Using Kwant for finding tunneling probability from metal to vacuum
Hello Kwant experts, I have studied Kwant a bit, but before I delve deeper into it, I want to know if it can be applied to my problem. I have a 3d electron potential landscape for a metalvacuum surface calculated with DFT. (More detailed info and pictures in [1]). Can I use Kwant to calculate the transmission probability of electrons incident on the surface? And if kwant is not suitable for this problem, perhaps you know of any other numerical software or method that would allow this calculation? [1]  http://physics.stackexchange.com/questions/295329/quantumtunnelingprobabil... Best regards, Kristjan
Hello Kristjan, Probably you can model the vacuum as a dense square lattice making its hopping such as to mimic free electron dispersion. And the hopping between your material and the "vacuum" lattice must be defined from the DFT atomic orbitals of the band you are considering. The vacuum lattice will overlap with your real lattice and will share the same potentials. Does this seem to make sense? Best wishes, Sergey On 29/11/16 13:36, Kristjan Eimre wrote:
Hello Kwant experts,
I have studied Kwant a bit, but before I delve deeper into it, I want to know if it can be applied to my problem.
I have a 3d electron potential landscape for a metalvacuum surface calculated with DFT. (More detailed info and pictures in [1]). Can I use Kwant to calculate the transmission probability of electrons incident on the surface?
And if kwant is not suitable for this problem, perhaps you know of any other numerical software or method that would allow this calculation?
[1]  http://physics.stackexchange.com/questions/295329/quantumtunnelingprobabil...
Best regards, Kristjan
Kristjan Eimre wrote:
I have a 3d electron potential landscape for a metalvacuum surface calculated with DFT. (More detailed info and pictures in [1]). Can I use Kwant to calculate the transmission probability of electrons incident on the surface?
This looks to me like Kwant's first tutorial [1], only with a nonconstant potential. In order to have periodic boundary conditions across your sample just add appropriate "long range" hoppings. Let us know if you encounter problems. [1] https://kwantproject.org/doc/1/tutorial/tutorial1
Hello again, Thanks for the ideas. I have fiddled around with Kwant a bit now, and I'm trying to recreate quantum tunneling through rectangular potential barrier (see [1] for analytical solution). I have also added a figure of the analytical solution (rect_trans.png), which shows the transmission probability dependence on electron energy. I modified the 1st tutorial code to include the rectangular barrier (see my code in [2]). I have attached the figures i got from the code (conductance.png and rect_trans.png). If i have understood correctly, then Kwant outputs the conductance depending on excitation energy. And the conductance is the sum of the transmission probabilities of the conduction channels, right? I probably should delve deep into books on quantum transport to understand some of these things, but perhaps you could give me some answers for a good start. I have also calculated the number of conduction channels, but this goes to 0 at higher energies. Why does this occur? In the simple analytical case, there is no upper bound on electron energies. Is it even possible to recreate the analytical quantum tunneling result for the rectangle barrier in Kwant? Best regards, Kristjan [1]  https://en.wikipedia.org/wiki/Rectangular_potential_barrier [2]  http://pastebin.com/jq7CKfSP
Hi,
I modified the 1st tutorial code to include the rectangular barrier (see my code in [2]). I have attached the figures i got from the code (conductance.png and rect_trans.png). If i have understood correctly, then Kwant outputs the conductance depending on excitation energy. And the conductance is the sum of the transmission probabilities of the conduction channels, right?
Yes, this is correct. You may also get transmission probablities between individual channels directly from the scattering matrix.
I have also calculated the number of conduction channels, but this goes to 0 at higher energies. Why does this occur? In the simple analytical case, there is no upper bound on electron energies.
This is because the tightbinding model corresponds to a discretization of the continuum model. If you do the analytical calculation for the actual (discretized) case that you have, you will find the dispersion relation `E(k) = 2 * t (1  cos(k))` (t the hopping), which reduces to E(k) = k**2 (the continuum result) in the limit k>0 as it should. This is natural, as at large enough k, the wavelength of your solution will be of comparable size to the lattice spacing, and will start to "see" the discretization. In addition, as you have created a quasi1D system, as opposed to a purely 1D (chain) system, there will be an energy offset to the above dispersion relation for each band.
