Mailman 3 November 2020 - Kwant-discuss

Realistic values of hopping parameters
by Yu Li 19 Oct '23

19 Oct '23

Dear all, Now I’m studying some transport property based on multilayered system, with nonmagnetic layer/ferromagnetic layer (NM/FM) stack. May I ask when creating a tight binding system, and setting hoppings by the following function: syst[(lat(1, 0), lat(1, 1))] = t, how to associate the hopping with a realistic system? Such as hoppings between FM and FM atoms, or between FM and NM atoms? I found in many literatures people just simply put t=1, but I assume the value should depend on the type of atoms. My another question is, is it possible to define more than one type of hopping between two sites? For the tight-binding approximation of p electrons, there are two types of hopping integrals: Vppσ, and Vppπ, so I’m wondering if it’s possible to simulate the effect of both hoppings? Thanks a lot for reading my questions! Cheers, Yu Li

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Gate-defined Nanowire Quantum Dot
by Jonathan Fields 15 Sep '22

15 Sep '22

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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current-voltage diagram
by loojoo026＠gmail.com 08 Apr '22

08 Apr '22

dear all I have done my project using Kwant and I have drawn a conductance-energy diagram for the system. The question I have is how can I get the current-voltage diagram? thanks Leo

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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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Spin polarized transport calculation
by L.M J 06 Mar '22

06 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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To to separate spins, electron and hole
by Khani Hosein 22 Jun '21

22 Jun '21

Dear all, I am using Kwant to study a system with both spins and electron and hole, so the hopping matrix is 4X4. Now I want to know how to separate the spins, electron and hole. I study this through the "2.6. Superconductors: orbital degrees of freedom, conservation laws and symmetries" smatrix = kwant.smatrix(syst, energy) data.append(smatrix.submatrix((0, 0), (0, 0)).shape[0] -smatrix.transmission((0, 0), (0, 0)) +smatrix.transmission((0, 1), (0, 0))) I do not find more information about this for kwant. For my system, if I have 3 leads, and the hopping and onsite energy is set in this order: (e↑,0,0,0)- spin up electron,first row of the 4X4matrix (0,e↓,0,0)- spin down electron,second row of the 4X4matrix (0,0, h↑ ,0)- spin up hole,third row of the 4X4matrix (0,0, 0, h ↓ )- spin down hole,fourth row of the 4X4matrix I want to obtain the transmissions from 0 →2. smatrix = kwant.smatrix(syst, energy) The transmission from h↑ to e↑ is: smatrix.transmission((2, 1), (0, 2)) The transmission from h ↓ to e↑ is: smatrix.transmission((2, 1), (0, 3)) Is my understanding correct? Thanks very much in advance! Hosein Khani

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Online Kwant tutorial session
by Xavier Waintal 27 Nov '20

27 Nov '20

Dear colleagues, We are organizing a 4 hour tutorial session where we will introduce the basic concepts of quantum transport and how to use Kwant to make practical calculations. This mini-workshop is aimed at experimental or theoretical physicists who want to learn how to simulate coherent nanoelectronics devices The hands-on sessions will cover several examples including graphene devices, quantum Hall effect or devices with superconducting contacts. The workshop will take place online on December 17th, 2020 from 15:00 to 19:00 CET (9:00 to 13:00 ET). More information and (free but mandatory) registration can be found at https://virtualscienceforum.org/#/quantum-transport-workshop We would be very grateful if you could forward this email to people potentially interested in this tutorial. Also, we are putting together a team of people to help supervising the hands-on sessions. Please join us if you can. You can let us know that you're interested in helping us introduce new users to Kwant by simply replying to this list (or drop us a note at authors(a)kwant-project.org <mailto:authors@kwant-project.org>). Yours sincerely, Anton, Christoph, Michael and Xavier

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Tkwant issue
by Olivier Leblanc 25 Nov '20

