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For spintronic applications, it is highly desirable to realize 100%-spin-polarized two-dimensional (2D) electron systems in field-controllable epitaxial ultrathin films or 2D materials on semiconductor substrates. Through systematic first-principles investigation, we find that one epitaxial TcO 2 bilayer on rutile TiO 2 (001) substrate is an antiferromagnet-like narrow-gap semiconductor, and its electronic and magnetic properties can be manipulated through electric field. When electric field reaches 0.03 V/A, it transits to a half-metallic ferrimagnet with 100% spin polarization. Our analysis indicates that in both phases the magnetization density and the electronic states near the Fermi level originate mainly from the TcO 2 bilayer, and across the interface the bond lengths and angles quickly converge to the corresponding values of the bulk TiO 2 . Therefore, the heterostructure actually hosts a 2D electron system determined by the TcO 2 bilayer and the TiO 2 substrate. Because the half-metallic phase can be achieved from a nonmetallic phase, such epitaxial 2D electron systems should be usable in spintronic devices.

Two-dimensional electron gas due to semiconductor interfaces can have high mobility and exhibits superconductivity, magnetism, and other exotic properties that are unexpected in constituent bulk materials. We study crystal structures, electronic states, and magnetism of short-period (BTO)m/(GTO)2 (m=2 and 4) superlattices consisting of insulating BaTiO3 (BTO) and GdTiO3 (GTO) by first principles calculations. Our investigation shows that the middle Ti-O monolayer in the GTO layer becomes metallic. Although both GTO and BTO share a common constituent, the Ti-O atomic layer, the M-O (M=Ba, Ti, and Gd) displacements along the c axis in each monolayer reflect the asymmetry of two interfacial Ti-O monolayers and the large distortion of the TiO6 octahedra in such superlattices, against electronic reconstruction at the interfaces and thus differentially changing the d energy levels of the three Ti-O monolayers related with the GTO layer. Such superlattices are interesting for potential spintronics applications because of their unique asymmetrical two-dimensional electron-gas properties and possible useful spin-orbit effects.

Structures of the two-dimensional atomic nuclei on ZnO (0001)-Zn and ( 000 1 ¯ )-O polar surfaces were studied by first principles density functional theory. The polarity-dependent nucleation dynamics was investigated by simulating two-dimensional (2D) nuclei consisting of 1-8 ZnO monomers on both polar surfaces. According to total energy calculations, average binding energy per ZnO monomer of the surface nuclei was analyzed to investigate if the nucleation and growth will proceed reasonably in physics. We found nucleation on (0001)-Zn surface was easier than that on ( 000 1 ¯ )-O surface. By using atomistic thermodynamics analysis, we calculated the Gibbs free energy of formation of these nuclei and made a comparison between the two polar surfaces. On (0001)-Zn surface, the critical Gibbs free energy of formation is much lower than that on ( 000 1 ¯ )-O surface under the same supersaturation, which leads to a much larger ZnO growth rate and rougher morphology, in accordance with experimental results. In addition, energetic analysis of nucleation at real thermodynamic conditions was achieved by introducing the temperature- and pressure-dependent chemical potentials of ZnO precursors.

Recent temperature-dependent x-ray diffraction and Raman spectroscopy experiment proved that single-crystalline BiMnO 3 assumes a centrosymmetric monoclinic structure ( C 2/ c space group). Here we investigate magnetic structure and electronic structure of this centrosymmetric BiMnO 3 phase by using the modified Becke-Johnson (mBJ) exchange functional within the density functional theory (DFT). Our mBJ calculated semiconductor gap, magnetic moment, and other aspects of the electronic structure, in contrast with previous DFT results, are in good agreement with recent experimental values. This satisfactory description of the electronic structure and magnetism of the BiMnO 3 is because mBJ reasonably captures the kinetic property and correlation of electrons. Our calculated results with mBJ approach are both useful to study such Bi-based perovskite oxide materials for spintronics applications.

