nextnano^{3}  Tutorial
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1D Tutorial
Tightbinding band structure of graphene
Note: This tutorial's copyright is owned by Stefan Birner,
www.nextnano.com.
Author:
Stefan Birner,
Reinhard
Scholz
If you are interested in the input files that are used within this tutorial, please contact stefan.birner@nextnano.de.
> 1D_TightBinding_graphene.in
Tightbinding band structure of graphene
Nearestneighbor tightbinding approximation
In this tutorial we calculate the bulk band structure of graphene which is a
twodimensional crystal (i.e. a monolayer of graphite) using a standard
tightbinding approach.
For more details, see for example the article
Optical Properties and Raman Spectroscopy of Carbon Nanotubes
R. Saito, H. Kataura
in
Carbon Nanotubes  Synthesis, Structure, Properties, and Applications
M.S. Dresselhaus, G. Dresselhaus, P. Avouris (Eds.)
Topics in Applied Physics, Vol. 80, Springer (2001)
The following figure shows the conduction band pi* (upper part) and valence
band pi (lower part) of graphene along special highsymmetry directions in the twodimensional
hexagonal Brillouin zone (k space).
The high symmetry points that are used in this graph (from left to right)
are:
 K: k = (k_{x},k_{y})
= ( 0 , 2/3 ) 2 pi/a
 Gamma: k = (k_{x},k_{y}) = ( 0
, 0 )
 M: k = (k_{x},k_{y})
= ( 1/3^{1/2} , 0 ) 2 pi/a
 K': k = (k_{x},k_{y})
= ( 1/3^{1/2} , 1/3 ) 2 pi/a
Two lines correspond to the case where the s_{0} parameter is set to
zero, i.e. in that case the dispersion of both pi* and pi is the same (apart
from the sign, i.e. they are symmetric with respect to the Fermi level E_{F}
= 0 eV). In this case the splitting energy at Gamma is three times as large as
at the M point:
 splitting at Gamma: 6 gamma_{0}
(for s_{0} = 0)
 splitting at M:
2 gamma_{0} (for s_{0} = 0)
Two lines correspond to the case where s_{0} = 0.129. pi* and pi are
then nonsymmetric and are close to calculations from first principles and
experimental data.
The data points are contained in this file:
TightBinding/BandStructureGraphene.dat
The first column contains integers which refer to the x axis (i.e.
numbering of k points), the second column contains the eigenvalue of pi* in
units of [eV] (conduction band), the third column contains the
eigenvalue of pi in units of [eV] (valence band).
The file TightBinding/k_vectors.dat contains information about which
integer corresponds to which k_{x} and k_{y} value.
The general formula for these lines read:
E_{+,} = [ E_{2p}  + gamma_{0}
w(k) ] / [ 1  + s_{0} w(k) ]
The parameters can be specified in the input file:
$numericcontrol
...
(nearestneighbor approximation, i.e. 3
parameters)
tightbindingparameters = 0d0
! [eV] E_{2p}: site energy of the 2p_{z}
atomic orbital (orbital energy)
3.013d0 ! [eV] gamma_{0}: CC
transfer energy (usually it holds:  3 eV < gamma_{0}
<  2.5 eV)
0.129d0 ! [] s_{0}
= 0.129: denotes the overlap of the electronic wave function on adjacent sites
! 0d0 ! []
s_{0} = 0: denotes the
overlap of the electronic wave function on adjacent sites
!
(usually it holds: s_{0} < 0.1. Since this value is small, very
often, it is neglected.)
Then there are two lines that are linear around the K point (k.p
approximation or linear expansion). Their linear dispersion is independent of
the parameter s_{0}. Thus for small values of k (i.e. with respect to
the K point), the energy dispersion
can be approximated by a linear dispersion relation.
E(k) = E_{2p} + h_{bar} v_{F} 
k  = E_{2p} + 3^{1/2} gamma_{0} k
a / 2
where a is the lattice constant of graphene (a = 0.24612 nm) and
the Fermi velocity of the charge carriers is given by v_{F} = 3^{1/2}
 gamma_{0}

a / (2 h_{bar}) ~= 0.98 * 10^{6} m/s ~= 0.003 c
where c is the velocity of light.
These data points are contained in this file:
TightBinding/BandStructureGraphene_kp.dat
At the K point, the band gap is zero.
The following figure shows the energy dispersion E(k_{x},k_{y})
of graphene for s_{0} = 0.129.
At the K points, the band gap is zero.
The point in the middle is the Gamma point.
k_{x} is from [ 2/3,2/3] 2 pi/a, the same holds for k_{y}.
