We investigate the possibility of band structure engineering in the recently predicted 2D layered form of blue phosphorus via an electric field (Ez) applied perpendicular to the layer(s). Using density functional theory, we study the effect of a transverse electric field in monolayer, as well as three differently stacked bilayer structures of blue phosphorus. We find that, for Ez > 0.2 V/Å the direct energy gap at the Γ point, which is much larger than the default indirect band gap of mono-and bilayer blue phosphorus, decreases linearly with the increasing electric field; becomes comparable to the default indirect band gap at Ez ≈ 0.45 (0.35) V/Å for monolayer (bilayers) and decreases further until the semiconductor to metal transition of 2D blue phosphorus takes place at Ez ≈ 0.7 (0.5) V/Å for monolayer (bilayers). Calculated values of the electron and hole effective masses along various high symmetry directions in the reciprocal lattice suggests that the mobility of charge carriers is also influenced by the applied electric field.

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Novel magnetic topological materials pave the way for studying the interplay between band topology and magnetism. However, an intrinsically ferromagnetic topological material with only topological bands at the charge neutrality energy has so far remained elusive. Using rational design, we synthesized MnBi8Te13, a natural heterostructure with [MnBi2Te4] and [Bi2Te3] layers. Thermodynamic, transport, and neutron diffraction measurements show that despite the adjacent [MnBi2Te4] being 44.1 Å apart, MnBi8Te13 manifests long-range ferromagnetism below 10.5 K with strong coupling between magnetism and charge carriers. First-principles calculations and angle-resolved photoemission spectroscopy measurements reveal it is an axion insulator with sizable surface hybridization gaps. Our calculations further demonstrate the hybridization gap persists in the two-dimensional limit with a nontrivial Chern number. Therefore, as an intrinsic ferromagnetic axion insulator with clean low-energy band structures, MnBi8Te13 serves as an ideal system to investigate rich emergent phenomena, including the quantized anomalous Hall effect and quantized magnetoelectric effect.

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We predict SnP 3 to be an easily exfoliable and dynamically stable twodimensional (2D) material with thickness-dependent electronic properties. On the basis of density functional theory calculations, we show that mono-and bilayer SnP 3 has relatively low cleavage energies of 0.71 and 0.45 J m −2 , lower than several other 2D materials and comparable to that of graphene (0.32 J m −2 ). Mono-and bilayer SnP 3 have an indirect band gap of 0.83 and 0.55 eV, respectively, and the magnitude of the gap can be tuned by applying strain. Remarkably, pristine monolayer SnP 3 has a relatively high carrier mobility in the range of 3000−7000 cm 2 V −1 s −1 , at par with well-known 2D semiconductors such as MoS 2 , phosphorene, and other phosphorus-based layered materials such as GeP 3 and InP 3 . Mono-and bilayer SnP 3 also show large optical absorption, resulting from the existence of the van-Hove singularities in the electronic density of states. The combined properties of layered SnP 3 , in particular, its high carrier mobility and tunable band gap, along with large optical absorption coefficient, open up interesting possibilities for nanoelectronic and nanophotonic applications.

Transition-metal dichalcogenides showing type-II Dirac fermions are emerging as innovative materials for nanoelectronics. However, their excitation spectrum is mostly unexplored yet. By means of high-resolution electron energy loss spectroscopy and density functional theory, here, we identify the collective excitations of type-II Dirac fermions (3D Dirac plasmons) in PtTe_{2} single crystals. The observed plasmon energy in the long-wavelength limit is ∼0.5  eV, which makes PtTe_{2} suitable for near-infrared optoelectronic applications. We also demonstrate that interband transitions between the two Dirac bands in PtTe_{2} give rise to additional excitations at ∼1 and ∼1.4  eV. Our results are crucial to bringing to fruition type-II Dirac semimetals in optoelectronics.

In this article, we explore the anisotropic electron energy loss spectrum (EELS) in monolayer phosphorene based on ab-initio time dependent density functional theory calculations. Similar to black phosphorous, the EELS of undoped monolayer phosphorene is characterized by anisotropic excitonic peaks for energies in vicinity of the bandgap, and by interband plasmon peaks for higher energies. On doping, an additional intraband plasmon peak also appears for energies within the bandgap. Similar to other two dimensional systems, the intraband plasmon peak disperses as ω pl ∝ √ q in both the zigzag and armchair directions in the long wavelength limit, and deviates for larger wavevectors. The anisotropy of the long wavelength plasmon intraband dispersion is found to be inversely proportional to the square root of the ratio of the effective masses: ω pl (qŷ)/ω pl (qx) = mx/my.

Using spin-and angle-resolved photoemission spectroscopy (spin-ARPES) together with ab initio calculations, we demonstrate the existence of a type-II Dirac semimetal state in NiTe2. We show that, unlike PtTe2, PtSe2, and PdTe2, the Dirac node in NiTe2 is located in close vicinity of the Fermi energy. Additionally, NiTe2 also hosts a pair of band inversions below the Fermi level along the Γ − A high-symmetry direction, with one of them leading to a Dirac cone in the surface states. The bulk Dirac nodes and the ladder of band inversions in NiTe2 support unique topological surface states with chiral spin texture over a wide range of energies. Our work paves the way for the exploitation of the low-energy type-II Dirac fermions in NiTe2 in the fields of spintronics, THz plasmonics and ultrafast optoelectronics.