Another mechanism for the insulator-metal transition observed in Mott insulators

The two widely accepted mechanisms of the insulator-metal Mott--Hubbard transitions which have been considered up until now are driven by the band-filling or bandwidth effects. We found a different mechanism of the Mott--Hubbard insulator-metal transition, which is controlled instead by the changes in the Mott--Hubbard energy U. In contrast to the changes in the bandwidth W in the ``bandwidth control'' scenario or to the variations of the band-filling n parameter in the ``band-filling'' scenario, a dramatic decrease in the Mott--Hubbard energy U plays the key role in this mechanism. We have experimentally observed this type of the insulator metal transition in the transition metal oxide BiFeO₃. The decrease in the Mott--Hubbard energy is caused by the high-spin--low-spin crossover in the electronic d shell of $3d$ transition metal ion Fe³⁺ with d⁵ configuration under high pressure. The pressure-induced spin crossover in BiFeO₃ was investigated and confirmed by synchrotron x-ray diffraction, nuclear forward scattering, and x-ray emission methods. The insulator-metal transition at the same pressures was found by the optical absorption and dc resistivity measurements.