Kwant-discuss June 2016

kwant-discuss@python.org

17 participants
17 discussions

by ZHANG Bing

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

4 months, 2 weeks

LDOS and added lead sites to the scattering region

by Tibor Sekera

Dear all, I want to plot the local density of states (LDOS) for a honeycomb system of a rectangular shape modeled by Hamiltonian H. Because the Bravais lattice vectors are non-orthogonal, the honeycomb leads (but modeled by different Hamiltonian H') are attached in such a way, that sites have to be added to the scattering region. This added part, however, is modeled by H' whereas the rest (original scattering region) by H. I know that I can just say that the added part is actually part of the leads (matter of definition). But, if I want to plot LDOS using "kwant.ldos(sys, energy)", it plots LDOS also on this added part modeled by H', which obscures the LDOS of my interest on the original scattering region modeled by H. How to make KWANT not to plot the LDOS on the artificially added part to the scattering region? Thanks for an advice, Tibor

5 years, 7 months

CNT lead

by Roohollah Hafizi

Dear all Hello I want to introduce a deformed Carbon Nanotube (with no symmetry) lead. I should note that a CNT supercell consists of at least (in my case, exactly) four rows of Carbon atoms in the unit cell. I appreciate general remarks on how to do it on KWANT? ?Hafizi

5 years, 7 months

Hooping from sites does not match the dimensions of onsite

by james edward hernandez

Hi. I tried to modify the tutorial for spin orbit coupling by replacing the system with a honeycomb lattice. However, I got this error, which came from bands: "Hopping from site 0 to site 12 does not match the dimensions of onsite Hamiltonians of these sites."Part of the code is attached. sys[lat.shape(rectangle, (1, 1))] = 0 #next nearest sys[lat.neighbors()] = 1.04 #nearest #### Define the left lead. #### sym = kwant.TranslationalSymmetry((-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) def lead1_shape(pos): x, y = pos return 0 < y < 10 for j in range(W): lead[lat.shape(lead1_shape, (0,j))] = 4 * t * identitymat + e_z * sigma_z lead[lat.neighbors()] = 0 #lead[(lat(0, j) for j in range(W))] = 4 * t * sigma_0 + e_z * sigma_z #hoppings in x-direction #lead[kwant.builder.HoppingKind((1, 0), 1, 1)] = \ # -t * identitymat - 1j * alpha * sigma_y # hoppings in y-directions #lead[kwant.builder.HoppingKind((0, 1), 1, 1)] = \ # -t * identitymat + 1j * alpha * sigma_x sys.attach_lead(lead) sys.attach_lead(lead.reversed()) return sys,lead def plot_bandstructure(sys, momenta): bands = kwant.physics.Bands(sys) energies = [bands(k) for k in momenta] plt.figure() plt.plot(momenta, energies) plt.xlabel("momentum [(lattice constant)^-1]") plt.ylabel("energy [t]") plt.show() def main(): sys,lead = make_system() honeycomb = sys.finalized() kwant.plot(sys) momenta = [0.01* pi * i for i in range(200)] plot_bandstructure(honeycomb.leads[1], momenta) #leads because of the attribute of silicene # We should see non-monotonic conductance steps. #plot_conductance(sys, energies=[0.01 * i - 0.3 for i in range(100)]) if __name__ == '__main__': main() James

5 years, 7 months

Gamma, M and K points in Kwant

by james edward hernandez

Hello, I saw in the tutorial 'Band structure calculations' that Kwant is able to display band structures for each lead. This applies also to Graphene nanoribbons. I was wondering if kwant.plotter.Bands() could also plot the band structure of graphene (or any honeycomb lattice), noting specific points in the Brillouin Zone, particularly Gamma->M->K->K'->Gamma? Regards, James

5 years, 7 months

conductance in large-voltage regime

by Sergey Slizovskiy

Dear all, Kwant is by default computing the linear Ohmic conductance in the regime where left and right chemical potentials differ infinitesimally. Do I get it right that to obtain ballistic conductance in the case of finite difference of chemical potentials, I can simply integrate the Kwant conductance result over a range of chemical potentials between the left and right contacts? This seems like a very natural thing to do to me. Am I right? Also, one can smoothen this integration with derivative of Fermi function to account for temperature. Best wishes, Sergey

