Mailman 3 June 2017 - Kwant-discuss

Gate-defined Nanowire Quantum Dot
by Jonathan Fields Sept. 15, 2022

Sept. 15, 2022

Dear Reader, I am new to KWANT and trying to figure out if it is suitable for calculating transport properties of gate-defined quantum dots made out of nanowires, along the lines of the structure used in https://arxiv.org/abs/cond-mat/0609463. Now, I understand that the mailing list is not a place to ask people to do my work for me, and my question is also not for someone to do this. What I am looking for is some insights into if this is possible (and not all that difficult) with the package. At first sight (and going through the tutorials as well as the APS March Meeting material) this does not seem straightforward; what I struggle with is how to define this type of dot in KWANT. Now, one way would of course be to built the entire 3D geometry of the system, but this will be computationally expensive, and probably also rather tricky to do in terms of defining everything, from the hexagonal wire itself to all of the gates and oxide layers and such. Some abstraction would be preferred, perhaps even reducing the dimensionality and making a 2D system, such as often done in the tutorial. Is this a good idea? My problem is that if one does this, then I run into the problem of how to model the 'half wrap-around' type gates typically used in these devices. In the transport through barrier section of the APS March Meeting material there is a section on how to model realistic potentials, but this would not work all that well for these wrap around type gates. On the other hand, in a way they are perhaps not so different from the QPC in that section. One could model the three gates as something like https://i.imgur.com/WZC983c.png as they do essentially form channels in the wire. Apart from if this sacrifices too much of the nanowire nature in favor of a 2DEG system, I do suppose such a system should in principle be able to be treated as a quantum dot. I haven't been able to confirm this as I am not yet sure how one can apply a source-drain voltage in KWANT; I first thought that this would probably be related to the energy of the modes, but then again I have not been able to produce Coulomb diamonds in this way. The question is getting rather lenghty, and also a bit unclear at this point. Perhaps I should finish up by stating it in a concise form; would you think that it is possible to simulate the transport of such a gate defined nanowire quantum dot device with KWANT, and if so, is the approach I am suggesting above a viable one, or would you go about it very differently? Kind regards Jonathan <http://aka.ms/weboutlook>

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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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lead_info
by Prof. CHAN Kwok Sum Sept. 20, 2017

Sept. 20, 2017

Dear Kwant developer, Thank you for your effort in developing Kwant. I have a question about the definition of the direction of momentum in the smatrix.lead_info. Assume a lead lies along the y direction with primitive vector (0,1). The symmetry operation of a lead is (0,1) if the lead goes towards the positive direction and is (0,-1) if the lead goes towards the negative direction. For the momenta contained in smatrix.lead_info for these two leads , do they use the same coordinate system? Positive momenta mean the direction (0,1), negative momenta mean the direction (0,-1). For velocity, in smatrix.lead_info, it has nothing to do with the coordinate system. Positive means outgoing, negative means incoming in the lead. Thank you KS Chan Disclaimer: This email (including any attachments) is for the use of the intended recipient only and may contain confidential information and/or copyright material. If you are not the intended recipient, please notify the sender immediately and delete this email and all copies from your system. Any unauthorized use, disclosure, reproduction, copying, distribution, or other form of unauthorized dissemination of the contents is expressly prohibited.

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distinguishing bands belonging to different orbitals
by Tibor Sekera June 30, 2017

June 30, 2017

Hi all! I would like to plot bandstructure of a graphene lead with electron and hole structure (i.e. norbs=2), while distinguishing (e.g. with color) between electron and hole bands. The lattice is defined as lat = kwant.lattice.general([(sqrt(3)*1/2, 1/2), (0, 1)], [(0, 0), (1/(2*sqrt(3)),1/2)], norbs=2) the lead is created as lead0 = kwant.Builder(sym_lead) lead0[a.shape(lead0_shape,(0,0))] = -EF * tau_z lead0[b.shape(lead0_shape,(0,0))] = -EF * tau_z lead0[lat.neighbors()] = -t * tau_z Then the bandstructure is obtained as bands = kwant.physics.Bands(lead0.finalized()) momenta = numpy.linspace(-pi, pi, 201) energies = [bands(k) for k in momenta] Is there an easy way to get the bands just for electrons? Something like "bands = kwant.physics.Bands(lead0.finalized(), orbital = 0)" or for holes "bands = kwant.physics.Bands(lead0.finalized(), orbital = 1)" Thanks, Tibor Sekera

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Feedback on Kwant FAQ
by Kwant Authors June 29, 2017

June 29, 2017

Dear Kwant users, The Kwant team in Grenoble has recently hosted a masters student, Paul Clisson, who has taken the initiative and written a Kwant "FAQ" that is meant to complement the existing Kwant tutorial and documentation. The aim of this document is to be very explicit in explaining common pitfalls and misunderstandings that people new (and not so new!) to Kwant make. As Paul was new to Kwant at the start of his internship, he was in a good position to identify what was not clear and what could be explained in more detail. You can see a preliminary version of the FAQ on a test deployment of the Kwant website [1]. At this stage we are eager to collect feedback to see what could be improved. You can either reply by email in this thread, or join the discussion on the open merge request on the Kwant gitlab [2]. Happy Kwanting, Kwant team [1]: https://test.kwant-project.org/doc/doc-faq/tutorial/FAQ [2]: https://gitlab.kwant-project.org/kwant/kwant/merge_requests/148

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Documentation about scattering theory
by Татьяна Матусевич June 28, 2017

June 28, 2017

Do you have any additional documentation about scattering theory besides official documentation on your site? -- Tatsiana Matusevich

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Multiple CPUs when calculating S-matrix
by Sverre Gulbrandsen June 27, 2017

June 27, 2017

Hi, When I calculate the scattering matrix (kwant.smatrix()) it seems that only one CPU (I have 12 in total) is working, while the others are idle. Is there a (simple) way to make all the CPUs work together when using kwant.smatrix ? Maybe similar improvements can be done for kwant.wave_function ? I am using Ubuntu 16.10. I know this has been discussed before a long time ago, but I was wondering if there was some progress on this, now that Kwant 1.3 is released. All the best, Sverre Gulbrandsen

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Fw: Kwant-graphene adatom
by narjes khosravian June 22, 2017

June 22, 2017

Dear Kwant user, I am beginner at using kwant and I need to calculate DOS of graphene adatom. I was wondering whether these is possibility to set the potential function for this system? Best regards, Narjes

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