Theoretical Study Of Topological Characters Of Tetradymite Compounds

Topological insulators(TIs)constitute a new class of quantum materials with insulating bulk but metallic boundary states,which are essentially different from traditional metals and insulators.Protected by time-reversal symmetry,those boundary states possess spin-momentum locked Dirac structure in which all the backscattering channels are suppressed,favoring dissipationless electronic conduction.This salient property renders TIs immense application potentials in new electronic devices.Nevertheless,there are relatively few stable TIs that feature optimal properties for the specific applications.In this thesis,using tetradymite compounds as a prototypical example class of materials,we have performed first-principles calculations to obtain reliable information on their topological natures.Based on the band inversion induced by spin-orbit coupling or the compressed sensing technique,we have proposed several simple and efficient criterions that allow ready screening of potential TIs.The underlying design principles of our work could shed a light on searching novel topological quantum materials.The main contents of our works include:We demonstrate through a comparative first-principles study that Sb₂Se₃ is actually also an intrinsic TI,as characterized by its topologically protected surface states and Z₂ invariant.The underlying reason is the inclusion of the ubiquitous van der Waals attraction between the quintuple layers,effectively narrowing the bulk band gap,allowing the spin-orbit coupling effect to induce a band inversion around the Fermi level.This paper also points to the importance of the seemingly weak dispersion forces in defining the topological properties of quantum materials.We proposed a simple and efficient criterion that allows ready screening of potential topological materials.The criterion is inherently tied to the band inversion induced by spin-orbit coupling,and is uniquely defined by a minimal number of two elemental physical properties of the constituent elements:the atomic number and Pauling electronegativity.The validity and predictive power of the criterion is demonstrated by rationalizing many known topological insulators and potential candidates in the tetradymite and half-Heusler families,and the underlying design principle is naturally also extendable to predictive discoveries of other classes of topological materials.In order to consider different orbital contributions to the band inversion of tetradymites,we have optimized the above-mentioned?criterion and proposed a weighted?_W criterion.Such a new criterion explicitly includes the site contribution of five atoms in the primitive cell,and is also defined by the atomic number and the Pauli electronegativity of the constituent elements.It is interesting to find that?_W criterion can correctly identify the topological characters of those tetradymites with the same chemical formula but different atomic configurations.Furthermore,we see from the weighted coefficients that the anions at the center site make the smallest contributions to the band inversion.If we want to optimize certain topological features of the systems,it is therefore necessary to consider constituent atoms at other sites,which may provide a new idea for the theoretical and experimental studies of TIs.We have performed extensive,high-level first-principles calculations to obtain reliable information on the topological natures of 243 tetradymites.From this data set,we obtain a simple and predictive 2D descriptor for the TI/NI classifications using the SISSO approach.The descriptor is identified which requires only the atomic number and Pauling electronegativity of the constituent elements.Such a simple descriptor is dimensionless,physically interpretable,and exhibits a strong predicting power.Most importantly,the implementation of the 2D descriptor provides a phase diagram by which one can quickly identify possible TIs with arbitrary mutation of atoms and/or stoichiometry.Using this descriptor we readily scanned over 4 million compound materials in the tetradymite family,with nearly 1.97 millions identified as hitherto unknown topological insulators.Our study therefore offers a major step forward in exploration of a much larger materials space far beyond the existing databases.