Irradiation-induced magnetism in carbon nanostructures

Nitrogen (15N) and carbon (12C) ion implantations with implant energy of 100 keV for different doses were performed on nanosized diamond (ND) particles. Magnetic measurements on the doped ND show ferromagnetic hysteresis behavior at room temperature. The saturation magnetization (M(s)) in the case of 15N implanted samples was found to be higher compared to the 12C implanted samples for dose sizes greater than 10(14) cm(-2). The role of structural modification or defects along with the carbon-nitrogen (C-N) bonding states for the observed enhanced ferromagnetic ordering in 15N doped samples is explained on the basis of x-ray photoelectron spectroscopy measurements.     

Effects of the substrate on graphone magnetism: A density functional theory study

The magnetism of graphone, a single-side-hydrogenated graphene derivative, has been related to the localized and unpaired p-electrons associated with the unhydrogenated carbon atoms. In the present density functional theory study, the effects the adhesion to either Cu(111) or α-quartz (0001) surface on the magnetic properties of graphone have been investigated. The total magnetization of the graphone adsorbed to copper and quartz surface is reduced by four and two times, respectively, with respect to the isolated graphone. We have shown there is electronic charge transfer from surface towards three-fold coordinated C atoms of graphone, but the main role in the partial magnetism quenching is played by bond formation and the consequent electron pairing of p-electrons. The critical temperature has been investigated on the basis of the mean field theory to evaluate the stability of the magnetism at ordinary temperature.     

Ferromagnetism in GaN:Gd: a density functional theory study

First-principle calculations of the electronic structure and magnetic interaction of GaN:Gd have been performed within the generalized gradient approximation (GGA) of the density functional theory with the on-site Coulomb energy U taken into account (also referred to as GGA+U). The ferromagnetic p-d coupling is found to be over 2 orders of magnitude larger than the s-d exchange coupling. The experimental colossal magnetic moments and room-temperature ferromagnetism in GaN:Gd reported recently are explained by the interaction of Gd 4f spins via p-d coupling involving holes introduced by intrinsic defects such as Ga vacancies.     

Density functional study of LaFeAsO(1-x)F(x): a low carrier density superconductor near itinerant magnetism

Density functional studies of 26 K superconducting LaFeAs(O,F) are reported. We find a low carrier density, high density of states, N(E(F)), and modest phonon frequencies relative to T(c). The high N(E(F)) leads to proximity to itinerant magnetism, with competing ferromagnetic and antiferromagnetic fluctuations and the balance between these controlled by the doping level. Thus LaFeAs(O,F) is in a unique class of high T(c) superconductors: high N(E(F)) ionic metals near magnetism.     

硅团簇自旋电子器件的理论研究

利用过渡金属掺杂的硅基团簇, 构建了一种自旋分子结; 并利用第一性原理方法, 对其电子自旋极化输运性质进行了研究. 计算表明, 通过过渡金属掺杂可以有效地产生自旋极化电流, 磁性金属Fe和非磁性金属Cr和Mn掺杂的体系呈现出较明显的自旋极化透射现象, 但分子结的自旋极化输运能力与团簇孤立状态下的磁矩无一致性.从Sc到Ni的掺杂, 体系的自旋极化透射能力先增大后迅速减小, 在Fe掺杂的Si₁₂团簇中出现最大值. 关键词： 硅团簇 自旋极化输运 密度泛函理论 非平衡格林函数     

Two dimensional XY type magnetism in intercalated graphite: an elastic and inelastic neutron scattering study

Stage-2 CoCl₂-GIC approximates a two-dimensional easy-plane (XY) ferromagnet on a triangular lattice. It has been found in prior work to order in two steps, with the intermediate phase showing long-ranged ferromagnetic correlations within the intercalate plane, but no correlations between neighboring planes. We have probed the wave vector and temperature dependence of the static and dynamic spin correlations in detail, including measurements of the critical scattering, the quasielastic scattering from vortex diffusion and the spin wave excitations with and without an external magnetic field. Some of the predictions for a Kosterlitz-Thouless type transition are met in this compound, at least qualitatively, including an apparent jump in the spin stiffness at the critical point and the existence of a diffusive central peak in the scattering function possibly originating from vortex autocorrelations. However, there are some inconsistencies between our observations and recent analytical studies as well as Monte Carlo-molecular dynamics simulations of the vortex dynamics that prevent unambigous assignment of the upper critical temperature as a vortex-binding transition.     

