Enhancement of Kondo Temperature in Nanometer-Size Point Contacts

Abstract

Recently, size effects in scattering of conduction electrons off magnetic impurities has gained a renewed interest [1, 2, 3, 4, 5, 6, 7]. The estimate for the characteristic size of Kondo interaction around paramagnetic impurities embedded in a normal metal equals in order of magnitude ξ _K ∼ υ _F /T _K where T _K is the characteristic energy scale and υ _F is the Fermi velocity. In noble metals with dissolved impurities such as Mn, Cr, and Fe, T _K can be quite small (down to ∼ 10⁻¹³ K in AuMn alloys [8]), leading to a macroscopic ξK which is easily accessible in experiments. The theory predicts that at low temperatures (T < T _K ) the conduction electron spins create a “cloud” around each impurity which compensates the spin of the impurity [9]. At higher temperatures (T ≫ T _K ), one might think that the same spatial scale determines the logarithmic behavior of the magnetic part of the resistivity as a function of temperature. It is unimportant that the average distance between impurities, even in the most dilute alloys, is much less than ξ _K , since the wave functions of spin-screening conduction electrons at each impurity are mutually orthogonal. There are a number of experiments aiming to discover the changes of Kondo interaction across this characteristic spatial scale [2, 3, 10]. In spite of some controversy in experimental interpretation