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The non-equilibrium approach to correlated electron systems is often based on the paradigm that different degrees of freedom interact on different timescales. In this context, photo-excitation is treated as an impulsive injection of electronic energy that is transferred to other degrees of freedom only at later times. Here, by studying the ultrafast dynamics of quasi-particles in an archetypal strongly correlated charge-transfer insulator (La 2 CuO 4 þ d ), we show that the interaction between electrons and bosons manifests itself directly in the photo-excitation processes of a correlated material. With the aid of a general theoretical framework (Hubbard-Holstein Hamiltonian), we reveal that sub-gap excitation pilots the formation of itinerant quasi-particles, which are suddenly dressed by an ultrafast reaction of the bosonic field.

International audienceThe intercalation ofEu underneathGr on Ir(111) is comprehensively investigated by microscopic, magnetic, and spectroscopic measurements, as well as by density functional theory. Depending on the coverage, the intercalated Eu atoms form either a (2x2) or a (√3x√3)R30° superstructure with respect to Gr. We investigate the mechanisms of Eu penetration through a nominally closed Gr sheet and measure the electronic structures and magnetic properties of the two intercalation systems. Their electronic structures are rather similar. Compared to Gr on Ir(111), the Gr bands in both systems are essentially rigidly shifted to larger binding energies resulting in n doping. The hybridization of the Ir surface state S1 with Gr states is lifted, and the moir´e super periodic potential is strongly reduced. In contrast, the magnetic behavior of the two intercalation systems differs substantially, as found by x-ray magnetic circular dichroism. The (2x2) Eu structure displays plain paramagnetic behavior, whereas for the (√3x√3)R30° structure the large zero-field susceptibility indicates ferromagnetic coupling, despite the absence of hysteresis at 10 K. For the latter structure, a considerable easy-plane magnetic anisotropy is observed and interpreted as shape anisotropy

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X-ray-absorption near-edge spectroscopy ͑XANES or NEXAFS͒ has been used to obtain information on the orientation, corrugation, and cross-linking of graphitic carbon nitride planes, structural parameters that determine the mechanical properties of the material. The contribution of p electrons from carbon and nitrogen atoms to bonding in graphitic carbon nitride has been studied with elemental and angular sensitivity by XANES. The density of * states from nitrogen is composition dependent and presents angular anisotropy, while the density of * states from carbon is isotropic and independent of composition. Both observations are consistent with a model of the superstructure of basal planes.