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Using density functional theory plus self-consistent Hubbard $U$ (DFT$+U_{sc}$) calculations, we have investigated the structural and electronic properties of the rare-earth cobaltites \textit{R}CoO$_3$ (\textit{R} = Pr -- Lu). Our calculations show the evolution of crystal and electronic structure of the insulating low-spin (LS) \textit{R}CoO$_3$ with increasing rare-earth atomic number (decreasing ionic radius), including the invariance of the Co-O bond distance ($d_{Co-O}$), the decrease of the Co-O-Co bond angle ($\Theta$), and the increase of the crystal field splitting ($\Delta_{CF}$) and band gap energy ($E_g$). Agreement with experiment for the latter improves considerably with the use of DFT$+U_{sc}$ and all trends are in good agreement with experimental data. These trends enable a direct test of prior rationalizations of the trend in spin-gap associated with the spin crossover in this series, which is found to expose significant issues with simple band based arguments. We also examine the effect of placing the rare-earth \textit{f}-electrons in the core region of the pseudopotential. The effect on lattice parameters and band structure is found to be small, but distinct for the special case of \textit{Pr}CoO$_3$ where some \textit{f}-states populate the middle of the gap, consistent with recent reports of unique behavior in Pr-containing cobaltites. Overall, this study establishes a foundation for future predictive studies of thermally induced spin excitations in rare-earth cobaltites and similar systems.

We provide accurate projected augmented wave (PAW) datasets for rare-earth (RE) elements with some suggested Hubbard U values allowing efficient plane-wave calculations. Solid state tests of generated datasets were performed on rare-earth nitrides. Through density of state (DOS) and equation of state (EoS) comparisons, generated datasets were shown to yield excellent results comparable to highly accurate all-electron full-potential linearized augmented plane-wave plus local orbital (FLAPW+LO) calculations. Hubbard U values for trivalent RE ions are determined according to hybrid functional calculations. We believe that these new and open-source PAW datasets will allow further studies on rare-earth materials., Comment: accepted (http://dx.doi.org/10.1016/j.commatsci.2014.07.030)

We determined frictional figures of merit for a pair of layered honeycomb nanostructures, such as graphane, fluorographene, MoS$_2$ and WO$_2$ moving over each other, by carrying out ab-initio calculations of interlayer interaction under constant loading force. Using Prandtl-Tomlinson model we derived critical stiffness required to avoid stick-slip behavior. We showed that these layered structures have low critical stiffness even under high loading forces due to their charged surfaces repelling each other. The intrinsic stiffness of these materials exceed critical stiffness and thereby avoid the stick-slip regime and attain nearly dissipationless continuous sliding. Remarkably, tungsten dioxide displays much better performance relative to others and heralds a potential superlubricant. The absence of mechanical instabilities leading to conservative lateral forces is also confirmed directly by the simulations of sliding layers.

Using first-principles calculations within density functional theory we demonstrate that the adsorption of single oxygen atom results in significant electron transfer from graphene to oxygen. This strongly disturbs the charge landscape of the C-C bonds at the proximity. Additional oxygen atoms adsorbing to graphene prefer always the C-C bonds having highest charge density and consequently they have tendency to form domain structure. While oxygen adsorption to one side of graphene ends with significant buckling, the adsorption to both sides with similar domain pattern is favored. The binding energy displays an oscillatory variation and the band gap widens with increasing oxygen coverage. While a single oxygen atom migrates over the C-C bonds on graphene surface, a repulsive interaction prevents two oxygen adatoms from forming an oxygen molecule. Our first-principles study together with finite temperature ab-initio molecular dynamics calculations concludes that oxygen adatoms on graphene cannot desorb easily without influence of external agents., Comment: under review

We demonstrate that graphene coating can provide an efficient protection from oxidation by posing a high energy barrier to the path of oxygen atom, which could have penetrated from the top of graphene to the reactive surface underneath. Graphene bilayer, which blocks the diffusion of oxygen with a relatively higher energy barrier provides even better protection from oxidation. While an oxygen molecule is weakly bound to bare graphene surface and hence becomes rather inactive, it can easily dissociates into two oxygen atoms adsorbed to low coordinated carbon atoms at the edges of a vacancy. For these oxygen atoms the oxidation barrier is reduced and hence the protection from oxidation provided by graphene coatings is weakened. Our predictions obtained from the state of the art first-principles calculations of electronic structure, phonon density of states and reaction path will unravel how a graphene can be used as a corrosion resistant coating and guide further studies aiming at developing more efficient nanocoatings., Comment: under review in PRB; http://link.aps.org/doi/10.1103/PhysRevB.85.155445

