Mailman 3 December 2015 - Kwant-discuss

How the spin is positioned in the Smatrix?
by Qingtian Zhang 06 Mar '22

06 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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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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New instructions for Windows installation
by Michael Wimmer 15 Jan '16

15 Jan '16

Dear kwant users, we have posted new instructions for installing kwant on Windows on the kwant website: http://www.kwant-project.org/install The old instructions do not work any more, as the format of the python packages on Christoph Gohlke's webpage have changed. Best regards, the kwant team

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Installing Kwant on Windows
by Jerzy Wrobel 04 Jan '16

04 Jan '16

Hi All, The command, which is suggested on Kwant webpage pip3 install --no-deps *.whl generates (python 3.4 and win7) the following error message: Requirement '*.whl' looks like a filename, but the file does not exist *.whl is not a valid filename. Are there any other possibilities to install a bunch of packages with a single command on Windows? Thanks in advance, Jerzy

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How resource dependent is Kwant?
by siddheshwar chopra 03 Jan '16

03 Jan '16

Dear Users, Can anybody tell me HOW resource intensive is KWant? I mean, as compared to DFT calculations. For DFT, computer has to be quite powerful. Any suggestion or explanation is welcomed. Especially in terms of number of atoms. Regards, -- *Dr. Siddheshwar chopra,* *M.Sc., Ph.D (Physics)Assistant Professor (Physics),* *Amity University, Noida, India.*

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Hall effect (4-contacts measurement)
by Cabosart Damien 03 Jan '16

03 Jan '16

Dear all, I read some advices about the Hall effect (4-leads measurement) in previous posts referring to the code on the website: http://nbviewer.ipython.org/github/topocm/topocm_content/blob /master/w3_pump_QHE/Laughlinargument.ipynb However, I'm wondering if the following code is well correct in order to get the potential of each lead when a current I=1 is "imposed" between the contacts 4 and 5. r = np.linalg.inv(G) V = r.dot(np.array([0, 0, 0, 1, -1])) where G is the conductance matrix ( G[i,j] gives the conductance between lead i and j) and hence, r is the resistance matrix. V[0] is the potential of the lead 0 V[1] is the potential of the lead 1 V[2] is the potential of the lead 3 This would mean that, for example, V[0] = r[3,0] - r[4,0] However, is it more correct to write V[0] = r[3,0] - r[0,4] ? Indeed, r[0,4] is not equal to r[4,0] when we apply a magnetic field (the time reversal symmetry is broken). I think it is better to use r[0,4] in order to set a current flowing from lead 4 to lead 5. Am I right? Thank you in advance for your help. Best regards, D. Cabosart

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kwant and Anaconda distribution
by Jessica Alfonsi 27 Dec '15

27 Dec '15

Hi all, I have a more Python related question, however since it's related with Kwant installation I hope you can help me. I have just installed the latest Kwant update (using python 3) via apt-get on Ubuntu 14.04 using the ppa. However, since I'd like to have the latest scipy, numpy matplotlib etc. packages, I also installed Anaconda 3 in my home directory: my bashrc is set with Anaconda path. I'd like to use the latest jupyter/ipython shell with kwant, but when I try to import kwant it gives me an error probably because of Python path issues. I'm not an expert with Python, so I'd like to know how I should import the kwant module which is installed system-wide. The other way could be obviously to install kwant from conda repositories but it looks like they're not updated yet to the latest version. Thank you in advance, Jessica

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Plot eigenvectors of a closed system2
by Weiyuan Tong 23 Dec '15

23 Dec '15

Dear all, For the plot eigenvectors of a closed system in Tutorial 2.7.1. def plot_data(sys, n): import scipy.linalg as la sys = sys.finalized() ham = sys.hamiltonian_submatrix() evecs = la.eigh(ham)[1] wf = abs(evecs[:, n])**2 .... What is the value for n, how to choose the appropriate one? I have tried different values and it gives completely different figures. Thanks in advance Weiyuan

