2D Topological Crystalline Insulator Research Articles

Switchable quantum anomalous Hall effect in a ferromagnetic topological crystalline insulating NpSb monolayer

The combination of band topology and magnetic order provides an exciting platform for exploring prominent topological quantum effects, such as the quantum anomalous Hall (QAH) effect, which is well known to emerge when the ferromagnetic (FM) order is introduced in two-dimensional (2D) topological (crystalline) insulators. However, a topological phase transition between QAH insulators and magnetic topological (crystalline) insulators in 2D intrinsic magnets is seldom reported. Here, in contrast to previous magnetic doping, we predict that a switchable topological phase transition between a QAH insulator and a 2D topological crystalline insulator can emerge in an intrinsic FM NpSb monolayer via strain engineering. With the spin-up and spin-down manifolds being oppositely tunable, the calculated mirror Chern number, Chern number, and nontrivial edge states clearly confirm the topological phase transition as well as the QAH effect. Moreover, we reveal the feasibility of attaining the QAH effect while strain is introduced by an appropriate substrate. These results are of great scientific interest for exploring and understanding nontrivial topology in 2D magnets.

A novel topological crystalline insulator in planar pentacoordinate OsS2 monolayer

Topological crystalline insulators have been confirmed to be topological insulators whose surface states are protected by the crystalline symmetry. A new type of 2D pentagonal rings anisotropic structure OsS2 is confirmed as 2D topological crystalline insulator with gap of 44.3 meV by two pairs of spin filtering edge states and mirror Chern number CM = 2. The dxz and dyz orbitals of Os atom cross linearly between S and Γ points to form a Dirac cone without spin–orbit coupling effect. Carrier mobility of OsS2 is as high as 3.74 × 104cm2 V−1 s- 1for electrons and 3.33 × 104cm2 V−1 s−1 for holes. The OsS2 monolayer is a high-strength 2D material with the minimum Young’s modulus 153.51 N·m−1. Furthermore, OsS2 shows strong optical absorption in the deep ultraviolet region. The OsS2 monolayer is expected to play important applications in fields of nanoelectronics, spintronics and mechanics owing to its TCI and high carrier mobility.

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

Topological materials with both insulating and semimetal phases can be protected by crystalline (e.g. mirror) symmetry. The insulating phase, called topological crystalline insulator (TCI), has been intensively investigated 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 insulator, while so far considered 2D TCIs almost exclusively possess a square lattice structure with the mirror Chern number $\mathcal C_{M} =-2$. Here, we predict theoretically that hexagonal monolayer of Dirac semimetal Na$_3$Bi is a 2D TCI with a mirror Chern number $\mathcal C_{M} =-1$. 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 $\mathcal C_{S} =-1$ is obtained, indicating the coexistence of 2D topological insulator and 2D TCI, i.e. the dual topological character. Remarkably, a spin-valley polarization is revealed in Na$_3$Bi monolayer due to the breaking of crystal inversion symmetry. The dual topological character is further explicitly confirmed via unusual edge states' behavior under corresponding symmetry breaking.

Two-dimensional topological crystalline quantum spin Hall effect in transition metal intercalated compounds

While most of the two-dimensional (2D) topological crystalline insulators (TCIs) belong to group IV-VI narrow-band-gap semiconductors in a square lattice, in the present work we predict a TCI family based on transition metal intercalated compounds in a hexagonal lattice. First-principles calculations combined with a substrate-fixed globally optimal structural search technique show that a layer of Os prefers a uniform distribution between two graphene sheets. Band dispersion calculations reveal a Dirac point and a Dirac nodal ring near the Fermi level. The Dirac point is ascribed to the hybridization of ${e}_{2}$ and ${e}_{2}{}^{*}$ orbitals, and the Dirac ring is formed due to dispersion of $s$ and ${e}_{1}{}^{*}$ orbitals. Upon inclusion of spin-orbit coupling, these Dirac states open topologically nontrivial local band gaps, which are characterized by nonzero mirror Chern numbers. The quantum spin Hall effect is also observed by integrating the spin Berry curvature in the Brillouin zone. In contrast to the 2D group IV-VI TCIs whose band inversions at $X$ and $Y$ points are ``locked'' by ${C}_{4}$ rotation symmetry, here the relative energy of two local band gaps can be manipulated by in-plane biaxial strains. Some other similar intercalation compounds are also shown to be topologically nontrivial. Our work extends the 2D TCI family into a hexagonal lattice composed of transition metals.

