Frustration controlled metal-insulator transition

This is a schematic phase diagram associated with the Mott-Hubbard metal-insulator transit

ion, taken from a widely cited review article by Imada, Fujimori, and Tokura.

The Mott insulating phase occurs at half filling and when the on-site Coulomb repulsion U is much larger than the hopping energy t and the associated band width. A transition to a metallic phase occurs either by doping away from half filling [FC-MIT= filling controlled metal-insulator transition] or by decreasing the ratio U/t [BC-MIT bandwidth controlled metal-insulator transition]. In the cuprates FC-MIT occurs whereas in the organic charge transfer salts such as (BEDT-TTF)2X one might argue that the BC-MIT occurs.

On the other hand, perhaps one should consider a third co-ordinate, the frustration, in addition to the filling and band width. This would lead to the notion of an FrC-MIT, a frustration controlled transition.

In the Hubbard model on the anisotropic triangular lattice at half-filling for fixed U/t increasing t'/t can drive an insulator to metal transition, as shown in the Figure below taken from

this 2006 review of the organic charge transfer salts, by Ben Powell and myself. [See also references 16 to 23 of this recent paper].