Mailman 3 January 2023 - 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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How to add magnetic field (Peierls substitution) for the continuum model
by dielectric_hjzhao＠126.com March 16, 2023

March 16, 2023

Dear kwanters, I am using kwant for handle a k.p Hamiltonian, for the conductance calculations. I start from a k.p model, and transform it to a tight-binding model using kwant.continuum tools [procedures listed here https://kwant-project.org/doc/1/tutorial/discretize]. Now I want to study the conductance of the system under magnetic field. I try to do Peierls substitution for hoppings between site i and site j (i-j being parallel to the lead). However, I find that using the kwant.continuum scheme, there are no information regarding the lattice information such as "lat = kwant.lattice.honeycomb() ". This prevents the application of Peierls substitution by the following way: _____________________________ def hopx(site1, site2, B): y = site1.pos[1] return - np.exp(-1j * B * y) syst[kwant.builder.HoppingKind((1, 0), lat.a, lat.b)] = hopx ____________________________ My question is how to add magnetic field during the calculations for the systems generated from a continuum model by kwant.continuum? Thank you very much!

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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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Quasienergy spectrum of a periodically driven chain
by valentin.fadeev＠open.ac.uk Jan. 8, 2023

Jan. 8, 2023

Hello, I have been trying to find a way to replicate the quasienergy spectrum of a periodically driven dimers chain, as depicted in Fig 3 of https://doi.org/10.1103/PhysRevLett.110.200403, using tkwant. I started with a basic time-independent model import kwant from matplotlib import pyplot import numpy as np a = 1.0 b = 0.2 * a lat = kwant.lattice.general([(0, a), (a, 0)], [(1, 0), (b, 0)]) lat_a, lat_b = lat.sublattices sym = kwant.TranslationalSymmetry((-a, 0)) dimers = kwant.Builder(sym) dimers[lat_a(-1, 0)] = 5 dimers[lat_b(-1, 0)] = 3 dimers[[kwant.builder.HoppingKind((0, 0), lat_a, lat_b)]] = -1.5 dimers[[kwant.builder.HoppingKind((0, 1), lat_a, lat_b)]] = -1.7 kwant.plot(dimers) dimers = dimers.finalized() bands = kwant.physics.Bands(dimers) momenta = np.linspace(-np.pi, np.pi, 101) energies = [bands(k) for k in momenta] pyplot.plot(momenta, energies) pyplot.grid("on") Now I want to introduce the time-dependent vector potential def vec_potential(time, freq, e_field): q = -1 A_0 = q * e_field / freq return A_0 * np.sin(freq * time) however, I have been stuck over how exactly to couple the two together and correctly modify the hoppings. I am also uncertain whether the existing kwant/tkwant API will allow me to extract quasi-energy spectrum directly. Any hints will be greatly appreciated. Best wishes, Valentin

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how can I get latest version 1.5.0?
by 1137689418＠qq.com Jan. 8, 2023

Jan. 8, 2023

I need to calculate Landua level and add a mangeitc field. They can only be realized with Kwant latest version 1.5.0. So, how can I get latest version 1.5.0 on Windows or Linux cluster? Many thanks.

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How to set the grid in kwant.continuum.discretize?
by dielectric_hjzhao＠126.com Jan. 6, 2023

Jan. 6, 2023

Dear Kwanters, The kwant.continuum.discretize helps transform a continuous k.p Hamiltonian to a tight-binding Hamiltonian. During this discretizing process, we can use the "grid" parameter to control the "mesh" of such a discretization. In the tutorial, I see that different "grid" result in different approximations of the tight-binding Hamiltonian with respect to the continuous Hamiltonian. See https://kwant-project.org/doc/1/tutorial/discretize My question is: which value of grid should we use during the discretizing process? Should we decrease the "grid" from a larger value to a smaller value, monitoring the band structure and conductance behavior, until the band structure and conductance behavior remain almost unchanged when further decreasing the "grid"? Thanks a lot for your answer.

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Full Quantum point contact Simulation
by Mohit Gupta Jan. 4, 2023

Jan. 4, 2023

I have been trying to implement full Conductance measurements in bias and gate space of a quantum point contact. As I understand gate implementation is rather straightforward by generating on site potential. But when scanning this potential the energy parameter is fixed. What does the chosen fixed value of energy mean? Ideally one can integrate out the energy to get Conductance as a function of bias using Landauer formalism, is this approach correct? Or one can avoid this somehow in Kwant.

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