Pb1-xSnxSe is a highly tunable material where a topological phase transition from a normal insulator to a topological insulator can be induced by increasing Sn content above a critical composition x=0.16. [1] We thus study magnetotransport at pulsed magnetic fields in Pb1-xSnxSe for x varying between 0.1 and 0.3 across the topological phase transition. We report the observation of a negative longitudinal magnetoresistance (NLMR) only in samples that are in the topological regime (x > 0.16). This NLMR is only observed when the samples reach the quantum limit. In trivial control samples (x < 0.16), the MR is robustly positive up to 60T, suggesting that the NLMR may be a hallmark of the topological insulating state. We argue that this effect can be due to the anomalous behavior of the N=0 Landau levels, predicted to converge and cross, leading to a magnetic field induced closure of the energy gap in topological materials. [2] We thus tie the occurrence of the NLMR to the non-trivial topological character of Pb1-xSnxSe. [3]

[1] B. A. Assaf, T. Phuphachong, V. V Volobuev, G. Bauer, G. Springholz, L.-A. De Vaulchier, and Y. Guldner, Arxiv 1608.08912.

[2] B. A. Bernevig and T. L. Hughes, Topological Insulator and Topological Superconductors (Princeton University Press, 2013).

[3] B.A. Assaf, T. Phuphachong, E. Kampert, V.V. Volobuev, G. Bauer, G. Springholz, L.A.  

de Vaulchier, and Y. Guldner. Arxiv 1704.02021