Effect of Pressure on the Kondo Temperature of Cu:Fe—Existence of a Universal Resistivity Curve

The electrical resistivity of Cu---110-ppm Fe has been measured for temperatures from 1.3---40 K at pressures to 82 kbar. The Kondo temperature TK is observed to increase with pressure, having doubled in value by 82 kbar. When plotted versus TTK, the spin-resistivity curves are found to accurately overlap for all measured pressures, thus confirming the existence of a universal resistivity curve ρ=ρ(TTK) for Cu:Fe. Within the experimental uncertainty of 1.7%, the saturation value of the resistivity ρ(T=0 K) does not change with pressure. This indicates that to this accuracy the spin S and the potential scattering at the magnetic impurity remain constant. From the pressure dependence of TK one obtains the volume dependence of the effective exchange parameter Jeff. Approximately the same volume dependence is found for a series of CuAu:Fe alloys using their known average atomic volume. The Cu:Fe high pressure and the CuAu:Fe-alloy data are discussed within the context of a simple Fermi-gas model based on the Schrieffer-Wolff transformation. The pressure dependence of the resistivity of pure copper, ρₚₕₒₙₒₙ, has also been studied and can be understood using the Bloch-Gr\"uneisen formula with known values of the compressibility and the Gr\"uneisen parameter. A method for experimentally determining deviations Δ from Matthiessen's rule in Kondo alloys is also presented. Such deviations can be very large; in fact, for $T<30$ K, we find that Δ=1.3ρₚₕₒₙₒₙ.