Mailman 3 August 2015 - 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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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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New instructions for Windows installation
by Michael Wimmer Jan. 15, 2016

Jan. 15, 2016

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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QZ Iteration fails
by Maximilian Trescher Nov. 9, 2015

Nov. 9, 2015

Dear kwant users and developers, this is my first post to this list, I'm a PhD student from Berlin. Currently I'm trying to implement a lattice model of a pyrochlore slab in kwant. I found a (more or less straightforward) way to introduce periodic boundary conditions in the system in the maillinglist archive. This works very nicely as long as I keep the periodicity along a lattice vector. But I run into problems when I choose different directions: Most of the time all solvers fail with a "LinAlgError: QZ iteration failed to converge in zgges". I know about this post http://comments.gmane.org/gmane.comp.science.kwant.user/372 but in my case it occurs not only for a band bottom, but for a range of energies. To give you all the details I prepared a Ipython notebook that you can view here: http://nbviewer.ipython.org/gist/faucon/729050da5e3f2eae1bb8# (Note that the inclusion of complex next-nearest-neighbor hopping is crucial to produce the error, I tried to create the shortest example which reproduces my problem). Do you have any idea how to circumvent this problem? Is it somehow related to http://netlib.org/lapack/Errata/vrac/lapack_known_issues.html#QZ this problem in Lapack? I would be thankful for any hint, Best, Maximilian

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Package for ubuntu vivid
by David Abergel Sept. 17, 2015

Sept. 17, 2015

Hi all, firstly, thanks so much for your work in developing and maintaining kwant. Hugely appreciated. Is there any chance you could package it for {,k,x}ubuntu 15.04? I tried installing kwant in that linux today, using the 14.10 package, and I am getting a bunch of errors from Tkinter and matplotlib when I use kwant's inbuilt plotter. Many thanks for your help David

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Carbon nanotube hoppings
by LaGasse, Samuel Sept. 5, 2015

Sept. 5, 2015

Hi everyone, I am trying to implement a carbon nanotube lattice inside of Kwant and I am not sure if my approach is correct. The CNT is slightly different, I think, than the other examples of lattices I have seen implemented in Kwant. My approach, which I have used in the past in my own NEGF code, is to generate the atoms of a single unit cell (just a ring for a (10, 0) tube) and then translate them using the translation vector for the ring. For a (10, 0) tube oriented along the z-direction the vector would be approximately (0, 0, 4.26) Angstrom. So, we have a 3D structure, but really only one lattice translation vector. The problem I am observing in my implementation of this in Kwant is that the Polyatomic lattice code seems to expect three primitive vectors for a 3D structure. In the case of the CNT, this does not really apply. When I run my code (an example for the (10, 0) tube is pasted below) I get the following error when I try to get the 1st nearest neighbors of the lattice: File "C:\Python27\lib\site-packages\kwant\lattice.py", line 282, in neighbors shortest_hopping = sls[0].n_closest(sls[0](*([0] * sls[0].dim)).pos, File " C:\Python27\lib\site-packages\kwant\builder.py", line 155, in __call__ return Site(self, tag) File "C:\Python27\lib\site-packages\kwant\builder.py", line 71, in __new__ raise t(msg.format(repr(tag), repr(family), v)) ValueError: Tag (0, 0, 0) is not allowed for site family kwant.lattice.Monatomic([[0.0, 0.0, 4.26]], [3.86624196507323, 0.6123525697973589, -0.355], '0'): Dimensionality mismatch. Does anyone have any advice for how I might get around this? With shape functions I am able to successfully translate the CNT to create/plot a system inside of Kwant- the only issue I have is with defining hoppings right now. I'm thinking that it might be possible to manually define the hoppings inside of the unit cell, but I am not exactly sure how to go about it for a problem of this size. Hopefully the way I've described my problem makes some sense! I am really hoping to get this working in Kwant- my NEGF code served my purposes for a while, but it is really quite terrible compared to Kwant, haha. ##Code snip: import kwant t_vec = [(0,0,4.26)] #(10,0) translation vector #unit cell coordinates for (10, 0) coords = [(3.86624196507323, 0.6123525697973589, -0.355), (3.86624196507323, -0.6123525697973589, 0.355), (3.48778726384012, 1.7771163733490807, 1.775), (3.86624196507323, 0.6123525697973589, 2.485), (2.7679236443795503, 2.76792364437955, -0.355), (3.48778726384012, 1.7771163733490807, 0.355), (1.7771163733490811, 3.4877872638401195, 1.775), (2.7679236443795503, 2.76792364437955, 2.485), (0.6123525697973591, 3.86624196507323, -0.355), (1.7771163733490811, 3.4877872638401195, 0.355), (-0.6123525697973586, 3.86624196507323, 1.775), (0.6123525697973591, 3.86624196507323, 2.485), (-1.7771163733490805, 3.48778726384012, -0.355), (-0.6123525697973586, 3.86624196507323, 0.355), (-2.76792364437955, 2.7679236443795503, 1.775), (-1.7771163733490805, 3.48778726384012, 2.485), (-3.4877872638401195, 1.7771163733490811, -0.355), (-2.76792364437955, 2.7679236443795503, 0.355), (-3.8662419650732294, 0.6123525697973593, 1.775), (-3.4877872638401195, 1.7771163733490811, 2.485), (-3.86624196507323, -0.6123525697973583, -0.355), (-3.8662419650732294, 0.6123525697973593, 0.355), (-3.48778726384012, -1.7771163733490802, 1.775), (-3.86624196507323, -0.6123525697973583, 2.485), (-2.7679236443795507, -2.76792364437955, -0.355), (-3.48778726384012, -1.7771163733490802, 0.355), (-1.7771163733490813, -3.4877872638401195, 1.775), (-2.7679236443795507, -2.76792364437955, 2.485), (-0.6123525697973595, -3.8662419650732294, -0.355), (-1.7771163733490813, -3.4877872638401195, 0.355), (0.6123525697973581, -3.86624196507323, 1.775), (-0.6123525697973595, -3.8662419650732294, 2.485), (1.7771163733490802, -3.4877872638401204, -0.355), (0.6123525697973581, -3.86624196507323, 0.355), (2.7679236443795494, -2.7679236443795507, 1.775), (1.7771163733490802, -3.4877872638401204, 2.485), (3.4877872638401195, -1.7771163733490816, -0.355), (2.7679236443795494, -2.7679236443795507, 0.355), (3.8662419650732294, -0.6123525697973597, 1.775), (3.4877872638401195, -1.7771163733490816, 2.485)] nanotube = kwant.lattice.Polyatomic(t_vec,coords) print nanotube.neighbors(1) #this is the line which gives an error ##End code snip Any assistance that anyone could offer would be hugely appreciated! Thanks very much, Sam LaGasse

