Since condensed-matter and materials are often thought to Since physics deals with welldefined and understood things (atoms) interacting with a well-defined and understood force, there are no fundamental intellectual difficulties remaining to find, actually, the opposite is true. Insulators have an energy gap for charge excitations, such that a collection of copper atoms with large interstices between them cannot conduct electricity. This is because the atoms are not strongly coupled to one another, hence their individual spectra are preserved. Solid copper is an efficient electrical conductor because its electrons "melt" into a new "liquid" phase when the atoms are compressed to absolute zero, even though this liquid phase has no excitation gap. Very little variations in atomic characteristics transform aluminium from a regular metal into a superconductor when the same experiment is carried out with aluminium atoms. The unexpected features of the large families of complex materials with many atoms per unit cell are still very difficult to anticipate a priori. In the next decade, this will be a crucial problem for theoretical researchers to solve.

Aluminum, Atoms, Materials, Superconductor