The discovery of topological states and topological phase transitions has reshaped our understanding of physics and materials over the past few years. The topological transition from a normal insulator to a topological insulator (TI) can be realized by chemical or mechanical pressure. The topological transition was first predicted and demonstrated in Tl-based chalcogenides TlBi(S1−xSex)2 by increasing the Se component where the chemical pressure increases the amplitude of the spin-orbit coupling (SOC) with the critical transition point at x=0.5, that is, TlBiSSe, which is a 3D Dirac semimetal. Instead, mechanical pressure pushes the bandgap of a normal insulator to close and re-open, leading to a nontrivial topological state. Furthermore, superconductivity may emerge with pressure and interplay with topological surface states, leading to possible topological superconductivity on the surface by the self-proximity effect.
Recently, a joint research team applied hydrostatic pressure to the thallium-based materials TlBi(S1−xSex)2 and investigated the effects of chemical and mechanical pressures on the phase transition and superconductivity induced by high pressure in three thallium-based materials, covering the phase transition from a normal insulator (TlBiS2) to a topological insulator (TlBiSe2) through a Dirac semimetal (TlBiSeS). By increasing the pressure up to 60GPa, the researchers observe superconductivity phase diagrams with maximal critical temperature (Tc) values at 6.0~8.1K. The researchers also formulated density-functional theory calculations that revealed topological surface states in superconductivity phases for all three compounds. This work paved the path to explore topological superconductivity and topological phase transitions.
Schematic of chemical pressure and mechanical pressure effect on TlBi(S1−xSex)2 compounds
Phase diagram for TlBiS2, TlBiSeS, and TlBiSe2
The research was conducted by SPST Assistant Professor Qi Yanpeng’s group, in collaboration with Professor Liu Hanyu from Jilin University, Professor Yao Yugui and Research Professor Wang Zhiwei from Beijing Institute of Technology, Professor Yan Binghai from Weizmann Institute of Science, Professor Claudia Felser from Max Planck Institute for Chemical Physics of Solids, and Professor Chen Yulin from University of Oxford. On October 4, their research results were published in a paper entitled “Pressure-induced superconductivity extending across the topological phase transition in thallium-based topological materials”, in the journal Cell Reports Physical Science, and ShanghaiTech University is the primary affiliation. This work got great support from the Analytical Instrumentation Center and Center for High resolution Electron Microscopy (CℏEM) of SPST.
**The news article is provided by Prof. Qi Yanpeng