Table of contents

We have reformulated the quantum Monte Carlo (QMC) technique so that a large part of the calculation scales linearly with the number of atoms. The reformulation is related to a recent alternative proposal for achieving linear-scaling QMC, based on maximally localized Wannier orbitals (MLWO), but has the advantage of greater simplicity. The technique we propose draws on methods recently developed for linear-scaling density functional theory. We report tests of the new technique on the insulator MgO, and show that its linear-scaling performance is somewhat better than that achieved by the MLWO approach. Implications for the application of QMC to large complex systems are pointed out.

We suggest that ferroelectric relaxor glasses represent an important class of materials in which intrinsic spatial heterogeneity and multiscale dynamics can arise from the formation of local modes due to inherent nonlinearity in the polarizable medium. Specifically, the phenomenology of relaxor ferroelectrics in terms of the spherical random bond–random field (Blinc et al 1999 Phys. Rev. Lett. 83 424) model is explained microscopically by the formation of discrete breathers embedded in a soft but silent medium, which form in-gap local modes (IGLM) where charge and lattice are intrinsically coupled. Complete mode softening is inhibited by the IGLM and soft elasticity a prerequisite for their existence.

Zinc oxide is a unique material that exhibits semiconducting and piezoelectric dual properties. Using a solid–vapour phase thermal sublimation technique, nanocombs, nanorings, nanohelixes/nanosprings, nanobelts, nanowires and nanocages of ZnO have been synthesized under specific growth conditions. These unique nanostructures unambiguously demonstrate that ZnO probably has the richest family of nanostructures among all materials, both in structures and in properties. The nanostructures could have novel applications in optoelectronics, sensors, transducers and biomedical sciences. This article reviews the various nanostructures of ZnO grown by the solid–vapour phase technique and their corresponding growth mechanisms. The application of ZnO nanobelts as nanosensors, nanocantilevers, field effect transistors and nanoresonators is demonstrated.

The spin–lattice relaxation processes of the ³⁹K and ²⁷Al nuclei in KAl(SO₄)₂·12H₂O crystals were studied; we found that these processes can be described by linear combinations of two and three exponential functions, respectively. From these results, we conclude that the discontinuity in the curve of the spin–lattice relaxation rate near T_c (= 360 K) corresponds to the first-order phase transition of the crystal. In the case of the ³⁹K nucleus, the relaxation rate increases as the temperature increases. The relaxation rate of the ²⁷Al nucleus decreases below T_c as the temperature increase, while above T_c its relaxation rate increases with increasing temperature. The temperature dependences of these spin–lattice relaxation rates can be described with the power law . Least square fits of the relaxation rates at temperatures below T_c indicate that k = 2 for the ³⁹K nucleus and k = 7 for the ²⁷Al nucleus. From these results, we conclude that the ³⁹K and ²⁷Al spin–lattice relaxations occur from Raman relaxation processes.

The pressure-induced phase transition in silicon carbide is studied using a constant-pressure ab initio technique. The reversible transition between the zinc-blende structure and the rock-salt structure is successfully reproduced through the simulation. The transformation mechanism at the atomistic level is characterized, and it is found that the transition is based on a tetragonal and an orthorhombic intermediate state. The space groups of the intermediate states are determined as and Imm 2.

I study the buckling transition under compression of a two-dimensional, hexagonal, regular elastic honeycomb. Under isotropic compression, the system buckles to a configuration consisting of a unit cell containing four of the original hexagons. This buckling pattern preserves the sixfold rotational symmetry of the original lattice but is chiral, and can be described as a combination of three different elemental distortions in directions rotated by 2π/3 from each other. Non-isotropic compression may induce patterns consisting of a single elemental distortion or a superposition of two of them. The numerical results compare very well with the outcome of a Landau theory of second-order phase transitions.

The electronic structure of the praseodymium monopnictides and monochalcogenides is studied using the self-interaction corrected (SIC) local spin density (LSD) approximation. This method allows for a description of the Pr ions with some f electrons localized in atomic like orbitals, while other f degrees of freedom are forming hybridized bands. In this way different valency configurations of the Pr ion may be compared. The ground state configuration is obtained from the global energy minimum. With trivalent Pr ions, corresponding to two localized f electrons per Pr ion, the experimental lattice constants and bulk moduli of all these compounds are well reproduced. In contrast, the conventional LSD band treatment of the Pr pnictides and chalcogenides yields too small lattice constants. With applied pressure, the Pr monopnictides and monochalcogenides undergo simple B1 to B2 structural transitions which are well reproduced by the present theory without destabilization of the localized f² shells.

