Negative magnetoresistance in strained $\alpha$-Sn and $\alpha$-SnGe films in an in-plane magnetic field
To test the hypothesis that the chiral anomaly is responsible for negative magnetoresitance (MR) in \atn{}, we have studied magnetotransport in strained, epitaxial films of pure \aSn{} and the alloy \aSnGe{} that are in the Dirac semimetal and 3D topological insulator state, respectively. We have observed for both states a negative MR with current either parallel or transverse to the in-plane magnetic field, but with a different dependence of MR on $\vec{B}$ strength. Our results are inconsistent with the chiral anomaly and suggest that other mechanisms may be responsible for negative MR in the Dirac/Weyl semimetal phase of \aSn{}. We also discuss several factors in sample design and material quality that may be contributing to the incongruous observations of MR reported in studies of strained \atn{} films.