Electron tunneling measurements of the density of states (DOS) in ultrathin Be films reveal that a correlation gap mediates their insulating behavior. In films with sheet resistance $R<\mathrm{}5000\Omega$ the correlation singularity appears as the usual perturbative $\ln \left(V\right)$ zero bias anomaly (ZBA) in the DOS. As R is increased further, however, the ZBA grows and begins to dominate the DOS spectrum. This evolution continues until a nonperturbative $|V|$ Efros-Shklovskii Coulomb gap spectrum finally emerges in the highest R films. Transport measurements of films which display this gap are well described by a universal variable range hopping law $R\left(T\right)=\left(h/{2e}^2\right)\exp (T_0/T{)}^{1/2}$.

• Received 10 September 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.1543