An international team of researchers has shown that in response to compressive strain, a form of tin known as gray tin (α-Sn) exhibits rare electronic properties and topological phase transitions. As reported in Physical Review Letters, this makes it just the third topological Dirac semimetal (TDS) identified, a category of semimetals commonly called the three-dimensional (3D) analog of graphene.  

As in graphene, the electrons in a TDS are governed by the Dirac equation and therefore act as massless Dirac fermions with especially high mobility. In graphene, this motion is limited to a two-dimensional plane, but in a TDS the electron motion occurs in three dimensions. This can lead to interesting properties such as giant linear magnetoresistance and the breaking of chiral symmetry. The α-Sn now joins Na₃Bi and Cd₃As₂ as the only simple-element TDS discovered to-date.

The research was led by Cai-Zhi Xu, a graduate student working under Tai-Chang Chiang at the University of Illinois at Urbana–Champaign. “We have been interested in emergent properties of simple materials under physical perturbations or modifications,” says Xu. “Prior studies showed that α-Sn under a positive (tensile) in-plane strain became a topological insulator. We were curious what it would become under a negative (compressive) in-plane strain.”

To explore the properties of α-Sn under negative strain, the researchers grew thin films of α-Sn on a substrate of indium antimonide (InSb) by physical vapor deposition. The α-Sn and InSb have similar crystalline lattice structures, but a slight mismatch in the lattice constants results in a compressive strain of 0.14% on the α-Sn. After introducing n-type doping into the strained α-Sn by depositing potassium onto its surface, the team used photoemission spectroscopy to map the electronic band structure. Surprisingly, they saw cone-shaped regions in all three momentum directions, the quintessential sign of a TDS in which the conduction and valence bands touch at discrete points and regress linearly in three dimensions.

First-principles calculations of the electronic band structure support the TDS nature of α-Sn under negative strain and its ordinary semimetal nature when not under strain. In addition, theoretical calculations reconciled the outcome of this study with previous experiments that showed α-Sn to be a topological insulator under positive strain.

“By changing the strain from compressive to tensile (squeezing to stretching), we can change the topological phase of the thin film of α-Sn—from topological Dirac semimetal to topological insulator,” says team member Sung-Kwan Mo from the Advanced Light Source at Lawrence Berkeley National Laboratory. This means that α-Sn can potentially be tuned from an ordinary semimetal (OS) to a topological insulator (TI) or a TDS by strain engineering.

“This work demonstrates the existence of a 3D Dirac semimetal in a surprisingly simple elemental material, Sn,” says Binghai Yan, an expert in topological materials at the Weizmann Institute of Science in Israel. Yan was part of a collaboration who found that an atomically thin Sn layer, called stanene, is a topological insulator that may be promising for room temperature applications (Physical Review Letters). “In this work, Sn films bring us another surprise. It will enable us to test the relativistic electrons in a single element and perhaps design future electronic and spintronic devices,” Yan says.

Moving forward, Xu and his colleagues will focus on further exploring theoretical predictions experimentally. “Theoretical calculations show that the separation of the two 3D Dirac cones of strained α-Sn in the TDS phase will increase at higher strain. We plan to prepare α-Sn films on a series of substrates with different lattice constants, which will allow us to tune the epitaxial strain,” says Xu.  They also plan to grow strained α-Sn films along crystallographic directions other than the (111) used in this work.

Read the abstract in Physical Review Letters.