Analysis of the spin exchange interactions and the ordered magnetic structures of lithium transition metal phosphates LiMPO4 (M = Mn, Fe, Co, Ni) with the olivine structure

The olivine-type compounds LiMPO4 (M = Mn, Fe, Co, Ni) consist of MO4 layers made up of corner-sharing MO6 octahedra of high-spin M2+ ions. To gain insight into the magnetic properties of these phosphates, their spin exchange interactions were estimated by spin dimer analysis using tight binding calculations and by electronic band structure analysis using first principles density functional theory calculations. Three spin exchange interactions were found to be important for LiMPO4, namely, the intralayer superexchange J1, the intralayer super-superexchange Jb along the b-direction, and the interlayer super-superexchange J2 along the b-direction. The magnetic ground state of LiMPO4 was determined in terms of these spin exchange interactions by calculating the total spin exchange interaction energy under the classical spin approximation. In the spin lattice of LiMPO4, the two-dimensional antiferromagnetic planes defined by the spin exchange J1 are antiferromagnetically coupled by the spin exchange J2, in agreement with available experimental data.     

Chiral molecular magnets: synthesis, structure, and magnetic behavior of the series [M(L-tart)] (M = Mn(II), Fe(II), Co(II), Ni(II); L-tart = (2R,3R)-(+)-tartrate)

A new series of layered magnets with the formula [M(L-tartrate)] (M = Mn(II), Co(II), Fe(II), Ni(II); L-tartrate = (2R,3R)-(+)-tartrate) has been prepared. All of these compounds are isostructural and crystallize in the chiral orthorhombic space group I222, as found by X-ray structure analysis. Their structure consists of a three-dimensional polymeric network in which each metal shows distorted octahedral coordination bound to four L-tartrate ligands, two of which chelate through an alcohol and a carboxylate group and the other two bind terminally through a monodentate carboxylate group. The chirality of the ligand imposes a Delta conformation on all metal centers. Magnetically, the paramagnetic metal centers form pseudotetragonal layers in which each metal is surrounded by four other metals, with syn,anti carboxylate bridges. These salts show intralayer antiferromagnetic or ferromagnetic interactions, depending on the electronic configuration of the metal, and weak interlayer antiferromagnetic interaction. In all cases the magnetic properties are strongly affected by the anisotropy of the system, and the presence of magnetic canting has been found. The Mn derivative behaves as a weak ferromagnet with a critical temperature of 3.3 K. The Ni derivative shows very unusual magnetic behavior in that it exhibits antiferromagnetic ordering below 6 K, the onset of spontaneous magnetization arising from spin reorientation into a canted phase below 4.5 K, and a field-induced ferromagnetic state above 0.3 T at 2 K, behavior typical of metamagnets. The Fe and Co derivatives show antiferromagnetic interactions between spin carriers, but do not order above 2 K.     

Orientation of a benzene molecule inside a carbon nanotube

Benzene molecules confined in carbon nanotubes of varying radii are employed as semiconductors in electronic nanodevices, and their orientation determines the electrical properties of the system. In this paper, we investigate the interaction energy of all the possible configurations of a benzene molecule inside various carbon nanotubes and then we determine the equilibrium configuration. We adopt the continuous approach together with the semi-empirical Lennard-Jones potential function to model van der Waals interaction between a benzene molecule and a carbon nanotube. This approach results in an analytical expression, which accurately approximates the interaction energy and can be readily used to generate numerical data. We find that horizontal, tilted and perpendicular configurations on the axis of the carbon nanotube are all possible equilibrium configurations of the benzene molecule when the radius of the carbon nanotube is less than 5.580 Å. However, when the radius of the carbon nanotube is larger than 5.580 Å an offset horizontal orientation is the only possible equilibrium configuration of the benzene molecule. In the limiting case, the orientation of a benzene molecule on a graphene sheet can be derived simply by letting the radius of the carbon nanotube tend to infinity.     

