Probing Dirac Fermions in Bi-based Semimetals by Cryomagnetic Scanning Tunneling and Point Contact Spectroscopy*

The topological semimetal Bi$_2$Se$_3$ is distinguished by the presence of two-dimensional Dirac fermions with strong spin-orbit coupling. The linear dispersion of Dirac fermions in Bi$_2$Se$_3$ was recently observed by scanning tunneling spectroscopy measurements of the Landau level spacing versus magnetic field. In this work we extend the field-dependent spectroscopy study of Dirac fermions to other Bi-based semimetals, for both topological and non-topological cases, using cryomagnetic scanning tunneling and point contact probes on single crystals down to 300 mK and up to 9 T. The spectral dependences on field strength and field direction are examined, in an effort to elucidate the role of spin-orbit coupling in each case.