Mailman 3 April 2022 - Kwant-discuss

To to separate spins, electron and hole
by Khani Hosein Oct. 11, 2024

Oct. 11, 2024

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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Realistic values of hopping parameters
by Yu Li Oct. 19, 2023

Oct. 19, 2023

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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Defining uniaxial strain in specified region of graphene
by shrushti.tapar07＠gmail.com Jan. 20, 2023

Jan. 20, 2023

I want to create the graphene strained superlattice-like structure, having uniaxial strain defined at the specified region. Please, someone can suggest to me how to define the strained region and interface between unstrained and strained graphene regions. Thanks in Advance Shrushti

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Re: How the spin is positioned in the Smatrix?
by Diego Bruno Sept. 27, 2022

Sept. 27, 2022

Can you disponibilize the code?

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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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Re: Problem in calculation of Current density
by Ajit Kumar Sahu April 11, 2022

April 11, 2022

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current-voltage diagram
by loojoo026＠gmail.com April 8, 2022

April 8, 2022

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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Problem in calculation of Current density
by sahu.ajit92＠gmail.com April 7, 2022

April 7, 2022

Dear all, I am new to kwant. I am trying to calculate the current density for different widths of zigzag graphene nanoribbon. But I am getting strange results for widths above 22 angstroms, there is slightly back propagation of current. I want to know, is there any mistake in my code or anything else to be considered for the calculation of current density. Here i am attaching the code with current density result for width 22 and 24 angstroms. In 24 angstroms, there is back propagation of current. Please help me. --------Code ---- import kwant import numpy as np from matplotlib import pyplot import matplotlib.pyplot as plt import numpy.linalg as npl import scipy.sparse.linalg as sla import os pi=np.pi ############################################## structure ######################## Wnr=22 lnr=180 ############# empty system kwant lat=kwant.lattice.honeycomb(a=2.46,norbs=1) a,b=lat.sublattices ########### functions for structure build def onsite(site, voltage): x, y = site.pos return voltage def rect(pos): x,y=pos return 0<x<=lnr and 0<y<=Wnr ################################ main structure i.e. rectangle #################### model1 = kwant.Builder() model1[lat.shape(rect,(1,1))] = 0 model1[lat.neighbors()] =2.64 model1.eradicate_dangling() kwant.plot(model1) ############### functions for lead structure build ################################ def lead1_shape(pos): x, y = pos return 0<y <=Wnr ############first lead codes########## sym = kwant.TranslationalSymmetry(lat.vec((-1,0))) #from the left 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) lead[lat.shape(lead1_shape, (0,1))]=onsite lead[lat.neighbors()] =2.64 #########attaching the lead to model1 model1.attach_lead(lead) model1.attach_lead(lead.reversed()) #second lead attach in reverse manner model1=model1.finalized() kwant.plot(model1) ####################### calculations ############################ ###functions for calculations params = dict(voltage=0) kwant.plotter.bands(model1.leads[1],params=params) #bandstructure of lead def plot_conductance(sys, energies,params): # Compute transmission as a function of energy data = [] for energy in energies: smatrix = kwant.smatrix(sys, energy, params=params) data.append(smatrix.transmission(0, 1)) pyplot.figure() pyplot.plot(energies, data) #pyplot.xlim([-2,2]) #pyplot.ylim([0,6]) pyplot.xlabel("energy [t]") pyplot.ylabel("conductance [e^2/h]") pyplot.show() return data ##########DOS KPM################# rho = kwant.kpm.SpectralDensity(model1,params=params) energies, densities = rho.energies, rho.densities plt.figure() plt.plot(energies, densities) ##################################### #######conductance w.r.t energy############## data=plot_conductance(model1,energies, params) ####################### ###########current density######## J_0 = kwant.operator.Current(model1) wf = kwant.wave_function(model1,energy=1,params=params) wfs=wf(0) wfs_of_lead_2 = wf(1) psi = wf(0)[0] current = J_0(psi) kwant.plotter.current(model1, current, colorbar=True) ################################################ please find the current density result in the link https://drive.google.com/file/d/1j8A6yZbZwaR1yAi3gKnuZalJYTJ8FQHc/view?usp=… https://drive.google.com/file/d/155-PxKLLCtGewqQL2fuitGWPI6KCReik/view?usp=…

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