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nextnano3 - Tutorial

next generation 3D nano device simulator

2D Tutorial

Landau levels of a bulk GaAs sample 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.
 -> 2DBulkGaAs_LandauLevels.in
 


Landau levels of a bulk GaAs sample in a magnetic field

In this tutorial we study the electron energy levels of a bulk GaAs sample that is subject to a magnetic field.

  • The GaAs sample extends in the x and y directions (i.e. this is a two-dimensional simulation) and has the size of 300 nm x 300 nm.
    At the domain boundaries we employ Dirichlet boundary conditions to the Schrödinger equation, i.e. infinite barriers.
     
  • The magnetic field is oriented along the z direction, i.e. it it perpendicular to the simulation plane which is oriented in the (x,y) plane).
    We calculate the eigenstates for different magnetic field strengths (1 T, 2 T, 3 T), i.e. we make use of the magnetic field sweep.

      $magnetic-field
       magnetic-field-on                    = yes
       magnetic-field-strength              = 1.0d0   !
    1 Tesla = 1 Vs/m2
       magnetic-field-direction             = 0 0 1   !
    [001] direction
       magnetic-field-sweep-active          = yes     !
       magnetic-field-sweep-step-size       = 1.0d0   !
    1 Tesla = 1 Vs/m2
       magnetic-field-sweep-number-of-steps = 2       !
    2 magnetic field sweep steps
      $end_magnetic-field

     
  • A useful quantitiy is the magnetic length (or Landau magnetic length) which is defined as:
        lB = [hbar / (me* wc)]1/2 = [hbar / (|e| B)]1/2
    It is independent of the mass of the particle and depends only on the magnetic field strength:
    - 1 T: 25.6556 nm
    - 2 T: 18.1413 nm
    - 3 T: 14.8123 nm
     
  • The electron effective mass in GaAs is me* = 0.067 m0. Another useful quantity is the cyclotron frequency:
        wc = |e| B / me*
    Thus for the electrons in GaAs, it holds for the different magnetic field strengths:
    - 1 T: hbarwc = 1.7279 meV
    - 2 T: hbarwc = 3.4558 meV
    - 3 T: hbarwc = 5.1836 meV

 

  • The energy spectra for different magnetic fields (1 T, 2 T, 3 T) look as follows:
     


 


 

 
 
  • The Landau levels are given by En = (n - 1/2) hbarwc where n = 1,2,3,...
     
  • The number of states for each Landau level can be calculated as follows (see P.Y. Yu, M. Cardona, Fundamentals of Semiconductors, p. 536, 3rd ed.):
        N = LxLy |e| B / h = 1/(2pi)  LxLy / lB2        (ignoring spin)
    where Lx and Ly are the lengths in the x and y directions (300 nm in this example) and lB is the magnetic length.

    N(1 T) = 21.76 ==> ~22 states per Landau level (in the figure above: 22 as it should be)
    N(2 T) = 43.52 ==> ~44 states per Landau level (in the figure above: 44 as it should be)
    N(3 T) = 65.29 ==> ~66 states per Landau level (in the figure above: 66 as it should be)

    Note that N is independent of n.
  • For the calculations, we used the symmetric gauge A = - 1/2 r x B = 1/2 B x r
    leading to the following energies (see J.H. Davies, The Physics of Low-Dimensional Semiconductors, p. 222):
        En,l = (n + 1/2 l + 1/2 |l| - 1/2) hbarwc
    One can see that all states having a negative value of 'l' are degenerate with the states with l=0, i.e. the allowed energies are independent of l if l < 0 (for the same n).
    The energies increase if l increases (for l > 0 and for the same n).
     
  • The motion in the z direction is not influenced by the magnetic field and is thus that of a free particle with energies and wave functions given by:
       Ez = hbar2 kz2 / (2 m*)
       psi(z) = exp (+- i kz z)
    For that reason, we did not include the z direction into our simulation domain, and thus only simulate in the (x,y) plane (two-dimensional simulation).

 

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