Mott physics in the multiflavored age

In 1937, de Boer and Verwey discovered that nickel oxide, which should be a metal according to band theory, was an insulator 1 . The first detailed theory of this family of systems, known as Mott insulators, was put forward by Mott more than a decade later 2 . If the number of electrons per unit cell is an odd integer, and if the hopping between sites is small enough, Coulomb repulsion can stabilize a state in which electrons are localized. This theory has two testable consequences: First, it should be possible to induce a transition to a metallic state by applying pressure since this will increase the hopping; Secondly, contrary to band insulators, Mott insulators do not have a gap to all excitations, but only to charge excitations. The spins of the electrons are not frozen, and the minimal model to describe their fluctuations is the Heisenberg model