Table of contents

Density functional theory (DFT) is widely used in surface science, but for some properties the predictions depend strongly on the approximation used for exchange–correlation energy. We note recent suggestions that the widely used generalized gradient approximation (GGA) is inferior to the local density approximation (LDA) for the surface formation energy σ of both transition metals and oxides. We report quantum Monte Carlo calculations of σ for the MgO(001) surface which support the accuracy of LDA for this case, and indicate that GGA is too low by ∼30%. We point out the potentially important implications of this result for nanoscience modelling.

The functionality of magnetic heterostructures is strongly affected by the interface structure and morphology as well as by the crystalline quality. Using Fe/V superlattices, serving as model systems for exchange coupling and for superconducting spin valves, we show that the excellent interfacial properties can be further improved by hydrogen assisted growth. The role of the hydrogen thereby is twofold. It reduces thickness fluctuations and improves the crystalline quality. Sputter deposited Fe/V superlattices grown at a partial pressure of 2 × 10⁻⁶ mbar are atomically flat and possess a residual resistivity ratio which is significantly improved as compared to those of conventionally grown samples.

Many naturally occurring microbiological and synthetic polymer systems have soft interfaces due, e.g., to the presence of polymer brushes at the surfaces. The initial interactions (before contact) in such systems are typically physical rather than chemical in origin, e.g. electrostatic or van der Waals forces. We calculate the form of the summed van der Waals interaction energies U_HS and U_SS between two bulk systems separated by a distance D, when one (U_HS) or both (U_SS) of their surfaces are soft. We find that the summed interactions diverge weakly as , U_HS∼−1/D and , and comment upon the applicability of these results.

The high symmetry of a single-wall nanotube made of a rectangular lattice sheet rolled up into a cylinder is described by a line group. Such a tube is characterized by helical symmetry, while it has translational periodicity if the squares of a rectangular lattice vector are commensurate or if its chiral vector is orthogonal to a rectangular lattice vector. Possible additional symmetry elements are consequences of the non-translational symmetries of the layer (e.g. rotational axis and mirror planes). Except for a two-fold axis, they appear only if a chiral vector matches specific, rather restrictive, conditions.

In this paper we present a theoretical analysis of the effect of local geometrical structure of the Fermi surface on the surface impedance of a metal under the anomalous skin effect. We show that when the Fermi surface includes nearly cylindrical and/or flattened segments it may significantly change both the magnitude and frequency dependence of the surface impedance. Being observed in experiments these unusual frequency dependences could bring additional information concerning fine geometrical features of the Fermi surfaces of metals.

The ab-plane reflectivity of YBa₂Cu₃O_6+x thin films was measured in the 500–25 000 cm⁻¹ range in samples with x = 1 (T_c = 90 K), x = 0.85 (T_c = 82 K), x = 0.6 (T_c = 55 K), x = 0.45 (T_c = 35 K) and x = 0.38 (T_c = 20 K) in normal state. The effective charge carrier number obtained from the spectral weight increases with increasing x. Variation is consistent with higher d_c resistivity for x<0.45. On the other hand the scattering frequency dependent rate shows significant differences between optimally oxygen doped and underdoped material. Results show that the high-T_c superconductor normal state deviates in many fundamental aspects from conventional superconductors.

A Green's function technique is applied for the Heisenberg model to study the influence of the magnetic surface single-ion anisotropy on the spin wave spectrum including damping effects in ferromagnetic thin films. It is shown that the magnetic surface anisotropy strongly affects the thickness dependence of different quantities: for strong surface anisotropy, the magnetization, the spin-wave energy and the phase transition temperature T_C are larger in thin films than in the bulk, whereas the opposite is true for small surface anisotropy. The magnetic surface anisotropy enhances the spin-wave damping. There is some competition between the surface single-ion anisotropy and the surface exchange interaction. The effect of an external magnetic field is discussed, too.

