Project Details
Projekt
Topological Insulators in Bismuth-Halogen and Related Systems: Design, Synthesis, Optimization and Properties
Subject Area
Experimental Condensed Matter Physics
Term
from 2013 to 2021
Project identifier
Deutsche Forschungsgemeinschaft (DFG) - Project number 237598131
Employing the concept of "confined metals" in the first funding period, we have established rough guidelines towards the directed search of new topological insulators based on crystal-structure features. A 2D TI fragment embedded as a low-dimensional structural fragment into the bulk structures of bismuth-rich metal-salt hybrids can account for weak or strong 3D TI properties of the entire compound. The salt-like part can be constructed from the highly polar iodide anions that favor for more defect-free, stoichiometric compounds as opposed to chalcogenides. This approach is by now exemplified by two 3D weak topological insulators (Bi14Rh3I9, Bi2TeI) and a 3D strong topological insulator, beta-Bi4I4, which electronic structure is in proximity of both the weak 3D TI phase and the trivial insulator phase. These bulk materials are built by two different 2D TI fragments: a decorated honeycomb intermetallic layer that structurally resembles graphene and a bismuth bilayer, which is a building unit of the elemental bismuth structure. Bi14Rh3I9 is so far the only known 3D weak TI, for which the occurrence of topological edge states was experimentally confirmed. beta-Bi4I4 is the only representative of the strong topological class Z2=1;(1,1,0) and features a highly anisotropic Dirac cone. Our theoretical consideration of the Bi2TeI electronic structure predicts exotic topological surface states beyond the Z2-formalism. Our goal within the second funding period of the Priority Program is to synthesize, explore and functionalize new families of TIs, taking the above mentioned compounds as a starting point. The availability of real materials will promote theoretical and experimental advances for 3D weak TIs that were so far overshadowed by the strong counterparts also due to the lack of specimens. The essential steps in the research program to realize these goals are: 1) Fine-tuning of the available large Bi14Rh3I9 single-crystals for intrinsic electronic and magneto-electronic transport by means of iodine doping, as well as experimental and theoretical assessment of exfoliation, gating and contacting of thin Bi14Rh3I9 layers in order to get access to first simple devices based on a 3D weak TI. 2) Exploration and expansion of a next generation of strong TIs derived from beta-Bi4I4 by synthetic chemistry, DFT-based calculations of electronic structure and topological invariants. 3) Dedicated efforts to push single-crystal growth of Bi2TeI to the highest attainable level in order to investigate the possible co-existence of the weak 3D TI phase and the topological-crystalline-insulator phase that is protected by the mirror crystal symmetry. 4) Identification of new TI candidates with differing structural types via application of the "confined metals"-concept toward the crystal structure and via first-principle electronic structure calculations and direct calculation of topological invariants.
DFG Programme
Priority Programmes
