Possible new phase of commensurate insulators with disorder: The Mott glass

A new thermodynamic phase resulting from the competition between a commensu rate potential and disorder in interacting fermionic or bosonic systems is predicted. It requires interactions of finite extent. This phase, intermedi ate between the Mott insulator and the Anderson insulator, is both incompre ssible and has no gap in the conductivity. The corresponding phase is also predicted for commensurate classical elastic systems in the presence of cor related disorder.