Is it even possible to recreate the analytical quantum tunneling result for the rectangle barrier in Kwant?
Of course. You only need to make sure that your discretization is fine enough for the range of energies (viz. wavelengths) that you wish to look at, if you wish to approximate well the continuum result. Hope that helps, Joe
" And the conductance is the sum of the transmission probabilities of
Dear Kristjan,
the conduction channels, right? "
Yes this is correct.
The number of conducting channels goes to zeros at higher energies because
you work in a lattice: the dispersion relation is not anymore E=k^2 .
In a lattice, the dispersion relation for a mode 'm' is
E_m=4te_m2*cos(k) (where e_m=2*cos(m*pi/(W+1)))
in order to recover the results of the continuum, you need to work at low
energies.
in your case, you need to change the theoretical relation by putting a
shift 22*cos(pi/(W+1)) for the energies and this will work for the first
mode. you need to do the same thing (shift 2e_m) for the other modes and
make a sum of the transmissions to obtain the results for higher modes.
a script is included below to show how we recover the exact result.
I hope that this helps
Adel
import kwant
from numpy import sqrt,sin,cos,pi,sinh
# For plotting
from matplotlib import pyplot
def make_system(a=1, t=1.0, W=10, L=30,V0=0):
lat = kwant.lattice.square(a)
sys = kwant.Builder()
sys[(lat(x, y) for x in range(L) for y in range(W))] = 4*t+V0
sys[lat.neighbors()] = t
#### Define and attach the leads. ####
lead = kwant.Builder(kwant.TranslationalSymmetry((a, 0)))
lead[(lat(0, j) for j in range(W))] = 4 * t
lead[lat.neighbors()] = t
sys.attach_lead(lead)
sys.attach_lead(lead.reversed())
return sys
def plot_conductance(sys, energies):
# Compute conductance
data = []
for energy in energies:
smatrix = kwant.smatrix(sys, energy)
data.append(smatrix.transmission(1, 0))
# pyplot.figure()
pyplot.plot(energies, data,lw=2,label="numerics")
pyplot.xlabel("well depth [t]")
pyplot.ylabel("conductance [e^2/h]")
pyplot.legend()
pyplot.show()
def main():
V0=0.1
sys = make_system(V0=V0)
# Check that the system looks as intended.
# kwant.plot(sys)
# Finalize the system.
sys = sys.finalized()
e1=22*cos(pi/11) #energy of the first transverse mode
def Cond(E,V0,L=30):
if Ee1V0<=0:
k=sqrt(E+e1+V0)
return 1/(1+((V0*sinh(k*L))**2 )/(4*(Ee1)*(E+e1+V0)))
else:
k=sqrt(Ee1V0)
return 1/(1+((V0*sin(k*L))**2 )/(4*(Ee1)*(Ee1V0)))
energies=[0.1+0.001 * i for i in range(300)]
Cond_analytic=[Cond(E,V0) for E in energies]
pyplot.plot(energies,Cond_analytic,"ro",label="analytic")
# We should see conductance steps.
plot_conductance(sys, energies)
# Call the main function if the script gets executed (as opposed to
imported).
# See http://docs.python.org/library/__main__.html.
if __name__ == '__main__':
main()
On Mon, Dec 19, 2016 at 4:25 PM, Kristjan Eimre
Hello again,
Thanks for the ideas. I have fiddled around with Kwant a bit now, and I'm trying to recreate quantum tunneling through rectangular potential barrier (see [1] for analytical solution). I have also added a figure of the analytical solution (rect_trans.png), which shows the transmission probability dependence on electron energy.
I modified the 1st tutorial code to include the rectangular barrier (see my code in [2]). I have attached the figures i got from the code (conductance.png and rect_trans.png). If i have understood correctly, then Kwant outputs the conductance depending on excitation energy. And the conductance is the sum of the transmission probabilities of the conduction channels, right?
I probably should delve deep into books on quantum transport to understand some of these things, but perhaps you could give me some answers for a good start. I have also calculated the number of conduction channels, but this goes to 0 at higher energies. Why does this occur? In the simple analytical case, there is no upper bound on electron energies.
Is it even possible to recreate the analytical quantum tunneling result for the rectangle barrier in Kwant?