Gapped ferromagnetic graphene nanoribbons

We theoretically design a graphene-based all-organic ferromagnetic semiconductor by terminating zigzag graphene nanoribbons (ZGNRs) with organic magnets. A large spin-split gap with a 100% spin polarized density of states near the Fermi energy is obtained, which is of potential application in spin transistors. The interactions among electron, spin and lattice degrees of freedom are studied using the first-principles calculations including non-collinear spin orientations. All of the calculations consistently demonstrate that although no d electrons existing, the antiferromagnetic π-π exchange together with the strong electron-lattice interactions between organic magnets and ZGNRs make the ground state ferromagnetic.     

Effects of Edge Oxidation on the Stability and Half‐Metallicity of Graphene Quantum Dots

A comprehensive first‐principles theoretical study of the electronic properties and half‐metallic nature of zigzag edge‐oxidized graphene quantum dots (GQDs) is carried out by using density functional theory (DFT) with the screened exchange hybrid functional of Heyd, Scuseria and Ernzerhof (HSE06). The oxidation schemes include ‐OH, ‐COOH and ‐COO groups. We identify oxidized GQDs whose opposite spins are localized at the two zigzag edges in an antiferromagnetic‐type configuration, showing a spin‐polarized ground state. Oxidized GQDs are more stable than the corresponding fully hydrogenated GQDs. The partially hydroxylated and carboxylated GQDs with the same size exhibit half‐metallic state under almost the same electric‐field intensity whereas fully oxidized GQDs behave as spin‐selective semiconductors. The electric‐field intensity inducing the half metal increases with the length of the partially oxidized GQDs, ranging from M=4 to 7.     

Electronic structure of octagonal boron nitride nanotubes

The effect of an octagonal lattice configuration on a boron nitride nanotube is explored using first principle calculations. Calculations show that the formational energy of an octagonal boron nitride nanotube (o‐BNNT) is an exothermic reaction. Boron and nitrogen atoms within an o‐BNNT have an average of 2.88 electrons and 9.09 electrons, respectively, indicating ionic‐like bonding. In addition, the electronic structure of the octagonal boron nitride nanotube shows semiconductive properties, while h‐BNNT is reported to be an insulator. Additional o‐BNNTs with varying diameters are calculated where the results suggest that the diameter has an effect on the binding energy and bandgap of the o‐BNNT. The defect sites of the o‐BNNT are reactive against hydrogen where a boron defect is particularly reactive. Thus, this work suggests that physical and chemical properties of a boron nitride nanotube can be tailored and tuned by controlling the lattice configuration of the nanotube.     

On the applicability of cluster models to study the chemical reactivity of carbon nanotubes

We have performed a comparative study on the reactivity of metallic and semiconducting nanotubes using infinite and finite models. Infinite models were created using periodic boundary conditions while finite ones were constructed by means of hydrogen terminated nanotubes sections. Cluster models systematically underestimate the reactivity of metallic single wall carbon nanotube (SWCNT)s. We have confirmed that metallic nanotubes are more reactive than semiconducting species, in disagreement with previous works. The differences can be attributed to the presence of an instability in the singlet ground state of the wavefunction corresponding to semiconducting nanotubes clusters. When lower electronic states of the pristine cluster are considered, semiconducting nanotubes become less reactive as compared with metallic SWCNTs. Particularly, if an antiferromagnetic solution is considered for the semiconducting (10,0) SWCNT cluster, it becomes less reactive than the (5,5) SWCNT, as observed for infinite models. Because semiconducting nanotubes are less reactive than metallic counterparts, their reaction energies converge faster to the values observed for graphene. For a 1.6-nm diameter semiconducting nanotube, the addition energy is comparable with graphene. Thus, semiconducting nanotubes with diameters larger than 1.6 nm are going to be as reactive as graphene and the effects of curvature will be unimportant.     