Influence of unlike dispersive interactions on methane adsorption in graphite: a grand canonical Monte Carlo simulation and classical density functional theory study

Activated carbons are popular adsorbents due to their large micro- and mesoporous volumes and high specific surface areas. Modeling adsorption behaviour using molecular computations is frequently undertaken, but the influence of the unlike intermolecular interactions on adsorption behaviour is often not well understood. This study employed grand canonical Monte Carlo simulations, and classical density functional theory coupled with a simple lattice gas model to study the influence of unlike intermolecular interactions on adsorption behaviour, with a focus on the dispersive interactions. Both approaches yielded qualitative agreement with experimental data from the literature, although only a fitted classical density functional theory approach agreed quantitatively. Changing the potential energy well depth of the methane-carbon interaction did not change the Langmuir-type adsorption behaviour observed, however, there was some dependence of the adsorption behaviour on the unlike interactions, depending on the thermodynamic conditions.     

Application of the density functional method for investigating hydrogen adsorption in a plane-parallel graphite pore

Local density profiles in a Lennard-Jones adsorption layer, corresponding to molecular hydrogen in the two-dimensional continuum, which simulates graphene have been calculated using the density functional method (approximation with weight multipliers). In addition to this, the density distribution and absolute adsorption in the plane-parallel pore of a graphite adsorbent have been calculated. It has been found that a certain characteristic width of the pore exists starting from which the density profiles correspond to profiles in two separate graphene layers. At a lesser pore width the force fields from the pore walls overlap which leads to a change in the density profile patterns. It has also shown that porous graphites with a pore width corresponding to five to six removed graphenes are the most optimal for hydrogen storage. The determined absolute adsorption values are in agreement with the results of independent thermodynamic calculations.     

Defect-induced photoconductivity in layered manganese oxides: a density functional theory study

Enhanced photoconductivity of layered Mn(IV)O2 containing protonated Mn(IV) vacancy defects has been recently demonstrated, suggesting new technological possibilities for photoelectric conversion based on visible light harvesting. Using spin-polarized density functional theory, we provide the first direct evidence that such defects can indeed facilitate photoconductivity by (i) reducing the band-gap energy and (ii) separating electron and hole states. Our results thus support the proposition that nanosheet MnO2 offers an attractive new material for a variety of photoconductivity applications.     

Mixed Si-Ge nanoparticle quantum dots: a density functional theory study

We present a systematic study of small mixed composition Si and Ge nanoparticle quantum dots performed using density functional theory (DFT) and time-dependent density functional theory (TDDFT) with real space grids and norm-conserving pseudopotentials. Quantum dot models are obtained by distributing Si and Ge atoms within basic models containing a specified number of atomic sites, tetrahedrally-coordinated about a central site, with H atoms added to terminate the surfaces. We consider models containing up to 35 semiconductor atoms, and a number of subsets of the total number of possible configurations (e.g. alternating elements in successive shells). Following calculation of structural and energetic properties, the optical absorption spectra are determined using time-dependent DFT within Casida’s linear-response approach. With respect to Si, the inclusion of Ge moves spectral weight to lower energies. We observe that the relative fractions of Si and Ge have stronger effects on the absorption spectra than the structural distribution of those atoms within the nanoparticle, which tends to have quite small effects.     

Imogolite类纳米管直径单分散性密度泛函理论研究

采用密度泛函理论方法研究了三种imogolite类(未取代、NH₂取代和F取代)纳米管的直径单分散性及表面电荷的分布情况, 并从键长方面定性地解释了直径单分散性的原因. 我们给出了IMO, IMO_NH₂和IMO_F的应变能曲线, 结果表明三种纳米管结构的最稳定管径值按照IMO < IMO_NH₂ < IMO_F的顺序递增, 而imogolite类纳米管直径单分散性是由于管径的增大导致内部Si–O, Al–O键与外部Al-OH键键长变化趋势相反造成的, 总之是内部Si–O, Al–O 键和外部Al–OH键相互作用的结果. 此外, 对三种稳定的纳米管结构做了Mulliken布局分析, 并总结了纳米管直径变化对表面电荷的影响. 结果表明正电荷主要积聚在外表面, 而内表面则感应出负电荷, 同时随着纳米管直径的增大表面电荷逐渐增加, 揭示了表面电荷与管径大小的关系. 研究表明, 可以通过改变imogolite内表面不同的官能化取代来控制纳米管直径, 进而调节表面电荷的分布情况, 这在imogolite类材料的分子设计及应用方面有着重要意义.     

Molecules relevant for organic photovoltaics: a range-separated density functional study

Accurate determination of both fundamental and optical gap is necessary for designing molecules relevant for organic photovoltaics. Here, we study how range-separated density functionals reproduce frontier orbital energies, HOMO (highest occupied molecular orbital)–LUMO (lowest unoccupied molecular orbital) gaps, and optical gaps for molecules relevant for organic photovoltaics. In this study, we consider 12 different range-separated density functional for computing HOMO energy, HOMO–LUMO gap, and optical gap which are compared with available experimental and reported GW values. We found that the reproduction of desired photovoltaic properties primarily depend on range separation parameter. Moreover, the tested functionals are comparable with OT-BNL functional.     

