Dear Sir,
I am a PhD student of Hong Kong University of Science and Technology. I
want to use KWANT to caculate Hall resistance of a Hall bar structure.We
can get the conductance between 6 electrodes, but how to get hall
resistance? Can you give me some help? Thank you very much.
Best Regards,
Zhang Bing

Dear all,
I am having a problem defining a system which matches what I want.
I start by defining a rectangular graphene lattice of size 0<=x<L in the x
direction and 0<=x<W in the y direction. (By "rectangular" I mean in the
real space coordinates, not the crystallographic coordinates.)
I want to attach a lead to the left-had end of this rectangle, going to
minus infinity. Therefore, I define a lead with translational symmetry
(-1,0) and the appropriate hopping. I attach the lead and plot the system.
When I plot the system, I find that the lead has been attached along the
(0,1) crystallographic direction (so, that is along the (1/2, sqrt(3)/3)
real space vector). A triangle of extra sites have been added for x<0 (real
space) so that the total shape of the scattering region is now not rectangular.
If I attach another lead with lead.reversed(), a similar thing happens on
the right of the sample so that my scattering region is now a parallelogram.
As I understand it, this should not affect the physics in any way, since the
lead is semi-infinite. But, if I want to draw pictures, plot functions over
the scattering region, and gain physical understanding, it is a bit of a
pain. So, my question is whether there is any way to make the lead attach
along the (0,1) real space direction (which is the same as the (-1,2)
crystallographic direction) and yet maintain the (-1,0) translational symmetry?
If you require a sample program which reproduces this behavior then I can
easily provide that, but I thought I should not extend an already long post
unnecessarily.
Thanks in advance.
David

Hi,
Firstly, thanks for creating Kwant - it's so nice to use physics code
written by people who understand software-engineering as well as physics :)
I've got a few questions about units and density-of-states in Kwant, please
respond if you know anything about any of them; don't feel the need to
respond to them all at once.
I'm trying to add self-consistent electrostatics to my Kwant system (using
FiPy as a finite-element/volume Poisson-solver). Obviously, I need to
calculate the electron-carrier-density of the system by integrating over
the Fermi-Dirac occupation, then feed that (via real-space basis functions)
to my Poisson solver. I'm not quite sure how to handle the density of
states in the context of Kwant:
I assume that just summing over the LDOS (integrating over all space) will
give the density of states as a function of energy, D(E). Plotting it seems
to produce reasonable bands, but I'm not quite sure about the units, or how
it scales with system size. In the system I'm modelling (low-temperature
p-donors in silicon), every lattice site adds an electron to the system
(the temperatures are low enough that the silicon is frozen-out as a
conductor and can just be treated as a background dielectric constant). I
should be able to integrate over the density-of-states until the total
equals the (known) number of electrons, but the density of states obtained
by summing over the LDOS calculated by Kwant does not scale properly with
the number of sites in the system; larger systems always need higher Fermi
energies, which isn't physical at all.
What am I missing here? Are the units of the LDOS Kwant calculates somehow
normalised? How can I get a density-of-states which scales appropriately
with the total number of electrons/sites in my system?
The lead unit-cell of my system will need to be solved self-consistently
too; how can I calculate the local density of states (and thus, via
Fermi-Dirac, the electron-density) of a lead?
Am I correct in assuming that the LDOS produced by Kwant is equivalent to
summing over the state-density-weighted scattering-wavefunctions from the
modes in all leads (and thus that integrating it over the occupied-energies
will produce a sensible total electron-density)?
Finally, and slightly unrelated, do my chosen energy-units need to be
accounted-for anywhere in Kwant's Schroedinger-solutions? I'm writing my
Hamiltonian terms in meV; will bands, LDOS etc. all naturally scale to make
this choice transparent? Similarly, does effective electron-mass need to be
accounted for at all?
Thanks so much for your help,
Daniel R-P

Dear all,
Can we really use temperature variable in KWANT? If yes,then with
what function.Also, How to apply magnetic field perpendicular to the sample
like I am having the graphene sheet and I want to apply per mag. field on
it.this is what I am trying!
################################################
def hopx(site1, site2, B=0):
# The magnetic field is controlled by the parameter B
y = site1.pos[1]
return -t * exp(-1j * B * y)
sys[lat.shape(circle, (0, 0))] = 4 * t
# hoppings in x-direction
sys[kwant.builder.HoppingKind((1, 0), lat, lat)] = hopx
# hoppings in y-directions
sys[kwant.builder.HoppingKind((0, 1), lat, lat)] = -t
# It’s a closed system for a change, so no leads
##################################################