The electronic structure and magnetic properties of poly(m-phenylcarbene)

The electronic structure and the magnetic properties of the ferromagnetic organic polymer poly (m-phenylcarbene ) was studied by application of the unrestricted Hartree-Fock (UHF) crystal orbital (CO) method. In comparison with the restricted Hartree-Fock (RHF) result, it was revealed that the ferromagnetic state is more stable than the non-magnetic state. According to a detailed energy analysis, the stability originates from both the triplet spin configuration at the carbene centre and the delocalized π spins in an antiferromagnetic fashion over the phenyl ring.     

Synthesis, crystal structure, magnetic properties, and electronic structure of the new ternary vanadate CuMnVO4

A new magnetic oxide, CuMnVO4, was prepared, and its crystal structure was determined by single-crystal X-ray diffraction. The magnetic properties of CuMnVO4 were characterized by magnetic susceptibility and specific heat measurements, and the spin exchange interactions of CuMnVO4 were analyzed on the basis of spin-polarized electronic band structure calculations. CuMnVO4 contains MnO4 chains made up of edge-sharing MnO6 octahedra containing high-spin Mn2+ cations. Our work shows that CuMnVO4 undergoes a three-dimensional antiferromagnetic transition at approximately 20 K. Both the intrachain and interchain spin exchanges are antiferromagnetic, and the interchain spin exchange is not negligible compared to the intrachain spin exchange.     

Structure and magnetic properties of oxychalcogenides A2F2Fe2OQ2 (A = Sr, Ba; Q = S, Se) with Fe2O square planar layers representing an antiferromagnetic checkerboard spin lattice

The oxychalcogenides A2F2Fe2OQ2 (A = Sr, Ba; Q = S, Se), which contain Fe2O square planar layers of the anti-CuO2 type, were predicted using a modular assembly of layered secondary building units and subsequently synthesized. The physical properties of these compounds were characterized using magnetic susceptibility, electrical resistivity, specific heat, (57)Fe Mossbauer, and powder neutron diffraction measurements and also by estimating their exchange interactions on the basis of first-principles density functional theory electronic structure calculations. These compounds are magnetic semiconductors that undergo a long-range antiferromagnetic ordering below 83.6-106.2 K, and their magnetic properties are well-described by a two-dimensional Ising model. The dominant antiferromagnetic spin exchange interaction between S = 2 Fe(2+) ions occurs through corner-sharing Fe-O-Fe bridges. Moreover, the calculated spin exchange interactions show that the A2F2Fe2OQ2 (A = Sr, Ba; Q = S, Se) compounds represent a rare example of a frustrated antiferromagnetic checkerboard lattice.     

Magnetic exchange couplings in the single-molecule magnet of Mn(12)-Ac

Four types of isotropic exchange interactions of Mn(12)-Ac are obtained by using the classical Monte Carlo simulations. The equilibrium susceptibilities are well reproduced in the temperature range between 10 and 100 K. The calculated effective spin at 0.1 K coincides with the ground-state spin. Our results show that J(1) and J(2) are strong antiferromagnetic, but the magnitude of J(2) is much smaller than that of J(1). Both J(3) and J(4) favor weakly antiferromagnetic couplings. The effects of the exchange couplings on the magnetic properties and ground-state spin are investigated too. The magnetic susceptibilities below 100 K depend more on J(2) rather than on the stronger J(1). The weak exchange couplings J(3) and J(4) have significant frustration effects on the ground-state configuration.     