We use first-principles methods to study double perovskite Pb2CrMO6 (M: some 4d and 5d transition metals), and find that a magnetic I 4 / m structure is lowest in total energy per formula unit for M=Mo, W, and Re. Our electronic structure investigation shows that the three double perovskite compounds are half-metallic ferrimagnets with total magnetic moments of 2 μ B , 2 μ B and 1 μ B per formula unit, respectively. With the spin–orbit coupling included, our calculations show the high magnetocrystalline anisotropy (MCA) in Pb2CrReO6. Fortunately, the spin polarization of Pb2CrMO6 at the Fermi level remains 90% ~ 99 % . The orbital moments of 3d Cr and 4d Mo are small, but the 5d ions of W and Re have large unquenched orbital moments, which can be attributed stronger spin–orbit coupling in the 5d orbitals than in 3d/4d ones. In addition, these results are confirmed by using improved exchange–correlation functionals. Furthermore, we calculate the spin exchange constants of the three Pb2CrMO6, and our Monte Carlo simulated Curie temperatures for them are at least 480 K. These materials, when synthesized, should be useful in spintronics.

We investigate the electronic structures of the cubic and tetragonal phases of Sr 2 MgWO 6 . Calculated electronic structures show that the two phases are both wide indirect band gap semiconductors. Our phonon calculations indicate that the structural phase transition from cubic to tetragonal phase is because the phonon frequencies of Γ 4 + modes become imaginary. Our variable cell-shape relaxation results imply that a symmetry-allowed intermediate trigonal phase could be realized.

We use a modified Becke–Johnson (mBJ) exchange plus local density approximation correlation functional within the density functional theory (DFT) [semi-local, orbital-independent, producing accurate semiconductor gaps, see Tran and Blaha, Phys. Rev. Lett., 102 (2009) 226401] to investigate the electronic structures of CdCr 2 S 4 and CdCr 2 Se 4 as two ferromagnetic semiconductors. Our results show that mBJ makes the empty Cr d-eg character bands higher by about 1.1 eV for CdCr 2 S 4 and 0.7 eV for CdCr 2 Se 4 for the majority-spin channel with respect to the generalized gradient approximation (GGA). For the minority-spin channel, mBJ raises the Cr d character bands by about 0.8 eV for CdCr 2 S 4 and CdCr 2 Se 4 . The mBJ gap is enhanced by about 104% for CdCr 2 S 4 and 170% for CdCr 2 Se 4 compared to GGA results, and is consistent with experiment in both of the cases. Our mBJ dielectric functions and the zero frequency refractive index n(0) for CdCr 2 S 4 and CdCr 2 Se 4 are also consistent with experimental results available. The improved DFT results can be understood in terms of mBJ-enhanced spin exchange splitting and the reduced band width of Cr d character bands.

We investigate electronic structure and optical properties of LaMnO3 through density-functional-theory calculations with a recent improved exchange potential. Calculated gaps are consistent with recent experimental values, and calculated optical conductivities and dielectric constants as functions of photon energy are in excellent agreement with low-temperature experimental results. These lead to a satisfactory theoretical understanding of the electronic gaps and optical properties of LaMnO3 without tuning atomic parameters and can help elucidate electronic structures and magnetic properties of other manganite materials. (C) 2011 Elsevier B.V. All rights reserved.

We investigate dynamical magnetization of rectangular lattices of interacting tiny magnets with strong perpendicular uniaxial anisotropy using a nonequilibrium Monte Carlo method. Our simulated results show that the magnetism is a nonequilibrium dynamical ferromagnetism unique to such nanoscale systems under changing fields. Such simulations are efficient and reliable to interacting small magnets with strong anisotropy and relatively weak exchange interaction, with the magnetic moment sizes ranging from several Bohr magnetons to hundreds or more.