The well-known theory of quantum transport is extended to an approach that allows the description of quantum diffusion in strongly biased multi-band semiconductors and semiconductor superlattices. The longitudinal diffusion coefficient is identified and expressed by a quantity that satisfies a different quantum-kinetic equation than the carrier distribution function, from which the drift velocity is calculated. Results are obtained for a simple two-band model at the interband (intersubband) tunnelling resonance. In contrast to the drift velocity, the diffusion coefficient may exhibit a Zener antiresonance.

The spin-polarized transport is investigated in a new type of magnetic tunnel junction which consists of two ferromagnetic electrodes separated by a magnetic barrier and a nonmagnetic metallic spacer. Based on the transfer matrix method and the nearly-free-electron approximation the dependence of the tunnel magnetoresistance (TMR) and electron-spin polarization on the nonmagnetic layer thickness and the applied bias voltage are studied theoretically. The TMR and spin polarization show an oscillatory behaviour as a function of the spacer thickness and the bias voltage. The oscillations originate from the quantum well states in the spacer, while the existence of the magnetic barrier gives rise to a strong spin polarization and high values of the TMR. Our results may be useful for the development of spin electronic devices based on coherent transport.

By applying the simple and effective method developed to study the gauge-invariant fermion Green function in 2+1 dimensional non-compact QED, we study the gauge-invariant Green function in 3+1 dimensional QED and 2+1 dimensional non-compact Chern–Simon theory. We also extend our results to the corresponding SU(M) non-Abelian gauge theories. Implications for the fractional quantum Hall effect are briefly discussed.

We consider an array of l_b Cooper pair boxes, each of which is coupled to a superconducting reservoir by a capacitive tunnel junction. We discuss two effects that probe not just the quantum nature of the islands, but also of the superconducting reservoir coupled to them. These are analogues to the well-known quantum optical effects 'superradiance' and 'revival'. When revival is extended to multiple systems, we find that 'entanglement revival' can also be observed. In order to study the above effects, we utilize a highly simplified model for these systems in which all the single-electron energy eigenvalues are set to be the same (the strong coupling limit), as are the charging energies of the Cooper pair boxes, allowing the whole system to be represented by two coupled quantum spins, one finite, which represents the array of boxes, and one representing the reservoir, which we consider in the limit of infinite size. Although this simplification is drastic, the model retains the main features necessary to capture the phenomena of interest. Given the progress in superconducting box experiments over recent years, it is possible that experiments to investigate both of these interesting quantum coherent phenomena could be performed in the foreseeable future.

We consider the phase diagram of the BCS (Bardeen–Cooper–Schrieffer)–BE (Bose–Einstein) crossover in the ground state (T = 0 K) of a d_x²−y²-wave superconductor, with a nearest neighbour tight binding structure, when we take into account the Debye (phononic) frequency around the chemical potential, μ. This approach is a continuation of the work of den Hertog (1999 Phys. Rev. B 60 559) and that of Soares et al (2002 Phys. Rev. B 65 174506). The latter authors considered the influence of the second-nearest neighbours, but neither set of authors took into account the effect of the Debye frequency, ω_D, or the influence of the next nearest neighbour matrix hopping element. We have found the following results: (1) there is not a metallic phase—that is, when , , , and , where n is the carrier density per site, V is the attractive interaction, t is the nearest neighbour hopping integral, and is the next nearest neighbour hopping ratio; (2) the BCS–BE crossover line is strongly affected by the presence of ω_D/4t and that of —actually, the values of V/4t needed to achieve the Bose–Einstein regime become extremely large for small values of ω_D/4t; and (3) both Δ/4t and μ/4t strongly depend on the values of ω_D/4t and . The results (1) are in agreement with the ones found by Perali et al (2003 Phys. Rev. B 68 066501 (Preprint cond-mat/0211132)) and Rodríguez-Núñez et al (2003 Phys. Rev. B 68 066502), and in disagreement with those of den Hertog and Soares et al.

