Nonlocal response of a small coated sphere

A theory for the response of a small spherical particle with a coating is developed using the semiclassical infinite-barrier model. Both the core and the outer layer are described by a nonlocal dielectric function. The theory is applied to an Al₂{O}₃$-coated Al sphere and to an Sn-coated ${Al}₂O₃ sphere, and we find the coating to dominate the high-multipole response of the particle in either case. The optical-absorption and scanning tunneling electron microscopy spectra of the above systems are discussed.