Medium effects on the van der Waals force

We consider the van der Waals interaction between two ground-state atoms embedded in adjacent semi-infinite magnetodielectric media, with emphasis on medium effects on it. We demonstrate that, in this case, at small atom-atom distances the van der Waals interaction is screened by the surrounding media in the same way as in an effective (single) medium. At larger atomic distances, however, its dependence on the material parameters of the system and the positions of the atoms is more complex. We also calculate the Casimir-Polder potential of an atom A arising from a uniform distribution of atoms B in the medium across the interface. Comparison of this potential with the corresponding result deduced from the Casimir force on a thin composite slab in front of a composite semi-infinite medium, both obeying the Clausius-Mossotti relation, suggests a hint on how to improve a well-known formula for the van der Waals potential with respect to the local-field effects.