Mailman 3 August 2024 - 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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Alternative procedure to calculate the quantum Hall conductivity?
by adiptapal7＠gmail.com Oct. 9, 2024

Oct. 9, 2024

Hello Kwant users, I have been trying to calculate the Hall conductivity in a time reversal broken Weyl semimetal (Weyl axis z) by using a 6-terminal Hall bar geometry with two current leads along z and 4 voltage leads perpendicular to z and then injecting unit current through the left current lead and calculate the p.d. across the voltage leads by inverting the conductance matrix as shown in qhe.py. However, it seems for some cases in thin film systems (thin film along x), the conductance matrix is singular and I cannot get results. Is there an alternative way to calculate the Hall conductivity in this case where one does not need to invert the conductance matrix? Thanks in advance.

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Fwd: Query on Conductance Quantization in Kwant Simulations within the Integer Quantum Hall effect
by Henrique Veiga Aug. 18, 2024

Aug. 18, 2024

Dear Kwant community, I have been studying the integer quantum Hall effect using the Kwant software, through which I computed the longitudinal and transverse conductance of a central sample (without disorder) in a four-terminal setup. I model the central device, which is subject to a strong magnetic field, as a square of lateral length, L. Both the sample and the four ideal leads are described by a square-lattice tight-binding model with nearest-neighbor hoppings. By computing the transverse conductance as a function of the Fermi energy, so that the first two Landau levels are crossed, I observe the expected behavior: the conductance appears quantized in integer steps of e²/h. However, upon zooming in on each of the two plateaus, I notice that the conductance does not converge immediately to the expected value. Instead, it exhibits a rippling effect, which seems to be dependent on the leads' cross-section, L_{Lead}. This phenomenon is illustrated in the attached .pdf file, and I have also included the code used to evaluate the conductance. My question is whether this behavior is a numerical issue, or if it is expected within the context of charge transport simulations in four-terminal setups. Thank you in advance, Henrique Veiga

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