What is the energy gap in a Mott insulator called?

The energy gap in a Mott insulator is called the Mott gap.

A Mott insulator is unusual because ordinary band theory predicts that some such materials should conduct electricity: their electronic bands can appear partially filled. Strong electron–electron repulsion changes that outcome by making it energetically costly for electrons to move and by splitting the relevant states into lower and upper Hubbard bands.

The separation between those correlated states produces the Mott gap. It is not simply the ordinary band gap of a conventional semiconductor, where filled and empty bands are separated primarily by the crystal’s one-electron structure. In a Mott system, Coulomb interaction and electron localization are central.

The term “charge gap” is also used in technical discussions, especially for the energy required to create charge excitations, so the two expressions can overlap in some contexts. “Mott gap” is the distinctive name for the correlation-driven gap associated with Mott-insulating behavior. Changing pressure, composition, temperature, or carrier concentration can close the gap and drive a Mott transition toward metallic behavior.