Two Types of Mott Transitions

One type of Mott transition (MTl) is characterized by diverging enhancement of the charge effective mass when one approaches a Mott insulator from the side of the paramagnetic metal. Another fundamentally different type of Mott transition (MT2) is shown to exist when a spin gap opens. The Unear coefficient of the specific heat y decreases and then vanishes at the transition point of MT2 in sharp contrast with MTl. As an example, a dimerized t-J model is shown to undergo MT2. The underlying pairing mechanism determines the character of MT2. Recent controversial experimental results on the Mott transitions in copper oxides and other strongly correlated systems are discussed from the above viewpoint.