What’s new in Kwant 1.1

This article explains the user-visible changes in Kwant 1.1.0, released on 21 October 2015. See also the full list of changes up to the most recent bugfix release of the 1.1 series.

Harmonize Bands with modes

Kwant’s convention is that momenta are positive in the direction of TranslationalSymmetry. While the momenta returned by modes did respect this convention, the momenta read off the band structure as given by Bands had the wrong sign. This has been fixed now.

New option add_cells of attach_lead

Before actually attaching a lead to a builder, the method attach_lead of Builder prepares a “nice” interface by adding “missing” sites such that the first unit cell of the lead is completely connected with the system under construction. These sites and their hoppings are taken over from the lead.

By setting the new option add_cells, attach_lead can now be told to add in addition any number of complete unit cells of the lead to the system before attaching it. Among other things, this can be useful for

  • controlling the hopping between the lead and the system (Leads are always attached with their inter-unit-cell hopping to the system, but absorbing one lead unit cell into the system allows to control this),
  • creating a buffer for long range disorder present in the system to die away before the translation-invariant lead begins.

To support these applications, attach_lead now returns a list of all the sites that have been added to the system. Creating a buffer for disorder can be thus done as follows:

syst[syst.attach_lead(lead, add_cells=10)] = onsite

Note how we set the onsite Hamiltonians of the sites that have been added to the value used in the system.

New method conductance_matrix

SMatrix and GreensFunction have each gained a method conductance_matrix that returns the matrix \(G\) such that \(I = GV\) where \(I\) and \(V\) are, respectively, the vectors of currents and voltages for all the leads. This matrix is useful for calculating non-local resistances. See Section 2.4 of the book by S. Datta.

Deduction of transmission probabilities

If smatrix or greens_function have been called with check_hermicity=True (on by default) and a restricted number of leads in the out_leads and in_leads parameters, calls to transmission and conductance_matrix will work whenever it is possible to deduce the result from current conservation.

This allows leaving out one lead (preferably the widest) from out_leads and in_leads, and still to calculate all transmission probabilities. Doing so has been measured to speed up computations by 20% in some cases.

Clearer error messages

The error messages (exceptions) that appear when the Kwant interface is used incorrectly have been improved in many cases. For example, if instead of

builder[lattice(0, 1)] = 1

one writes

builder[(0, 1)] = 1

the error message will be more helpful now.

Please continue reporting confusing error messages on the Kwant mailing list.

New option pos_transform of kwant.plotter.map

This option which already existed for kwant.plotter.plot is now also available for kwant.plotter.map.