An alternative view at the zero-bias anomaly of metallic point contacts

Abstract

We have studied the zero-bias anomaly of point contacts as function of contact size for a wide range of materials from simple normal metals, superconductors, and magnets to highly correlated heavy-fermion compounds. The size δR of the zero-bias anomaly varied in a universal manner proportional to the square of the contact resistance R between 1Ω and 10kΩ, that is for contact radii from about 15nm down to 0.2nm. Magnetic impurities, two-level systems, or a tunneling barrier are unlikely to be the main source of this anomaly. We suggest instead Kondo-type scattering of the conduction electrons due to a spontaneous electron spin polarization at the point contact.