Size Dependence of Kondo Scattering in Point Contacts

By utilizing a mechanically controllable break junction, point contact spectra $\frac{\mathrm{dV}}{\mathrm{dI}}$ were determined for various noble metals (Cu; Au) doped with magnetic impurities (Mn). The contact diameter $d$ could be varied continuously between ca. 2 and 50 nm. The zero bias maximum in $\frac{\mathrm{dV}}{\mathrm{dI}}$ due to the Kondo scattering exhibits a large broadening and increase of relative amplitude as $d$ is decreased. By using the theoretical expression for the logarithmic slope of $\frac{\mathrm{dV}}{\mathrm{dI}}$ vs $V$ we find a significant enhancement of exchange coupling parameter $J$, which leads to an enormous increase in the apparent Kondo temperature $T_K$ for small contact diameters.