In this paper we present a unified phase-field model for non-equilibrium growths of various three-dimensional metal islands on insulating surfaces. We introduce a phase-field variable to distinguish the island from the non-island regions and substrate and a density variable to describe local density of deposited adatoms. Two partial differential equations with appropriate boundary conditions, as the governing equations, are used to describe the evolution of the three-dimensional metal islands and the diffusion of adatoms. We solve the equations by using an adaptive mesh refinement method so that we can simulate the non-equilibrium growth of three-dimensional metal islands from tens of nanometers to several micrometers. We investigate the dependence of simulated results on the model parameters and experimental conditions. Equilibrium shape of such islands can be obtained through sufficient post-deposition relaxation. Experimental trends of island size and shape on various scales are obtained with reasonable parameters. This method should be a good approach to non-equilibrium growths of multi-scale three-dimensional metal islands.

Anti ferromagnetic Co3O4 nanoparticles with diameter around 30nm have been synthesized by a solution-based method. The phase identification by the wide-angle X-ray powder diffraction indicates that the CO3O4 nanoparticle has a cubic spinel structure with a lattice constant of 0.80843(2) nm. The image of field emission scanning electron microscope shows that the nanoparticles are assembled together to form nanorods. The magnetic properties of Co3O4 fine particles have been measured by a superconducting quantum interference device magnetometer. A deviation of the Neel temperature from the bulk is observed, which can be well described by the theory of finite-size scaling. An enhanced coercivity as well as a loop shift are observed in the field-cooled hysteresis loop. The exchange bias field decreases with increasing temperature and diminishes at the Neel temperature. The training effect and the opening of the loop reveal the existence of the spin-glass-like surface spins. (C) 2008 Elsevier B.V. All rights reserved.

We use a simple magnetization model to determine domain structures of ultrathin magnetic nanobelts. A train of alternate domains are formed along the length direction. Optimal domain length decreases with belt width. Experimental domain length distributions of Fe bilayer nanobelts can be naturally explained. This approach should be applicable to similar nanomagnets.

We report systematical density-functional calculations of ternary transition-metal compounds based on zincblende ZnTe and CdTe semiconductor. Some of them are half-metallic at their optimized cell volumes. The effect of atomic position optimization (and cell volume re-optimization) on the electronic structures is to widen the energy bands near the Fermi energy and to reduce the density of states there. As a result, the Fermi level moves upward and the energy gap of the minority-spin bands at the Fermi energy becomes narrower. Therefore, the half-metallic gaps are reduced, two of them even being closed. These compounds are compatible with the II–VI semiconductors, and could be useful in spin-based electronics.

We study hard photon production in a chemically equilibrating quark-gluon plasma at finite baryon density, and find that the effect of the system evolution on the photon production and large contribution of the bremsstrahlung and Compton qg -> gamma q processes make the total photon yield as a strongly increasing function of the initial quark chemical potential. (c) 2005 Elsevier B.V. All rights reserved.

We obtained exact analytic solutions of one-dimensional two-particle systems with Dirac-delta repulsive interaction in two typical potentials. It was shown that in these potentials there exists not any wave function symmetrical with respect to the particle exchange. There exist only anti-symmetrical solutions to these systems. Applied to electronic quantum dots, they imply that there exist spin triplet states only and spin singlet states are not allowed for this electronic repulsive interaction.

For decades research on thin-film growth has attracted a lot of attention as these kinds of materials have the potential in new generation device application. It is known that the nuclei at initial stage of the islands are more stable than others and certain atoms are inert while others are active. In this paper, we will show that, when a surfactant layer is used to mediate the growth, a counter-intuitive fractal-to-compact island shape transition can be induced by increasing deposition flux or decreasing growth temperature. Specifically, we introduce a reaction-limited aggregation (RLA) theory, where the physical process controlling the island shape transition is the shielding effect of adatoms stuck to stable islands on incoming adatoms. Also discussed are the origin of a transition from triangular to hexagonal then to inverted triangular and the decay characteristics of 3D islands on surface, and connections of our unique predictions with recent experiments. Furthermore, we will present a novel idea to make use of the condensation energy of adatoms to control the island evolution along special direction.

