How are heavy and itinerant electrons born in a dilute Kondo alloy?

We report a continuous evolution of Fermi surface properties with temperature in a dilute Kondo alloy of Ce _0.02La _0.98Ru ₂Si ₂ via measurements of the de Haas-van Alphen (dHvA) effect. The temperature variation of resistivity in this alloy exhibits a typical impurity Kondo behavior, i.e., the resistivity increases with decreasing temperature proportionally to - log T and becomes nearly constant below about 0.3 K. The Kondo temperature of this alloy is estimated to be about 1.3 K. The frequency of the dHvA oscillation from the main hole surface decreases with decreasing temperature implying that the hole surface shrinks with decreasing temperature. The temperature dependence of the frequency change is found to be similar to that of resistivity. The temperature dependence of this dHvA oscillation amplitude deviates largely from that expected from the Lifshitz-Kosevich formula conventionally employed to determine the effective mass from the temperature dependence. By assuming that the scattering of the conduction electron increases with decreasing temperature in the same manner as that of resistivity and that the effective mass increases with decreasing temperature, the anomalous temperature dependence can be explained. From these observations we argue that the f electron nature changes from localized at high temperatures to itinerant at low temperatures resulting in the change of the Fermi surface volume and the increase in the effective mass. The effective mass of this oscillation is determined from the temperature dependence at the lowest temperatures below 0.3 K to be about 3.8m ₀, which is considerably larger than 1.6m ₀ of the corresponding oscillation in LaRu ₂Si ₂ but seems to be much smaller than that expected from the Kondo temperature of this alloy.