Mailman 3 July 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

4 5

Package announcement: Pymablock
by Anton Akhmerov Nov. 12, 2023

Nov. 12, 2023

Hello fellow Kwanters, I am happy to announce Pymablock, a Python package useful to combine with Kwant. Its main purpose is constructing effective models using perturbation theory, which may help you in different ways. Pymablock allows you to analyse the behavior of a few levels of a large Hamiltonian, or to construct a symbolic k.p model. We originally wrote the code for one of our projects, but then saw that this tool is useful almost for the entire group, so we decided to turn it into a package. We spent countless hours generalizing, optimizing, and testing it so that it is efficient and reliable. Now it handles multiple perturbations to any order, and it is compatible with symbolic and numerical calculations. We hope that Pymablock will save you a lot of effort and computation time. To see where it can help, check out our its documentation (https://pymablock.rtfd.io/), and in particular the tutorials about the k.p model of bilayer graphene (https://pymablock.readthedocs.io/en/latest/tutorial/bilayer_graphene.html) or the induced superconducting gap (https://pymablock.readthedocs.io/en/latest/tutorial/induced_gap.html). Just like Kwant, Pymablock is open source (https://gitlab.kwant-project.org/qt/pymablock), and you are welcome to follow and contribute to its development. Let us know if you have any questions or comments! Also if there's interest we can organize an online introduction of the package Best, Anton Akhmerov and the Pymablock maintainers: Isidora Araya Day and Sebastian Miles

3 3

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

4 3

Release of Tkwant v1.1
by Thomas KLOSS July 27, 2023

July 27, 2023

Dear colleagues, we are pleased to announce the release of Tkwant v1.1. Besides new tutorial sections, improvements and bug fixes, the new version offers two new main features: First, a new generic solver class for self-consistent mean-field calculations, and second, a class for calculating non-equilibrium Green functions. A more detailed list can be found in the [ https://tkwant.kwant-project.org/doc/stable/pre/whats_new.html | Whats new ] section in the documentation at https://tkwant.kwant-project.org/doc/stable/. Kind regards, Xavier and Thomas on behalf of the Tkwant authors.

1 0

Is Tkwant capable of simulating nonlinear optical responses
by X.-X. Zhang July 26, 2023

July 26, 2023

Hello Kwant community, The Kwant package deals with the (dc) linear response for tight-binding models while TKwant is time-dependent. But it is not obvious to me from the documentation whether TKwant is capable of simulating nonlinear responses (at finite frequencies). Instead of naively assuming that it does not go beyond linear Kwant, it is probably worth asking here. I'm particularly interested in finding nonlinear conductivities like a second-harmonic sigma(2w; w, w) and a dc response sigma(0; w, -w) with w the driving frequency. They are generated, e.g., by applying the ac bias driving voltage/current through the attached leads. If Tkwant does capture all such nonlinear effects, I presume one can simulate in the real time and transform the current/voltage response to the frequency space and obtain these results. Any relevant comment will be appreciated!

3 3

Scattering region and lead with different kind of lattice
by Gabriel Garcia July 11, 2023

July 11, 2023

Hi devs, I am learning kwant alone, but i have doubts. I am trying to construct a system with a diatomic chain as scattering region and the leads as a monoatomic chain. But i am doing something wrong. Someone else can help me? Bellow i will show the code: ##################################################################################################################### from matplotlib import pyplot as plt from matplotlib import backends import kwant # Defining the parameters L = 5 # range of scattering region C = 2 # primitive vector # Definindo o parametro de Hopping t = 5 # hopping parameter lat = kwant.lattice.general([(C,0),(0,1)],[(0,0),(1,0)], norbs=1) a, b = lat.sublattices syst = kwant.Builder() # Scattering region for i in range(L): syst[a(i, 0)] = 0 syst[b(i, 0)] = 0 syst[kwant.builder.HoppingKind((0, 0), a, b)] = -t syst[kwant.builder.HoppingKind((1, 0), b, a)] = -t kwant.plot(syst) plt.show() # leaft lead (lead 1) lat_lead1 = kwant.lattice.chain(C, norbs=1) sym_lead1 = kwant.TranslationalSymmetry((-C, 0)) lead1 = kwant.Builder(sym_lead1) lead1[lat_lead(0,0)] = 4*t lead1[lat_lead1.neighbors()] = -t syst[(lat_lead1(-C, 0), a(0, 0))] = -t syst.attach_lead(lead1) kwant.plot(syst) --------------------------------------------------------------------------- ValueError Traceback (most recent call last) ~\anaconda3\lib\site-packages\kwant\builder.py in __new__(cls, family, tag, _i_know_what_i_do) 75 try: ---> 76 tag = family.normalize_tag(tag) 77 except (TypeError, ValueError) as e: ~\anaconda3\lib\site-packages\kwant\lattice.py in normalize_tag(self, tag) 475 if len(tag) != self.lattice_dim: --> 476 raise ValueError("Dimensionality mismatch.") 477 return tag ValueError: Dimensionality mismatch. During handling of the above exception, another exception occurred: ValueError Traceback (most recent call last) <ipython-input-46-3d36b05d2870> in <module> 8 lead1[lat_lead1.neighbors()] = -t 9 ---> 10 syst[(lat_lead1(-C, 0), a(0, 0))] = -t 11 12 syst.attach_lead(lead1) ~\anaconda3\lib\site-packages\kwant\builder.py in __call__(self, *tag) 195 raise ValueError('Use site_family(1, 2) instead of ' 196 'site_family((1, 2))!') --> 197 return Site(self, tag) 198 199 ~\anaconda3\lib\site-packages\kwant\builder.py in __new__(cls, family, tag, _i_know_what_i_do) 77 except (TypeError, ValueError) as e: 78 msg = 'Tag {0} is not allowed for site family {1}: {2}' ---> 79 raise type(e)(msg.format(repr(tag), repr(family), e.args[0])) 80 return tuple.__new__(cls, (family, tag)) 81 ValueError: Tag (-2, 0) is not allowed for site family kwant.lattice.Monatomic([[2.0]], [0.0], '', 1): Dimensionality mismatch. ############################################################################################# Thanks! Gabriel Garcia

2 8