  Germanene, the germanium analogue of graphene, shares many properties with
its carbon counterpart. Both materials are two-dimensional materials that host
Dirac fermions. There are, however, also a few important differences between
these two materials: (1) graphene has a planar honeycomb lattice, whereas
germanene's honeycomb lattice is buckled and (2) the spin-orbit gap in
germanene is predicted to be about three orders of magnitude larger than the
spin-orbit gap in graphene (24 meV for germanene versus 20 $\mu$eV for
graphene). Surprisingly, scanning tunneling spectra recorded on germanene
layers synthesized on different substrates do not show any sign of the presence
of a spin-orbit gap in germanene. To date the exact origin of the absence of
this spin-orbit gap in the scanning tunneling spectra of germanene has remained
a mystery. In this work we show that the absence of the spin-orbit can be
explained by germanene's exceptionally low work function of only 3.8 eV. The
difference in work function between germanene and the scanning tunneling
microscopy tip (the work functions of most commonly used STM tips are in the
range of 4.5 to 5.5 eV) gives rise to an electric field in the tunnel junction.
This electric field results in a strong suppression of the size of the
spin-orbit gap.
