Consequences of breaking time reversal symmetry in LaSb: a resistivity plateau and extreme magnetoresistance

Time reversal symmetry (TRS) protects the metallic surface modes of topological insulators (TIs). The transport signature of robust metallic surface modes of TIs is a plateau that arrests the exponential divergence of the insulating bulk with decreasing temperature. This universal behavior is observed in all TI candidates ranging from Bi2Te2Se to SmB6. Recently, several topological semimetals (TSMs) have been found that exhibit extreme magnetoresistance (XMR) and TI universal resistivity behavior revealed only when breaking TRS, a regime where TIs theoretically cease to exist. Among these new materials, TaAs and NbP are nominated for Weyl semimetal due to their lack of inversion symmetry, Cd3As2 is nominated for Dirac semimetal due to its linear band crossing at the Fermi level, and WTe2 is nominated for resonant compensated semimetal due to its perfect electron-hole symmetry. Here we introduce LaSb, a simple rock-salt structure material without broken inversion symmetry, without perfect linear band crossing, and without perfect electron-hole symmetry. Yet LaSb portrays all the exotic field induced behaviors of the aforementioned semimetals in an archetypal fashion. It shows (a) the universal TI resistivity with a plateau at 15 K, revealed by a magnetic field, (b) ultrahigh mobility of carriers in the plateau region, (c) quantum oscillations with a non-trivial Berry phase, and (d) XMR of about one million percent at 9 tesla rivaled only by WTe2 and NbP. Due to its dramatic simplicity, LaSb is the ideal model system to formulate a theoretical understanding of the exotic consequences of breaking TRS in TSMs.