[Wien] Slab dielectric function vs. bulk dielectric function

[phlhj phlhj]

Dear all,

I was trying to calculate the optical properties of Al(111) slab. For the
bulk FCC Al, I can reproduce the dielectric functions and plasma frequency
very precisely reported in literature before.  However, I did find some
difference between the slab dielectric functions and the corresponding bulk
values.

Especially even though I used very thick slab, say 39MLs, in the low photo
energy range (<1eV), the imaginary part is much larger than the bulk. I
doubt this may be related to the band-folding and symmetry reduction in the
direction normal to the surface.

Also, I found the plasma frequency of the slab is smaller than the bulk
plasma frequency.

Mathematically, this behavior of the imaginary parts of the interband and
intraband transitions contributions seems to be able to be understood from
the f-sum rule.

1) However, physically it's hard to believe, because when the slab
thickness is very thick for example the 39MLs used in my test calculation,
the slab wavefunctions should be very very close to the bulk wavefunction
except in the very thin slab/vacuum interface region. This should give us
the dielectric functions for the slab which are very very close to the bulk
values. This argument should be also true for the slab plasma frequency.

2) If the different values are because of the surface slab structure we
used in the calculation, which indeed breaks the translational symmetry in
the normal direction. Then the question is that in real experiment because
the sample always is finite with the boundary surface, how can we get the
dielectric information really for the ideal bulk rather than the slab
similar as that mentioned above. Or in calculating dielectric function,
when should we use bulk geometry? when should we use slab geometry?

Thanks a lot for any idea.

Wenmei
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