25 Nov '20

Answer below : -----Message d'origine----- De : Christoph Groth <christoph.groth(a)cea.fr<mailto:christoph.groth@cea.fr>> Envoyé : mercredi 25 novembre 2020 20:21 À : Olivier Leblanc <o.leblanc(a)uclouvain.be<mailto:o.leblanc@uclouvain.be>> Cc : kwant-discuss(a)kwant-project.org<mailto:kwant-discuss@kwant-project.org> Objet : Re: [Kwant] tkwant issue solved Hello Olivier, We are aware that tkwant is not available for Python 3.8 on conda-forge. Unfortunately, as far as I understand, the underlying problem is beyond our control. Olivier Leblanc wrote: > My problem was that even when activating the virtual environment in > cmd, launching "jupyter notebook" did not use the virtual environment > because jupyter notebook was not part of it. So, in other words, you installed tkwant in Python environment A and then you were surprised not to find it in Python environment B. Perhaps this can be seen as a design issue of Python virtual environments, or of Jupyter, but it's certainly not a problem specific to Tkwant or Kwant. No, I installed tkwant in Python environment B, and launching jupyter notebook with environment B activated did not really use environment B but was keeping the environment A. Thus tkwant was not found. This is indeed a problem from python, but adding the last line I suggest wouldn't change anything for users that don't obtain the issue, and would solve the one I had because of Python and Jupyter notebook. > Hoping this will be added on your website. In my opinion listing possible generic problems that one may encounter when using Tkwant is beyond the scope of our web site. I think that this would render the instructions less useful by overloading them. This means I do not ask you to make a generic problems listing, but simply add a command line in your installation tutorial. Cheers Christoph Thank you, Olivier

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tkwant issue solved
by Olivier Leblanc 25 Nov '20

25 Nov '20

Hello, I am a new PhD student at UCLouvain (Belgium). Trying to install tkwant, I lost a lot of time with an issue I did not understand. For windows users : I suggest you to add "pip install jupyter" As : conda create -n env-tkwant python=3.7 conda activate env-tkwant conda install -c intel mpi4py conda install tkwant -c conda-forge pip install jupyter My problem was that even when activating the virtual environment in cmd, launching "jupyter notebook" did not use the virtual environment because jupyter notebook was not part of it. Thus the python version was kept to 3.8 (uncompatible with the current tkwant package), and I was receiving a "module not found error". Hoping this will be added on your website. Kind regards, Olivier Leblanc

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extracting scattering states momenta with Kwant
by Vardan Kaladzhyan 17 Nov '20

17 Nov '20

Dear Kwant developers, I was wondering if there were any examples of extracting the momenta of the scattering states? I have found several functions in the documentation that seem to cover what I would like to do, however, I have not managed to implement them, unfortunately. To be more specific, my problem is such that there are 'bands' in the left lead, and I would like to see how many scattering states (+their momenta) Kwant finds at a given value of the chemical potential in the lead. Thank you in advance for your help! Cheers, Vardan.

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how to plot the Charge density with multiple orbitals per site
by Xiangru Kong 13 Nov '20

13 Nov '20

Hi all, I want to plot the the Charge density using kwant.plot method, I want to know how to plot the Charge density in a system with multiple orbitals per site. Here is some pesuo codes: ..... syst = kwant.Builder() ..... model= kwant.wraparound.wraparound(syst).finalized() ham=model.hamiltonian_submatrix (eigen,evecs) = la.eigh(ham) def site_size(i): return 2 * wf[i] / wf.max() kwant.plot(model, site_size=site_size, site_color=(0, 0, 1, 0.3),fig_size=(10,10)); ********* In my codes, I just found len(model.sites)=6, that is the number of sites in the unit cell, however, due to there are multiple orbitals per site, the len(evecs[:,i])=22 orbitals. So I want to know the real infortion when using kwant.plot? Like what I just posted, it only plot evecs[:6,i]**2 functions? Thanks Xiangru

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questions for wraparound in Kwant
by dae han Park 10 Nov '20

10 Nov '20

Dear all I want to draw a contour plot of the first Brillouin zone and band structure along the path of high symmetry points by using Kwant. In Kwant, I know that "kwant.wraparound.plot_2d_bands" function shows the Brillouin zone in 3D. I read a kwant official documentary to find the information that I want, but I couldn't find the solution. My questions are as follows; 1) are there functions in kwant to draw 2D contour plot of the Brillouin zone and the band structure along the path of high symmetry points? 2) If it is not, could I extract data shown from kwant.wraparound.plot_2d_bands? Thank you for reading my email. Sincerely, Daehan Park. ===================================== Department of Physics, Soongsil University, 369, Sangdo-ro, Dongjak-gu, Seoul 156-743, Republic of Korea (South Korea) mail: m31phy(a)gmail.com ===================================== ᐧ

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