Two -dimensional semimetal AlSb monolayer with multiple nodal-loops and extraordinary transport properties under uniaxial strain.

Two-dimensional (2D) nodal-loop semimetal (NLSM) materials have attracted much attention for their high-speed and low-consumption transporting properties as well as their fantastic symmetry protection mechanisms. In this paper, using systematic first-principles calculations, we present an excellent NLSM candidate, a 2D AlSb monolayer, in which the conduction and valence bands cross with each other forming fascinating multiple nodal-loop (NL) states. The NLSM properties of the AlSb monolayer are protected by its glide mirror symmetry, which was confirmed using a symmetry-constrained six-band tight-binding model. The transport properties of the AlSb monolayer under in-plane uniaxial strains are also studied, based on a non-equilibrium Green's function method. It is found that both compressive and tensile strains from -10% to 10% improve the transporting properties of AlSb, and it is interesting to see that flexure configurations are energetically favored when compressive uniaxial strains are applied. Our studies not only provide a novel 2D NLSM candidate with a new symmetry protection mechanism, but also raise the novel possibility for the detection of out-of-plane flexure in 2D semimetal materials.

Quantum anomalous Hall effect in germanene by proximity coupling to a semiconducting ferromagnetic substrate NiI2.

Interfaces between materials are ubiquitous in materials science, especially in devices. As device dimensions continue to be reduced, understanding the physical characteristics that appear at interfaces is crucial to exploit them for applications, spintronics in this case. Here, based on first-principles calculations, we propose a general and tunable platform to realize an exotic quantum anomalous Hall effect (QAHE) with the germanene monolayer by proximity coupling to a semiconducting ferromagnetic NiI2 (Ge/NiI2). Through analysis of the Berry curvature and band structure with spin-orbit coupling, the QAHE phase with an integer Chern number (C = -1), which is induced by band inversion between Ge-p orbitals, can achieve complete spin polarization for low-dissipation electronic devices. Also, the proximity coupling between germanene and the NiI2 substrate makes the non-trivial bandgap reach up to 85 meV, and the Curie temperature of the Ge/NiI2 heterostructure (HTS) is enhanced to 238 K, which is much higher than that of pristine NiI2. An effective k·p model is proposed to clarify the quantum phenomena in the Ge/NiI2 HTS. These findings shed light on the possible role of magnetic proximity effects on condensed matter physics in germanene and open new perspectives for multifunctional spin quantum devices in spintronics.

Anisotropic nodal loop in NiB2 monolayer with nonsymmorphic configuration.

Two-dimensional (2D) materials featuring a nodal-loop (NL) state have been drawing considerable attention in condensed matter physics and materials science. Owing to their structural polymorphism, recent high-profile metal-boride films have great advantages and the potential to realize a NL. Herein, a 2D NiB2 monolayer with an anisotropic NL nature is proposed and investigated using first-principles calculations. We show that the NiB2 monolayer has excellent thermal dynamics stability, suggesting the possibility of its synthesis in experiments. Remarkably, the NL with a considerable Fermi velocity is demonstrated to be protected by nonsymmorphic glide mirror symmetry, instead of the widely known horizontal mirror symmetry. Accompanied by the proper preservation of the NL, strain engineering can not only regulate the anisotropy of the NL but also give rise to a self-doping phenomenon characterized by effective modulation of the carrier type and concentration. Moreover, this NL state is robust against the correlation effect. These findings pave the way for exploring nonsymmorphic-symmetry-enabled NL nature in 2D metal-borides.

Emergence of 2D high-temperature nodal-line half-metal in monolayer AgN.

Nodal-line half-metals (NLHMs) are highly desirable for future spintronic devices due to their exotic quantum properties. However, the experimental realization in spin-polarized materials is nontrivial to date. Herein we perform first-principles calculations to demonstrate a 2D honeycomb, AgN, as a promising candidate of NLHMs, which is thermodynamically and dynamically stable. Band structure analysis reveals that two concentric NLs coexist centered at a Γ point near EF, accompanied by the electron and hole pockets that touch each other linearly with single-spin components. Inclusion of SOC can enrich the electronic structures of AgN, sensitive to the protection of mirror reflection symmetry: the NLHM survives if the spin is perpendicular to the Mz mirror plane, while it tunes into Wyle nodal-points by rotating spins from the out-of-plane to the in-plane direction. The characteristics of HM and NL can be well maintained on semiconducting h-BN and is immune to mechanical strains. These tunable magnetic properties render 2D AgN suitable for exotic quantum transports in nodal fermions as well as related spintronic devices.