Best regards, Kristjan
[1]  https://en.wikipedia.org/wiki/Rectangular_potential_barrier
 Abbout Adel
Hello once again,
Thanks for all the help so far.
In my initial problem, I wanted to calculate the tunneling probability from
bulk metal to "bulk" vacuum. The potential landscape is periodic in the
"transverse" directions and assuming the free electron model, plane waves
should come from the bulk and try to tunnel through the barrier. The main
result I want is the tunneling probability dependence on the wave vector or
D(k).
At first, I naively though that I could just point the finitewidth leads
at different angles (different k vectors) at the surface and have a big
lead in the vacuum as the output. This doesn't work very well, as the
result depends heavily on the size of the leads, orientation of the vacuum
lead and on the shape of the scattering region. In the ideal case, I would
like the result to be independent of these parameters.
For example, the tunneling probability through the rectangular barrier
matches with the analytical case if the scattering region is the same width
as the leads, but if I change the shape and/or add periodic boundary
conditions, the result doesn't match with the analytical case any more.
I have looked around and found the wraparound module (
https://gitlab.kwantproject.org/cwg/wraparound), which looks like it's
exactly the thing I need. However the demo code doesn't really illustrate
how to calculate transmission with arbitrary k vectors. How should I attach
the leads and how to make them also periodic? As the first step, I would
like to use this wraparound code on a system of infinite plane of
rectangular barrier and reproduce the analytical result (when the incident
plane wave is normal to the barrier). Could you give me some advice on how
to achieve this?
Best regards,
Kristjan
On Mon, Dec 19, 2016 at 8:15 PM, Abbout Adel
Dear Kristjan,
" And the conductance is the sum of the transmission probabilities of the conduction channels, right? "
Yes this is correct.
The number of conducting channels goes to zeros at higher energies because you work in a lattice: the dispersion relation is not anymore E=k^2 . In a lattice, the dispersion relation for a mode 'm' is E_m=4te_m2*cos(k) (where e_m=2*cos(m*pi/(W+1))) in order to recover the results of the continuum, you need to work at low energies.
in your case, you need to change the theoretical relation by putting a shift 22*cos(pi/(W+1)) for the energies and this will work for the first mode. you need to do the same thing (shift 2e_m) for the other modes and make a sum of the transmissions to obtain the results for higher modes.
a script is included below to show how we recover the exact result.
I hope that this helps Adel
import kwant from numpy import sqrt,sin,cos,pi,sinh # For plotting from matplotlib import pyplot
def make_system(a=1, t=1.0, W=10, L=30,V0=0):
lat = kwant.lattice.square(a)
sys = kwant.Builder()
sys[(lat(x, y) for x in range(L) for y in range(W))] = 4*t+V0 sys[lat.neighbors()] = t
#### Define and attach the leads. #### lead = kwant.Builder(kwant.TranslationalSymmetry((a, 0))) lead[(lat(0, j) for j in range(W))] = 4 * t lead[lat.neighbors()] = t sys.attach_lead(lead) sys.attach_lead(lead.reversed())
return sys
def plot_conductance(sys, energies):
# Compute conductance data = [] for energy in energies: smatrix = kwant.smatrix(sys, energy) data.append(smatrix.transmission(1, 0))
# pyplot.figure() pyplot.plot(energies, data,lw=2,label="numerics") pyplot.xlabel("well depth [t]") pyplot.ylabel("conductance [e^2/h]") pyplot.legend() pyplot.show()
def main(): V0=0.1 sys = make_system(V0=V0)
# Check that the system looks as intended. # kwant.plot(sys)
# Finalize the system. sys = sys.finalized()
e1=22*cos(pi/11) #energy of the first transverse mode def Cond(E,V0,L=30): if Ee1V0<=0: k=sqrt(E+e1+V0) return 1/(1+((V0*sinh(k*L))**2 )/(4*(Ee1)*(E+e1+V0))) else: k=sqrt(Ee1V0) return 1/(1+((V0*sin(k*L))**2 )/(4*(Ee1)*(Ee1V0))) energies=[0.1+0.001 * i for i in range(300)] Cond_analytic=[Cond(E,V0) for E in energies] pyplot.plot(energies,Cond_analytic,"ro",label="analytic")
# We should see conductance steps. plot_conductance(sys, energies)
# Call the main function if the script gets executed (as opposed to imported). # See http://docs.python.org/library/__main__.html. if __name__ == '__main__': main()
On Mon, Dec 19, 2016 at 4:25 PM, Kristjan Eimre
wrote: Hello again,
Thanks for the ideas. I have fiddled around with Kwant a bit now, and I'm trying to recreate quantum tunneling through rectangular potential barrier (see [1] for analytical solution). I have also added a figure of the analytical solution (rect_trans.png), which shows the transmission probability dependence on electron energy.
