Theory of hopping conductivity of a suspension of nanowires in an insulator

We study the hopping conduction in a composite made of straight metallic nanowires randomly and isotropically suspended in an insulator. Uncontrolled donors and acceptors in the insulator lead to random charging of wires and, hence, to a finite bare density of states at the Fermi level. Then the Coulomb interactions between electrons of distant wires result in the soft Coulomb gap. At low temperatures the conductivity σ is due to variable range hopping of electrons between wires and obeys the Efros-Shklovskii (ES) law ln σ - (TES T)1 2. We show that TES 1 (n L3) 2, where n is the concentration of wires and L is the wire length. Due to enhanced screening of Coulomb potentials, at large enough n L3 the ES law is replaced by the Mott law.