In topological insulators (TIs), metallic surface conductance saturates the insulating bulk resistance with decreasing temperature, resulting in resistivity plateau at low temperatures as a transport signature originating from metallic surface modes protected by time reversal symmetry (TRS). Such a characteristic has been found in several materials including ${\mathrm{Bi}}_2{\mathrm{Te}}_2\mathrm{Se}$, ${\mathrm{SmB}}_6$ etc. Recently, similar behavior has been observed in metallic compound LaSb, accompanying an extremely large magnetoresistance (XMR). Shubnikov-de Hass (SdH) oscillation at low temperatures further confirms the metallic behavior of the plateau region under magnetic fields. LaSb [Tafti et al., Nat. Phys. 12, 272 (2015)] 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 ${\mathrm{NbAs}}_2$/${\mathrm{TaAs}}_2$ 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 ($1.0\times {10}^5\%$ for ${\mathrm{NbAs}}_2$ and $7.3\times {10}^5\%$ for ${\mathrm{TaAs}}_2$ at 2.5 K and 14 T). With temperature decreased, a resistivity plateau emerges after the insulatorlike regime, and SdH oscillation has also been observed in ${\mathrm{NbAs}}_2$ and ${\mathrm{TaAs}}_2$.

• Received 8 March 2016
• Revised 13 June 2016

DOI:https://doi.org/10.1103/PhysRevB.94.041103