Imaging the ``Hidden Order'' Transition in URu$_{2}$Si$_{2}$

In URu$_{2}$Si$_{2}$, bulk measurements indicating the formation of heavy bands begin at temperatures around 55 K but are interrupted by an unidentified electronic phase transition, the ``hidden order,'' at $T_{o}$ = 17.5 K. Heavy bands in a Kondo lattice are expected to form due to strong hybridization between electrons localized in real space on magnetic ions and those delocalized in momentum space. Why the ``hidden order'' appears has been an outstanding question in heavy fermion physics. We use spectroscopic imaging scanning tunneling microscopy (SI-STM) to image the electronic structure of URu$_{2}$Si$_{2}$ though $T_{o}$. Above $T_{o}$ we find the Fano spectra expected for Kondo screening of a magnetic lattice, while below $T_{o}$ a partial energy gap opens. Heavy-quasiparticle interference imaging shows that the gap forms due to a light momentum-space band splitting below $T_{o}$ into two heavy fermion bands. Our observations of the ``hidden order'' transition are thus consistent with a sudden alteration in both the hybridization at each U atom and the associated heavy bands.