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Zusammenfassung:
Analyzing temperature dependent photoemission (PM) data of the ferromagnetic kondo-lattice (KL) system YbNiSn in the light of the periodic Anderson model (PAM) we show that the KL behavior is not limited to temperatures below a temperature (T) over bar (K), defined empirically from resistivity and specific heat measurements. As characteristic for weakly hybridized Ce and Yb systems, the PE spectra reveal a 4f -derived Fermi level peak, which reflects contributions from the Kondo resonance and its crystal electric field (CEF) satellites. In YbNiSn this peak has an unusual temperature dependence: With decreasing temperature a steady linear increase of intensity is observed which extends over a large interval ranging from 100 K down to 1 K without showing any peculiarities in the region of (T) over bar (K) similar to T-C = 5.6 K. In the light of the single-impurity Anderson model (SIAM) this intensity variation reflects a linear increase of 4f occupancy with decreasing temperature, indicating an onset of Kondo screening at temperatures above 100 K. Within the PAM this phenomenon could be described by a non-Fermi-liquid-like T - linear damping of the self-energy which accounts phenomenologically for the feedback from the closely spaced CEF states.