The structure of a steady wave system is considered which is admitted by the continuum equations for materials that undergo phase transformations with exothermic chemical reaction. In particular, the dynamic phase front structures between liquid and gas phases and solid and liquid phases are computationally investigated. With its theoretical basis in one-dimensional continuum shock structure analysis, the present approach estimates the micro-width of waves associated with phase transformation phenomena. For illustration purposes, n-heptane is used in the evaporation and condensation analysis and HMX is used in the melting and freezing analysis of energetic materials of interest. The estimated thickness of the evaporation–condensation front of n-heptane is on the order of 10⁻² µm while the HMX melting–freezing front thickness is estimated at 1 µm. The present structure analysis is being extended to account for a broader range of energetic materials whose phase front thickness measurements are neither available nor experimentally attainable.

Scanning tunnelling spectroscopy measurements were performed on thin films of La_0.7Sr_0.3MnO₃ both at room temperature and liquid nitrogen temperature. While no inhomogeneities were recorded at liquid nitrogen temperature on any sample, clear evidence of spectroscopic inhomogeneities was evident in tunnelling conductance maps collected at room temperature. The investigated films exhibit a transition from a ferromagnetic-metallic to a paramagnetic-insulating state at around room temperature, so the observed spectroscopic features can be interpreted within a phase separation scenario. A quantitative analysis of the observed spectroscopic features is reported, pointing out the occurrence of phase modulation and its possible correlation with the properties of the system.

We have synthesized polycrystalline Sr₂IrO₄ and measured its magnetic susceptibility, electrical resistivity, specific heat, Seebeck coefficient, and thermal conductivity. The magnetic susceptibility χ(T) shows a ferromagnetic transition at 250 K while the behaviour above the transition temperature is well described by a Curie–Weiss fit with a small effective moment μ_eff = 0.33 μ_B and a paramagnetic Curie–Weiss temperature, θ_CW = +251 K, consistent with previous studies on this compound. However, specific heat, Seebeck coefficient, and thermal conductivity are all dominated by the phonon contribution and show no anomalies at the ferromagnetic transition. Electrical resistivity, unlike the single crystal, shows a huge increase, three orders of magnitude, with decreasing temperature. The temperature dependence of resistivity is logarithmic at high temperatures (210 K<T<350 K), Arrhenius type in intermediate temperatures (110 K<T<190 K) with a small energy gap of Δ = 37 meV, and variable-range-hopping type at low temperatures (35 K<T<80 K), in contrast to the data reported for single crystals. These results can be understood if localized states are present near the Fermi level due to structural oxygen disorder in this low dimensional compound.

The nonlocal dynamical polarizability of two arbitrarily shaped objects coupled by long-range Coulomb forces, when overlap of their electron densities is negligible, is calculated by expressing it in terms of the response functions of separate objects. That enables us to extend the density functional theory approach within the Kohn–Sham scheme to such systems. The method is applied to the system consisting of two metallic slabs within the jellium model. The obtained response function of the total system is used to calculate the correlation energy of a static test charge placed in the system.

The bulk and surface phonon–polaritons in ternary mixed crystals are investigated in the random-element-isodisplacement model and Born–Huang approximation. The numerical results of the polariton frequencies and oscillator strengths as functions of the wavevector and the composition for ternary mixed crystals Al_xGa_1−xAs, Zn_xCd_1−xS, and Ga_xIn_1−xN are obtained and discussed. It is shown that there are three propagated bands separated by two forbidden bands for the phonon–polaritons in bulk materials and two branches of surface phonon–polaritons in semi-infinite systems. The 'two-mode' and 'one-mode' behaviours of phonon–polaritons are also shown in the dispersion curves of bulk and surface phonon–polaritons.