Superconducting first‐order pairing: Finite‐temperature simulations

A recently developed first‐order mechanism for superconducting pairing has been extended from T = 0 K to finite temperatures. On the basis of quantum statistical considerations, we have suggested a direct pairing interaction that does not necessarily involve second‐order elements, such as the electron–phonon coupling or specific magnetic interactions submitted by spin fluctuations. The driving force for the (energy‐driven) first‐order pairing is an attenuation of the destabilizing influence of the Pauli antisymmetry principle (PAP). Only the moves of unpaired fermions are controlled by the PAP, while the moves of superconducting Cooper pairs are not. The quantum statistics of Cooper pairs is of a mixed type, as it combines fermionic on‐site and bosonic intersite properties. The strong correlation between the strength of PAP constraints and system topology in combination with the electron number has been discussed for some larger clusters. Detailed finite‐temperature simulations on first‐order pairing have been performed for four‐center–four‐electron clusters with different topologies. A canonical ensemble statistics has been employed to derive the electronic energy, the electronic configuration entropy, and the free energy of paired and unpaired states in thermal equilibrium. The simulations show that pairing can be caused by either the electronic energy or the electronic configuration entropy. The coexistence of two different sets of quantum particles in paired states (i.e., the Cooper pairs and the unpaired electrons) can lead to an enhanced configuration entropy. In this context, we discuss the possibility of an entropy‐driven high‐temperature superconductor emerging from a low‐temperature unpaired state. The charge and spin degrees of freedom of the four‐center–four‐electron systems have been studied with the help of the charge and spin fluctuations. The spin fluctuations are helpful in judging the validity of pairing theories based on magnetic interactions. The charge fluctuations are a measure for the carrier delocalization in unpaired and paired states. The well‐known proximity between Jahn–Teller activity and superconductivity is analyzed in the zero‐temperature limit. It is demonstrated that both processes compete in their ability to reduce PAP constraints. All theoretical results have been derived within the framework of the simple Hubbard Hamiltonian. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Graphene-Ni (001) interface study

The interfacial properties for a carbon nanotube on a Ni (001) surface are modeled by a piece of vertical graphene standing on a Ni (001) surface. The interaction between the graphene and the nickel (001) surface is investigated using density functional theory (DFT) calculations. Zigzag type graphene can stand on the hollow sites of the Ni (001) surface along the [linear span]110[linear span] direction. For such a configuration, Ni (001)-graphene interfacial mechanical properties are studied, and we find that Ni-Ni bonds near the interface will break first under tensile strain. C-C bond lengths near the interface are longer than the C-C bonds of graphene, and the charge density of those bonds decrease due to the formation of interfacial Ni-C bonds. It suggests that C-C bonds near the interface may break during the carbon nanotube growth processes.     

Large Hexagonal Bi‐ and Trilayer Graphene Single Crystals with Varied Interlayer Rotations

Bi‐ and trilayer graphene have attracted intensive interest due to their rich electronic and optical properties, which are dependent on interlayer rotations. However, the synthesis of high‐quality large‐size bi‐ and trilayer graphene single crystals still remains a challenge. Here, the synthesis of 100 μm pyramid‐like hexagonal bi‐ and trilayer graphene single‐crystal domains on Cu foils using chemical vapor deposition is reported. The as‐produced graphene domains show almost exclusively either 0° or 30° interlayer rotations. Raman spectroscopy, transmission electron microscopy, and Fourier‐transformed infrared spectroscopy were used to demonstrate that bilayer graphene domains with 0° interlayer stacking angles were Bernal stacked. Based on first‐principle calculations, it is proposed that rotations originate from the graphene nucleation at the Cu step, which explains the origin of the interlayer rotations and agrees well with the experimental observations.     

层间共价增强石墨烯材料的构筑、性能与应用

大批量石墨烯可控制备技术的逐渐成熟为实现其宏观组装和应用提供了基础。在众多的组装策略中,调节石墨烯层间的界面相互作用可以直接影响组装体的力学、电学、热学以及渗透等性质,具有重要的意义。石墨烯片层间以共价键连接的层间共价石墨烯材料以其可调的层间距、较强的层间作用力、丰富的功能化、以及可能的原子构型重排等特性,受到了广泛的关注和深入的研究。相比于其他非共价的键合手段,共价连接是一种更为牢固的枢纽。本文中我们将总结讨论层间共价石墨烯材料的构筑方法、性能以及应用。在构筑方法中,依据石墨烯本身的制备方法分为氧化还原法以及化学气相沉积法,而在氧化还原法中,以其宏观材料的形貌分为纸状和纤维状来讨论。接着,我们重点介绍了层间共价对其力学和电学性能的影响,并概述了此类宏观组装体材料的应用。层间共价石墨烯材料继承了石墨烯自身优异的特性,同时也具有宏观组装所赋予的性能,有望在多个领域得到广泛的应用。     