These data points are contained in the files:
 TightBinding/BandStructureGraphene_cb.dat / *.coord / *.fld
 TightBinding/BandStructureGraphene_vb.dat / *.coord / *.fld
The following figure shows the contour plot of the energy dispersion E(k_{x},k_{y})
of graphene for s_{0} = 0 for the conduction band.
(Note that the valence band dispersion is identical, apart from the sign, for s_{0}
= 0.)
These data points are contained in the file:
TightBinding/BandStructureGraphene_cb.dat / *.coord / *.fld
The following figure shows the contour plot of the energy dispersion E(k_{x},k_{y})
of graphene for s_{0} = 0.129 for the conduction band.
These data points are contained in the file:
TightBinding/BandStructureGraphene_cb.dat / *.coord / *.fld
The following figure shows the contour plot of the energy dispersion E(k_{x},k_{y})
of graphene for s_{0} = 0.129 for the valence band.
These data points are contained in the file:
TightBinding/BandStructureGraphene_vb.dat / *.coord / *.fld
nextnanomat screenshot for the conduction band dispersion E(k_{x},k_{y}).
The six dark areas correspong to the Dirac points in graphene.
Output options
!!
$outputkpdata
...
bulkkpdispersion3D = yes ! In
this case, this refers to the bulk 2D tightbinding energy dispersion E(kx,ky).
! If
yes , then the twodimensional energy
dispersion E(kx,ky) is written out.
$tighten
destinationdirectory = TightBinding/
!!
! This parameter defines the resolution of the E(kx,ky) energy
dispersion.
! E.g. if 'numberofkpoints = 100' then
the kx gridding has a total of
! [100,...,0,...,100] grid points,
! i.e. a total of 2 * 100 + 1 = 201 grid points along the kx direction.
! The same applies for ky direction.
!!
numberofkpoints = 100
! ==> 2 * n + 1
Thirdnearestneighbor tightbinding approximation
The following figure shows the band structure of graphene.
All lines are identical to the ones shown already above with the exception of
the blue lines which is the thirdnearestneighbor tightbinding approximation.
The thirdnearestneighbor tightbinding approximation is described in the
following paper:
[Reich]
Tightbinding description of graphene
S. Reich, J. Maultzsch, C. Thomsen, P. Ordejon
Physical Review B 66, 035412 (2002)
The following parameters have been used:
(thirdnearestneighbor approximation,
i.e. 7 parameters)
tightbindingparameters = 0.28d0
! [eV] E_{2p}: site energy of the 2p_{z}
atomic orbital (orbital energy)
2.97d0 ! [eV] gamma_{0}: CC
transfer energy (nearestneighbor, nn)
0.073d0 ! [] s_{0}:
denotes the overlap of the electronic wave function on adjacent sites (nn)
0.073d0 ! [eV] gamma_{1}: (2^{nd}nn)
0.018d0 ! [] s_{1}:
(2^{nd}nn)
0.33d0 ! [eV] gamma_{2}: (3^{rd}nn)
0.026d0 ! [] s_{2}:
(3^{rd}nn)
(The parameters are taken from Reich et al. Note
that the band gap at the K point is not exactly zero when using these
parameters.)
More options...
Inside the code several options, i.e. several algorihms, exist to setup the
tightbinding Hamiltonian:
tightbindingmethod = bulkgrapheneSaitonn
! nn =
nearestneighbor
= bulkgrapheneScholznn !
nn =
nearestneighbor
= bulkgrapheneScholz3rdnn !
3rdnn = thirdnearestneighbor
= bulkgrapheneReich3rdnn !
3rdnn = thirdnearestneighbor
Special option for thirdnearestneighbor tightbinding approximation in
graphene:
tightbindingcalculateparameter = no
! (default) use 7
parameters (E_{2p},gamma_{0},gamma_{1},gamma_{2},s_{0},s_{1},s_{2})
= E2p !
use 6 parameters ( gamma_{0},gamma_{1},gamma_{2},s_{0},s_{1},s_{2})
= gamma1 !
use 6 parameters (E_{2p},gamma_{0}, gamma_{2},s_{0},s_{1},s_{2})
E_{2p} or gamma_{1} can be calculated internally in order to
force E(K) = 0 eV where K is the K point in the Brillouin zone. In that case,
only 6 parameters are used (although 7 parameters have to be present in the
input file). The parameter to be calculated is simply ignored inside the code.
The k space resolution, i.e. the number of grid points on the axis of these
plots can be adjusted.
$tighten
calculatetightbindingtighten = no
!
destinationdirectory
= TightBinding/
numberofkpoints
= 50
! This corresponds to 50 k
points between the Gamma point and the M point.
! The number of k points along the other directions are scaled
correspondingly.
 Please help us to improve our tutorial. Send comments to
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[at] nextnano.com .