I modified the 1st tutorial code to include the rectangular barrier (see my code in [2]). I have attached the figures i got from the code (conductance.png and rect_trans.png). If i have understood correctly, then Kwant outputs the conductance depending on excitation energy. And the conductance is the sum of the transmission probabilities of the conduction channels, right?
I probably should delve deep into books on quantum transport to understand some of these things, but perhaps you could give me some answers for a good start. I have also calculated the number of conduction channels, but this goes to 0 at higher energies. Why does this occur? In the simple analytical case, there is no upper bound on electron energies.
Is it even possible to recreate the analytical quantum tunneling result for the rectangle barrier in Kwant?
Best regards, Kristjan
[1]  https://en.wikipedia.org/wiki/Rectangular_potential_barrier
 Abbout Adel
Dear Kristjan,
First, I would like to say that your problem is not very clear. If what you
want is to calculate the conductance in the x direction for a system
infinite in the x and y directions, you should know that the result is
infinite.
you can though define a conductance per mode T/M but still, to my
knowledge, you can not do it with kwant (wraparound module will help you
just to have the 2D band structure).
In your case, you are studying the scattering by a simple unifirme rectangle
On Mon, Feb 13, 2017 at 3:28 PM, Kristjan Eimre
Hello once again,
Thanks for all the help so far.
In my initial problem, I wanted to calculate the tunneling probability from bulk metal to "bulk" vacuum. The potential landscape is periodic in the "transverse" directions and assuming the free electron model, plane waves should come from the bulk and try to tunnel through the barrier. The main result I want is the tunneling probability dependence on the wave vector or D(k).
At first, I naively though that I could just point the finitewidth leads at different angles (different k vectors) at the surface and have a big lead in the vacuum as the output. This doesn't work very well, as the result depends heavily on the size of the leads, orientation of the vacuum lead and on the shape of the scattering region. In the ideal case, I would like the result to be independent of these parameters.
For example, the tunneling probability through the rectangular barrier matches with the analytical case if the scattering region is the same width as the leads, but if I change the shape and/or add periodic boundary conditions, the result doesn't match with the analytical case any more.
I have looked around and found the wraparound module ( https://gitlab.kwantproject.org/cwg/wraparound), which looks like it's exactly the thing I need. However the demo code doesn't really illustrate how to calculate transmission with arbitrary k vectors. How should I attach the leads and how to make them also periodic? As the first step, I would like to use this wraparound code on a system of infinite plane of rectangular barrier and reproduce the analytical result (when the incident plane wave is normal to the barrier). Could you give me some advice on how to achieve this?
Best regards, Kristjan
On Mon, Dec 19, 2016 at 8:15 PM, Abbout Adel
wrote: Dear Kristjan,
" And the conductance is the sum of the transmission probabilities of the conduction channels, right? "
Yes this is correct.
The number of conducting channels goes to zeros at higher energies because you work in a lattice: the dispersion relation is not anymore E=k^2 . In a lattice, the dispersion relation for a mode 'm' is E_m=4te_m2*cos(k) (where e_m=2*cos(m*pi/(W+1))) in order to recover the results of the continuum, you need to work at low energies.
in your case, you need to change the theoretical relation by putting a shift 22*cos(pi/(W+1)) for the energies and this will work for the first mode. you need to do the same thing (shift 2e_m) for the other modes and make a sum of the transmissions to obtain the results for higher modes.
a script is included below to show how we recover the exact result.
