Orbital selective gap opening in Ca$_{3}$Ru$_{2}$O$_{7}$ observed by ARPES

We present a high--resolution photoemission study of the bilayered ruthenate Ca$_{3}$Ru$_{2}$O$_{7}$, which is right on the verge of a Mott--Hubbard metal-insulator transition. The low carrier density, inferred from transport measurements is manifested in a very small quasiparticle weight, and a characteristic broadening of the valence band states. Coincident with the first order structural phase transition at 48 K, we observe the orbital selective opening of a weak, mean--field like gap. This sheds new light on the mysterious low--temperature metallic state of Ca$_{3}$Ru$_{2}$O$_{7}$, and on the interplay of charge, lattice, and orbital degrees of freedom.