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

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. Based on 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

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.Topological metals and semimetals have become a focus of current physics research [1,2]. These materials feature nontrivial band-crossings in their low-energy band structures, around which the quasiparticles behave drastically different from the usual Schrödinger-type fermions. Depending on its dimensionality, the crossing manifold may take zero-dimensional (nodal point), onedimensional (nodal loop), or two-dimensional (nodal surface) form [3]. There has already been extensive studies on nodal points, especially on so-called Weyl and Dirac semimetal materials [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. Recently, nodal loops begin to attract considerable interest: several nodal-loop materials have been proposed, with interesting physical consequences revealed [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33].Consider the generic case of a nodal loop formed by the linear crossing between two bands in a three-dimensional system. Close to any point P on the loop, the dispersion is linear along the two transverse directions of the loop, and is at least quadratic along the tangential direction. The low-energy effective model near P can be expressed as (set = 1)up to first order in the wave-vector q measured from P . Here q i 's (i = 1, 2) are the components of q along two orthogonal transverse directions [see Fig. 1(a)], v i 's are

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.

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 Ta3SiTe6 and Nb3SiTe6, enabled by nonsymmorphic symmetries. We show that in the absence of spin-orbit coupling (SOC), these threedimensional (3D) bulk materials possess accidental Dirac loops and essential fourfold nodal lines.In the presence of SOC, there emerges an hourglass Dirac loop-a fourfold degenerate nodal loop, on which each point is a neck-point of an hourglass-type dispersion. We show that this interesting type of band-crossing is protected and dictated by the nonsymmorphic space group symmetries, and it gives rise to drumhead-like surface states. Furthermore, we also investigate these materials in the monolayer form. We show that these two-dimensional (2D) monolayers host nodal lines in the absence of SOC, and the nodal lines transform to essential spin-orbit Dirac points when SOC is included. Our work suggests a realistic material platform for exploring the fascinating physics associated with nonsymmorphic band-crossings in both 3D and 2D systems. arXiv:1710.08376v2 [cond-mat.mtrl-sci]