Anisotropic Negative Magnetoresistance in Strained α-Sn Films

We present magnetoresistance measurements in strained, ~200 nm thick pure and Ge-doped α-Sn (gray Tin) films grown via MBE on CdTe substrates. α-Sn is a zero-gap semiconductor with an inversion-symmetric diamond lattice and an electronic structure that possesses the s-p band inversion property, closely related to that of binary compounds such as HgTe. Recently, α-Sn has been studied as a simple single-element compound with electronic properties tunable by strain. In strained α-Sn films, the zero-gap semiconductor phase is replaced with a Dirac semimetal phase, where the Weyl phase can be further induced by applying an external magnetic field. A growing number of studies have recently focused on the Weyl semimetal phase of α-Sn as a candidate for observing a solid state analog of the Adler-Bell-Jackiw chiral anomaly. The effect is predicted to produce a strong enhancement of conductance, i.e. a negative magnetoresistance MR, in the direction parallel to the magnetic field. However, there is not yet a consensus regarding the role of chiral effects in recent transport studies.

We have measured MR in lithographically patterned 3000 x 600 microns Hall bars at T~ 5K, and varied the in-plane magnetic field B between 0 and 8.5 T. With B parallel to the current, MR decreases monotonically with B. Surprisingly, MR also decreases up to a large B applied perpendicularly to the current flow and only begins to increase in the upper range of B. We compare our observations to the findings of several other groups and discuss mechanisms competing with the chiral effects that may also lead to negative magnetoresistance in α-Sn..