Anomalous suppression of valley splittings in lead salt nanocrystals without inversion center

We demonstrate that confinement-induced intervalley splittings of electron energy levels in PbSe and PbS nanocrystals are sensitive to the arrangement of atoms within a nanocrystal. The splittings are strongly suppressed for stoichiometric nanocrystals of T _d point symmetry lacking a center of inversion as opposed to nonstoichiometric nanocrystals of O _h point symmetry having an inversion center. Our findings are supported by both atomistic sp3d5s ^* tight-binding calculations and a symmetry analysis.