The conductance of granular Al films shows a non-monotonous temperature dependence when approaching the metal to insulator transition. Upon cooling it is first metallic like, then becomes insulating-like, and finally again metallic-like at low temperatures. Eventually these films become superconducting. We will show that this behavior bears a striking resemblance to that predicted for the conductance of quantum dots in a regime where broadening of the discrete energy levels, due to coupling to the leads reservoirs, is larger than the level separation (1).
We believe that this multi-level Kondo resonance model for single dots may apply to our 3D network of nanoscale grains. The multi-level resonance prevents the insulating state from setting -in already at high temperatures. This interpretation of the experimental conductance data (2) is in line with the previously reported presence of magnetic moments in these films (3), which appear clearly in the Coulomb dominated regime. We conjecture that the enhanced critical temperature of these films is due to the high density of states at the Fermi level in the single level Kondo resonance regime when it sets in at a sufficiently high temperature.
1) S. Florens and A. Georges, Phys. Rev. B68, 245311 (2003).
2) A. Moshe, N. Bachar, S. Lerer, Y. Lereah and G. Deutscher, J. Supercond. Nov. Magn. 31, 733 (2018).
3) N. Bachar et al. , Phys. Rev. B91, 041123 (2015).