Electron spin resonance studies at the phase transition in YbInCu4

In the intermetallic compound YbInCu4 the Gd3+ -ESR and the static susceptibility were measured in the temperature range 1.5 K - 300 K, i.e. both below and above the valence phase transition which occurs in this material at T-ph approximate to 50 K. The Gd3+ resonance is mainly determined by exchange interaction of Gd3+ ions with fluctuating Yb3+ ions via conduction electrons (RKKY coupling) both below and above this transition. Arguments are presented that at low temperature YbInCu4 is a dense Kondo system (mixed valent state).