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nextnano3 - Tutorial

next generation 3D nano device simulator

2D Tutorial

Fock-Darwin states of a 2D parabolic, anisotropic (elliptical) potential in a magnetic field

Author: Stefan Birner

If you want to obtain the input files that are used within this tutorial, please contact stefan.birner@nextnano.de.
 -> 1DGaAs_ParabolicQW_infinite_4_6meV.in
 -> 1DGaAs_ParabolicQW_infinite_6_1meV.in
 -> 2DGaAs_BiParabolicEllipticQD_Austing.in
 


Fock-Darwin states of a 2D parabolic, anisotropic (elliptical) potential in a magnetic field

In this tutorial we study the electron energy levels of a two-dimensional parabolic, anisotropic (elliptical) confinement potential that is subject to a magnetic field.
Such a potential can be constructed by surrounding GaAs with an AlxGa1-xAs alloy that has a parabolic alloy profile in the x and y directions.

 

First, it is necessary to study the energy states of a 1D parabolic confinement.

 

1D parabolic confinement along the x direction with hbarw0 = 4.6 meV (1D simulation)

 -> 1DGaAs_ParabolicQW_infinite_4_6meV.in

For similar results and a discussion, we refer to this tutorial: Parabolic Quantum Well (GaAs / AlAs)

 

1D parabolic confinement along the y direction with hbarw0 = 6.1 meV (1D simulation)

 -> 1DGaAs_ParabolicQW_infinite_6_1meV.in

For similar results and a discussion, we refer to this tutorial: Parabolic Quantum Well (GaAs / AlAs)

 

First, it is a good idea to get familiar with the results of a 2D parabolic and isotropic confinement: Fock-Darwin states of a 2D parabolic potential in a magnetic field

 

Now we turn to the anisotropic confinement...

 

2D parabolic, anisotropic (elliptical) confinement with hbarwx = 4.6 meV and and hbarwy = 6.1 meV -
Fock-Darwin-like spectrum  (2D simulation)

 -> 2DGaAs_BiParabolicEllipticQD_Austing.in
 

  • The electron effective mass in GaAs is me* = 0.067 m0. We assume this value for the effective mass in the whole region (i.e. also inside the AlGaAs alloy).
     
  • The following figure shows the parabolic, anisotropic (elliptical) conduction band edge confinement potential, as well as the ground state wave function (psi2) at B = 0 T.
    In the middle of the sample the conduction band edge is at 0 eV and at the boundary region the conduction band edge has the value 0.84 eV. The radii of the ellipse are 300 nm along the x axis and 226 nm along the y axis.
    The parabolic confinement along the x direction is: hbarwx = 4.6 meV
    The parabolic confinement along the y direction is: hbarwy = 6.1 meV
    Thus the ellipticity is roughly 4/3.


     

At zero magnetic field, the eigenvalues for such a system are given by:
    Enx,ny = (nx + 1/2) hbarwx + (ny + 1/2) hbar wy
    nx = n + 1/2 |l| - 1/2 |l|
    ny = n + 1/2 |l| + 1/2 |l|
          for n = 0,1,2,3,... and l = 0,+-1,+-2,...
(n = radial quantum number, l = angular momentum quantum number, wx and wy = oscillator frequencies)
For more details, see A.V. Madhav, T. Chakraborty, Physical Review B 49, 8163 (1994).

The eigenvalue spectrum of a 2D parabolic and isotropic potential shows a shell-like structure: Energy levels of an "artificial atom" - 2D harmonic potential
For the anisotropic elliptical potential, this degeneracy at B = 0 T is lifted.

The following figure shows the calculated Fock-Darwin-like spectrum, i.e. the eigenstates as a function of magnetic field magnitude.
Here, each of these states is two-fold spin-degenerate. However, a magnetic field lifts this degeneracy (Zeeman splitting) but this effect is not taking into account in this tutorial.

  • Such a spectrum can be related to experimental transport measurements which give insight into the single-particle energy spectrum of a quantum dot.

    The rectangles in the above figure are related to the figures of the following publications:

    cyan rectangle: Fig. 2 of
         Two-level anti-crossings high up in the single-particle energy spectrum of a quantum dot
         C. Payette, D.G. Austing, G. Yu, J.A. Gupta, S.V. Nair, B. Partoens, S. Amaha, S. Tarucha
         arXiv:0710.1035v1 [cond-mat.mes-hall] (2007)

    green rectangle: Fig. 2(b) of
    red rectangle: Fig. 3(a) of
         Probing by transport the single-particle energy spectrum up to high energy of
         one quantum dot with the ground state of an adjacent weakly coupled quantum dot
         D.G. Austing, G. Yu, C. Payette, J.A. Gupta, M. Korkusinski, G.C. Aers
         physica status solidi (a), 508 (2007)

    (Comments red rectangle: In Fig. 3(a) of the publication by Austing et al., the ground state energy has been subtracted from the excited states. Thus the slope of the energy spectrum look slightly different.)

    It is interesting to note that there are exact crossings in the calculated spectrum whereas the experiment reveals anti-crossings.

 

  • Please help us to improve our tutorial. Send comments to support [at] nextnano.com.