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

Abstract

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 $T_K$ is observed to increase with pressure, having doubled in value by 82 kbar. When plotted versus $\frac{T}{T_K}$, the spin-resistivity curves are found to accurately overlap for all measured pressures, thus confirming the existence of a universal resistivity curve $\rho =\rho \left(\frac{T}{T_K}\right)$ for Cu:Fe. Within the experimental uncertainty of 1.7%, the saturation value of the resistivity $\rho (T=0\mathrm{} \mathrm{K})$ 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 $T_K$ one obtains the volume dependence of the effective exchange parameter $J_{\mathrm{eff}}$. 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, ${\rho }_{\mathrm{phonon}}$, has also been studied and can be understood using the Bloch-Grüneisen formula with known values of the compressibility and the Grüneisen parameter. A method for experimentally determining deviations $\Delta$ from Matthiessen's rule in Kondo alloys is also presented. Such deviations can be very large; in fact, for $T<\mathrm{}30\mathrm{}$ K, we find that $\Delta =1.3\mathrm{}{\rho }_{\mathrm{phonon}}$.