Nuclear magnetism of sub-monolayer solid He on graphite

Nuclear magnetization has been measured with NMR down to temperatures around 0.1 mK for sub-monolayer solid ³He formed on graphite preplated with two layers of HD. The data for the commensurate phase show no evidence of a magnetic transition and a rather slow increase of the magnetization even at ultra-low temperature (0.15 mK), much lower than the antiferromagnetic exchange interaction of −6.3 mK.     

Local spin density total energy study of surface magnetism: V (100)

Results of self-consistent all-electron local (spin) density functional studies of the electronic and magnetic properties of vanadium (100) 1-, 3-, 5- and 7-layers films are reported using our full-potential linearized augmented plane wave (FLAPW) method. The calculated work function, 4.2 eV, agrees very well with the experimental value of 4.12 eV. From both Stoner factor analyses and spin-polarized total energy calculations, it is concluded that V(100) undergoes a ferromagnetic phase transition only for the monolayer system. The magnetic moment is found to be 3.09μ_B per atom of this monolayer film and to have a total energy 57 mRy below that of the paramagnetic structure. For multilayer V(001) systems, the sharp surface density-of-states peak which is characteristic of the occurrence of surface magnetism in the 3d transition metals is located 0.3 eV above the Fermi level. As a result, the paramagnetic state is stable. In addition, no enhancement of the exchange-correlation integral is found for the surface atoms compared with the bulk value. The lower energy of the paramagnetic structure is further supported by total energy investigations of the multilayer relaxation of V(100) — the calculated interlayer spacings for the paramagnetic surface with a 9% contraction of the topmost interlayer spacing and a 1% expansion of the second interlayer spacing with respect to its bulk value are in good agreement with LEED measurements. It is suggested that the surface magnetism of V(100) may be associated with surface oxygen or caused by impurity induced surface reconstructions.     

The NMR response of boroxol rings: a density functional theory study

Cluster models of boron oxide glasses are studied computationally using density functional theory. It is shown that the isotropic chemical shielding of boron in boroxol rings is about 5 ppm less than for boron in non-ring BO3/2 units, and that the quadrupole coupling in ring sites is about 0.1 MHz larger than in non-ring sites, confirming assignments made in glasses and crystalline model compounds. The chemical shielding anisotropy of these sites is computed and shown to be in agreement with recent experimental measurements. Furthermore, it is shown that the reason for the different responses is not the co-planarity of BO3/2 groups bound in rings, but rather the contraction in the B-O-B bond angle from about 134 degrees in relaxed structures to 120 degrees as found in rings.     

Polar behavior in a magnetic perovskite from a-site size disorder: a density functional study

We elucidate a mechanism for obtaining polar behavior in magnetic perovskites based on A-site disorder and demonstrate this mechanism by density functional calculations for the double perovskite (La,Lu)MnNiO6 with Lu concentrations at and below 50%. We show that this material combines polar behavior and ferromagnetism. The mechanism is quite general and may be applicable to a wide range of magnetic perovskites.     

Trapping of helium atom by vacancy in tungsten: a density functional theory study

The interaction between helium (He) atom and vacancy defect in tungsten (W) has been investigated by using first-principles simulations. We have obtained that the most stable site for He in tungsten is the substitutional position because He can keep its own electronic structure at this position. In the studied tungsten system, vacancy can act as a trapping center for surrounding He atom with negative trapping energy. The migration behaviors of He atom at tetrahedral interstitial site in W, which can be trapped by vacancy but the final position is almost unchanged comparing with its initial position through structural relaxation, have been predicted and discussed. It is also found that single He atom prefers to go through an octahedral site rather than through a direct path to the vacancy, and the stronger the interaction between He atom and vacancy is, the lower the migration barrier will be.     

Structure, magnetism, and adhesion at Cr/Fe interfaces from density functional theory

Properties of the Cr(1 0 0)/Fe(1 0 0) and Cr(1 1 0)/Fe(1 1 0) interfaces are investigated with spin-polarized density functional theory within the generalized gradient approximation (DFT-GGA) for electron exchange and correlation. Contrary to earlier predictions for a monolayer of Cr on bulk Fe, we find intermixing of Cr and Fe at the interface of thick films to be endothermic; hence here we focus on characterizing abrupt, unalloyed interfaces. The ideal work of adhesion for both the (1 0 0) and (1 1 0) abrupt interfaces is predicted to be ∼5.4 J/m². We propose that this anomalously strong adhesion between heterogeneous interfaces is derived from significant spin correlations and d-d bonding at the interface.