  One of the early triumphs of quantum physics is the explanation why some
materials are metallic whereas others are insulating. While a treatment based
on single electron states correctly predicts the character of most materials
this approach can fail spectacularly, when the electrostatic repulsion between
electrons causes strong correlations. Not only can these favor new and subtle
forms of order in metals, such as magnetism or superconductivity, they can even
cause the electrons in a half-filled energy band to lock into position
altogether, producing a correlated, or Mott insulator. Arguably the most
extreme manifestation of electronic correlations in dense electronic matter,
the transition into the Mott insulating state raises a number of fundamental
questions. Foremost among these is the fate of the electronic Fermi surface and
the associated charge carrier mass, as the Mott transition is approached at low
temperature, which have been particularly controversial in the high temperature
superconducting cuprates. We report the first direct observation of the Fermi
surface on the metallic side of a Mott insulating transition by high pressure
quantum oscillatory measurements in NiS2. Our results point at a large Fermi
surface consistent with Luttinger's theorem and a strongly enhanced carrier
effective mass, suggesting that electron localization occurs via a diverging
effective mass and concomitant slowing down of charge carriers as predicted
theoretically.