Using first-principle plane wave method we investigate the effects of static charging on structural, elastic, electronic and magnetic properties of suspended, single layer graphene, graphane, fluorographene, BN and MoS2 in honeycomb structures. The limitations of periodic boundary conditions in the treatment of charged layers are clarified. Upon positive charging the band gaps between the conduction and valence bands increase, but the single layer materials become metallic owing to the Fermi level dipping below the maximum of valence band. Moreover, their bond lengths increase and their in-plane stiffness decreases. As a result, phonons are softened and frequencies of Raman active modes are lowered. High level of charging leads to instability. We showed that wide band gap BN and MoS2 slabs are metallized as a result of electron removal and their outermost layers are exfoliated once the charging exceeds a threshold value., Comment: http://link.aps.org/doi/10.1103/PhysRevB.85.045121

Based on first-principles calculations we revealed fundamental properties of infinite and finite size monatomic chains of carbon atoms in equilibrium and under an applied strain. Distributions of bond lengths and magnetic moments at atomic sites exhibit interesting even-odd disparity depending on the number of carbon atoms in the chain and on the type of saturation of carbon atoms at both ends. It was shown that, the $\pi$-bands of carbon atomic chains behave as a one dimensional free electron system. A local perturbation created by a small displacement of the single carbon atom at the center of a long chain induces oscillations of atomic forces and charge density, which are carried to long distances over the chain. These long ranged oscillations are identified as Friedel oscillations showing $1/r$ decay rate in one dimensional systems., Comment: Please find the published version in http://link.aps.org/doi/10.1103/PhysRevB.82.195444

We present a study of mechanical, electronic and magnetic properties of two dimensional (2D), monolayer of silicon carbide (SiC) in honeycomb structure and its quasi 1D armchair nanoribbons using first-principles plane wave method. In order to reveal dimensionality effects, a brief study of 3D bulk and 1D atomic chain of SiC are also included. Calculated bond-lengths, cohesive energies, charge transfers and band gaps display a clear dimensionality effect. The stability analysis based on the calculation of phonon frequencies indicates that 2D SiC monolayer is stable in planar geometry. We found that 2D SiC monolayer in honeycomb structure and its bare and hydrogen passivated nanoribbons are ionic, non-magnetic, wide band gap semiconductors. The band gap is further increased upon self-energy corrections. The mechanical properties are investigated using the strain energy calculations. The effect of various vacancy defects, adatoms and substitutional impurities on electronic and magnetic properties in 2D SiC monolayer and in its armchair nanoribbons are also investigated. Some of these vacancy defects and impurities, which are found to influence physical properties and attain magnetic moments, can be used to functionalize SiC honeycomb structures., Comment: please see published version. http://prb.aps.org/abstract/PRB/v81/i7/e075433

We present a first-principles study of bare and hydrogen passivated armchair nanoribbons of the puckered single layer honeycomb structures of silicon and germanium. Our study includes optimization of atomic structure, stability analysis based on the calculation of phonon dispersions, electronic structure and the variation of band gap with the width of the ribbon. The band gaps of silicon and germanium nanoribbons exhibit family behavior similar to those of graphene nanoribbons. The edges of bare nanoribbons are sharply reconstructed, which can be eliminated by the hydrogen termination of dangling bonds at the edges. Periodic modulation of the nanoribbon width results in a superlattice structure which can act as a multiple quantum wells. Specific electronic states are confined in these wells. Confinement trends are qualitatively explained by including the effects of the interface. In order to investigate wide and long superlattice structures we also performed empirical tight binding calculations with parameters determined from \textit{ab initio} calculations., Comment: please find the published version in http://link.aps.org/doi/10.1103/PhysRevB.81.195120

Based on first-principles calculations, we resent a method to reveal the elastic properties of recently synthesized monolayer hydrocarbon, graphane. The in-plane stiffness and Poisson's ratio values are found to be smaller than those of graphene, and its yielding strain decreases in the presence of various vacancy defects and also at high ambient temperature. We also found that the band gap can be strongly modified by applied strain in the elastic range., Comment: accepted version at: http://link.aip.org/link/?APL/96/091912

