  In topological insulators (TIs), metallic surface conductance saturates the
insulating bulk resistance with de- creasing temperature, resulting in
resistivity plateau at low temperatures as a transport signature originating
from metallic surface modes protected by time reversal symmetry (TRS). Such
characteristic has been found in several materials including Bi2Te2Se, SmB6
etc. Recently, similar behavior has been observed in metallic com- pound LaSb,
accompanying an extremely large magetoresistance (XMR). Shubnikov-de Hass (SdH)
oscillation at low temperatures further confirms the metallic behavior of
plateau region under magnetic fields. LaSb[1] has been proposed by the authors
as a possible topological semimetal (TSM), while negative magnetoresistance is
absent at this moment. Here, high quality single crystals of NbAs2/TaAs2 with
inversion symmetry have been grown and the resistivity under magnetic field is
systematically investigated. Both of them exhibit metallic behavior under zero
magnetic field, and a metal-to-insulator transition occurs when a nonzero
magnetic field is applied, resulting in XMR (1.0*105% for NbAs2 and 7.3*105%
for TaAs2 at 2.5 K & 14 T). With tempera- ture decreased, a resistivity plateau
emerges after the insulator-like regime and SdH oscillation has also been
observed in NbAs2 and TaAs2.

Materials Science