Abnormal selection rules of interface modes in ultrathin GaAs/AlAs superlattice

We observed a violation of the normal Raman selection rule in the resonant Raman spectra of interface (IF) phonon modes of the ultrathin (GaAs)4/(AlAs)2 superlattice. Contrary to the prediction of conventional theories, all four IF modes were observed in both (XX) and (XY) geometries. The result can be interpreted as a consequence of the deep penetration of the electron wave function in the GaAs wells into the AlAs barriers and a lack of definite parity of the electron wave function. Furthermore, our result indicates that conventional theory for bulk (thicker) systems may need to be modified and further developed to be applicable to ultrathin systems.