5 years, 7 months

Adding leads on rectangular sheet

by james edward hernandez

Hi. I tried to add leads on the rectangular honeycomb scattering region. I was able to add two leads (the first one is from the left from 0 < y < 10, and the second one is from the right 0 < y < 4). But when I tried add another lead on the right from 7 < y < 10, it gives an error: ValueError: No sites close to (1, 1) are inside the desired shape. Here is the code for the setup: import matplotlib.pyplot import kwant lat = kwant.lattice.honeycomb() sys = kwant.Builder() def rectangle(pos): x, y = pos return 0 < x < 20 and 0 < y < 10 sys[lat.shape(rectangle, (1, 1))] = 0 #next nearest sys[lat.neighbors()] = -1 #nearest sym = kwant.TranslationalSymmetry((-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) def lead1_shape(pos): x, y = pos return 0 < y < 10 lead[lat.shape(lead1_shape, (1,1))] = 0 lead[lat.neighbors()] = -1 sym2 = kwant.TranslationalSymmetry((1, 0)) #to the right, botttom sym2.add_site_family(lat.sublattices[0], other_vectors=[(-1, 2)]) sym2.add_site_family(lat.sublattices[1], other_vectors=[(-1, 2)]) def lead2_shape(pos): x, y = pos return 0 < y < 4 # bottom lead2 = kwant.Builder(sym2) lead2[lat.shape(lead2_shape, (1,1))] = 0 lead2[lat.neighbors()] = -1 sym3 = kwant.TranslationalSymmetry(lat.vec((1, 0))) #to the right, top sym3.add_site_family(lat.sublattices[0], other_vectors=[(-1, 2)]) sym3.add_site_family(lat.sublattices[1], other_vectors=[(-1, 2)]) def lead3_shape(pos): #tried to add this lead and gives an error x, y = pos return 7 < y < 10 # top lead3 = kwant.Builder(sym3) lead3[lat.shape(lead3_shape, (1,1))] = 0 lead3[lat.neighbors()] = -1 sys.attach_lead(lead) sys.attach_lead(lead2) sys.attach_lead(lead3) kwant.plot(sys); Thanks, James

5 years, 7 months

kwant applicability

by Bibek Bhandari

I was doing some project on thermoelectric transport in quantum dots. I wanted to know whether I can use the kwant package to study the transport properties. If so, is there any materials that may support my project. Thanks a lot. Most Sincerely, Bibek Bhandari

5 years, 7 months

QHE code

by T.C. Wu

Hi Kwanters, I recently come across the file in http://topocondmat.org/w3_pump_QHE/Laughlinargument.html. When I try to run the code and plot pho_xx (which is E_x in the code) against B, I can't get a similar plot with the experimental data. Maybe it's a silly question, but do anyone have any clue? Thanks. Best, Johnny

5 years, 7 months

Leads with greater than NN hopping

by Matthew Daniels

Hello all, I’m interested in simulating a tight-binding system with matrix elements beyond nearest neighbor hopping (which I’ll call extended hopping, for short). I initially realized that the InfiniteSystem finalizer did not allow this construction when I tried to plot the band structure of an infinite lead. It is not clear to me why this restriction exists; I can see that the code in physics/dispersion.py only accounts for nearest neighbor hopping anyway (which could easily be changed), but since the error I encountered occurs in builder.py, I suspect that there’s a deeper reason for disallowing extended hopping terms. Forging ahead, it occurred to me that I could simply enlarge the unit cell so that all of my hopping terms only extended by a single unit cell away. Unfortunately, this also has the effect of band folding (quite a lot in my case), so the dispersion plot generated by Kwant is somewhat useless for me. But that’s of little consequence, because I’m more interested in transport. Now, since the error I encountered happens during finalization, I imagine I cannot even attach a lead with extended hopping to my finite sample. I *could* enlarge the unit cell (i.e. enlarge the translational symmetry vector) like I did for the band structure calculation, and I suspect that Kwant would happily (i.e. without runtime errors) carry out the calculation in this case. However, I worry that my artificial restriction of the leads’ translational symmetry could have some unintended consequences in the transport calculation. I’d appreciate any comments on the codebase or from experience that might suggest whether this is a safe manipulation to make or not. I’m also somewhat curious why this restriction exists, if anyone has insight to that. Thanks! Cheers, Matthew Daniels

5 years, 7 months

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Kwant-discuss June 2016