  Low temperature scanning tunneling spectroscopy of HfNiSn shows a V^m(m < 1)
zero bias anomaly around the Fermi level. This local density of states with a
fractional power law shape is well known to be a consequence of electronic
correlations. For comparison, we have also measured the tunneling conductances
of other half-Heusler compounds with 18 valence electrons. ZrNiPb shows a
metal-like local density of states, whereas ZrCoSb and NbFeSb show a linear and
V^2 anomaly. One interpretation of these anomalies is that a correlation gap is
opening in these compounds. By analyzing the magnetoresistance of HfNiSn, we
demonstrate that at low temperatures, electron-electron scattering dominates.
The T^m(m < 1) temperature dependence of the conductivity confirms that the
electronic correlations are a bulk rather than a surface property.