I hope that this helps Adel
import kwant from numpy import sqrt,sin,cos,pi,sinh # For plotting from matplotlib import pyplot
def make_system(a=1, t=1.0, W=10, L=30,V0=0):
lat = kwant.lattice.square(a)
sys = kwant.Builder()
sys[(lat(x, y) for x in range(L) for y in range(W))] = 4*t+V0 sys[lat.neighbors()] = t
#### Define and attach the leads. #### lead = kwant.Builder(kwant.TranslationalSymmetry((a, 0))) lead[(lat(0, j) for j in range(W))] = 4 * t lead[lat.neighbors()] = t sys.attach_lead(lead) sys.attach_lead(lead.reversed())
return sys
def plot_conductance(sys, energies):
# Compute conductance data = [] for energy in energies: smatrix = kwant.smatrix(sys, energy) data.append(smatrix.transmission(1, 0))
# pyplot.figure() pyplot.plot(energies, data,lw=2,label="numerics") pyplot.xlabel("well depth [t]") pyplot.ylabel("conductance [e^2/h]") pyplot.legend() pyplot.show()
def main(): V0=0.1 sys = make_system(V0=V0)
# Check that the system looks as intended. # kwant.plot(sys)
# Finalize the system. sys = sys.finalized()
e1=22*cos(pi/11) #energy of the first transverse mode def Cond(E,V0,L=30): if Ee1V0<=0: k=sqrt(E+e1+V0) return 1/(1+((V0*sinh(k*L))**2 )/(4*(Ee1)*(E+e1+V0))) else: k=sqrt(Ee1V0) return 1/(1+((V0*sin(k*L))**2 )/(4*(Ee1)*(Ee1V0))) energies=[0.1+0.001 * i for i in range(300)] Cond_analytic=[Cond(E,V0) for E in energies] pyplot.plot(energies,Cond_analytic,"ro",label="analytic")
# We should see conductance steps. plot_conductance(sys, energies)
# Call the main function if the script gets executed (as opposed to imported). # See http://docs.python.org/library/__main__.html. if __name__ == '__main__': main()
On Mon, Dec 19, 2016 at 4:25 PM, Kristjan Eimre
wrote: Hello again,
Thanks for the ideas. I have fiddled around with Kwant a bit now, and I'm trying to recreate quantum tunneling through rectangular potential barrier (see [1] for analytical solution). I have also added a figure of the analytical solution (rect_trans.png), which shows the transmission probability dependence on electron energy.
I modified the 1st tutorial code to include the rectangular barrier (see my code in [2]). I have attached the figures i got from the code (conductance.png and rect_trans.png). If i have understood correctly, then Kwant outputs the conductance depending on excitation energy. And the conductance is the sum of the transmission probabilities of the conduction channels, right?
I probably should delve deep into books on quantum transport to understand some of these things, but perhaps you could give me some answers for a good start. I have also calculated the number of conduction channels, but this goes to 0 at higher energies. Why does this occur? In the simple analytical case, there is no upper bound on electron energies.
Is it even possible to recreate the analytical quantum tunneling result for the rectangle barrier in Kwant?
Best regards, Kristjan
[1]  https://en.wikipedia.org/wiki/Rectangular_potential_barrier
 Abbout Adel
 Abbout Adel
(sorry I send the previous email accidentally before finishing)
...In your case, you are studying the scattering by a simple uniform
rectangle, so the modes d not mix. you just need to diagonalize your system
in the y direction, and you will get infinite number of 1D systems, each
one you can solve it easily. your conductance is the sum of all the
conductances (Actually the sum is an integral (you need to divide over
the number of modes to go from the sum to the Riemann integral))
I hope that this helps
Adel
On Tue, Feb 14, 2017 at 2:36 PM, Abbout Adel
Dear Kristjan,
First, I would like to say that your problem is not very clear. If what you want is to calculate the conductance in the x direction for a system infinite in the x and y directions, you should know that the result is infinite. you can though define a conductance per mode T/M but still, to my knowledge, you can not do it with kwant (wraparound module will help you just to have the 2D band structure).
In your case, you are studying the scattering by a simple unifirme rectangle
On Mon, Feb 13, 2017 at 3:28 PM, Kristjan Eimre
wrote: Hello once again,
Thanks for all the help so far.
