1.3.rst

What's new in Kwant 1.3

This article explains the user-visible changes in Kwant 1.3. Please consult the full list of changes in Kwant for all the changes up to the most recent bugfix release.

Calculating charges and currents using the operator module

Often one may wish to calculate quantities that are defined over sites of the system (such as charge density, spin density along some axis etc), or over hoppings of the system (such as current or spin current). With the introduction of the operator module it has now become much easier to calculate such quantities. To calculate the regular density and current everywhere in a system due to a wavefunction wf, one only needs to do the following:

syst = make_system().finalized()
wfs = kwant.wave_function(syst)
wf = wfs[0]
# create the operators
Q = kwant.physics.LocalOperator(syst)
J = kwant.physics.Current(syst)
# evaluate the operator matrix elements
q = Q(wf)
j = J(wf)

See the Kwant tutorial for more details.

Sites in finalized builders have consistent ordering

In Python 3 the internal ordering of dictionaries is not deterministic. This meant that running a Kwant script twice would produce systems with different ordering of sites, which lead to non-reproducible calculations. Now, sites in finalized builders are always ordered first by their site family, then by their tag.

Coincidentally, this means that you can plot a wavefunction in a simple 1D system by just saying:

lattice_1D = chain()
syst = make_system(lattice_1D)
h = syst.hamiltonian_submatrix()
pyplot.plot(np.eigs(h)[1][0])

Improved build configuration

The name of the build configuration file, build.conf by default, is now configurable with the --configfile=PATH option to setup.py. (This makes build configuration usable with the pip tool.) The build configuration as specified in this file is now more general, allowing to modify any build parameter for any of the compiled extensions contained in Kwant. See the :ref:`Installation instructions <build-configuration>` for details.

Scattering states with discrete symmetries

Three discrete symmetries are especially relevant in condensed matter physics, namely time-reversal symmetry, particle-hole symmetry and chiral (or sublattice) symmetry. When one or more of these symmetries are present, and it can be useful to have leads with scattering states that reflect this. It is now possible to these discrete symmetries in Kwant, which then uses them to construct scattering states that are accordingly symmetric.

Pickling support

It is now possible to pickle and unpickle Kwant Builder and System.

Using Builders as templates with Builder.fill()

Builders now have a fill() method that fills the Builder with copies of a template Builder (passed as a parameter). This can be used to "cut out" shapes from high-symmetry models, or to increase the symmetry period of a lead.

attach_lead() can now handle leads with greater than nearest-neighbor hoppings

When attaching a lead with greater than nearest-neighbor hoppings, the symmetry period of the finalized lead is suitably extended and the unit cell size is increased.