Materials Science-Poland, Vol. 25, No. 2, 2007 Observation of non-Fermi liquid behaviour in new Yb-based alloys V.H. TRAN * W. Trzebiatowski Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wrocław 2, Poland In this contribution, after presenting the existing theoretical models, we discuss the evidence for non- Fermi liquid behaviour in Yb(Cu 1–x Si x ) 2–y that have been investigated for the last years in our Institute. The alloys crystallize in the hexagonal AlB 2 type structure and exhibit a dramatic change in the electronic properties upon change of the electron concentration. Undoped YbSi 2–y is an intermediate valent system, whereas the doping with 10% Cu ensures χ(T) ∝ T n , ρ(T) ∝ T and C p ∝ 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. Key words: non Fermi liquid; Yb-based intermetallic; Hall effect 1. Introduction For over half a century, the Landau Fermi liquid (LFL) theory has been success- fully applied to describe the low-temperature properties of metals and intermetallic compounds [1]. In particular, this theory is useful in understanding the behaviour of strongly correlated electron systems (SCES) in which the particle–particle interaction is significant [2]. For instance, the application of LFL theory helps us to explain the enormous enhancement of effective electron masses m * and other coefficients of physical quantities, observed in heavy fermion (HF) compounds. Other predictions of the theory (quadratic temperature dependence of the electrical resistivity, linear tem- perature dependence of the electronic specific heat and temperature independence of the magnetic susceptibility) are also valid in a number of SCES. There is, however, a growing number of systems showing an anomalous behaviour which distinctly devi- ates from the behaviour of the Landau theory. These non-Fermi liquid (NFL) systems __________ * E-mail: V.H.Tran@int.pan.wroc.pl