The high-pressure behaviour of scheelite-structured ZrGeO₄ and HfGeO₄ has been investigated with the help of angle-dispersive powder x-ray diffraction measurements. Our results show that these compounds do not undergo any phase transition up to the pressures of 20.7 and 19.0 GPa, respectively. The isothermal bulk modulus and its pressure derivatives are found to be 238 GPa and 4.5 for ZrGeO₄, and 242 GPa and 4.8 for HfGeO₄, implying that these germanates are highly incompressible.

We have studied Heisenberg antiferromagnets on two-dimensional frustrated lattices—triangular and Kagomé lattices, using linear spin-wave theory. A collinear ground state ordering is possible if one of the three bonds in each triangular plaquette of the lattice becomes weaker or frustrated. We study spiral order in the Heisenberg model along with Dzyaloshinskii–Moriya interaction and in the presence of a magnetic field. The quantum corrections to the ground state energy and sublattice magnetization are calculated analytically in the case of a triangular lattice with nearest neighbour interaction. The corrections depend on the DM interaction strength and the magnetic field. We find that the DM interaction stabilizes the long-range order, reducing the effect of quantum fluctuation. Similar conclusions are reached for the Kagomé lattice. We work out the linear spin-wave theory at first with only the nearest-neighbour terms for the Kagomé lattice. We find that the near-neighbour interaction is not sufficient to remove the effects of low energy fluctuations. The flat branch in the excitation spectrum becomes dispersive on inclusion of further neighbour interactions. The ground state energy and the excitation spectrum have been obtained for various cases.

A theoretical electronic structure study is performed on two crystalline phases of the mixed valence, charge ordering Fe₂OBO₃ warwickite and on a V substituted compound, Fe_1.91V_0.09OBO₃, known to exhibit no charge ordering or structural transitions. By using the extended Hückel method applied to the high spin band (hsb) filling scheme, calculations on bulk and on several crystal sub-units have shown that local monoclinic distortions lead to a decrease in Fe–Fe dimer interaction strength. It is suggested that changes in metal–metal interactions between two Fe sub-lattices stabilize local charge arrangements in the monoclinic phase, possibly set in through other effects such as electrostatic long range interactions, discussed in the literature. The importance of electron–lattice interactions in charge localization and structural transition of Fe₂OBO₃ is further corroborated by calculations in the substituted compound, which show that V acts so as to hinder inter-ribbon Fe–Fe interactions. It is also shown that the use of hsb gives very good results for the prediction of charge distribution in the pure and substituted warwickites.

The effect of NiO doping on the structure, magnetic and magnetotransport properties of La_0.8Sr_0.2MnO₃(LSMO)/xNiO has been investigated. Two types of LSMO/xNiO composites were prepared by different processes using the solid-state reaction method. In the first type, the LSMO powders are mixed with small particle size NiO (SA_x samples) and in the second type the LSMO powders are mixed with as-powder NiO (SB_x samples). These studies show that some part of the Ni goes into the perovskite lattice, substituting Mn in LSMO, and the remainder segregates as a separate phase at the grain boundaries and grain surfaces. The presence of NiO at the grain boundaries increases the number of disordered states at the surface of the grains and therefore the low-temperature resistivity increases very quickly with the increase of NiO doping level. Results also show that the NiO doping has an effect on the low-field magnetoresistance (LFMR). The value of magnetoresistance (MR) increases for a low doping level of 0≤x≤3 and decreases at a high doping level of x≥5. The spin-dependent tunnelling and scattering at the interfaces of the grain boundaries are responsible for the increasing of LFMR at x≤3, while reduction of the LFMR at x≥5 originates from substitution of Ni²⁺ for the Mn³⁺ ions, which introduces magnetic dilution into the samples. Also it can be found that, as T<230 K, the MR value of the SB₃ sample is larger than that of the SA₃ sample due to the presence of a larger amount of NiO at the grain boundaries and surfaces.

