We measured low temperature conduction characteristics of amorphous indium/indium-oxide composite films deep in the insulating regime. Upon lowering temperature, resistance crosses over from an Efros-Shklovskii variable-range-hopping conduction to a simple activation conduction, with parameters in good agreement with the picture of hardening of a parabolic Coulomb gap near the Fermi level. In contrast to the previous observations in magnetic materials, however, the hard gap in our films is extremely insensitive to a magnetic field, which strongly indicates a nonmagnetic nature of the hard gap.

• Received 2 December 1992

DOI:https://doi.org/10.1103/PhysRevLett.70.2798