Topological crystalline insulator states in the Ca$_2$As family

TL;DR

This paper identifies new topological crystalline insulator states in the Ca$_2$As family using density functional theory, revealing unique surface states protected by crystalline symmetries and proposing material modifications for topological phase control.

Contribution

The study discovers novel TCI states in Ca$_2$As family materials and demonstrates how lattice distortions can manipulate topological surface states, advancing the understanding of symmetry-protected topological phases.

Findings

Identification of TCI states in Ca$_2$As family

Observation of topological surface states protected by symmetries

Proposal of topological phase transitions in related systems

Abstract

Topological crystalline insulators (TCI) are insulating electronic phases of matter with nontrivial topology originating from crystalline symmetries. Recent theoretical advances have provided powerful guidelines to search for TCIs in real materials. Using density functional theory, we identify a class of new TCI states in the tetragonal lattice of the Ca$_2$As material family. On both top and side surfaces, we observe topological surface states protected independently by rotational and mirror symmetries. We show that a particular lattice distortion can single out the newly proposed topological protection by the rotational symmetry. As a result, the Dirac points of the topological surface states are moved to generic locations in momentum space away from any high symmetry lines. Such topological surface states have not been seen before. Moreover, the other family members, including Ca$_2$Sb, Ca$_2$Bi and Sr$_2$Sb, feature different topological surface states due to their distinct topological invariants. We thus further propose topological phase transitions in the pseudo-binary systems such as (Ca$_{1-x}$Sr$_x$)$_2$As and Ca$_2$As$_x$Sb$_{1-x}$. Our work reveals rich and exotic TCI physics across the Ca$_2$As family of materials, and suggests the feasibility of materials database search methods to discover new TCIs.