windows. This is a very hard task, and I fear none of the Kwant developers has enough knowledge of it right now (our Windows packages are built by Christoph Gohlke, see [1] for the build environment description). However if you are using windows 10, I suggest to try to install Kwant using the windows subsystem for linux. That way the standard Ubuntu build procedure should work for you. Best, Anton [1]: http://www.lfd.uci.edu/~gohlke/pythonlibs/ On Mon, Jan 16, 2017 at 9:45 AM, Camilla Espedal wrote: > Thanks a lot. I tried to install the cons_laws_combined, but I get the following error message: > > "LINK: fatal error LNK1181: cannot open input file 'lapack.lib'" > > Is there some package or installation I am missing? > > Best regards, > Camilla > > -----Original Message----- > From: anton.akhmerov(a)gmail.com [mailto:anton.akhmerov@gmail.com] On Behalf Of Anton Akhmerov > Sent: 8. januar 2017 16:35 > To: Tómas Örn Rosdahl > Cc: Camilla Espedal ; kwant-discuss(a)kwant-project.org > Subject: Re: [Kwant] Regarding smatrix and spin > > Hi Camilla, everyone, > > I've slightly modified Tómas's example to a case where the spins do get coupled, check it out: > http://nbviewer.jupyter.org/url/antonakhmerov.org/misc/spin_conductance.ipy… > > I've also provided more detailed installation instructions in the notebook. > > Cheers, > Anton > > On Sun, Jan 8, 2017 at 2:45 PM, Tómas Örn Rosdahl wrote: >> Dear Camilla, >> >> For a Hamiltonian with degeneracies due to a conservation law, the >> scattering states will in general not have a definite value of the >> conservation law. In your case, Kwant returns scattering states that >> are arbitrary linear combinations of spin up and down, so it is not >> possible to label the amplitudes in the scattering matrix by spin. >> >> However, in Kwant 1.3 a feature will be added that allows for the >> construction of scattering states with definite values of a >> conservation law. See here for an explanation of the basic idea behind the algorithm. >> >> We're currently working on implementing this feature in Kwant itself. >> The good news is that we're practically done - here is a link to a git >> repo with a functioning implementation. After you clone the repo, >> check out the branch cons_laws_combined, which contains a version of >> Kwant with conservation laws implemented. This notebook contains a >> simple example to illustrate how to work with conservation laws and the scattering matrix. >> >> I invite you and anyone else who is interested to give it a try. We'd >> appreciate any feedback! >> >> In your case specifically, there would be two projectors in the new >> implementation - P0 which projects out the spin up block, and P1 that >> projects out the spin down block. If they are specified in this order, >> then the spin up and down blocks in the Hamiltonian have block indices >> 0 and 1, respectively. In the new implementation, it is possible to >> ask for subblocks of the scattering matrix relating not only any two >> leads, but also any two conservation law blocks in any leads. To get >> the reflection amplitude of an incident spin up electron from lead 0 >> into an outgoing spin down electron in lead 0, you could simply do >> smat.submatrix((0, 1), (0, 0)). Here, the arguments are tuples of indices (lead index, block index). >> >> Best regards, >> Tómas >> >> On Fri, Jan 6, 2017 at 3:46 PM, Camilla Espedal >> >> wrote: >>> >>> Hi again, >>> >>> >>> >>> This question is basically the same as this: >>> https://www.mail-archive.com/kwant-discuss@kwant-project.org/msg00076 >>> .html >>> >>> >>> >>> I want to calculate some things using the scattering matrix. I >>> started out with a very simple system, most basic two-terminal >>> system. For some energy there is one propagating mode. I now add >>> matrix structure to the mix (just multiply by s_0 everywhere) and >>> there are now 2 propagating modes (which makes sense). >>> >>> >>> >>> Now, if I look at the reflection coefficients for lead 0 by using >>> submatrix(0,0), it is now a 2x2 matrix after I introduced the >>> matrices. How are the elements ordered? Is it >>> >>> >>> >>> [[r_upup, r_updown],[r_downup, r_downdown]] >>> >>> >>> >>> I know that I could make two lattices, but since I do not plan to use >>> the other functions such as transmission. I just want the smatrix. >>> >>> >>> >>> Hope you can help me, and thanks in advance. >>> >>> >>> >>> Best regards, >>> >>> Camilla >> >>

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Re: [Kwant] putting disorder (kwant.digest.uniform) in graphene with shape function
by Sudin Ganguly Jan. 14, 2017

Jan. 14, 2017

Dear Sir, That helps a lot. Thank you very much. With Regards, Sudin

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Re: [Kwant] putting disorder (kwant.digest.uniform) in graphene with shape function
by SUDIN GANGULY Jan. 13, 2017

Jan. 13, 2017

Dear Sir, Thank you for the quick reply. I think my institute mail is not working properly, so I used this mail-id. By using the line: lambda site: sys_2['sys'][site] I got another error massage, which says that, "TypeError: 'int' object is not callable". With regards, Sudin -- সুদিন

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putting disorder (kwant.digest.uniform) in graphene with shape function
by Sudin Ganguly Jan. 13, 2017