From Open‐Shell Singlet Diradicaloid to Closed‐Shell Global Antiaromatic Macrocycles

A dithieno[a,h]‐s‐indacene‐ (DTI‐) based diradicaloid DTI‐2Br was synthesized and its open‐shell singlet diradical character was validated by magnetic measurements. On the other hand, its macrocyclic trimer DTI‐MC3 and tetramer DTI‐MC4 turned out to be closed‐shell compounds with global antiaromaticity, which was supported by X‐ray crystallographic analysis and NMR spectroscopy, assisted by ACID and 2D‐ICSS calculations. Such change can be explained by a subtle balance between two types of antiferromagnetic spin–spin coupling along the π‐conjugated macrocycles. The dications of DTI‐MC3 and DTI‐MC4 turned out to be open‐shell singlet diradical dications, with a singlet–triplet energy gap of ?2.90 and ?2.60 kcal mol^?1, respectively. At the same time, they are both global aromatic. Our studies show that intramolecular spin–spin interactions play important roles on electronic properties of π‐conjugated macrocycles.     

Enhancement of hydrogen physisorption on graphene and carbon nanotubes by Li doping

Density-functional calculations of the adsorption of molecular hydrogen on a planar graphene layer and on the external surface of a (4,4) carbon nanotube, undoped and doped with lithium, have been carried out. Hydrogen molecules are physisorbed on pure graphene and on the nanotube with binding energies about 80-90 meV/molecule. However, the binding energies increase to 160-180 meV/molecule for many adsorption configurations of the molecule near a Li atom in the doped systems. A charge-density analysis shows that the origin of the increase in binding energy is the electronic charge transfer from the Li atom to graphene and the nanotube. The results support and explain qualitatively the enhancement of the hydrogen storage capacity observed in some experiments of hydrogen adsorption on carbon nanotubes doped with alkali atoms.     

On the correct spin lattice for the spin-gapped magnetic solid NH4CuPO4·H2O

NH₄CuPO₄·H₂O is a spin-gapped compound that has been described in terms of an isolated antiferromagnetic spin dimer model. To explore the origin of this spin gap, we examined the spin exchange interactions of NH₄CuPO₄·H₂O by performing qualitative spin dimer analysis based on extended Hückel tight binding calculations and also by carrying out quantitative mapping analysis based on first principles density functional theory electronic band structure calculations. Our study indicates that, to a first approximation, the magnetic properties of NH₄CuPO₄·H₂O should be described by an antiferromagnetic and ferromagnetic alternating chain.     

Ab initio study of structural and electronic properties of zigzag graphene nanoribbons on hexagonal boron nitride

Results of the study of structural and electronic properties of the 8-ZGNR/h-BN(001) heterostructure by the pseudopotential method using plane waves within density functional theory are presented. Within one approximation the features of the spin state at the Fermi level are studied along with the role of the edge and substrate effects in the opening of the energy gap in the 8-ZGNR/h-BN(001) heterostructure in both ferromagnetic and antiferromagnetic orderings. The effect of a substrate made of hexagonal boron nitride was found for the first time. It consists in the opening of the energy gap in the π electron spectrum of the 8-ZGNR/h-BN(001) heterostructure for the ferromagnetic spin ordering. It is shown that the gap was 30 meV. Contributions of the edge effects of the graphene nanoribbon and the substrate to the energy gap formation are differentiated for the first time. It is found that in the 8-ZGNR/h-BN(001) heterostructure the dominant role in the opening of the energy gap at the Fermi level is played by the edge effects. However, when the nanoribbon width decreases, e.g., to six dimmers the substrate role in the gap opening increases and amounts to 45%. Local magnetic moments of carbon atoms are estimated. It is shown that small magnetic moments are induced on boron and nitrogen atoms at the interface.