Chemically driven surface effects in polar intermetallic topological insulators A3Bi

Interplay of Topological States on TI/TCI Interfaces

Based on first-principles calculations, we study electronic structure of interfaces between a Z2 topological insulator (TI) SnBi2Te4 and a topological crystalline insulator (TCI) SnTe. We consider two interface models characterized by the different atomic structure on the contact of the SnTe(111) and SnBi2Te4(0001) slabs: the model when two materials are connected without intermixing (abrupt type of interface) and the interface model predicted to be realized at epitaxial immersion growth on topological insulator substrates (smooth interface). We find that a strong potential gradient at the abrupt interface leads to the redistribution of the topological states deeper from the interface plane which prevents the annihilation of the Γ¯ Dirac states, predicted earlier. In contrast, a smooth interface is characterized by minor charge transfer, which promotes the strong interplay between TI and TCI Γ¯ Dirac cones leading to their complete annihilation.The M¯ topologically protected Dirac state of SnTe(111) survives irrespective of the interface structure.

Unique topological nodal line states and associated exceptional thermoelectric power factor platform in Nb3GeTe6 monolayer and bulk

To date, ideal topological nodal line semimetal (TNLS) candidates in high dynamically stable and high thermally stable two-dimensional (2D) materials are still extremely scarce. Herein, by performing first-principles calculations, on the one hand, we found that three-dimensional Nb₃GeTe₆ bulk possesses a single closed TNL in the k_x = 0 plane and a fourfold TNL in the S-R direction without considering spin-orbit coupling (SOC). Under the SOC effect, a new topological signature, i.e., hourglass-like Dirac nodal line, occurs in Nb₃GeTe₆ bulk. On the other hand, we found that the 2D Nb₃GeTe₆ monolayer features a doubly degenerate TNL along surface X-S paths. Importantly, this monolayer enjoys the following advantages: (i) it has high thermal stability at room temperature and above; (ii) its TNL is nearly flat in energy and is very close to the Fermi level (E_F), which provides a fantastic maximum value platform of the thermoelectric power factor around the E_F; and (iii) no extraneous bands are close to the TNL, near the Fermi level. Moreover, we explore the entanglement between the topological states and thermolectric properties for the 2D Nb₃GeTe₆ monolayer. Our work not only reports the discovery of a novel TNL material, but also builds the link between the TNL and thermoelectric properties.

Rich novel zero-dimensional (0D), 1D, and 2D topological elements predicted in the P63/m type ternary boride HfIr3B4

Topological semimetals, including topological nodal point semimetals (TNPSs), topological nodal line state semimetals (TNLSs), and topological nodal surface semimetals (TNSSs), featuring zero-dimensional (0D), one-dimensional (1D), and two-dimensional (2D) topological elements (TEs), respectively, have attracted widespread attention in recent years. In this work, based on first-principles calculations, we propose for the first time that three different (0D, 1D, and 2D) TEs are simultaneously present in a synthetic compound, HfIr₃B₄, with a P6₃/m type structure. In detail, HfIr₃B₄ hosts a Dirac point (DP) state at the K point, a TNL state in the k_z = 0 plane, and a 2D TNS state in the k_z = π plane, respectively. All sorts of topological elements, 0D, 1D, and 2D TEs, coexisting in the P6₃/m type HfIr₃B₄, provide an ideal platform to study the rich fermionic states and their related physical properties in this type of compound. In addition, because the 0D, 1D, and 2D TEs of HfIr₃B₄ are equally distributed in different energy ranges relative to the Fermi level, an approach is proposed to utilize individual TEs to build on-demand devices.