  Electrons in correlated insulators are prevented from conducting by Coulomb
repulsion between them. When an insulator-to-metal transition is induced in a
correlated insulator by doping or heating, the resulting conducting state can
be radically different from that characterized by free electrons in
conventional metals. We report on the electronic properties of a prototypical
correlated insulator vanadium dioxide (VO2) in which the metallic state can be
induced by increasing temperature. Scanning near-field infrared microscopy
allows us to directly image nano-scale metallic puddles that appear at the
onset of the insulator-to-metal transition. In combination with far-field
infrared spectroscopy, the data reveal the Mott transition with divergent
quasiparticle mass in the metallic puddles. The experimental approach employed
here sets the stage for investigations of charge dynamics on the nanoscale in
other inhomogeneous correlated electron systems.
