STM image of Graphene

[lahaye]

Dear All,

I hope to get some advice on this topic on this list.

A collegue of mine started taking STM images from graphite
(graphite surface or graphene layer). The obtained images have
bright spots for every other atom. This observation seems
to be supported by STM images in the literature.
These STM images are hexagons with an additional spot in
its centre.

Hence my approach to set up input files for graphite, with 2 atoms
in the unit celll, and calculate the charge density near the Fermi
energy. I expected a charge distribution that is asymmetric with
respect to the two atoms in the unit cell, in order to explain the
STM images that detect every other atom.
However, the charge density of the k-points at the fermi level is
perfectly symmetric with respect to the two atoms in the unit cell.
So now I wonder whether I'm doing something wrong and how
I can match theory with experiment.....

The POSCAR file is:

Code: Select all

Graphene sheet at 10 A layer separation
   2.46852
 0.866025404  -0.5  0.0
 0.866025404   0.5  0.0
 0.0           0.0  4.0510103219
2
cartesian coordinates of positions
 0.0          0.0  0.0
 0.577350269  0.0  0.0

The kpoints file is:

Code: Select all

k-points with Monckhorst-Pack
0
Monkhorst-Pack
15 15 1
0 0 0

The INCAR file is:

Code: Select all

 PREC  = Normal
 EDIFF = 1E-5
 LREAL=.FALSE.
 ISMEAR = -5
 LORBIT = 12

# use next lines after having generated WAVECAR file
 LPARD = .TRUE.
 NBMOD = -3
 EINT = -0.05 0.05
 LSEPK = .TRUE.

Thanks,
Rob.


PS: I have a copy of the paper by Tersoff and Hamann on the "Theory
of the scanning tunneling microscope" [PRB 31 (1985) 805], in which
equation (10) also states that the tunneling conductance scales with
the charge density at the Fermi energy.

[matt_dubya]

Did you run with symmetry?

[admin]

also, tunneling EITHER goes from the tip to the surface OR from the surface to the tip. One of the E-interval boundaries has to be the fermi level therefore. Please check if this is consistent with your EINT input. if it is not, you just plot the charge density of any energy interval of the spectrum of states, which may have nothing to do with the tunneling current (according to Tersoff and Haman)