Spin-polarized topological insulators under pressure

Despite the low compressibility of solids, the thermodynamic parameter pressure can notably influence the stability of compounds, their structure and properties. In this proposal we are going to tackle high-pressure modifications of materials that are in focus of the cutting-edge research in solid- state physics, namely topological insulators (TIs). These recently discovered non-magnetic materials represent a new state of matter that enables the quantum spin Hall state at normal conditions. This state supports spatially-separated spin-polarized channels on the bulk surface that can be controlled independently and can transmit information without decoherence. Upon magnetic doping, TIs can exhibit quantized electric current without dissipation, which promises breakthroughs in realization of quantum computing at standard conditions. The number of confirmed TIs is very limited. Some theoretical studies indicate a possibility to obtain new TIs under pressure, but the complementary experimental effort has so far been very scarce. As for magnetic doping of known TIs, experiments under normal pressure have so far yielded very modest substitution rates (25 at.%) under barely controlled synthetic conditions. In order to fill in these gaps we propose: to explore structural evolution of selected TI-candidates, e. g. layered heterostructures such as BiTeI, (Bi2)n(BiTeI)m, (Bi2)n(Bi2Te3)m, (BiTeI)n(Bi2Te3)m (m, n = 1) under pressure; to explore possibilities of magnetic substitution for Bi and magnetic intercalation in the confirmed TIs (Bi2Te3, Bi2Se3) and the abovementioned TI-candidates under pressure; 3d- metals (Cu, Mn, Fe, Cr, V) and rare-earth elements (Ce, Gd) will be investigated first; to search for new, probably metastable, structural modifications of BiMX and SbMX, M = S, Se, Te, X = Cl, Br, I, under high-pressure conditions. The proposed research would contribute significantly to solid-state chemistry and physics by increasing the number of TIs and, especially, magnetically-doped ones.

Bulk functional materials that can exhibit quantum effects under normal conditions are under intense spotlight nowadays, among them topological materials, 2D magnetic monolayers and frustrated magnets. Topological insulators (TIs) feature dissipationless electron transport on their surface thanks to protected spin-resolved surface states. First topological materials were discovered in 2009, and nowadays this is a burgeoning field of condensed-matter research. Particularly intriguing is a combination of non-trivial topology with magnetic order that could yield new types of topological (magneto)transport and possibilities to actively manipulate them. Material candidates are emerging hand in hand with theoretical advances. In our project, we have identified and experimentally characterized six new topological materials, namely, Bi2TeI, Bi3TeI, Bi2TeBr, Bi3TeBr, -Bi4I4 and MnBi2Te4. Each group of compounds exhibits its special topological "signature". The layered (Bi2)n(BiTeX)m compounds allow to trace the development of the topological properties from a trivial semiconductor (BiTeX) to a dual topological insulator (Bi2TeX) and a topological metal (Bi3TeX) via enhancement of the interlayer interactions (hybridization of the states). -Bi4I4 undergoes a transition into a superconducting phase under high pressure, which makes it an intriguing candidate for topological superconductivity. The layered MnBi2Te4 is the first antiferromagnetic topological insulator with a periodic crystal structure. Thanks to their versatile magnetic and electronic properties, MnBi2Te4 and its derivatives may become a material platform for the realization of tunable topological quantum phenomena and foster applications in prototype devices for AFM spintronics, 2D magnets, etc. During these investigations, the application of high-pressure conditions favored the formation of several novel manganese hydroxide halides, Mn(OH)X, X = Cl, Br, I, Mn5(OH)6Cl4, Mn5(OH)7I3 and Mn7(OH)10I4, that were first synthesized and structurally characterized in the current project. Calculations and magnetic measurements hint at competing magnetic ground states and proximity to magnetic frustration in some of these material that host Mn2+ cations on a triangular lattice. Extreme hygroscopicity challenges the physical measurements and calls for a specialized study. Furthermore, the new phases Mn5(BO3)3OH and Bi2(C3H5O3)2 with acentric crystal structures, as confirmed by X-ray diffraction and SHG measurements, were discovered as an unforeseen outcome of the studies.