Low-temperature resistivity of YbCu5-xAuxunder magnetic fields

YbCu5 shows a typical heavy-fermion behaviour while YbCu4Au exhibits an antiferromagnetic transition below 1 K. We have investigated the low-temperature electrical resistivity, ρ, of the pseudo-binary compound YbCu5-xAux (0≤x≤1.0) as functions of temperature (T) and magnetic field (H). The ρ(T) data indicate that the magnetic transition occurs for x = 1.0, 0.8, and 0.6 at Torder = 0.6, 0.4, 0.1 K, respectively, whereas magnetic ordering is absent for x≤0.2 down to the lowest temperature. ρ(H) for x = 0, 0.1, and 0.2 measured at 0.05 K shows a characteristic behaviour in the non-magnetic Kondo-lattice state. It has been found that the increase of the Au concentration x results in a rapid decrease of the Kondo temperature, TK, as well as the evolution of impurity Kondo behaviour, destroying the Kondo-lattice state for x = 0. This may imply that the Au substitution causes a significant change of the local electronic structures. ρ(T) for x = 0.6 varies nearly as T3/2 at 0.5 T below 0.15 K, while it shows a T2-power law at 1 T. This behaviour can be interpreted as the crossover caused by a magnetic field from a magnetic ordered state at zero field through a non-Fermi-liquid state to the Fermi-liquid state at high field.