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 leads 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 and conservation laws
Given a lead Hamiltonian that has a conservation law, it is now possible to construct lead modes that have definite values of the conservation law. This is done by declaring projectors that block diagonalize the Hamiltonian before the modes are computed. For a Hamiltonian that has one or more of the three fundamental discrete symmetries (time-reversal symmetry, particle-hole symmetry and chiral symmetry), it is now possible to declare the symmetries in Kwant. The symmetries are then used to construct scattering states that are properly related by symmetry. The discrete symmetries may be combined with conservation laws, such that if different blocks of the Hamiltonian are related by a discrete symmetry, the lead modes are computed to reflect this.
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.