Two-dimensional topological crystalline insulator phase in quantum wells of trivial insulators

The realization of two-dimensional (2D) topological insulators (TIs) in HgTe/CdTe quantum wells (QWs) has generated an explosion of research on TIs and novel topologically nontrivial phases. Here we predict, based on first-principles calculations, that the newly discovered 2D topological crystalline insulators (TCIs) phase exists even in the QWs of trivial insulators, e.g. (Sn/Pb)Te and Na(Cl/Br), with mirror Chern number . Tunable nontrivial energy gaps ranging from 4 to 238 meV are obtained, guaranteeing further room-temperature observations and applications. The combined effect of strain and electrostatic interaction that can be engineered by the cladding layers leads to a band inversion, resulting in the phase transition from trivial insulator to 2D TCIs. Our work provides a new strategy for engineering topological states in 2D materials.

A new topological crystalline insulator in two-dimensional PbPo with tunable large bulk gaps

On the basis of first principles calculations, we predict that the PbPo monolayer is a new 2D topological crystalline insulator (TCI) with crystalline-protected Dirac states at the edges.

Two-Dimensional Topological Crystalline Insulator and Topological Phase Transition in TlSe and TlS Monolayers.

The properties that distinguish topological crystalline insulator (TCI) and topological insulator (TI) rely on crystalline symmetry and time-reversal symmetry, respectively, which encodes different bulk and surface/edge properties. Here, we predict theoretically that electron-doped TlM (M = S and Se) (110) monolayers realize a family of two-dimensional (2D) TCIs characterized by mirror Chern number CM = -2. Remarkably, under uniaxial strain (≈ 1%), a topological phase transition between 2D TCI and 2D TI is revealed with the calculated spin Chern number CS = -1 for the 2D TI. Using spin-resolved edge states analysis, we show different edge-state behaviors, especially at the time reversal invariant points. Finally, a TlBiSe2/NaCl quantum well is proposed to realize an undoped 2D TCI with inverted gap as large as 0.37 eV, indicating the high possibility for room-temperature observation.

Topological crystalline insulator and quantum anomalous Hall states in IV-VI-based monolayers and their quantum wells

Different from the two-dimensional (2D) topological insulator, the 2D topological crystalline insulator (TCI) phase disappears when the mirror symmetry is broken, e.g., upon placing on a substrate. Here, based on a new family of 2D TCIs---SnTe and PbTe monolayers---we theoretically predict the realization of the quantum anomalous Hall effect with a Chern number $\mathcal{C}=2$ even when the mirror symmetry is broken. Remarkably, we also demonstrate that the considered materials retain their large-gap topological properties in quantum well structures obtained by sandwiching the monolayers between NaCl layers. Our results demonstrate that the TCIs can serve as a seed for observing robust topologically nontrivial phases.

Crystal field effect induced topological crystalline insulators in monolayer IV-VI semiconductors.

Two-dimensional (2D) topological crystalline insulators (TCIs) were recently predicted in thin films of the SnTe class of IV-VI semiconductors, which can host metallic edge states protected by mirror symmetry. As thickness decreases, quantum confinement effect will increase and surpass the inverted gap below a critical thickness, turning TCIs into normal insulators. Surprisingly, based on first-principles calculations, here we demonstrate that (001) monolayers of rocksalt IV-VI semiconductors XY (X = Ge, Sn, Pb and Y = S, Se, Te) are 2D TCIs with the fundamental band gap as large as 260 meV in monolayer PbTe. This unexpected nontrivial topological phase stems from the strong crystal field effect in the monolayer, which lifts the degeneracy between p(x,y) and p(z) orbitals and leads to band inversion between cation pz and anion px,y orbitals. This crystal field effect induced topological phase offers a new strategy to find and design other atomically thin 2D topological materials.

Highly tunable Berry phase and ambipolar field effect in topological crystalline insulator Pb(1-x)Sn(x)Se.

Recently, rock-salt IV-VI semiconductors, such as Pb(1-x)Sn(x)Se(Te) and SnTe, have been observed to host topological crystalline insulator (TCI) states. The nontrivial states have long been believed to exhibit ambipolar field effects and possess massive Dirac Fermions in two-dimension (2D) limit due to the surface hybridization. However, these exciting attributes of TCI remain previously inaccessible owing to the complicated control over composition and thickness. Here, we systematically investigate doping and thickness-induced topological phase transitions by electrical transport. We demonstrate the first evidence of the ambipolar properties in Pb(1-x)Sn(x)Se thin films. Surface gap opening is observed in 10 nm TCI originated from the strong finite-size effect. Importantly, magnetoconductance hosts a competition between weak antilocalization and weak localization, suggesting a strikingly tunable Berry phase evolution and strong electron-electron interaction. Our findings serve as a new probe to study electron behavior and pave the way for further exploring and manipulating this novel 2D TCI phase.