In my initial problem, I wanted to calculate the tunneling probability from bulk metal to "bulk" vacuum. The potential landscape is periodic in the "transverse" directions and assuming the free electron model, plane waves should come from the bulk and try to tunnel through the barrier. The main result I want is the tunneling probability dependence on the wave vector or D(k).
At first, I naively though that I could just point the finitewidth leads at different angles (different k vectors) at the surface and have a big lead in the vacuum as the output. This doesn't work very well, as the result depends heavily on the size of the leads, orientation of the vacuum lead and on the shape of the scattering region. In the ideal case, I would like the result to be independent of these parameters.
For example, the tunneling probability through the rectangular barrier matches with the analytical case if the scattering region is the same width as the leads, but if I change the shape and/or add periodic boundary conditions, the result doesn't match with the analytical case any more.
I have looked around and found the wraparound module ( https://gitlab.kwantproject.org/cwg/wraparound), which looks like it's exactly the thing I need. However the demo code doesn't really illustrate how to calculate transmission with arbitrary k vectors. How should I attach the leads and how to make them also periodic? As the first step, I would like to use this wraparound code on a system of infinite plane of rectangular barrier and reproduce the analytical result (when the incident plane wave is normal to the barrier). Could you give me some advice on how to achieve this?
Best regards, Kristjan
On Mon, Dec 19, 2016 at 8:15 PM, Abbout Adel
wrote: Dear Kristjan,
" And the conductance is the sum of the transmission probabilities of the conduction channels, right? "
Yes this is correct.
The number of conducting channels goes to zeros at higher energies because you work in a lattice: the dispersion relation is not anymore E=k^2 . In a lattice, the dispersion relation for a mode 'm' is E_m=4te_m2*cos(k) (where e_m=2*cos(m*pi/(W+1))) in order to recover the results of the continuum, you need to work at low energies.
in your case, you need to change the theoretical relation by putting a shift 22*cos(pi/(W+1)) for the energies and this will work for the first mode. you need to do the same thing (shift 2e_m) for the other modes and make a sum of the transmissions to obtain the results for higher modes.
a script is included below to show how we recover the exact result.
I hope that this helps Adel
import kwant from numpy import sqrt,sin,cos,pi,sinh # For plotting from matplotlib import pyplot
def make_system(a=1, t=1.0, W=10, L=30,V0=0):
lat = kwant.lattice.square(a)
sys = kwant.Builder()
sys[(lat(x, y) for x in range(L) for y in range(W))] = 4*t+V0 sys[lat.neighbors()] = t
#### Define and attach the leads. #### lead = kwant.Builder(kwant.TranslationalSymmetry((a, 0))) lead[(lat(0, j) for j in range(W))] = 4 * t lead[lat.neighbors()] = t sys.attach_lead(lead) sys.attach_lead(lead.reversed())
return sys
def plot_conductance(sys, energies):
# Compute conductance data = [] for energy in energies: smatrix = kwant.smatrix(sys, energy) data.append(smatrix.transmission(1, 0))
# pyplot.figure() pyplot.plot(energies, data,lw=2,label="numerics") pyplot.xlabel("well depth [t]") pyplot.ylabel("conductance [e^2/h]") pyplot.legend() pyplot.show()
def main(): V0=0.1 sys = make_system(V0=V0)
# Check that the system looks as intended. # kwant.plot(sys)
# Finalize the system. sys = sys.finalized()
e1=22*cos(pi/11) #energy of the first transverse mode def Cond(E,V0,L=30): if Ee1V0<=0: k=sqrt(E+e1+V0) return 1/(1+((V0*sinh(k*L))**2 )/(4*(Ee1)*(E+e1+V0))) else: k=sqrt(Ee1V0) return 1/(1+((V0*sin(k*L))**2 )/(4*(Ee1)*(Ee1V0))) energies=[0.1+0.001 * i for i in range(300)] Cond_analytic=[Cond(E,V0) for E in energies] pyplot.plot(energies,Cond_analytic,"ro",label="analytic")
# We should see conductance steps. plot_conductance(sys, energies)
# Call the main function if the script gets executed (as opposed to imported). # See http://docs.python.org/library/__main__.html. if __name__ == '__main__': main()
On Mon, Dec 19, 2016 at 4:25 PM, Kristjan Eimre
wrote:
Hello again,
Thanks for the ideas. I have fiddled around with Kwant a bit now, and I'm trying to recreate quantum tunneling through rectangular potential barrier (see [1] for analytical solution). I have also added a figure of the analytical solution (rect_trans.png), which shows the transmission probability dependence on electron energy.
