### Granular metals

In granular metals metallic nano-grains are embedded into a non-conducting matrix. The electronic properties of granular metals are the consequence of the non-trivial interplay of the diffusive charge transport inside the grains and the (thermally assisted) tunnel processes between the grains. Due to the sequential charging associated with the tunneling, the electronic transport is highly correlated. Delocalization effects due to tunneling and the tendency for localization because of the charging lead to very particular low-temperature transport properties. As a consequence, granular metals are model systems for the interplay of electronic correlation effects and disorder.

Recent theoretical work indicates that the granular metals should exhibit a universal low-temperature transport behavior in the limit of strong tunnel coupling - the granular Fermi liquid. At higher temperatures and for weaker coupling other properties are precicted, such as a universal logarithmic temperature dependence for the electrical conductivity, or logarithmic corrections to the thermopower and Hall coefficient. Experimentally, we focus on disordered granualr metals with finely tunable tunnel coupling strength in the vicinity of the criticial tunnel coupling at which at metal-insulator transition occurs in the three-dimensional case. Employing FEBID in conjunction with ALD, we also develop techniques to fabricate ordered granular metals in one and two dimensions. Single-electron transistors are the zero-dimensional endpoint of this sequence. The interplay of electronic correlation effects and disorder in homogeneous solid state materials close to a metal-insulator transition can also cause electronic properties which are quite similar to those of granular metals.