Abstract
Ultrafast electron delocalization induced by a fs laser pulse is a well-known process and is the initial step for important applications such as fragmentation of molecules or laser ablation in solids. It is well understood that an intense fs laser pulse can remove several electrons from an atom within its pulse duration. [1] However, the speed of electron localization out of an electron gas, the capture of an electron by ion, is unknown. Here, we demonstrate that electronic localization out of the conduction band can occur within only a few hundred femtoseconds. This ultrafast electron localization into 4f states has been directly quantified by transient x-ray absorption spectroscopy following photo-excitation of a Eu based correlated metal with a fs laser pulse. Our x-ray experiments show that the driving force for this process is either an ultrafast reduction of the energy of the 4f states, a change of their bandwidth or an increase of the hybridization between the 4f and the 3d states. The observed ultrafast electron localization process raises further basic questions for our understanding of electron correlations and their coupling to the lattice.
Highlights
IntroductionOccurrence of non-Bardeen-Cooper-Schrieffer-like superconductivity, and non-Fermi-liquid states [1]
This method is extremely sensitive for quantifying the timescale of an electron localization process in the Eu 4 f shell and allows us to provide a better understanding of the underlying mechanism on this EuNi2(Si0.21Ge0.79)2 intermetallic compound
We presented a series of pump-probe optical reflectivity, fs time-resolved x-ray absorption spectroscopy measurements at the Eu L3 edge, and x-ray diffraction collected on the (002) Bragg peak for the valence transition EuNi2(Si0.21Ge0.79)2 intermetallic compound
Summary
Occurrence of non-Bardeen-Cooper-Schrieffer-like superconductivity, and non-Fermi-liquid states [1]. Valence transitions and their fluctuations are still not well understood, even for a prototypical system such as elemental cerium, which exhibits a first order valence transition as a function of pressure [2]. Many correlated metals are based on the fact that the f electron shell is partly occupied with 4 f states lying at the Fermi surface. Hybridization with the conduction electrons plays a significant role in the structure, electronic, and magnetic properties, and it has been a field of condensed matter which reveals novel materials. We drawn our attention to the EuNi2(Si1–xGex ) system, which exhibits an excellent playground to investigate valence transition [3], due to its dependences on temperature
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