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Re: [Kwant] Kwant-discuss Digest, Vol 23, Issue 11
by Lee Kwok-Long 22 Dec '15

22 Dec '15

Dear Joe, Can we use this method to calculate spin currents between two leads for a three dimensional system? Spin is implemented by 2x2 matrix elements for each site, but other couplings (ie. couplings between layers) are also included. For example, we have a bilayer graphene; for each layer, we have 2x2 matrix elements for each site, but we also have a coupling between the two layers. Can we use this method for this system? Best wishes, Kwok-Long Lee On Sat, Jul 18, 2015 at 8:02 PM, <kwant-discuss-request(a)kwant-project.org> wrote: > Send Kwant-discuss mailing list submissions to > kwant-discuss(a)kwant-project.org > > To subscribe or unsubscribe via the World Wide Web, visit > https://mailman-mail5.webfaction.com/listinfo/kwant-discuss > or, via email, send a message with subject or body 'help' to > kwant-discuss-request(a)kwant-project.org > > You can reach the person managing the list at > kwant-discuss-owner(a)kwant-project.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Kwant-discuss digest..." > > > Today's Topics: > > 1. Spin Currents using Greens Functions (Joseph Weston) > 2. Re: Spin Currents using Greens Functions (Joseph Weston) > 3. Re: Spin Currents using Greens Functions (Anton Akhmerov) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Fri, 17 Jul 2015 19:42:01 +0200 > From: Joseph Weston <joseph.weston(a)cea.fr> > To: kwant-discuss(a)kwant-project.org > Subject: [Kwant] Spin Currents using Greens Functions > Message-ID: <55A93E69.80602(a)cea.fr> > Content-Type: text/plain; charset="utf-8" > > Dear all, > > I have noticed that several people have posted on the mailing list > asking how to calculate spin currents between two leads when an orbital > representation is used for the spin degree of freedom (i.e. spin > is implemented by 2x2 matrix elements for each site, as opposed to > a separate lattice for each spin). > > The arbitrary choice of spin quantization axis in the leads, in the case > where the lead Hamiltonian is spin-rotation invariant, renders direct > use of the scattering matrix cumbersome. The attached recipe > (spin_conductance.py) calculates the spin current aligned along the α > direction using the Greens functions and the Landauer formula: > > G_{pq} = (e/h) Tr[ σ_{α} Γ_{q} G_{qp} Γ_{p} G^+_{qp} ] > > where Γ_{q} is the coupling matrix to lead q ( = i[Σ - Σ^+] ) > and G_{qp} is the submatrix of the system green's function > connecting sites which interfaces to leads q and p, σ_{α} > is the pauli matrix along direction α and Tr denotes the trace. > > This was discussed in a non-mailing-list email thread with > Branislav Nikolic, Xavier Waintal and Christoph Groth but I > thought it would be useful to post the recipe here. The above relation > has been derived in reference [1]. > > > Any thoughts/discussion welcome, > > Joseph Weston > > [1]: http://dx.doi.org/10.1103/PhysRevB.89.195418 > >

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Kwant 1.2 has been released
by Christoph Groth 19 Dec '15

19 Dec '15

Hello, We are happy to announce the release of Kwant 1.2 [1]. Kwant 1.2 is identical to Kwant 1.1 except that it has been updated to run on Python 3.4 and above. Bugfix releases for the 1.1 and 1.2 series will mirror each other, i.e. 1.1.3 and 1.2.3 will fix the same bugs. Starting with Kwant 1.2, all Kwant development will target Python 3. We plan, however, to maintain Python 2 support with the 1.1 series for several years. At this moment, Kwant 1.2 packages are available [2] for: Debian, Ubuntu & Windows. Arch Linux and Mac are not ready yet, but will hopefully follow soon. Please do report any problems (especially with installation) that you may encounter with the new version. Thanks to everyone who contributed to this release! Happy kwanting, Christoph on behalf of the Kwant team [1] http://kwant-project.org/doc/1/pre/whatsnew/1.2 [2] http://kwant-project.org/install

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