Alpha-Tin Exhibits a Novel Electronic Phase and Joins a Unique New Class of 3D Materials

Physics Professor Tai-Chang Chiang and graduate student Caizhi Xu pose in Chang's laboratory at the Frederick Seitz Materials Research Lab at the University of Illinois at Urbana-Champaign. Image courtesy of Physics Illinois.

A recent discovery highlighted that a novel electronic phase is exhibited by alpha-tin, generally known as gray tin, when its crystal structure is strained.

This places it in a unique new class of 3D materials called topological Dirac semimetals (TDSs). The existence of only two other TDS materials was discovered in 2013. Now, Alpha-tin is included in this class as the only simple-element member.

This discovery is promising for novel physics and several other potential applications in technology. These results are the work of Caizhi Xu, a physics graduate student at the University of Illinois at Urbana-Champaign, working under U of I Professor Tai-Chang Chiang and in partnership with scientists at the Advanced Light Source at the Lawrence Berkeley National Laboratory and six other institutions internationally.

TDSs exhibit electronic properties similar to those of the topological insulators (TIs) on their surfaces. Study on these TIs is currently gaining much importance. The interior or “bulk” of the TIs acts as an insulator, and the surfaces allow electrons to conduct freely like a metal.

The surface electrons act as 2D massless spin-polarized Dirac fermions that are strong against non-magnetic impurities, which produce potential applications in fault-tolerant quantum computing and spintronic devices.

The bulk electrons present in TDSs act as massless Dirac fermions in all three dimensions, which results in extra possibilities for novel physical behaviors.

TDSs are of profound interest to condensed matter physicists, primarily because they exhibit a number of novel physical properties, including ultrahigh carrier mobility, giant linear magnetoresistance, chiral anomaly, and novel quantum oscillations. Secondly, this class of materials can realize many interesting topological phases—under controlled conditions, the material can undergo phase transitions and can become a topological insulator, a Weyl semimetal, or a topological superconductor.

Caizhi Xu, University of Illinois

Tin is known to comprise of two well-established allotropes: at 13.2 °C and above, beta-tin, or white tin, is metallic. The material becomes alpha-tin, or gray tin, which is semi-metallic, and the atomic structure of tin transitions when the temperature is below 13.2 °C.

However, in thin films grown on a substrate like indium antimonide (InSb) the transition temperature of tin increases up to 200 °C. This explains that alpha-tin continues to be stable even above room temperature.

An ordinary semi-metallic phase is generally exhibited by alpha-tin’s diamond-cubic crystal structure, and the material currently does not have any common applications. Gray tin can actually become problematic in several applications involving tin—the so-called “tin pest” problem referring to the formation of gray tin that leads to disintegration of parts made up of white tin.

A strain on the material was engineered in the experiment conducted by Xu et al. This was performed by growing alpha-tin samples in layers on a substrate of InSb, another crystalline material comprising of a slightly different lattice constant.

That lattice mismatch leads to strain, or compression, in the alpha-tin. It was believed that strain would open a band gap in gray tin and turn it into a TI. In a few recent studies researchers observed topological surface states in strained tin, but they didn’t observe the strain-induced band gap because they were not able to access the conduction band. In this study, we used potassium doping and with this simple method were able to reach the conductance band. We were able to see the gapless and linear band dispersion that is the hallmark of a Dirac semimetal. This discovery is kind of unexpected. I decided to study the material because of its known TI phase. Once I dug into the experimental results and performed some theoretical calculations, what I found is that alpha-tin under a compressive strain is not an insulator, as had been thought. It turns out to be a Dirac semimetal. Our calculations also show that it is only under a tensile strain that alpha-tin becomes a TI.

Caizhi Xu, University of Illinois

Chiang hopes that these findings will make room for new research areas: “Caizhi Xu’s work illustrates that interesting new physics can still be found in simple common materials, such as gray tin, which has been known and studied for decades.”

“It’s clear from this study that strain engineering can open up many possibilities,” Chiang continues. “My group is currently exploring a different way to apply strain, by mechanically stretching a sample. The strain will be uniaxial—along one direction only—and it will be tunable, but limited by sample breakage.”

Extraction and utilization of tin in alloys has been carried out by mankind since the Bronze Age, c. 3000 BC. Before the invention of aluminum cans, tin cans were used to preserve food. These tin cans were in fact steel lined with tin. This discovery could help in establishing alpha-tin to be an extremely useful material in future technologies.

Potential applications of alpha-tin as a topological Dirac semimetal could include taking advantage of its high carrier mobility to generate ultrafast electronic devices. Additionally, the giant magneto resistance could be useful in developing ultra-compact storage devices, like computer hard disks. Furthermore, this material could be a platform for further fundamental research related to optical properties, or to transport properties, including superconductivity. There is even potential that it could be used as a platform to realize Majorana fermions. I believe our new finding will be of interest to many physicists.

Caizhi Xu, University of Illinois

The April 4, 2017 issue of Physical Review Letters has published a report on these findings in the article “Elemental topological Dirac semimetal α-Sn on InSb,” which is highlighted as a PRL Editor’s Suggestion.

The U. S. Department of Energy and the National Science Foundation supported this research along with several other international institutions. The samples were grown at Lawrence Berkeley National Laboratory’s Advanced Light Source, supported by the U.S. DOE.

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