Topological materials (TMs) showcase intriguing physical properties defying expectations based on conventional materials, and hold promise for the development of devices with new functionalities. While several theoretically proposed TMs have been experimentally confirmed, extensive experimental exploration of topological properties as well as applications in realistic devices have been held back due to the lack of excellent TMs in which interference from trivial Fermi surface states is minimized. We tackle this problem in the present work by applying our recently developed method of symmetry indicators to all non-magnetic compounds in the 230 space groups. An exhaustive database search reveals thousands of TM candidates. Of these, we highlight the excellent TMs, the 258 topological insulators and 165 topological crystalline insulators which have either noticeable full band gap or a considerable direct gap together with small trivial Fermi pockets. We also give a list of 489 topological semimetals with the band crossing points located near the Fermi level. All predictions obtained through standard generalized gradient approximation (GGA) calculations were cross-checked with the modified Becke-Johnson (MBJ) potential calculations, appropriate for narrow gap materials. With the electronic and optical behavior around the Fermi level dominated by the topologically non-trivial bands, these newly found TMs candidates open wide possibilities for realizing the promise of TMs in next-generation electronic devices.

Towards ideal topological materials: Comprehensive database searches using symmetry indicators

Discoveries of topological states and topological materials reshape our understanding of physics and materials over the last 15 years. First-principles calculations have been playing a significant role in bridging the theory of topology and experiments by predicting realistic topological materials. In this article, we overview the first-principles methodology on topological quantum materials. First, we unify different concepts of topological states in the same band inversion scenario. Then, we discuss the topology using first-principles band structures and newly-established topological materials databases. We stress challenges in characterizing symmetry-independent Weyl semimetals and calculating topological surface states, closing with an outlook on the exciting transport and optical phenomena induced by the topology.

First Principles Calculations for Topological Quantum Materials

The successful commercial applications as thermoelectric devices and, due to their exotic electronic properties, as topological insulators of bismuth telluride (Bi2Te3) and bismuth selenide (Bi2Se3) have stimulated research interest on Bi2Se3/Bi2Te3-based chemical compounds. Based on the first-principles calculations, we investigate the electronic, optical, vibrational and transport properties of new monolayer Bi2TeSe2 obtained by transmuting one Se atom into its neighboring Te atom in the same group from Bi2Se3. We find that the monolayer Bi2TeSe2 maintains a stable hexagonal structure up to 700 K. Monolayer Bi2TeSe2 possesses a direct bandgap of 0.29 eV due to the strong spin–orbit coupling effects, and it remains a direct semiconductor for strains in a moderate range. The optical absorption covers a wide range from the green region to the ultraviolet region, which may lead to applications in optoelectronic devices like saturable absorbers. An extremely high electron mobility of 20 678 cm2 V−1 s−1 along the zigzag direction can be achieved by strain engineering with −6% compressive strain, which is nearly ten times larger than the intrinsic mobility. These indicate that monolayer Bi2TeSe2 is a promising candidate for future high-speed (opto)electronic devices.

Ultrahigh electron mobility induced by strain engineering in direct semiconductor monolayer Bi2TeSe2.

Author(s): Inzani, K; Faghaninia, A; Griffin, SM | Abstract: New ideas for low-mass dark matter direct detection suggest that narrow band gap materials, such as Dirac semiconductors, are sensitive to the absorption of meV dark matter or the scattering of keV dark matter. Here we propose spin-orbit semiconductors - materials whose band gap arises due to spin-orbit coupling - as low-mass dark matter targets owing to their O(10 meV) band gaps. We present three material families that are predicted to be spin-orbit semiconductors using density functional theory (DFT), assess their electronic and topological features, and evaluate their use as low-mass dark matter targets. In particular, we find that the tin pnictide compounds are especially suitable having a tunable range of meV-scale band gaps with anisotropic Fermi velocities allowing directional detection. Finally, we address the pitfalls in the DFT methods that must be considered in the ab initio prediction of narrow-gapped materials, including those close to the topological critical point.

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Prediction of tunable spin-orbit gapped materials for dark matter detection

We show that compositions of time-reversal and spatial symmetries, also known as the magnetic-space-group symmetries, protect topological invariants as well as surface states that are distinct from those of all preceding topological states. We obtain, by explicit and exhaustive construction, the topological classification of electronic band insulators that are magnetically ordered for each one of the 1421 magnetic space groups in three dimensions. We have also computed the symmetry-based indicators for each nontrivial class, and, by doing so, establish the complete mapping from symmetry representations to topological invariants. 

/pdf/topological-classification-and-diagnosis-in-magnetically-26dd4jc7.pdf

Topological classification and diagnosis in magnetically ordered electronic materials

https://iopscience.iop.org/article/10.1088/1674-1056/28/8/087102/pdf

He Huang

Papers

SymTopo:An automatic tool for calculating topological properties of nonmagnetic crystalline materials