Universal Conductance Correction in a Tunable Strongly Coupled Nanogranular Metal

We present temperature-dependent conductivity data obtained on a sample set of nanogranular Pt-C with finely tuned intergrain tunnel coupling strength g. For samples in the strong-coupling regime g > g(C), characterized by a finite conductivity for T -> 0, we find a logarithmic behavior at elevated temperatures and a crossover to a root T behavior at low temperatures over a wide range of coupling strengths g(C) approximate to 0.25 < g <= 3. The experimental observation for g > 1 is in very good agreement with recent theoretical findings on ordered granular metals in three spatial dimensions. The results indicate a validity of the predicted universal conductivity behavior that goes beyond the immediate range of the approach used in the theoretical derivation.