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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.

We theoretically study the three-dimensional topological semimetals with nodal surfaces protected by crystalline symmetries. Different from the well-known nodal-point and nodal-line semimetals, in these materials, the conduction and valence bands cross on closed nodal surfaces in the Brillouin zone. We propose different classes of nodal surfaces, both in the absence and in the presence of spinorbit coupling (SOC). In the absence of SOC, a class of nodal surfaces can be protected by spacetime inversion symmetry and sublattice symmetry and characterized by a Z2 index, while another class of nodal surfaces are guaranteed by a combination of nonsymmorphic two-fold screw-rotational symmetry and time-reversal symmetry. We show that the inclusion of SOC will destroy the former class of nodal surfaces but may preserve the latter provided that the inversion symmetry is broken. We further generalize the result to magnetically ordered systems and show that protected nodal surfaces can also exist in magnetic materials without and with SOC, given that certain magnetic group symmetry requirements are satisfied. Several concrete nodal-surface material examples are predicted via the first-principles calculations. The possibility of multi-nodal-surface materials is discussed.arXiv:1712.09773v2 [cond-mat.mes-hall]

We show several distinct signatures in the magneto-response of type-II Weyl semimetals. The energy tilt tends to squeeze the Landau levels (LLs), and for a type-II Weyl node, there always exists a critical angle between the B-field and the tilt, at which the LL spectrum collapses, regardless of the field strength. Before the collapse, signatures also appear in the magneto-optical spectrum, including the invariable presence of intraband peaks, the absence of absorption tails, and the special anisotropic field dependence.PACS numbers: 73.61.Ph, 71.70.Di The exploration of solids with nontrivial band topologies has become a focus in current research [1,2]. Besides novel physical effects and application perspectives, the interest also comes from the possibility of simulating intriguing elementary particles phenomena in condensed matter systems. Notably, the Weyl fermion, which was originally proposed as a massless solution of the Dirac equation but remained elusive in high-energy experiments, could find its realization as low-energy quasiparticles [3][4][5][6][7][8][9][10][11][12][13][14][15] in the so-called Weyl semimetals (WSMs). In a WSM, the conduction and valence bands touch with linear dispersion at isolated Fermi points known as Weyl nodes. Each Weyl node is like a monopole in reciprocal space, carrying a topological charge of ±1 corresponding to its chirality. Weyl nodes of opposite chiralities appear or annihilate in pairs [16], and at the system boundary their projections are connected by surface Fermi arcs [3]. The recent progress in identifying several WSM materials [17][18][19][20][21][22][23][24][25] have driven a flurry of exciting researches trying to probe the various fascinating phenomena connected to Weyl fermions [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45].The energy dispersion at a Weyl node could generally be tilted along a certain direction in k-space. When the tilt is large enough, the Weyl cone could even be tipped over such that the Fermi surface transforms from a point to a line or a surface. Such Weyl nodes are referred to as type-II to be distinguished from the conventional ones, and have recently been proposed in a few materials [46][47][48][49][50][51][52]. The essential topology (like chirality) of the Weyl node is unchanged by the tilt, however, since the geometry of Fermi surface plays a key role in many material properties, the type-II WSMs are expected to exhibit signatures distinct from the conventional WSMs and also other materials, e.g., as manifested in the predicted anisotropic chiral anomaly and anomalous Hall effects [46,53].Under an external magnetic field, electrons' motion is typically quantized into discrete Landau levels (LLs). In a three-dimensional (3D) solid, these LLs become dispersive in the direction along the field, such is the case also for conventional WSMs. Here we show that the additional energy tilt tends to squeeze the Landau level spacing, and remarkably, for a type-II node, the squeezing can be so dramatic that t...