  The mixed-chalcogenide compound TlBiSSe realizes a three-dimensional (3D)
Dirac semimetal state. In clean, low-carrier-density single crystals of this
material, we found Shubnikov-de Haas oscillations to signify its 3D Dirac
nature. Moreover, we observed very large linear magnetoresistance (MR)
approaching 10,000% in 14 T at 1.8 K, which diminishes rapidly above 30 K. Our
analysis of the magnetotransport data points to the possibility that the linear
MR is fundamentally governed by the Hall field; although such a situation has
been predicted for highly-inhomogeneous systems, inhomogeneity does not seem to
play an important role in TlBiSSe. Hence, the mechanism of large linear MR is
an intriguing open question in a clean 3D Dirac system.