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Re: [Kwant] Spin Currents using Greens Functions
by Joseph Weston Sept. 1, 2015

Sept. 1, 2015

This time *with* the recipe attached Joe

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Honeycomb HE
by james edward hernandez Aug. 27, 2015

Aug. 27, 2015

Hi all. I would like to know how to compute Hall conductance in a honeycomb lattice using Kwant. In my attempt, I started with the construction of the honeycomb lattice: import kwantimport matplotlib.pyplot as pltimport numpyfrom cmath import expfrom kwant.digest import gauss lat = kwant.lattice.honeycomb()sys = kwant.Builder() def rectangle(pos): x, y = pos return 0 < x < 40 and 0 < y < 20 sys[lat.shape(rectangle, (1, 1))] = 0sys[lat.neighbors()] = -1 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)]) lead = kwant.Builder(sym) def lead_shape(pos): x, y = pos return 0 < y < 20 lead[lat.shape(lead_shape, (-1,1))] = 4lead[lat.neighbors()] = -1 sys.attach_lead(lead)sys.attach_lead(lead.reversed()) My questions are: How to attach another pair of leads at 13 < x < 15 and 23 < x < 25 with y = 10 and y =0? I tried # For the 2nd Lead sym2 = kwant.TranslationalSymmetry((-1, 0)) sym2.add_site_family(lat.sublattices[0], other_vectors=[(-1, 2)]) sym2.add_site_family(lat.sublattices[1], other_vectors=[(-1, 2)]) lead2 = kwant.Builder(sym) def lead2_shape(pos2): x, y = pos2 return 13 < x < 15, 10 < y < 10 lead[lat.shape(lead2_shape, (1,1))] = 4 lead[lat.neighbors()] = -1 sys.attach_lead(lead2) sys.attach_lead(lead2.reversed()) but nothing works. Also How to calculate hall conductance from these leads (assuming hopping is by Peierls substitution where t is replaced by an exponential)? I am very new to the kwant program, as this is for an undergraduate research.Regards, James

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Conductance in a 3D nanowire
by Bahareh Goldozian Aug. 26, 2015

Aug. 26, 2015

Hello, I have tried to find the conductance for a three dimensional nanowire, I got some current for the zero energy however the current goes to zero when the energy is "-2t" How can I fix this problem? Thank you! Bahareh

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Discussing kwant development
by Christoph Groth Aug. 19, 2015

Aug. 19, 2015

Dear all, We would like to make it easier for the community to see what is going on in terms of Kwant development. We hope that this will provide useful feedback and lower the barrier for participation. As a first step, we have created a new mailing list called “kwant-devel”. The new list will be used to discuss technical design and implementation issues of Kwant. The old list “kwant-discuss” will remain the place for general exchange about Kwant, Q&A, and bug reports. Over the next days we plan to discuss the design of an improved low-level system format for “kwant 2” on the new list. This new format should make the following possible: • Finalizing builders with more than a single translational symmetry vector. The additional symmetries could be used for periodic boundary conditions or multi-dimensional band structure calculations for example. • Better handling of parameters. Value functions should no longer be required to share the same parameters, and it should be possible to separately query the parts of the Hamiltonian that depend on a particular parameter. • Improved performance through vectorization of value functions. Low-level systems are at the core of Kwant. We would like to redesign them in a solid way and appreciate any feedback. If you would like to participate in the discussion (or just follow it), please subscribe to the “kwant-devel” at http://kwant-project.org/community. Everything works the same way as for the other lists (except that Gmane and Mail-archive do not just yet carry the new list). We hope to see many of you on kwant-devel. Christoph on behalf of the Kwant team

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