  We have investigated the effect of the long-range Coulomb interaction on the
one-particle excitation spectrum of n-type Germanium, using tunneling
spectroscopy on mechanically controllable break junctions. The tunnel
conductance was measured as a function of energy and temperature. At low
temperatures, the spectra reveal a minimum at zero bias voltage due to the
Coulomb gap. In the temperature range above 1 K the Coulomb gap is filled by
thermal excitations. This behavior is reflected in the temperature dependence
of the variable-range hopping resitivity measured on the same samples: Up to a
few degrees Kelvin the Efros-Shkovskii ln$R \propto T^{-1/2}$ law is obeyed,
whereas at higher temperatures deviations from this law are observed,
indicating a cross-over to Mott's ln$R \propto T^{-1/4}$ law. The mechanism of
this cross-over is different from that considered previously in the literature.
