Formation of coherent heavy fermion states at the hidden order transition in URu$_2$Si$_2$, as seen by ARPES

We present high-resolution angle-resolved photoemission (ARPES) spectra that allow us to delineate the evolution of the low energy electronic structure of the heavy-fermion superconductor URu$_2$Si$_2$ across the hidden order (HO) transition. By employing a range of excitation photon energies, we are able to disentangle various features in the electronic structure which, to date, have not been clearly identified. In contrast to the conventional Kondo lattice scenario, we find that precisely at T$_{\mathrm{HO}}$, the low energy electronic structure changes due to hybridization from incoherent and localized f states to a coherent heavy fermion liquid. We also observe a sharp drop in the scattering rate upon cooling through T$_{\mathrm{HO}}$, suggesting that the large scattering rate is caused by fluctuations in the order parameter. Our findings place clear constraints on the possible theoretical models for the HO state while clarifying a few of the apparently inconsistent observations of the previous ARPES measurements.