Kondo-like behavior near the metal-to-insulator transition of nano-scale granular aluminum


  N. Bachar  ,  S. Lerer  ,  S. Hacohen-Gourgy  ,  B. Almog  ,  G. Deutscher  
Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Tel Aviv, 69978, Israel

An enhanced Tc by a factor of 3 in granular Al thin films is known for more than 40 years. However, new experimental transport data shows also magnetic properties along with this elevation in Tc. We observe a resistivity minimum, a logarithmic dependence below this minimum and a negative curvature above this minimum which suggests a Kondo-like behavior in our films. A negative magnetoresistance is persistent up to high temperatures and high fields and scales as (H/T) which suggests the existence of a spin-flip scattering mechanism. These magnetic properties can be understood if one takes into account that nano scale grains have a shell electronic structure. In similar to dilute magnetic alloys, this shell electronic structure is at the origin of localized magnetic moments in the grain's volume. The same electronic structure may be also at the origin of the enhancement in Tc observed in these films. 

*Phys. Rev. Lett. Submitted, August 2012