Mailman 3 February 2016 - Kwant-discuss

Re: [Kwant] [PROVENANCE INTERNET] Re: Should we disable implicit overwriting in Builder?
by WAINTAL Xavier 11 Feb '16

11 Feb '16

I'd tend to vote with Anton : until it actually proves dangerous can't we suppose it is safe? I can see many places where it can be very handy to overwrite. For instance is you use a system constructed by someone else and want to see the effect of modifying this and that. ________________________________________ De : Kwant-discuss [kwant-discuss-bounces(a)kwant-project.org] de la part de GROTH Christoph Envoyé : jeudi 4 février 2016 17:43 À : kwant-discuss(a)kwant-project.org Objet : [PROVENANCE INTERNET] Re: [Kwant] Should we disable implicit overwriting in Builder? Anton Akhmerov wrote: > Firstly, overwriting hoppings is handy, and we even use it in > Kwant tutorial: > http://kwant-project.org/doc/1/tutorial/tutorial2#nontrivial-shapes Sure it is handy. That’s why I don’t want to disallow it, only to make it explicit in order to avoid things getting overwritten by accident. The relevant line in the tutorial would become something like: sys.rewrite(hops_across_cut, hopping_phase) One could argue that this is more explicit, though perhaps a bit ugly. The question is whether the gain in security outweighs the loss in simplicity. I think it could, especially with complex symmetries. > Second aspect of the problem I wanted to mention is that we > anyway discussed that the new low level system format would > allow to associate more than a single value with a site or a > hopping. That change would require a corresponding modification > of the builder interface, however not sure in which fashion. I actually encountered this problem today with Mathieu when we were writing a routine to wrap-around some hoppings (due to limitations of current Kwant low-level systems). An updated builder could behave (always?) as a mapping of sequences of values, perhaps similar to a collections.defaultdict(list). Then, adding a new value would work like this: sys[key].append(value) or sys[key] += [value] We could still accept values that are not sequences, so that sys[key] = number/function would be possible. But that might be problematic since matrices are sequences, too. I think a sequence is better than a set since it doesn’t leave the order of addition undefined. The order of addition doesn’t matter algebraically, but numerically it does. The above looks a bit unmathematical, but it has the advantage of being consistent with the rest of Python. One could also make sys[key] += value work, but that would necessitate twisting the semantics in rather ugly ways: sys[key] = value assert sys[key] is value # This would fail. What do you think? (Comments by people outside of the Kwant team are especially welcome!) Christoph

3 3

the phase of wave functions
by Fahriye Nur Gursoy 07 Feb '16

07 Feb '16

Hello all, My question is about modifying the phase of wave functions. I want to calculate derivative of scattering matrix with respect to energy and they come out to be discontinuous because of the phases. To remedy this I need to modify these phases. Following the suggestion of Anton Akhmerov in this mail group I understood how to modify the phase of modes in the leads. But I also need to change the phase of wave functions in the scattering region to obtain continuous scattering matrices at different energies. I wonder if there is an another way to call wave functions in the scattering region (apart from kwant.solvers.default.wave_function) or a method to modify these phases. Thank you in advance. Nur

2 3

Modifying Magnetic Field
by james edward hernandez 05 Feb '16

05 Feb '16

Hello. I would like to verify the Landau Levels in the Hall conductance of graphene nanoribbons. The code works for conductance, band structure and Hall conductance, but I would like to know why the plateaus are not visible. Also, I would like to know why the plot of wavefunctions does not return to the existence of edge states, which should be for zigzag graphene nanoribbons. Attached is the code. Regards, James

2 1

Should we disable implicit overwriting in Builder?
by Christoph Groth 04 Feb '16

04 Feb '16

Dear Kwant users, Right now it is possible to overwrite bits of a builder, i.e.: syst[lat(0, 0)] = 1 # Create a new site. syst[lat(0, 1)] = 1 # Create another site. syst[lat(0, 0)] = 2 # Overwrite the first site. Kwant builders behave like this because they are mimicking Python’s dictionaries. I wonder, however, whether this behavior is not both mostly useless and a bit dangerous, especially when a symmetry is involved. It will only get worse when Builder will understand other symmetries (e.g. rotational). What if Builder instances would not allow items to be overwritten when using the above syntax? (There would be a special method that provides the current behavior – it could be called “reset” or “overwrite”). I think that most Kwant scripts would not be affected by this behavior. If anyone every blindly assumes that a builder allows overwriting, the error message will quickly point out the problem and mention the “overwrite” method, so the potential for confusion seems very low. I do not intend to do this (backwards-incompatible) change right away. I’m just curious to hear whether anyone thinks that it’s a bad idea. Cheers, Christoph

2 2

superconducting leads
by anil 03 Feb '16

03 Feb '16

Hello, I am trying to how understand how to compute the wavefunctions of a system with superconducting leads as a function of the phase difference, just as in the case of normal leads, using fsys.wave_function() where I would do it as a function of the energy. However, the result is always an empty array. Is there a way to make it work ? Thanks in advance, Anil

2 4

lattice_general
by kamal ghosh 02 Feb '16

02 Feb '16

Dear All, There are inbuilt crystal lattices in Kwant. As little bit crazy I tried to use general( ) in constructing monatomic square or rectangular lattice instead of using lattice.square(). My code was: lat=kwant.lattice.general([(1,0),(0,1)],[(0,0)])instead of the inbuilt square lattice i.e. lat=kwant.lattice.square(1). Error given : polyatomic objects are not callable. I cant understand how this becomes polyatomic? I have not declared any sublattice. Please help me how to give a code for manatomic and polyatomic objects as well. Thanks to all of the discuss forum. K.K.Ghosh kk_ghosh(a)rediffmail.com

2 1