The spin-s quantum Heisenberg ferromagnetic model is investigated in the physical magnon theory. The effect of the extra unphysical magnon states on every site is completely removed in the magnon Hamiltonian and during approximation procedure so that the condition = 0(n greater than or equal to 2s + 1) is rigorously satisfied. The physical multi-approximation occupancy (1 less than or equal to n less than or equal to 2s) is proportional to T-3n/2 at low temperature and is equivalent to 1/(2s + 1) at the Curie temperature. The magnetization not only unified but also well-behaved from zero temperature to Curie temperature is obtained in the framework of the magnon theory for the spin-s quantum Heisenberg ferromagnetic model. The ill-behaved magnetizations at high temperature in earlier magnon theories are completely corrected. The relation of magnon (spin wave) theory with spin-operator decoupling theory is clearly understood. (C) 2001 Elsevier Science B.V. All rights reserved.

It was shown recently that in the submonolayer regime the shape of islands of exchanged adatoms can be changed substantially by temperature, deposition flux, or coverage in the surfactant-mediated heteroepitaxial growth systems. The shape transition from fractal islands to compact ones can be induced by decreasing temperature or increasing flux. This shape transition is completely contrary to the classic diffusion-limited aggregate (DLA) theory, but can be explained in the frame of a reaction-limited aggregate (RLA) model, in which a stable island consists of the exchanged (or dead) adatoms only, an adatom must overcome a large energy barrier to become the seed of a stable island, or overcome another little smaller barrier to join an existing island. We propose that the strain due to the mismatch in the heteroepitaxy plays the key role in the coverage-induced shape change. Our simulation shows that the strain always makes the islands more compact. With the strain taken into account, there is indeed an island shape change to more compact islands. Applied to the growth of Ge on the Si(111) substrate pre-covered by a monolayer of Pb, the coverage-induced shape change of Ge islands can be explained.

The energy spectra of heavy quark bound states are analyzed by using different quark binding potentials. The critical screening masses for J/Psi dissociation are calculated. The leptonic decay width of J/Psi in the hot and dense matter is obtained. (C) 1998 Published by Elsevier Science B.V. All rights reserved.

In this paper we study the effect of next-nearest-neighbor exchange on antiferromagnetic order and spin correlation of quasi-two-dimensional antiferromagnets in terms of our anisotropic nonlinear spin wave theory. We emphasize the Neel temperature, antiferromagnetic order parameter, and spin correlation as functions of the next-nearest-neighbor exchange parameter. We apply the theoretical results to the antiferromagnetic properties of insulating cuprates.

We propose a nonlinear spin wave theory of anisotropic 3D (quasi-2D) quantum Heisenberg antiferromagnets, which is equivalent to or reduces to known spin wave theories of isotropy parameter δ takes the values 0 or 1, respectively. With δ increasing from zero also the Neel temperature T N increases from zero. Applied to the antiferromagnetism of the layer-like cuprate La 2 CuO 4 it describes quite satisfactorily the existing experimental data of the Neel temperature, correlation length, and staggered magnetization of the material if J = 1034 K and δ = 4×10 −5 .

The full-potential augmented plane wave plus local orbitals method within density-functional theory is used to predict that MnBi in the zincblende phase is a true half-metallic ferromagnet with a magnetic moment of 4.000mu(B) per formula. This phase of MnBi is found to be robust against volume changes from -12% to +30% and remains qualitatively the same under various exchange-correlation approximations. (C) 2003 Elsevier Science B.V. All rights reserved.

BRST - and conformally invariant dilaton vertices of the closed superstring and the heterotic string are constructed, whose counter terms are consistent with the dilaton terms introduced in the non-linear sigma model approach of string theory.

New spontaneous compactification solutions of d = 10, N = 2 nonchiral supergravity are presented. The new solutions are AdS 4 ⊗ S 1 ⊗ M 5 , where M 5 can be identified with S 5 , SU (2) ⊗ SU (2) U (1) and SU (3) SU (2) Max . There does not remain any supersymmetry for all solutions.