Million-atom pseudopotential calculation of Gamma-X mixing in GaAs/AlAs superlattices and quantum dots

We have developed a ''linear combination of bulk bands'' method that permits atomistic, pseudopotential electronic structure calculations for similar to 10(6) atom nanostructures. Application to (GaAs)(n)/(AlAs)(n) (001) superlattices (SL's) reveals even-odd oscillations in the Gamma-X coupling magnitude V-Gamma x(n), which vanishes for n = odd, even for abrupt and segregated SL's, respectively. Surprisingly, in contrast with recent expectations, 0D quantum dots are found here to have a smaller Gamma-X coupling than equivalent 2D SL's. Our analysis shows that for large quantum dots this is largely due to the existence of level repulsion from many X states.