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Understanding the energy scales relevant for the valence transition in YbInCu4

Valence transition in YbInCu4 is interesting from both technological and fundamental aspect. A large magnetic entropy change at the transition could be used in thermomagnetic devices. In addition, the transition temperature TV can be tuned by doping. Knowing the relevant energy scales is crucial for understanding and modeling the transition. Our measurements and the data analysis of specific heat, magnetic susceptibility, electrical resistance and thermopower of the Y-doped YbInCu4 show that the low-temperature properties of the local moment phase can be explained by the CF effects but with a modified CF scheme, so that the Kondo interaction is very week. Our results support the Falicov-Kimball scenario for the onset of the valence transition. In this picture, the Fermi level EF is placed just below the dip in the DOS, and the hole delocalization at the transition pushes the EF deeper in the DOS where the density of states is higher. This suddenly sets up the Kondo interaction (with high characteristic temperature T_KVF = 400 K) which screens the Yb moments. A new equilibrium itinerant hole concentration (and Yb valence) is fixed by the equilibrium between the Kondo condensation energy gain and the magnetic entropy lose due to the quenching of the Yb3+ moments. The valence fluctuating (VF) phase is stabilized when about 15% of holes are delocalized. In the simplest picture, we assume the Kondo condensation energy EK is proportional to the product of the concentration x3+ of the Yb3+ ions and the concentration of delocalized itinerant holes 1 − x3+, EK = T_KVF K x3+(1 − x3+) . If we neglect the entropy of the fermy liquid, the entropy change at the transition is roughly T_V(x3+)ln(4). By equating these two expressions we obtain the valence change at the transition 1 − x3+ = T_Vln(4)/T_KVF = 42 K ln(4)/400 K = 0.15, that is the experimentally observed value.

copper alloys; crystal field interactions; ground states; indium alloys; Kondo effect; magnetic susceptibility; specific heat; ytterbium alloys; yttrium alloys

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