  Several prominent transport properties have been identified as key signatures
of topologicalmaterials. One is the resistivity plateau at low temperatures as
observed in several topological insulators (TIs), another is the negative
magnetoresistance (MR) when the applied magnetic field is parallel to the
current direction as observed in several topological semimetals (TSMs)
including Dirac semimetals (DSMs) and Weyl semimetals (WSMs). Usually, these
two exotic phenomena emerge in distinct materials with or without time reversal
symmetry (TRS), respectively. Here we report the discovery of a new member in
TSMs, TaSb2, which clearly exhibits both of these phenomena in a single
material. This compound crystallizes in a base-centered monoclinic,
centrosymmetric structure, and is metallic with a low carrier density in the
zero field. While applying magnetic field it exhibits insulating behavior
before appearance of a resistivity plateau below Tc =13 K. In the plateau
regime, the ultrahigh carrier mobility and extreme magnetoresistance (XMR) for
the field perpendicular to the current are observed as in DSMs and WSMs, in
addition to a quantum oscillation behavior with non-trivial Berry phases. In
contrast to the most known DSMs and WSMs, the negative MR in TaSb2 does not
saturate up to 9 T, which, together with the almost linear Hall resistivity,
manifests itself an electron-hole non-compensated TMS. These findings indicate
that the resistivity plateau could be a generic feature of topology-protected
metallic states even in the absence of TRS and compatible with the negative MR
depending on the field direction. Our experiment extends a materials basis
represented by TaSb2 as a new platform for future theoretical investigations
and device applications of topological materials.
