Shan-Shan Wang

Strain-Induced Isostructural and Magnetic Phase Transitions in Monolayer MoN2

The change of bonding status, typically occurring only in chemical processes, could dramatically alter the material properties. Here, we show that a tunable breaking and forming of a diatomic bond can be achieved through physical means, i.e., by a moderate biaxial strain, in the newly discovered MoN2 two-dimensional (2D) material. On the basis of first-principles calculations, we predict that as the lattice parameter is increased under strain, there exists an isostructural phase transition at which the N-N distance has a sudden drop, corresponding to the transition from a N-N nonbonding state to a N-N single bond state. Remarkably, the bonding change also induces a magnetic phase transition, during which the magnetic moments transfer from the N(2p) sublattice to the Mo(4d) sublattice; meanwhile, the type of magnetic coupling is changed from ferromagnetic to antiferromagnetic. We provide a physical picture for understanding these striking effects. The discovery is not only of great scientific interest in exploring unusual phase transitions in low-dimensional systems, but it also reveals the great potential of the 2D MoN2 material in the nanoscale mechanical, electronic, and spintronic applications.

Theoretical prediction of MoN2 monolayer as a high capacity electrode material for metal ion batteries

Benefiting from the advantages of environmental friendliness, easy purification, and high thermal stability, the recently synthesized two-dimensional (2D) material MoN2 shows great potential for clean and renewable energy applications. Here, through first-principles calculations, we show that monolayered MoN2 is promising as a high capacity electrode material for metal ion batteries. Firstly, identified by phonon dispersion and exfoliation energy calculations, MoN2 monolayer is proved to be a structurally stable material that can be exfoliated from its bulk counterpart in experiments. Secondly, all the studied metal atoms (Li, Na and K) can be adsorbed on MoN2 monolayer; both the pristine and doped MoN2 are metallic. Thirdly, the metal atoms possess moderate/low migration barriers on MoN2, which ensures excellent cycling performance as battery electrodes. In addition, the calculated average voltages suggest that MoN2 monolayer is a suitable cathode for Li-ion batteries and a suitable anode for Na-ion and K-ion batteries. Most importantly, as a cathode for Li-ion batteries, MoN2 possesses a comparable average voltage but a capacity 1 to 2 times larger (432 mA h g−1) than that of standard commercial cathode materials; as an anode for Na-ion batteries, the theoretical capacity (864 mA h g−1) of MoN2 is 2 to 5 times larger than that of typical 2D anode materials, such as MoS2 and most MXenes. Finally, we also provide an estimation of the capacities of other transition-metal dinitride materials. Our work suggests that the transition-metal dinitride MoN2 is an appealing 2D electrode material with high storage capacity.

Type-II nodal loops: Theory and material realization

Nodal loop appears when two bands, typically one electron-like and one hole-like, are crossing each other linearly along a one-dimensional manifold in the reciprocal space. Here we propose a new type of nodal loop which emerges from crossing between two bands which are both electron-like (or hole-like) along certain direction. Close to any point on such loop (dubbed as a type-II nodal loop), the linear spectrum is strongly tilted and tipped over along one transverse direction, leading to marked differences in magnetic, optical, and transport responses compared with the conventional (type-I) nodal loops. We show that the compound K4P3 is an example that hosts a pair of type-II nodal loops close to the Fermi level. Each loop traverses the whole Brillouin zone, hence can only be annihilated in pair when symmetry is preserved. The symmetry and topological protections of the loops as well as the associated surface states are discussed.

Hourglass Dirac chain metal in rhenium dioxide

Nonsymmorphic symmetries, which involve fractional lattice translations, can generate exotic types of fermionic excitations in crystalline materials. Here we propose a topological phase arising from nonsymmorphic symmetries—the hourglass Dirac chain metal, and predict its realization in the rhenium dioxide. We show that ReO2 features hourglass-type dispersion in the bulk electronic structure dictated by its nonsymmorphic space group. Due to time reversal and inversion symmetries, each band has an additional two-fold degeneracy, making the neck crossing-point of the hourglass four-fold degenerate. Remarkably, close to the Fermi level, the neck crossing-point traces out a Dirac chain—a chain of connected four-fold-degenerate Dirac loops—in the momentum space. The symmetry protection, the transformation under symmetry-breaking, and the associated topological surface states of the Dirac chain are revealed. Our results open the door to an unknown class of topological matters, and provide a platform to explore their intriguing physics.Exotic topological particles are reported to arise from special types of symmetry protection. Here, Wang et al. predict an hourglass Dirac chain metal protected by nonsymmorphic symmetries and its possible realization in ReO2.

Blue Phosphorene Oxide: Strain-Tunable Quantum Phase Transitions and Novel 2D Emergent Fermions

Tunable quantum phase transitions and novel emergent fermions in solid-state materials are fascinating subjects of research. Here, we propose a new stable two-dimensional (2D) material, the blue phosphorene oxide (BPO), which exhibits both. On the basis of first-principles calculations, we show that its equilibrium state is a narrow-bandgap semiconductor with three bands at low energy. Remarkably, a moderate strain can drive a semiconductor-to-semimetal quantum phase transition in BPO. At the critical transition point, the three bands cross at a single point at Fermi level, around which the quasiparticles are a novel type of 2D pseudospin-1 fermions. Going beyond the transition, the system becomes a symmetry-protected semimetal, for which the conduction and valence bands touch quadratically at a single Fermi point that is protected by symmetry, and the low-energy quasiparticles become another novel type of 2D double Weyl fermions. We construct effective models characterizing the phase transition and these novel emergent fermions, and we point out several exotic effects, including super Klein tunneling, supercollimation, and universal optical absorbance. Our result reveals BPO as an intriguing platform for the exploration of fundamental properties of quantum phase transitions and novel emergent fermions and also suggests its great potential in nanoscale device applications.

Ternary wurtzite CaAgBi materials family: A playground for essential and accidental, type-I and type-II Dirac fermions

Cong Chen, 2 Shan-Shan Wang, Lei Liu, Zhi-Ming Yu, ∗ Xian-Lei Sheng, 2, † Ziyu Chen, and Shengyuan A. Yang Department of Physics, Key Laboratory of Micro-nano Measurement-Manipulation and Physics (Ministry of Education), Beihang University, Beijing 100191, China Research Laboratory for Quantum Materials, Singapore University of Technology and Design, Singapore 487372, Singapore State Key Laboratory of Integrated Service Networks, Xidian University, Xi’an, China

Nonsymmorphic-symmetry-protected hourglass Dirac loop, nodal line, and Dirac point in bulk and monolayer X3SiTe6 ( X = Ta, Nb)

Nonsymmorphic space group symmetries can generate exotic band crossings in topological metals and semimetals. Here, based on symmetry analysis and first-principles calculations, we reveal rich band-crossing features in the existing layered compounds

Two-dimensional spin-orbit Dirac point in monolayer HfGeTe

{\mathrm{Ta}}_{3}{\mathrm{SiTe}}_{6}

Nodal Loop and Nodal Surface States in Ti3Al Family Materials

and

Monolayer Mg

{\mathrm{Nb}}_{3}{\mathrm{SiTe}}_{6}