I modified the 1st tutorial code to include the rectangular barrier (see my code in [2]). I have attached the figures i got from the code (conductance.png and rect_trans.png). If i have understood correctly, then Kwant outputs the conductance depending on excitation energy. And the conductance is the sum of the transmission probabilities of the conduction channels, right?
I probably should delve deep into books on quantum transport to understand some of these things, but perhaps you could give me some answers for a good start. I have also calculated the number of conduction channels, but this goes to 0 at higher energies. Why does this occur? In the simple analytical case, there is no upper bound on electron energies.
Is it even possible to recreate the analytical quantum tunneling result for the rectangle barrier in Kwant?
Best regards, Kristjan
[1]  https://en.wikipedia.org/wiki/Rectangular_potential_barrier
 Abbout Adel
 Abbout Adel
 Abbout Adel
Hi Abbout, Thanks for the reply. If what you want is to calculate the conductance in the x direction for a
system infinite in the x and y directions, you should know that the result is infinite.
In principle, when we have an infinite scattering system, electrons should
travel as plane waves with specific k vectors, but no different modes with
the same k vector should exist. I'm not entirely sure how the conductance
forms in this case, but it's not really important, as I want the tunneling
probability dependence on k vector and I don't need the "total" conductance.
In the uniform rectangular barrier, what I would expect is that the
tunneling probability will be given by the 1d analytical result, only that
the apparent width of the barrier depends on the direction of k vector.
Best,
Kristjan
On Tue, Feb 14, 2017 at 1:41 PM, Abbout Adel
(sorry I send the previous email accidentally before finishing)
...In your case, you are studying the scattering by a simple uniform rectangle, so the modes d not mix. you just need to diagonalize your system in the y direction, and you will get infinite number of 1D systems, each one you can solve it easily. your conductance is the sum of all the conductances (Actually the sum is an integral (you need to divide over the number of modes to go from the sum to the Riemann integral))
I hope that this helps Adel
On Tue, Feb 14, 2017 at 2:36 PM, Abbout Adel
wrote: Dear Kristjan,
First, I would like to say that your problem is not very clear. If what you want is to calculate the conductance in the x direction for a system infinite in the x and y directions, you should know that the result is infinite. you can though define a conductance per mode T/M but still, to my knowledge, you can not do it with kwant (wraparound module will help you just to have the 2D band structure).
In your case, you are studying the scattering by a simple unifirme rectangle
On Mon, Feb 13, 2017 at 3:28 PM, Kristjan Eimre
wrote: Hello once again,
Thanks for all the help so far.
In my initial problem, I wanted to calculate the tunneling probability from bulk metal to "bulk" vacuum. The potential landscape is periodic in the "transverse" directions and assuming the free electron model, plane waves should come from the bulk and try to tunnel through the barrier. The main result I want is the tunneling probability dependence on the wave vector or D(k).
At first, I naively though that I could just point the finitewidth leads at different angles (different k vectors) at the surface and have a big lead in the vacuum as the output. This doesn't work very well, as the result depends heavily on the size of the leads, orientation of the vacuum lead and on the shape of the scattering region. In the ideal case, I would like the result to be independent of these parameters.
For example, the tunneling probability through the rectangular barrier matches with the analytical case if the scattering region is the same width as the leads, but if I change the shape and/or add periodic boundary conditions, the result doesn't match with the analytical case any more.
I have looked around and found the wraparound module ( https://gitlab.kwantproject.org/cwg/wraparound), which looks like it's exactly the thing I need. However the demo code doesn't really illustrate how to calculate transmission with arbitrary k vectors. How should I attach the leads and how to make them also periodic? As the first step, I would like to use this wraparound code on a system of infinite plane of rectangular barrier and reproduce the analytical result (when the incident plane wave is normal to the barrier). Could you give me some advice on how to achieve this?
