Resistivity plateau and extreme magnetoresistance in LaSb

Time reversal symmetry (TRS) protects the metallic surface modes of topological insulators (TIs). The transport signature of such surface states is a plateau that arrests the exponential divergence of the insulating bulk with decreasing temperature. This universal behaviour is observed in all TI candidates ranging from Bi2Te2Se to SmB6. Recently, extreme magnetoresistance (XMR) has been reported in several topological semimetals which exhibit TI universal resistivity behaviour only when breaking time reversal symmetry, a regime where TIs theoretically cease to exist. Among these materials, TaAs and NbP are nominated as Weyl semimetals owing to their lack of inversion symmetry, Cd3As2 is known as a Dirac semimetal owing to its linear band crossing at the Fermi level, and WTe2 is termed a resonant compensated semimetal owing to its perfect electron-hole symmetry. Here we introduce LaSb, a simple rock-salt structure material that lacks broken inversion symmetry, perfect linear band crossing, and perfect electron-hole symmetry yet exhibits all the exotic field-induced behaviours of these more complex semimetals. It shows a field-induced universal TI resistivity with a plateau at roughly 15 K, ultrahigh mobility of carriers in the plateau region, quantum oscillations with the angle dependence of a two-dimensional Fermi surface, and XMR of about one million percent at 9 T. Owing to its structural simplicity, LaSb represents an ideal model system to formulate a theoretical understanding of the exotic consequences of breaking time reversal symmetry in topological semimetals.