# Synopsis: Nonmetallic Tin Behaves Like 3D Graphene

By applying strain to a form of tin, researchers make it behave like a 3D analog of graphene.

A rare class of materials, called topological Dirac semimetals (TDSs), exhibit electronic properties similar to graphene, but in three rather than two dimensions. Up until now, examples of TDSs have been mixtures of different elements, but new work has identified TDS features in a nonmetallic form of pure tin. Cai-Zhi Xu, from the University of Illinois at Urbana–Champaign, and colleagues show that changing the strain on the tin controls the appearance of TDS behavior.

In graphene and related materials, the electronic band structure harbors cone-shaped regions, around which the electronic states behave as if massless. These states, called Dirac fermions, are typically confined to two dimensions—be that a graphene sheet or the surface of a so-called topological insulator. But in a TDS, the Dirac fermions can move in all three dimensions. This freedom opens up a range of interesting properties, such as giant linear magnetoresistance and a unique pattern of quantum oscillations in the resistance. Previous work has identified just two members of the TDS class: ${\text{Na}}_{3}\text{Bi}$ and ${\text{Cd}}_{3}{\text{As}}_{2}$.

Recent studies of the nonmetallic form of tin, denoted $𝛼$-Sn, indicated that it might exhibit Dirac fermions when exposed to a strain. To investigate this, Xu and colleagues grew $𝛼$-Sn layers on a substrate of indium antimonide, which has nearly the same diamond-like lattice structure as $𝛼$-Sn. The slight mismatch in the two lattices induces a negative strain (or compression) in the tin. Using photoemission spectroscopy on their sample, the team identified the cone-shaped regions that are indicative of a TDS. Their theoretical computations showed that tuning the strain from negative to positive should switch the tin from a TDS to a topological insulator.

This research is published in Physical Review Letters.

–Michael Schirber

Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.

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