Spin-filtered edge states with an electrically tunable gap in a two-dimensional topological crystalline insulator

Abstract

Three-dimensional topological crystalline insulators were recently predicted and observed in the SnTe class of IV-VI semiconductors, which host metallic surface states protected by crystal symmetries. In this work, we study thin films of these materials and expose their potential for device applications. We demonstrate that thin films of SnTe and Pb1−xSnxSe(Te) grown along the (001) direction are topologically nontrivial in a wide range of film thickness and carry conducting spinfiltered edge states that are protected by the (001) mirror symmetry via a topological invariant. Application of an electric field perpendicular to the film will break the mirror symmetry and generate a band gap in these edge states. This functionality motivates us to propose a novel topological transistor device, in which charge and spin transport are maximally entangled and simultaneously controlled by an electric field. The high on/off operation speed and coupling of spin and charge in such a device may lead to electronic and spintronic applications for topological crystalline insulators. PACS numbers:Crystal structure and symmetry play a fundamental role in determining electronic properties of quantum materials. The interplay between crystallography and electronic topology[1, 2] has advanced our understanding of topological insulators [3][4][5]. More recently, a new type of topological phases termed topological crystalline insulators [6] has been predicted [7] and observed [8][9][10] in three-dimensional materials SnTe and Pb 1−x Sn x Se(Te). A key characteristic of topological crystalline insulators is the presence of metallic boundary states that are protected by crystal symmetry, rather than time-reversal [11]. As a consequence, these states can acquire a band gap under perturbations that break the crystal symmetry [7,12]. This novel property opens up an unprecedented functionality of tuning the charge and spin transport of topological boundary states with high on/off speed by applying an electric field. Here we theoretically demonstrate that thin films of SnTe and Pb 1−x Sn x Se(Te) grown along the (001) direction realize a new, two-dimensional topological crystalline insulator phase that supports spin-filtered edge states with a band gap tunable by electric field effect. Our work may thus enable electronic and spintronic device applications based on topological crystalline insulators.Topological crystalline insulators (TCI) have so far only been realized in three-dimensional materials [7][8][9][10]. In this work, we propose a material realization of a twodimensional (2D) topological crystalline insulator phase, which is topologically distinct from an ordinary insulator in the presence of mirror symmetry about the 2D plane.The topology here is mathematically characterized by two integer topological invariants N + and N − , which are Chern numbers of Bloch states with opposite mirror eigenvalues. While the sum N + + N − determines the quantized Hall conductance, the mirror Chern number [2] defined by N M ≡ (N + − N − )/2 serves as...

Crossed Surface Flat Bands of Weyl Semimetal Superconductors

“…Such crossed band structures strongly enhance the surface density of states, which might induce nontrivial low-energy phenomena. Our findings also extend the possibility of controlling zero energy flat bands [33].…”

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

“…This indicates that the Na 3 Bi monolayer is a 2D TCI. Interestingly, the C M = −1 case we consider here is topologically distinct from the previously reported 2D TCIs, such as (Sn/Pb)Te [10,23,24] and Tl(S/Se) [25] with…”

“…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 many cases, the complex structures and the lack of mirror symmetry in such materials prevent the formation of a 2D TCI phase. On the other hand, 2D TCIs have been limited thus far to theoretical predictions that are mainly restricted to SnTe multilayers [10], (Sn/Pb)(Se/Te) monolayers [22][23][24], Tl(S/Se) monolayers [25], and SnTe/NaCl quantum wells [26] with mirror Chern number C M = −2. The even number of band inversions leads to a vanishing Z 2 invariant.…”

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