Based on the spin fluctuation mechanism we have succeeded in deriving formulae for the magnetic entropy and the specific heat of itinerant electron ferromagnets that cover the wide range of the temperature and the external field strength. We show that it is necessary to include an extra term into the free energy for a thermodynamically consistent treatment. We are able to predict several new features on the temperature and the external field dependence, an extra enhancement of the T-linear coefficient at low temperature and the presence of a critical peak anomaly of the specific heat, for instance. They result from terms proportional to the second-order temperature derivative of the spontaneous magnetization. The presence of the terms is connected with the Maxwell relation of thermodynamics.

Nanowire-like iron oxides (α-Fe₂O₃, Fe₃O₄ and γ-Fe₂O₃) of diameter about 200 nm and length up to 8 µm were assembled into anodic aluminium oxide templates through electrodeposition followed by a heat-treating process. These wires have polycrystalline structures and consist of fine particles whose magnetic moments have preferred orientations in the templates. Some unique magnetic properties, such as perpendicular magnetic anisotropy in Fe₃O₄ wires, reduced transition temperature in α-Fe₂O₃ and Fe₃O₄ wires, and superparamagnetism in γ-Fe₂O₃ wires, were observed. The properties can be understood by their unique structures induced by reduced dimension and the competition of the shape and crystalline anisotropy.

The domain structures of Bi_4−xLa_xTi₃O₁₂ (BLT) ceramics (where x = 0, 0.5, 0.75, and 1.0) have been investigated by transmission electron microscopy. The morphology of the 90° domain walls were found to be curved due to a decrease of the spontaneous strain energy after doping with La. Movement of the 90° domain walls was observed in situ under electron beam irradiation. It is indicated that the irregular and active 90° domain walls in BLT0.75 result from a decrease in internal stress. It is suggested that the existence of irregular and active 90° domains in BLT0.75 has an important effect on the remanent polarization, and fatigue properties.

Two-dimensional tunable photonic crystals (TPCs) with movable components are proposed, which consist of a circular dielectric rod inserted into a square lattice of square dielectric cylinders in air. The band gap structures of TPCs can be tuned by adjusting the shift of the position of the circular rod in each unit cell. Band gap structures are calculated with the use of the plane-wave expansion method for various structural parameters, such as different filling fractions, ratio of the radius of the circular rod to the side-length of the square cylinder, different shifts or shifting orientations of the circular rod, etc. We find that there is a region of parameters in which the ratio of the gap width to the midgap is insensitive to the shift of the position of the circular dielectric rod. It is anticipated that the proposed TPC may become a favourable candidate of PCs owing to the benefit of facilitating fabrication, allowing a large tolerance.

Experiment unambiguously shows substitutional nickel is present in as-grown synthetic diamond where Ni is present in the solvent–catalyst. There is less convincing evidence for the presence of an interstitial species. We present the results of first-principles calculations of the properties of substitutional and interstitial Ni in diamond. The formation energy and mobility of the interstitial species suggest that nickel will be predominantly of the substitutional form. We also present possible models for the trigonal 1.404 eV optical centre correlated with the NIRIM-2 paramagnetic defect.

Infrared transmission and Raman scattering have been used to study Raman active phonons and crystal-field excitations in Yb³⁺-doped yttrium, lutetium and scandium orthosilicate crystals (Y₂SiO₅ (YSO), Lu₂SiO₅ (LSO) and Sc₂SiO₅ (SSO)), which belong to the same C_2h⁶ crystallographic space group. Energy levels of the Yb³⁺ ion ²F_5/2 manifold are presented. In the three hosts, Yb³⁺ ions experience high crystal field strength, particularly in Yb:SSO. Satellites in the infrared transmission spectra have been detected for the first time in the Yb³⁺-doped rare earth orthosilicates. They could be attributed to perturbed Yb³⁺ sites of the lattices or to magnetically coupled Yb³⁺ pairs.

