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Abstract 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 PdCoO 2 , which consists of alternating layers of metallic Pd and Mott-insulating CoO 2 sheets. To understand the nature of the electronic coupling between the layers that gives rise to the unique properties of PdCoO 2 , 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 VB spectrum into contributions from Pd and CoO 2 layers. Computationally, we find that many-body interactions in Pd and CoO 2 layers are highly different. Holes in the CoO 2 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. 

Distinct properties of multiple phases of vanadium oxide (VO_x) render this material family attractive for advanced electronic devices, catalysis, and energy storage. In this work, phase boundaries of VO_xare crossed and distinct electronic properties are obtained by electrochemically tuning the oxygen content of VO_xthin films under a wide range of temperatures. Reversible phase transitions between two adjacent VO_xphases, VO₂and V₂O₅, are obtained. Cathodic biases trigger the phase transition from V₂O₅to VO₂, accompanied by disappearance of the wide band gap. The transformed phase is stable upon removal of the bias while reversible upon reversal of the electrochemical bias. The kinetics of the phase transition is monitored by tracking the time‐dependent response of the X‐ray absorption peaks upon the application of a sinusoidal electrical bias. The electrochemically controllable phase transition between VO₂and V₂O₅demonstrates the ability to induce major changes in the electronic properties of VO_xby spanning multiple structural phases. This concept is transferable to other multiphase oxides for electronic, magnetic, or electrochemical applications.