One difficulty with your computational cell is that you actually have
two surfaces -- one on the left and one on the right. Remember that
there are periodic boundary conditions.
The solution is to intentionally have two equal surfaces that are
mirror symmetric along the y direction, and then use run-yeven to
avoid getting the same mode twice.
That is, have a computational cell like:
--> y direction
| o o o o o
o o o o |
| o o o o o
o o o o o |
| o o o o o
o o o o |
left
surface right surface
When you simulate this structure, if there is a surface state at the
left surface there will also be the same surface state at the right
surface, by symmetry. The exponential tails of these surface states
will cause them to couple into even (+ +) and odd (+ -)
combinations with slightly different frequencies. If the supercell
size is big enough, however, this coupling is negligible (decreasing
exponentially with the size of the crystal), so essentially you have
two copies of the same mode. To get just one, you use run-yeven (or
run-yodd, it doesn't matter), which will pick out one of the linear
combinations.
Steven
On Sep 2, 2009, at 10:39 PM, Chad Huskow wrote:
Dear Steven and MPB users,
I have solved the problem about the surface mode in 2D-PC
triangular structure of air holes in dielectric background. You can
see my PC structure in the attach. My problem is the surface mode
along Gamma-M direction (by changing the surface termination along
Gamma-M). In that case, the surface termination factor C=0.5. I want
to investigate the surface band structures for various the surface
termination factors.
I don't know how to define that structure MPB.
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