Best regards, Kristjan
On Mon, Dec 19, 2016 at 8:15 PM, Abbout Adel
wrote: Dear Kristjan,
> " And the conductance is the sum of the transmission probabilities of the conduction channels, right? "
Yes this is correct.
The number of conducting channels goes to zeros at higher energies because you work in a lattice: the dispersion relation is not anymore E=k^2 . In a lattice, the dispersion relation for a mode 'm' is E_m=4te_m2*cos(k) (where e_m=2*cos(m*pi/(W+1))) in order to recover the results of the continuum, you need to work at low energies.
in your case, you need to change the theoretical relation by putting a shift 22*cos(pi/(W+1)) for the energies and this will work for the first mode. you need to do the same thing (shift 2e_m) for the other modes and make a sum of the transmissions to obtain the results for higher modes.
a script is included below to show how we recover the exact result.
I hope that this helps Adel
import kwant from numpy import sqrt,sin,cos,pi,sinh # For plotting from matplotlib import pyplot
def make_system(a=1, t=1.0, W=10, L=30,V0=0):
lat = kwant.lattice.square(a)
sys = kwant.Builder()
sys[(lat(x, y) for x in range(L) for y in range(W))] = 4*t+V0 sys[lat.neighbors()] = t
#### Define and attach the leads. #### lead = kwant.Builder(kwant.TranslationalSymmetry((a, 0))) lead[(lat(0, j) for j in range(W))] = 4 * t lead[lat.neighbors()] = t sys.attach_lead(lead) sys.attach_lead(lead.reversed())
return sys
def plot_conductance(sys, energies):
# Compute conductance data = [] for energy in energies: smatrix = kwant.smatrix(sys, energy) data.append(smatrix.transmission(1, 0))
# pyplot.figure() pyplot.plot(energies, data,lw=2,label="numerics") pyplot.xlabel("well depth [t]") pyplot.ylabel("conductance [e^2/h]") pyplot.legend() pyplot.show()
def main(): V0=0.1 sys = make_system(V0=V0)
# Check that the system looks as intended. # kwant.plot(sys)
# Finalize the system. sys = sys.finalized()
e1=22*cos(pi/11) #energy of the first transverse mode def Cond(E,V0,L=30): if Ee1V0<=0: k=sqrt(E+e1+V0) return 1/(1+((V0*sinh(k*L))**2 )/(4*(Ee1)*(E+e1+V0))) else: k=sqrt(Ee1V0) return 1/(1+((V0*sin(k*L))**2 )/(4*(Ee1)*(Ee1V0))) energies=[0.1+0.001 * i for i in range(300)] Cond_analytic=[Cond(E,V0) for E in energies] pyplot.plot(energies,Cond_analytic,"ro",label="analytic")
# We should see conductance steps. plot_conductance(sys, energies)
# Call the main function if the script gets executed (as opposed to imported). # See http://docs.python.org/library/__main__.html. if __name__ == '__main__': main()
On Mon, Dec 19, 2016 at 4:25 PM, Kristjan Eimre < kristjaneimre@gmail.com> wrote:
Hello again,
Thanks for the ideas. I have fiddled around with Kwant a bit now, and I'm trying to recreate quantum tunneling through rectangular potential barrier (see [1] for analytical solution). I have also added a figure of the analytical solution (rect_trans.png), which shows the transmission probability dependence on electron energy.
I modified the 1st tutorial code to include the rectangular barrier (see my code in [2]). I have attached the figures i got from the code (conductance.png and rect_trans.png). If i have understood correctly, then Kwant outputs the conductance depending on excitation energy. And the conductance is the sum of the transmission probabilities of the conduction channels, right?
I probably should delve deep into books on quantum transport to understand some of these things, but perhaps you could give me some answers for a good start. I have also calculated the number of conduction channels, but this goes to 0 at higher energies. Why does this occur? In the simple analytical case, there is no upper bound on electron energies.
Is it even possible to recreate the analytical quantum tunneling result for the rectangle barrier in Kwant?
Best regards, Kristjan
[1]  https://en.wikipedia.org/wiki/Rectangular_potential_barrier
 Abbout Adel
 Abbout Adel
 Abbout Adel
participants (5)

Abbout Adel

Christoph Groth

Joseph Weston

Kristjan Eimre

Sergey Slizovskiy