Electron hybridization phenomena in Yb- and Eu-based heavy-fermion systems

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The main driving force of electron-correlation phenomena in rare-earth (RE) intermetalliccompounds are interactions of localized 4f and delocalized valence electrons. Uponincreasing the interaction strength, the ground state of these systems evolves from magneticorder via spin polarisation of the itinerant electrons (RKKY interaction) over screening of the 4fmagnetic moments (Kondo effect) to mixed-valent behavior (MV). Thereby a periodicarrangement of the 4f Kondo impurities (Kondo lattice) causes heavy-fermion (HF) propertiescharacterized by narrow hybrid bands with effective masses that are 1000 times larger thanthe free electron mass, me. Such systems attract a very vivid interest, since they are in aregime where electronic and magnetic subsystems are strongly coupled with each otherleading to a variety of fascinating quasi-particle phenomena. The possibility to tune thehybridization strength allows to explore quantum-phase transitions in detail.Subject of this project is a detailed investigation of the interactions of 4f and conductionelectrons in Yb- and Eu-based systems in the vicinity of quantum-phase transitions frommagnetically ordered to non-ordered ground states. Final goal is the understanding of therelation between electron spectroscopic results and the thermodynamic and transportproperties observed in these systems. Since the correlation properties depend crucially onhybridization of 4f and valence-band states, tuning is achieved by changing the relativeenergy positions of the bands with respect to those of the 4f states by means of alloying andoverlayer deposition. The project comprises the preparation and structural characterization ofrespective single crystalline samples, the determination of their low-energy excitationproperties like specific heat, magnetization and electrical conductivity and the electronspectroscopic investigation by means of ultra-high energy (down to 1 meV) and momentum(better than 0.01 1/Å) resolution photoemission. The obtained data will be analysed in theframework of a recently developed approach to the periodic Anderson model (PAM).