Correction: Prediction of topological property in TlPBr2 monolayer with appreciable Rashba effect.

Correction for 'Prediction of topological property in TlPBr2 monolayer with appreciable Rashba effect' by Min Yuan et al., Phys. Chem. Chem. Phys., 2018, 20, 4308-4316.

Prediction of topological property in TlPBr2 monolayer with appreciable Rashba effect.

A quantum spin Hall (QSH) insulator with high stability, large bulk band gap and tunable topological properties is crucial for both fundamental research and practical application due to the presence of dissipationless edge conducting channels. Recently, chemical functionalization has been proposed as an effective route to realize the QSH effect. Based on first-principles calculations, we predict that a two-dimensional TlP monolayer would convert into a topological insulator with the effect of bromination, accompanied by a large bulk band gap of 76.5 meV, which meets the requirement for room-temperature application. The topological nature is verified by the calculation of Z2 topological invariant and helical edge states. Meanwhile, an appreciable Rashba spin splitting of 77.2 meV can be observed. The bulk band gap can be effectively tuned with external strain and electric field, while the Rashba spin splitting shows a parabolic variation trend under an external electric field. We find that the topological property is available for the TlP film when the coverage rate is more than 0.75. BN and SiC are demonstrated as promising substrates to support the topological nature of TlPBr2 film. Our findings suggest that a TlPBr2 monolayer is an appropriate candidate for hosting the nontrivial topological state and controllable Rashba spin splitting, and shows great potential applications in spintronics.

Controllable electronic and magnetic properties in a two-dimensional germanene heterostructure.

The control of spin without a magnetic field is one of the challenges in developing spintronic devices. Here, based on first-principles calculations, we predict a new kind of ferromagnetic half-metal (HM) with a Curie temperature of 244 K in a two-dimensional (2D) germanene van der Waals heterostructure (HTS). Its electronic band structures and magnetic properties can be tuned with respect to external strain and electric field. More interestingly, a transition from HM to bipolar-magnetic-semiconductor (BMS) to spin-gapless-semiconductor (SGS) in a HTS can be realized by adjusting the interlayer spacing. These findings provide a promising platform for 2D germanene materials, which hold great potential for application in nanoelectronic and spintronic devices.

Controllable band structure and topological phase transition in two-dimensional hydrogenated arsenene.

Discovery of two-dimensional (2D) topological insulator such as group-V films initiates challenges in exploring exotic quantum states in low dimensions. Here, we perform first-principles calculations to study the geometric and electronic properties in 2D arsenene monolayer with hydrogenation (HAsH). We predict a new σ-type Dirac cone related to the px,y orbitals of As atoms in HAsH, dependent on in-plane tensile strain. Noticeably, the spin-orbit coupling (SOC) opens a quantum spin Hall (QSH) gap of 193 meV at the Dirac cone. A single pair of topologically protected helical edge states is established for the edges, and its QSH phase is confirmed with topological invariant Z2 = 1. We also propose a 2D quantum well (QW) encapsulating HAsH with the h-BN sheet on each side, which harbors a nontrivial QSH state with the Dirac cone lying within the band gap of cladding BN substrate. These findings provide a promising innovative platform for QSH device design and fabrication operating at room temperature.

Stanene cyanide: a novel candidate of Quantum Spin Hall insulator at high temperature.

The search for quantum spin Hall (QSH) insulators with high stability, large and tunable gap and topological robustness, is critical for their realistic application at high temperature. Using first-principle calculations, we predict the cyanogen saturated stanene SnCN as novel topological insulators material, with a bulk gap as large as 203 meV, which can be engineered by applying biaxial strain and electric field. The band topology is identified by Z2 topological invariant together with helical edge states, and the mechanism is s-pxy band inversion at G point induced by spin-orbit coupling (SOC). Remarkably, these systems have robust topology against chemical impurities, based on the calculations on halogen and cyano group co-decorated stanene SnXxX'1-x (X,X' = F, Cl, Br, I and CN), which makes it an appropriate and flexible candidate material for spintronic devices.

Tunable electronic properties in the van der Waals heterostructure of germanene/germanane.

It is challenging to epitaxially grow germanene on conventional semiconductor substrates. Based on first-principles calculations, we investigate the structural and electronic properties of germanene/germanane heterostructures (HTSs). The results indicate that the Dirac cone with nearly linear band dispersion of germanene is maintained in the band gap of the substrate. Remarkably, the band gaps opened in these HTSs can be effectively modulated by the external electric field and strain, and they also feature very low effective masses and high carrier mobilities. These results provide a route to design high-performance FETs operating at room temperature in nanodevices.