YbAl₃ is an intermediate valent compound with a large Kondo temperature T_K and moderately low conduction electron density. Because of this, YbAl₃ is a prime candidate for the observation of effects caused by low conduction electron density, where coherence sets in below T_coh rather than T_K (T_coh\ll T_K). For the first time, we have directly observed the cross\ over between the energy scales by the application of a magnetic field above\ B^\ast \approx 40 T (\approx k_BT_coh/\mu _B ). We also observe a reduction in the effective masses above B^\ast that is consistent with the energy scale crossover.

PACS numbers: 75.30.Mb, 75.20.Hr, 71.27.+a, 71.28.+d