  We propose that one of the best grounds for the materials design from the
viewpoint of {\it electron correlation} such as ferromagnetism,
superconductivity is the atomically controlled nanostructures and
heterointerfaces, as theoretically demonstrated here from three examples with
first-principles calculations: (i) Band ferromagnetism in a purely organic
polymer of five-membered rings, where the flat-band ferromagnetism due to the
electron-electron repulsion is proposed. (ii) Metal-induced gap states (MIGS)
of about one atomic monolayer thick at insulator/metal heterointerfaces,
recently detected experimentally, for which an exciton-mechanism
superconductivity is considered. (iii) Alkali-metal doped zeolite, a class of
nanostructured host-guest systems, where ferromagnetism has been experimentally
discovered, for which a picture of the "supercrystal" composed of "superatoms"
is proposed and Mott-insulator properties are considered. These indicate that
design of electron correlation is indeed a promising avenue for nanostructures
and heterointerfaces.
