Robust dual topological character with spin-valley polarization in a monolayer of the Dirac semimetal Na3Bi

Abstract

Topological materials with both insulating and semimetal phases can be protected by crystalline (e.g., mirror) symmetry. The insulating phase, called a topological crystalline insulator (TCI), has been investigated intensively and observed in three-dimensional materials. However, the predicted two-dimensional (2D) materials with TCI phase are explored much less than 3D TCIs and 2D topological insulators, while the 2D TCIs considered thus far possess almost exclusively a square-lattice structure with the mirror Chern number C M = −2. Here, we predict theoretically that a hexagonal monolayer of Dirac semimetal Na 3 Bi is a 2D TCI with a mirror Chern number C M = −1. The large nontrivial gap of 0.31 eV is tunable and can be made much larger via strain engineering, while the topological phases are robust against strain, indicating a high possibility for room-temperature observation of quantized conductance. In addition, a nonzero spin Chern number C S = −1 is obtained, indicating the coexistence of a 2D topological insulator and a 2D TCI, i.e., the dual topological character. Remarkably, a spin-valley polarization is revealed in the Na 3 Bi monolayer due to the breaking of crystal inversion symmetry. The dual topological character is further explicitly confirmed via the unusual behavior of the edge states under the corresponding symmetry breaking. DOI: 10.1103/PhysRevB.95.075404The discovery of topological insulators (TIs) [1,2] has triggered an explosion of novel topologically nontrivial phases, such as the topological crystalline insulator (TCI), for which the role of time-reversal symmetry is replaced by crystal (mirror) symmetry [3][4][5]. The hallmark of a TCI, similar to a TI, is the presence of gapless surface/edge states with Dirac points inside of the insulating bulk energy gap. In the presence of crystal mirror symmetry, the coexistence of TI and TCI phases has been predicted in three dimensions for Bi 1−x Sb x [6] and Bi chalcogenides [7][8][9], and thus they exhibit a dual topological character (DTC). Recently, unusual topological surface states for a three-dimensional (3D) DTC system have been observed experimentally [8,9]. In the 2D case, graphene may be a prototypical example of a DTC [10,11]. However, the extremely small band gap of graphene makes it very difficult to verify the DTC in this material experimentally For both 2D TIs and 2D TCIs, spin-orbit coupling (SOC) is known to play a vital role. In addition, SOC together with inversion symmetry breaking can lead to coupled spin and valley physics, in which the new degree of freedom offers a promising route to the eventual realization of valleytronic devices [28,29]. Spin-valley polarization has been observed experimentally in the MoS 2 monolayer [30], which is a topologically trivial insulator. Therefore, a natural question arises as to whether spin-valley polarization in nontrivial insulators, such as 2D TIs and 2D TCIs, is possible. Recently, thin films of the Dirac semimetal Na 3 Bi [31,32] were fabricated by molecular-beam epitaxy [33], and th...

Dimensional Crossover and Topological Phase Transition in Dirac Semimetal Na3Bi Films

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Electric-field-tuned topological phase transition in ultrathin Na3Bi

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