Kwant-discuss May 2019

kwant-discuss@python.org

17 participants
18 discussions

Re: [Kwant] About Calculating density of states in transport system
by Abbout Adel 17 May '19

17 May '19

Dea Yan, Kpm method is applied for a finite system as mentioned in the documentation: *hamiltonian* : FiniteSystem <https://kwant-project.org/doc/1/reference/generated/kwant.system.FiniteSyst…> or matrix Hamiltonian If a system is passed, it should contain no leads. Regards, Adel On Tue, May 14, 2019 at 6:00 AM X.F.Yan <yxf0125(a)aliyun.com> wrote: > Dear Anton and other Kwant users, > > > > Recently, I attempt to use Kwant to calculate the density of states in > transport system. I successfully calculated the dos of a large closed > system according to the manual. Unfortunately, I use the method to compute > the dos of an open system(i.e. a system composed of a scattering region and > leads): > > > > fsyst = make_syst().finalized() > > spectrum = kwant.kpm.SpectralDensity(fsyst) > > energies, densities = spectrum() > > > > But, the system generates a complexwarning : > > \Anaconda3\lib\site-packages\numpy\core\numeric.py:501: ComplexWarning: > Casting complex values to real discards the imaginary part > > return array(a, dtype, copy=False, order=order) > > > > Hence, there’s something wrong with the calculation. I don’t know how to > solve the warning. > > > > Please help me solve my bewilderment , Thanks! > > > > Best regard, > > > > Yan > > -- Abbout Adel

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Staggered Sublattice Potential
by SAYAN MONDAL 16 May '19

16 May '19

Respected Sir, I want to include the staggered sublattice potential in Kwant. The problem is, it is an onsite term which takes positive and negative values to two different sublattices of Graphene. how will I include this onsite potential? For reference you can check the fourth term on right hand side of equation (1) in the paper given in the following link. https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.95.146802. Thanks a lot. Sayan Mondal, IIT Guwahati, Guwahati, India <https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.95.146802.> <https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.95.146802.>

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Error with params SimpleNamespace
by jozef.houwman＠u-psud.fr 16 May '19

16 May '19

Dear Kwant members, I’ve been running into the same error message over and over when trying to pass a SimpleNamespace to the function hamiltonian_submatrix(). I am using Python 3.7.3 and Kwant 1.4.1. Could it be that this is due to a too recent Python version? Kind regards, Arfor sys = kwant.Builder() lat = kwant.lattice.square() r=10 def onsite(position,p): x,y = position return p.mu def hopping(x,y,p): return -p.t sys[(lat(x, y) for x in range(r) for y in range(r))] = onsite sys[sqlat.neighbors(1)]= hopping syst = sys.finalized() pars = SimpleNamespace(t=1, mu=-0.1) hamil = syst.hamiltonian_submatrix(params=pars) --------------------------- AttributeError Traceback (most recent call last) <ipython-input-5-70dbfa568582> in <module> 1 pars = SimpleNamespace(t=1, mu=-0.1) ----> 2 hamil = syst.hamiltonian_submatrix(params=pars) C:\ProgramData\Anaconda3\envs\lps\lib\site-packages\kwant\_common.py in inner(*args, **kwargs) 70 if sig.bind(*args, **kwargs).arguments.get(parameter_name): 71 warn() ---> 72 return f(*args, **kwargs) 73 74 return inner kwant/_system.pyx in kwant._system.hamiltonian_submatrix() C:\ProgramData\Anaconda3\envs\lps\lib\site-packages\kwant\builder.py in hamiltonian(self, i, j, params, *args) 1872 if isinstance(param_names, Exception): 1873 raise param_names -> 1874 args = map(params.__getitem__, param_names) 1875 try: 1876 value = value(site, *args) AttributeError: 'types.SimpleNamespace' object has no attribute '__getitem__'

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About Calculating density of states in transport system
by X.F.Yan 14 May '19

14 May '19

Dear Anton and other Kwant users, Recently, I attempt to use Kwant to calculate the density of states in transport system. I successfully calculated the dos of a large closed system according to the manual. Unfortunately, I use the method to compute the dos of an open system(i.e. a system composed of a scattering region and leads): fsyst = make_syst().finalized() spectrum = kwant.kpm.SpectralDensity(fsyst) energies, densities = spectrum() But, the system generates a complexwarning : \Anaconda3\lib\site-packages\numpy\core\numeric.py:501: ComplexWarning: Casting complex values to real discards the imaginary part return array(a, dtype, copy=False, order=order) Hence, there’s something wrong with the calculation. I don’t know how to solve the warning. Please help me solve my bewilderment , Thanks! Best regard, Yan

