What’s new in Kwant 1.0

This article explains the new features in Kwant 1.0 compared to Kwant 0.2. Kwant 1.0 was released on 9 September 2013. Please consult the full list of changes in Kwant for all the changes up to the most recent bugfix release.

Lattice and shape improvements

Lattices now have a method neighbors, which calculates all the n-th shortest possible hoppings on this lattice. This replaces the nearest attribute that some lattices used to have.

shape uses an improved flood-fill algorithm, making it work better on narrow ribbons (which were sometimes buggy before with non-square lattices). Additionally, it was made symmetry-aware: If shape is used with a lead, the shape does not have to be limited along the lead direction anymore. In fact, if the shape function does not have the same symmetry as the lead, the result may be unexpected, so it is highly recommended to use shape functions that have the same symmetry as the lead.

closest now returns an exact, and not approximate closest point. A new method n_closest was added, which returns the n closest lattice points.

possible_hoppings replaced by HoppingKind

The Builder method possible_hoppings has been rendered obsolete. Where previously one would have had

for kind in lat.nearest:
    syst[syst.possible_hoppings(*kind)] = t

now it suffices to write

syst[lat.neighbors()] = t

This is possible because Builder now accepts functions as keys in addition to Site objects and tuples of them (hoppings). These functions are expected to yield either sites or hoppings, when given a builder instance as the sole argument. The use of such keys is to implement sets of sites or hoppings that depend on what is already present in the builder, such as HoppingKind. In the above example, lat.neighbors() is a list of HoppingKind objects.

Some renames

  • site groups are now called site families. This affects all the names that used to contain “group” or “groups”.
  • lead slices are now referred to as lead cells: This affects all names that used to contain “slice” or “slices” in the context of leads.
  • self_energy has been renamed to selfenergy in all cases, most notably in kwant.physics.selfenergy.
  • wave_func has been renamed to wave_function,
  • MonatomicLattice has been renamed to Monatomic,
  • PolyatomicLattice has been renamed to Polyatomic.
  • solve was split into two functions: smatrix, and greens_function. The former calculates the scattering matrix, the latter the retarded Green’s function between the sites adjacent to the leads. It is temporarily not possible to mix self-energy and modes leads within the same system.
  • The object that contained the results, BlockResult was also split into SMatrix and GreensFunction.

Band structure plots

A convenience function bands for quick plotting of band structure was implemented.

Immutable site families

In order to make naming more consistent, kwant.make_lattice was renamed and can be found now as kwant.lattice.general. Classes Chain, Square, and Honeycomb from lattice were made functions chain, square, and honeycomb.

In previous versions if one executed a = kwant.lattice.square(); b = kwant.lattice.square() then a and b were actually different lattices. This often led to confusions in more convoluted use cases, so this behavior was changed. Now two site families created with the same parameters are actually indistinguishable by Kwant. If it is desired to make two site families which have the same geometry, but mean different things, as for instance in Superconductors: orbital degrees of freedom, conservation laws and symmetries, then the name argument has to be used when creating a lattice, e.g. a = kwant.lattice.square(name='a'); b = kwant.lattice.square(name='b').

Parameters to Hamiltonian

Kwant now allows the Hamiltonian matrix elements to be described with functions that depend on an arbitrary number of parameters in addition to the sites on which they are defined.

Previously, functions defining the Hamiltonian matrix elements had to have the following prototypes:

def onsite(site):
    ...

def hopping(site1, site2):
    ...

If the Hamiltonian elements need to depend on some other external parameters (e.g. magnetic field) then those had to be provided by some other means than regular function parameters (e.g. global variables).

Now the value functions may accept arbitrary arguments after the Site arguments. These extra arguments can be specified when smatrix is called by setting the arguments:

args
A tuple of values to be passed as the positional arguments to the Hamiltonian value functions (not including the Site arguments).

For example, if the hopping and onsite Hamiltonian value functions have the following prototype:

def onsite(site, t, B, pot):
    ...

def hopping(site1, site2, t, B, pot):
    ...

then the values of t, B and pot for which to solve the system can be passed to smatrix like this:

kwant.smatrix(syst, energy,
              args=(2., 3., 4.))

With many parameters it can be less error-prone to collect all of them into a single object and pass this object as the single argument. Such a parameter collection could be a dictionary, or a class instance, for example:

class SimpleNamespace(object):
    def __init__(self, **kwargs):
        self.__dict__.update(kwargs)
# With Python >= 3.3 we can have instead:
# from types import SimpleNamespace

def onsite(site, p):
    return p.mu * ...

def hopping(site1, site2, p):
    return p.t * exp(-1j * p.B * ...)

params = SimpleNamespace(t=1, mu=2)
for params.B in B_values:
    kwant.smatrix(syst, energy, args=[params])

Arguments can be passed in an equivalent way to wave_function, hamiltonian_submatrix, etc.

Calculation of modes separated from solving

The interface that solvers expect from leads attached to a FiniteSystem has been simplified and codified (see there). Similar to self-energy, calculation of modes is now the lead’s own responsibility.

The new class ModesLead allows to attach leads that have a custom way of calculating their modes (e.g. ideal leads) directly to a Builder.

Modes or self-energies can now be precomputed before passing the system to a solver, using the method precalculate. This may save time, when the linear system has to be solved many times with the same lead parameters.

Change of the modes and lead_info format

The function modes now returns two objects: PropagatingModes and StabilizedModes. The first one contains the wave functions of all the propagating modes in real space, as well as their velocities and momenta. All these quantities were previously not directly available. The second object contains the propagating and evanescent modes in the compressed format expected by the sparse solver (previously this was the sole output of modes). Accordingly, the lead_info attribute of SMatrix contains the real space information about the modes in the leads (a list of PropagatingModes objects).

New module for random-access random numbers

The module kwant.digest provides functions that given some input compute a “random” output that depends on the input in a (cryptographically) intractable way. This functionality is useful for introducing disorder, e.g.:

def onsite(site):
    return 0.3 * kwant.digest.gauss(repr(site)) + 4

New module for random matrix theory Hamiltonians

The module kwant.rmt supports the creation of random matrix theory Hamiltonians.

Improved plotting functionality

The plotting functionality has been extended. By default, symbols and lines in plots are now relative to the system coordinates, i.e. will scale accordingly if different zoom-levels are used. Different styles for representing sites and hoppings are now possible. 3D plotting has been made more efficient.