Observation of non-Fermi liquid behaviour in new Yb-based alloys

In this contribution, after presenting the existing theoretical models, we discuss the evidence for non-Fermi liquid behaviour in Yb(Cu1-xSix)(2-y) that have been investigated for the last years in our Institute. The alloys crystallize in the hexagonal AlB2 type structure and exhibit a dramatic change in the electronic properties upon change of the electron concentration. Undoped YbSi2-y is an intermediate valent system, whereas the doping with 10% Cu ensures chi(T) proportional to T-n, rho(T) proportional to T and C-p proportional to T-1/2 dependences. Comparison of the data with the Griffiths phase model shows that this model can describe some of the observed results. We present also a study of the Hall effect. The Hall coefficient at 7 T is found to follow a lnT dependence below 10 K, thus showing the behaviour opposite to that in Fermi-liquid heavy-fermion systems.