The uniaxial in-plane anisotropy of patterned arrays of micrometric magnetic stripes is observed through their magnetic Brillouin spectra: the analysis is based on the frequency variations and on the Stokes/anti-Stokes dissymmetry versus an external applied magnetic field H. The saturation field H_s is measured for H along the hard axis (H_s = 200 Oe with stripes of thickness 29 nm and of width 1 µm). For H along the easy axis a hysteresis loop is observed, allowing us to derive a coercive field approximately equal to 150 Oe for this array of stripes. A simple model provides a satisfactory interpretation: it stipulates that the anisotropy energy simply consists of a demagnetizing term related to an average demagnetizing field. The experimental determination lies within the interval provided by a panel of different plausible approximations.

The reflection spectra of a hexagonal CdS crystal in the region of S K-absorption edge were studied both experimentally and theoretically. It was found that the reflection spectrum fine structure as well as the absolute value of reflectivity of the crystal surface cut parallel to the crystal optical axis depends on the surface orientation with respect to the polarization of incoming radiation.

Femtosecond optical pump–probe measurements have been performed upon a 500 Å polycrystalline Al film at an interband transition at a wavelength of 800 nm. The dependence of the intensity and polarization of the reflected probe pulse upon the polarization of the pump pulse has been determined. A sharp peak was observed in the polarization signal at zero time delay that contains contributions from both the specular inverse Faraday effect (SIFE) and the specular optical Kerr effect (SOKE). The SIFE and SOKE are associated with the optical orientation of angular and linear momentum, respectively. The dependence of the peak height upon the pump polarization has allowed us to deduce values of χ_xxyy = (1+0.6i) × 10⁻⁸ rad cm³ erg⁻¹ and χ_xyyx = (0.5−0.1i) × 10⁻⁸ rad cm³ erg⁻¹ for the non-vanishing components of the local cubic susceptibility tensor. A distortion of the peak was observed when the pump was elliptically polarized. Its detailed shape shows that the characteristic relaxation time for the SOKE is about 30 fs longer than that for the SIFE in measurements of ellipticity. This suggests a similar difference in the relaxation times for the angular and linear momentum of the optically excited hot electrons. The initial peak was generally followed by a tail that decayed with a time constant of several picoseconds, which we attribute to an ultrafast anisotropic excitation of the lattice.

Ultraviolet photoemission spectra of C₆₀ deposited on Sm film were measured. The results indicate that C₆₀ and Sm can combine with each other to form fulleride in the interface at room temperature. The bonding is mainly ionic with some covalent contribution. Both the lowest unoccupied molecular orbital (LUMO) and the LUMO+1 orbital of C₆₀ are occupied at coverages less than about one monolayer. As for the second overlayer, only the LUMO orbital is occupied. The first and second C₆₀ overlayers are metallic and semiconducting, respectively.

Cd_1−xMn_xS quantum dots (QDs) with diameter  nm and with manganese concentrations from x = 0.001 to 0.15 have been prepared using aqueous solution precipitation. Absorption and fluorescence spectra at room temperature have been recorded. No monotonic variation of the band gap energy with increasing manganese concentration is observed. The electron paramagnetic resonance (EPR) measurements were performed on a 9.5 GHz spectrometer in the temperature range 20–290 K. Two structures in the EPR spectra are observed for all the manganese concentrations: a hyperfine structure and a broad EPR line. The hyperfine structure is described by allowed and forbidden hyperfine transitions of the Mn²⁺ ion central sextet with hyperfine splitting constant |A| = 9.6 mT and with axial-field splitting parameter D ranging from 11.1 to 13.5 mT. The hyperfine structure is attributed to the isolated Mn²⁺ ions at or near the surface of the nanocrystals. The width of the broad resonance and its temperature independence suggest that the wide line originates from the noncentral manganese transitions broadened by crystal-field effects, as in some disordered materials.

Nitrogen doped ZnO films are directly fabricated by the thermal oxidation of Zn₃N₂ films. Zn₃N₂ films are prepared by plasma-assisted metal-organic chemical vapour deposition (PA-MOCVD). By comparing with undoped ZnO photoluminescence spectra, a much stronger bound exciton emission due to a neutral nitrogen acceptor (A⁰X) is observed at low temperature. The neutral acceptor level is located at 130 meV above the valence band maximum. To demonstrate the quality of ZnO:N thin films as a p-type, a Zn₃N₂/n-Si heterojunction structure was first fabricated. With an increase of oxidation temperature, the structure has gradually shown p–n junction rectification characteristics from I–V measurements.