  Within a Kondo lattice, the strong hybridization between electrons localized
in real space (r-space) and those delocalized in momentum-space (k-space)
generates exotic electronic states called 'heavy fermions'. In URu2Si2 these
effects begin at temperatures around 55K but they are suddenly altered by an
unidentified electronic phase transition at To = 17.5 K. Whether this is
conventional ordering of the k-space states, or a change in the hybridization
of the r-space states at each U atom, is unknown. Here we use spectroscopic
imaging scanning tunnelling microscopy (SI-STM) to image the evolution of
URuSi2 electronic structure simultaneously in r-space and k-space. Above To,
the 'Fano lattice' electronic structure predicted for Kondo screening of a
magnetic lattice is revealed. Below To, a partial energy gap without any
associated density-wave signatures emerges from this Fano lattice.
Heavy-quasiparticle interference imaging within this gap reveals its cause as
the rapid splitting below To of a light k-space band into two new heavy fermion
bands. Thus, the URu2Si2 'hidden order' state emerges directly from the Fano
lattice electronic structure and exhibits characteristics, not of a
conventional density wave, but of sudden alterations in both the hybridization
at each U atom and the associated heavy fermion states.