Jan. 13, 2017

Dear Sir, I'm trying to put disorder in graphene with shape function. I got some help from the link: http://nullege.com/codes/show/src@k@w@kwant-1.0.0@kwant@tests@test_comprehe… But when I want to see the onsite values using the comment below, kwant.plotter.plot(sys_2['sys'],site_color=lambda site: sys_2['sys'][site],colorbar = True, cmap='gist_heat') It says, "cannot convert argument to rgb sequence". Is there any way to see the onsite values through plot? Also it seems that all the onsite values have the same disorder. Can I make them different using kwant.digest.uniform()? Here is my code. #==================================== import kwant from matplotlib import pyplot def onsite(site, salt): return kwant.digest.uniform(repr(site), salt) - 0.5 def make_sys(l, w): def rect(pos): x, y = pos return 0 < x < l and 0 < y < w sys=kwant.Builder() lat = kwant.lattice.honeycomb() a,b= lat.sublattices sys[lat.shape(rect, (1, 1))] = onsite sys[lat.neighbors()] = -1 # left lead sym = kwant.TranslationalSymmetry((-1,0)) sym.add_site_family(lat.sublattices[0], other_vectors=[(-1, 2)]) sym.add_site_family(lat.sublattices[1], other_vectors=[(-1, 2)]) def lead_shape(pos): x, y = pos return 0 < y < w lead = kwant.Builder(sym) lead[lat.shape(lead_shape, (1, 1))] = 0 lead[lat.neighbors()] = -1 sys.attach_lead(lead) sys.attach_lead(lead.reversed()) return {'sys': sys } def main(): sys_2 = make_sys(l=10, w=23) kwant.plotter.plot(sys_2['sys'],site_color=lambda site: sys_2['sys'][site],colorbar = True, cmap='gist_heat') # Call the main function if the script gets executed (as opposed to imported). # See <http://docs.python.org/library/__main__.html>. if __name__ == '__main__': main() #============================================ With Regards, Sudin

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Current density
by Harshad Sahasrabudhe Jan. 11, 2017

Jan. 11, 2017

Hi All, I am trying to calculate current density from the wavefunctions using the following code: for i in range(Np): for j in range(Nc): lat_idx_i = i-floor(Np/2) lat_idx_j = j-floor(Nc/2) site_i = lat(lat_idx_i, lat_idx_j) idx_i = sys.sites.index(site_i) if i < Np-1: site_j = lat(lat_idx_i+1, lat_idx_j) idx_j = sys.sites.index(site_j) H_ij = sys.hamiltonian(idx_i, idx_j, V, peierls_phase_factor) current_density_bond_x[i][j] = -2 * (wf_orb[idx_i].conjugate() \ * H_ij * wf_orb[idx_j]).imag if j < Nc-1: site_j = lat(lat_idx_i, lat_idx_j+1) idx_j = sys.sites.index(site_j) H_ij = sys.hamiltonian(idx_i, idx_j, V, peierls_phase_factor) current_density_bond_y[i][j] = -2 * (wf_orb[idx_i].conjugate() \ * H_ij * wf_orb[idx_j]).imag However, this code snippet takes about one and a half hours to run. The total number of sites in the system is about 201000. Is there any other way to write the code so that it runs faster? THanks, Harshad

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STM tip as lead
by Eric Mascot Jan. 8, 2017

Jan. 8, 2017

Hello I was wondering, is it possible to have a lead that hovers above a 2D system? I'm guessing that I have to use a 3D system where the system is in the XY plane and the lead has a translational symmetry in the Z direction. Thanks, Eric

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problem with installing kwant on supercomputer
by Shizeng Lin Jan. 4, 2017

Jan. 4, 2017

Hi, I try to install kwant on a supercomputer in my institute. I do not have root access, so I built the package from source. I am using python/3.5-anaconda-4.1.1. I installed tinyarray and kwant as a user module, with the following command pip install --user tinyarray/ pip install --user kwant/ The installation completed successfully. But when I try to run the test python -c 'import kwant; kwant.test()' I got the following error message: ========================Error message========================= Traceback (most recent call last): File "/turquoise/users/shizeng/kwant/kwant/__init__.py", line 14, in <module> from . import _system ImportError: cannot import name '_system' During handling of the above exception, another exception occurred: Traceback (most recent call last): File "<string>", line 1, in <module> File "/turquoise/users/shizeng/kwant/kwant/__init__.py", line 22, in <module> raise ImportError(msg) ImportError: Error importing Kwant: You should not try to import Kwant from its source directory. Please exit the Kwant source distribution directory, and relaunch your Python intepreter from there. ==========================End================================= I followed the instruction to launch the test at different directory. but still got the same error message. I am appreciated if someone can help me to figure out the problem. Thanks. Shizeng

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On-site potential only at the edge of the sample in Kwant
by Dongwook Go Jan. 3, 2017

Jan. 3, 2017

Dear developers, I’m interested in investigating the influence of the edge potential of the sample. For example, in an example by tutorial “quantum_wire.py”, I would like to put on-site potential only at the edge atoms. One could determine whether the site is edge or not by reading the number of neighbors of the site by using “degree”. For example, if the degree is 3 there is an on-site potentials , else 0. Is there any example code from which I can learn to use this function? Best, Dongwook.

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