1 0

How to calculate a few low-lying bands using kwant.physics.Bands
by 曹霑（CAO Zhan） 12 May '19

12 May '19

Dear Sir, I am trying to calculate a few low-lying bands of a two-dimensional model, which is infinite and periodic along the x-direction but finite along the y-direction. I find that kwant.physics.Bands gives me the whole bands costing a lot of time, see the codes below. My question is how to quickly calculate a few low-lying bands, rather than the whole bands. My codes are as follows. -------------------------------------------------------------------------------------- import kwant import kwant.continuum import numpy as np from matplotlib import pyplot a=10 periodic_x=10*a L_y=50*a hamiltonian = "k_x**2+k_y**2 + V(x,y)" # V(x,y) is a periodic potential along the x-direction template = kwant.continuum.discretize(hamiltonian, grid=a) def lead_shape(site): (x, y) = site.pos return 0 <= y <= L_y def potential(x, y): return np.sin(2*np.pi*x/periodic_x)+np.cos(y) lead = kwant.Builder(kwant.TranslationalSymmetry([periodic_x, 0])) lead.fill(template, lead_shape, (0, 0)) lead = lead.finalized() bands = kwant.physics.Bands(lead,params=dict(V=potential)) l_k=100 momenta = np.linspace(-np.pi, np.pi, l_k) energies = [bands(k*periodic_x) for k in momenta] pyplot.plot(momenta, energies) pyplot.show() -------------------------------------------------------------------------------------- When running above codes the time costed increases as periodic_x or L_y become large. Best regards Zhan Cao Southern University of Science and Technology of China

2 1

STM tip
by Antonio Lucas Rigotti Manesco 07 May '19

07 May '19

Dear all, I want to use KPM to calculate LDOS vs. energy for a given position. Apparently, I can do it using 'where' in kwant.operator.Density, but the input must be a crystal position. I wonder if there is any form to find the closest crystal position for a given position vector that does not necessarily belongs to the crystal. Alternatively, can I somehow integrate kwant.operator.Density in a specific region and not over the entire system? Thanks in advance. -- Antonio Lucas Rigotti Manesco PhD fellow - University of São Paulo, Brazil

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Peierls phase substitution on Tight-Binding Hamiltonian
by Naveen Yadav 07 May '19

07 May '19

Dear KWANT developers, I have an Hamiltonian *H(k) = tx*σx* sin kx + ty*σy*sin ky + mk*σz + λ*σ0 *sin kz,* *mk = tz(cos β − cos kz) + t'(2 − cos kx − cos ky)* I am trying to introduce magnetic field in *x-direction* for a gauge *A = (0, 0, By),* Can I make a *template* for this hamiltonian using KWANT discretizer or I have to discretize it by hand to get the onsite and hopping matrix? If I have to discretize it by hand then how should I proceed for that. And then how to add the peierls phase to the hopping matrix? Then I want to plot the spectrum as a function of k_z, with Wx =10 and Wy=50 Could you please help me, I tried a lot but failed. Thanks in advance. Naveen

2 11

Defining leads
by Luca Lepori 06 May '19

06 May '19

Dear Kwant developers, I write for a question about the definition of the leads. Suppose that I have constructed a certain scattering region (using kwant.lattice.general()), that hosts translational symmetry along a vector v1. Notably, this symmetry results from the final shape of the scattering region and it is not a property of some of the vectors, {r_i}, used in kwant.lattice.general(). More in detail, I can write (in short): lat = kwant.lattice.general({r_i}, ....) syst = kwant.Builder() syst[lat.shape(......)] = ...... Now I want to attach the scattering region to two leads, both obtained repeating the same region along v1. However, because of the feature of the translational symmetry described above, Kwant seems to encounter problems to define and to attach these leads in the usual way: sym = kwant.TranslationalSymmetry(v1) lead = kwant.Builder(sym) lead[lat.shape(......)] = ....... syst.attach_lead(lead) syst.attach_lead(lead.reversed()) Indeed, probably Kwant still "ignores" the existence of the translation symmetry along v1. Therefore, I think that, to solve the problem, the replica of the scattering region along v1 must be someway forced "by hand". I tried for instance: sym = kwant.TranslationalSymmetry(v1) lead = kwant.Builder(sym) lead = syst but it does not work, because translational symmetry is still not implemented (ValueError: Only builders with a 1D symmetry are allowed). Could you help me? Thank you very much and best regards L. L.

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Kwant-discuss May 2019