Using first-principles plane wave calculations, we investigate two dimensional honeycomb structure of Group IV elements and their binary compounds, as well as the compounds of Group III-V elements. Based on structure optimization and phonon mode calculations, we determine that 22 different honeycomb materials are stable and correspond to local minima on the Born-Oppenheimer surface. We also find that all the binary compounds containing one of the first row elements, B, C or N have planar stable structures. On the other hand, in the honeycomb structures of Si, Ge and other binary compounds the alternating atoms of hexagons are buckled, since the stability is maintained by puckering. For those honeycomb materials which were found stable, we calculated optimized structures, cohesive energies, phonon modes, electronic band structures, effective cation and anion charges, and some elastic constants. The band gaps calculated within Density Functional Theory using Local Density Approximation are corrected by GW0 method. Si and Ge in honeycomb structure are semimetal and have linear band crossing at the Fermi level which attributes massless Fermion character to charge carriers as in graphene. However, all binary compounds are found to be semiconductor with band gaps depending on the constituent atoms. We present a method to reveal elastic constants of 2D honeycomb structures from the strain energy and calculate the Poisson's ratio as well as in-plane stiffness values. Preliminary results show that the nearly lattice matched heterostructures of ..., Comment: 12 Pages, 7 Figures, 1 Table; http://link.aps.org/doi/10.1103/PhysRevB.80.155453

We present a first-principles study of the atomic, electronic, and magnetic properties of two-dimensional (2D), single and bilayer ZnO in honeycomb structure and its armchair and zigzag nanoribbons. In order to reveal the dimensionality effects, our study includes also bulk ZnO in wurtzite, zincblende, and hexagonal structures. The stability of 2D ZnO, its nanoribbons and flakes are analyzed by phonon frequency, as well as by finite temperature ab initio molecular-dynamics calculations. 2D ZnO in honeycomb structure and its armchair nanoribbons are nonmagnetic semiconductors but acquire net magnetic moment upon the creation of zinc-vacancy defect. Zigzag ZnO nanoribbons are ferromagnetic metals with spins localized at the oxygen atoms at the edges and have high spin polarization at the Fermi level. However, they change to nonmagnetic metal upon termination of their edges with hydrogen atoms. From the phonon calculations, the fourth acoustical mode specified as twisting mode is also revealed for armchair nanoribbon. Under tensile stress the nanoribbons are deformed elastically maintaining honeycomblike structure but yield at high strains. Beyond yielding point honeycomblike structure undergo a structural change and deform plastically by forming large polygons. The variation in the electronic and magnetic properties of these nanoribbons have been examined under strain. It appears that plastically deformed nanoribbons may offer a new class of materials with diverse properties., Comment: http://prb.aps.org/abstract/PRB/v80/i23/e235119

This paper presents a systematic study of two and one dimensional honeycomb structure of boron nitride (BN) using first-principles plane wave method. Two-dimensional (2D) graphene like BN is a wide band gap semiconductor with ionic bonding. Phonon dispersion curves demonstrate the stability of 2D BN flakes. Quasi 1D armchair BN nanoribbon are nonmagnetic semiconductors with edge states. Upon passivation of B and N with hydrogen atoms these edge states disappear and band gap increases. Bare zigzag BN nanoribbons are metallic, but become a ferromagnetic semiconductor when their both edges are passivated with hydrogen. However, their magnetic ground state, electronic band structure and band gap are found to be strongly dependent on whether B- or N-edge of the ribbon is saturated with hydrogen. Vacancy defects in armchair and zigzag nanoribbons affects also magnetic state and electronic structure. In order to reveal dimensionality effects these properties are contrasted with those of various 3D BN crystals and 1D BN atomic chain., Comment: 11 pages, 10 figures, 1 table

Using first-principles plane wave calculations we predict that electronic and magnetic properties of graphene nanoribbons can be affected by defect-induced itinerant states. The band gaps of armchair nanoribbons can be modified by hydrogen saturated holes. Defects due to periodically repeating vacancy or divacancies induce metallization, as well as magnetization in non-magnetic semiconducting nanoribbons due to the spin-polarization of local defect states. Antiferromagnetic ground state of semiconducting zigzag ribbons can change to ferrimagnetic state upon creation of vacancy defects, which reconstruct and interact with edge states. Even more remarkable is that all these effects of vacancy defects are found to depend on their geometry and position relative to edges. It is shown that these effects can, in fact, be realized without really creating defects., Comment: 5 pages, 5 figures

Based on first-principles calculations we predict that periodically repeated junctions of armchair graphene nanoribbons of different widths form superlattice structures. In these superlattice heterostructures the width and the energy gap are modulated in real space and specific states are confined in certain segments. Orientation of constituent nanoribbons, their width and length, the symmetry of the junction are the structural parameters to engineer electronic properties of these quantum structures. Not only the size modulation, but also composition modulation, such as periodically repeated, commensurate heterojunctions of BN and graphene honeycomb nanoribbons result in a multiple quantum well structure. We showed that these graphene based quantum structures can introduce novel concepts to design nanodevices., Comment: amended version