The seemingly ordinary and well-known material, tin, exhibits unique electronic properties under extreme but well controlled conditions. The discovery that it turns into Dirac and Weyl semimetals was made by an international team of researchers led by scientists from the International Research Center MagTop, at the Institute of Physics.
Tin (Sn, Latin: stannum) is a common chemical element, known since ancient times. Historically, it was used to produce pots, containers, and to protect iron surfaces against corrosion. Nowadays, it is mainly used in electronics in solders and for industrially important technical copper alloys. Tin comes in two allotropic varieties. In its commonly known form, it is a soft gray-silver metal, crystallizing in a tetragonal structure, with a density of 7.3 g/cm3, the so-called β (beta) tin. At temperatures below approximately 13 degrees Celsius, tin slowly transforms into the non-metallic α (alpha) form, the so-called gray tin, with a regular crystalline structure and a density of 5.85 g/cm3. This second variety of tin occurs in the form of a gray powder, which has hitherto seemed not very interesting. It is even considered detrimental, as per the common name of this phase transformation known as „tin pest” or tin disease. Research conducted by a team of scientists from the International Centre for Interfacing Magnetism and Superconductivity with Topological Matter – MagTop at the Institute of Physics, Polish Academy of Sciences (IF PAN) and just published in Materials Today reveals the fascinating electronic properties of this low-temperature α form.
By combining together a series of complementary experimental techniques they have shown that grey tin when subjected to biaxial compressive stress becomes a topological Dirac semimetal. Moreover, under the influence of an additional external magnetic field, it transforms into the Weyl semimetal phase. In both cases, these special properties result from relativistic phenomena characteristic of atoms with a high electric charge of the atomic nucleus and the associated high velocities of band electrons in the crystal, accelerated in the electric field of the nucleus. One example of such phenomena is negative magnetoresistance, which is a manifestation of the chiral anomaly.
These discoveries were made possible by the development of molecular beam epitaxial (MBE) technology for producing α-Sn layers with a thickness below 200 nm (approximately the size of a COVID-19 virus), stable at room temperature. It was also necessary to create microstructures using lithography techniques, similar to those employed in microprocessor production. The use of specially prepared, non-conductive hybrid CdTe/GaAs substrates enabled the direct examination of the electronic properties of grey tin. The large surface area of the α-Sn layers being fabricated (up to 3 inches in diameter) gives hope of translating these results into applied research and future applications in areas such as spintronics and quantum computing devices. The study was carried out in collaboration with research groups from Jagellonian University in Kraków (SOLARIS synchrotron), Johanees Kepler University in Linz, and Sorbonne Université in Paris, basing on the Institute’s decades of expertise in the growth of thin films and nanostructures, structural research, nanotechnology, and measurements of electronic properties.
