By using the analytical coupled cluster method, the numerical exact diagonalization method, and the numerical density matrix renormalization group method, we investigated the properties of the one-dimensional sawtooth chain. The results of the coupled cluster method based on Neel state demonstrate that the ground state is in the quasi-Neel-long-range order state when aac2. The results of the coupled cluster method and the density matrix renormalization group method both disclose that the type of the quantum phase transition occurring at ac2 belongs to the first-order transition.Comment: accepted versio

Two-dimensional transition metal dichalcogenides have been regarded as cheap and abundant catalysts for driving electrolysis of water. Using density functional theory methods, we systematically investigate the hydrogen evolution reduction of metal dichalcogenides/ graphene heterostructures (MX 2 /Gs, M = Mo, W; X = S, Se) with various defects, MX 2 /G_V X , MX 2 /G_V M , and MX 2 /G_V (M+X) . We find that such defected MX 2 /Gs show better hydrogen evolution reactive activities than pure MX 2 /Gs as well as freestanding MX 2 monolayers, due to the metallic states induced by the defects. Particularly, MX 2 / G_V X s with a S(Se) vacancy display catalytic performance comparable to that of Pt. Moreover, the catalytic performance for the hydrogen evolution reaction of most defected MX 2 /G_V M s and MX 2 /G_V (M+X) s varies with H coverage and the M vacancy concentration. Our results provide a feasible way to apply MX 2 /graphene heterostructures to water electrolysis for hydrogen production.

Graphene on a substrate will suffer an inversion-symmetry-breaking (ISB) lattice potential. Taking electron-electron interaction into account, we study in this paper the possibility of half-metallicity and noncollinear (NC) magnetic phase for graphene zigzag nanoribbons without inversion symmetry. At half-filling it is found that half-metallic(HM) state can be achieved at an intermediate value of the ISB potential due to its competition with the electron-electron interaction. Away from half-filling, the phase diagrams of doping versus ISB potential for different ribbon width are given, where the regimes for the HM states and NC magnetic state are clearly indicated and discussed. For ribbons with perfect edges, we predict a topological transition between two HM states with different magnetic structures, which is accompanied by an abrupt transition of electrical conductance along the ribbon from $2e^2/h$ to $e^2/h$.Comment: 7 pages, 7 figure

The energetics and electronic and magnetic properties of G/MS hybrid structures embedded with 3d transition metal atoms, TM@(G/MS) (G = graphene; M = W, Mo; TM = Sc-Ni), have been systematically studied using first-principles calculations. TM atoms were found to be covalently bound to two-sided graphene and MS layers with sizable binding energies of 4.35-7.13 eV. Interestingly, a variety of electronic and magnetic properties were identified for these TM@(G/MS) systems. Except for TM = Ni, all other systems were ferromagnetic, due to exchange splitting of the TM 3d orbitals. In particular, four TM@(G/MoS) systems (TM = V, Mn, Fe, Co) and three TM@(G/WS) systems (TM = Mn, Fe, Co) were half-metals or quasi half-metals, while Ni@(G/MoS) and Ni@(G/WS) were semiconductors with bandgaps of 33 and 37 meV, respectively. Further quasi-particle scattering theory analysis demonstrated that the origin of semiconducting or half-metallic properties could be well understood from the variation in on-site energy by the transition metal dichalcogenide substrate or the different on-site scattering potential induced by TM atoms. Our findings propose an effective route for manipulating the electronic and magnetic properties of graphene@MS heterostructures, allowing their potential application in modern spintronic and electronic devices.

The structural, electronic, and magnetic properties of transition metal atoms intercalated bilayer graphene, [GTMG] x/y , (x, y is integer, TM = Ti, Cr, Mn, Fe) with different TM/carbon hexagons ratios and insertion patterns, are systematically studied by density functional theory calculations. All the studied systems are thermodynamically stable and competitive ionic−covalent bonding characters are dominated in the TM−graphene interaction. Most studied systems are ferromagnetic; particularly, [GCrG] 1:1 8 , [GCrG] 1:9 , [GFeG] 1:6(1) , and [GTMG] 1:6(2) (TM = Cr, Mn, Fe) exhibit large magnetic moment of 4. 43, 5.60, 7.02, 10.85, 9.04, and 5.19 μ B per unit cell, respectively. In contrast, [GCrG] 1:8 and [GFeG] 1:8 are ferrimagnetic, while eight other [GTMG] x/y are nonmagnetic. Moreover, five intercalation nanostructures of [GTMG] 1:18 (TM = Ti, Mn), [GTMG] 1:9 (TM = Ti, Mn) and [GTiG] 1:6 are semiconductors with the gaps of 0.141/0.824 eV, 0.413/0.668 eV, and 0.087 eV, respectively. Comparison on different isomers with same TM/carbon hexagons ratios showed that the electronic and magnetic properties of these [GTMG] x/y are largely dependent on the TM atoms arrangement. For thus, an effective way to control the electronic and magnetic properties of graphene based nanostructures is proposed.