  When a three-dimensional material is constructed by stacking different
two-dimensional layers into an ordered structure, new and unique physical
properties can emerge. An example is the delafossite PdCoO2, which consists of
alternating layers of metallic Pd and Mott-insulating CoO2 sheets. To
understand the nature of the electronic coupling between the layers that gives
rise to the unique properties of PdCoO2, we revealed its layer-resolved
electronic structure combining standing-wave X-ray photoemission spectroscopy
and ab initio many-body calculations. Experimentally, we have decomposed the
measured valence band spectrum into contributions from Pd and CoO2 layers.
Computationally, we find that many-body interactions in Pd and CoO2 layers are
highly different. Holes in the CoO2 layer interact strongly with
charge-transfer excitons in the same layer, whereas holes in the Pd layer
couple to plasmons in the Pd layer. Interestingly, we find that holes in states
hybridized across both layers couple to both types of excitations
(charge-transfer excitons or plasmons), with the intensity of photoemission
satellites being proportional to the projection of the state onto a given
layer. This establishes satellites as a sensitive probe for inter-layer
hybridization. These findings pave the way towards a better understanding of
complex many-electron interactions in layered quantum materials.