We report the measurement of the a-axis fluctuation conductivity in zero field for Bi₂Sr₂CaCu₂O_8+x microcrystals. A complete geometrical characterization allows us to determine the absolute value of the excess conductivity and its temperature behaviour with high accuracy. A careful application of the complete fluctuation theory (Varlamov et al 1999 Adv. Phys. 48 655), which implements a minor correction for the factor and a suitable procedure for disentangling the influence of the different fit parameters, shows that data interpretations excluding the Maki–Thompson (MT) process are either not consistent with the crystal structure or not self-consistent. On the other hand, a data interpretation including the MT process appears to be both self-consistent and consistent with experimental measurements of the electron dephasing time τ_ϕ performed in other metallic or semiconducting systems. According to the latter scheme, the anomalous MT term could be a very important contribution to the excess conductivity throughout the temperature range of interest and thus the s-wave symmetry becomes an important component of the order parameter above T_c.

Magnetoelectronic properties of chiral carbon nanotubes and toroids are studied for any magnetic field. They are sensitive to the changes in the magnitude and the direction of the magnetic field, as well as the chirality. The important differences between chiral and achiral carbon nanotubes include band symmetry, band curvature, band crossing, band-edge state, state degeneracy, band spacing, energy gap, and semiconductor–metal transition. Carbon tori also exhibit the strong chirality dependence on the field modulation of discrete states. Chiral carbon tori might differ from chiral carbon nanotubes in energy-gap modulation, density of states, and state degeneracy.

Calculations of Raman spectra were performed in the case of four furanocoumarins: psoralen, bergapten, xanthotoxin, and isopimpinellin. The Raman bands were assigned by using the results for wavevectors also obtained through these calculations. It is shown that an easy distinction between the four compounds can be obtained by the analysis of the higher wavenumber region of the spectrum where the stretching modes of atoms pertaining to the radicals appear. The use of the other spectral regions is possible for this task, but at the expense of some additional reasoning.

This work is to study electric (E)-field-induced and temperature-induced phase transitions in (001)-cut Pb(Mg_1/3Nb_2/3)_0.67Ti_0.33O₃ (PMNT33%) single crystal, which are critical concerns for piezoelectric applications. Dielectric properties and domain structures (by polarizing microscope) are measured as functions of temperature and E field. The hysteresis loop of the polarization versus E field at room temperature is also measured. Without any E-field application, upon heating a first-order-type phase transition sequence rhombohedral (C) takes place near 350 and 430 K, respectively. Under a dc E-field application along [001] at room temperature, [001] tetragonal (T₀₀₁) phase domains are induced by various phase transition sequences, i.e. , , , and , as the E-field strength increases. In addition, E-field-induced microcracking is observed in this work.

The excitation gaps for the two-orbital degenerate Hubbard model are investigated by applying a generalized version of Lieb's spin-reflection positivity. Combining the known exact results on the ground state, and making use of symmetry properties, we rigorously show that, at half-filling, the charge gaps are always larger than the spin-excitation gaps and properly defined orbital gaps.

We present results of anisotropic thermal expansion and low temperature magnetostriction measurements on YNi₂B₂C single crystals grown by high temperature flux and floating zone techniques. Quantum oscillations of magnetostriction were observed at low temperatures for starting at fields significantly below H_c2 (H<0.7H_c2). Large irreversible, longitudinal magnetostriction was seen in both in-plane and along the c axis directions of the applied magnetic field in the mixed superconducting state. Anisotropic uniaxial pressure dependences of T_c were evaluated using results of zero field, thermal expansion measurements.

A hybrid Hartree–Fock and density functional theory, in which Hartree–Fock exchange is mixed with density functional theory exchange functionals, using Beckes' three-parameter method, combined with the non-local correlation functionals by Perdew and Wang, allows us to achieve the best agreement with experiment (11.00 eV) for the BaF₂ band gap (11.30 eV). The characterization of F centres in BaF₂ is still a question of debate. In order to understand the behaviour of the material, we performed ab initio calculations to determine their electronic structure, atomic geometry and formation energy. We also calculated the M centre, the simplest aggregation of two F centres, and the results show that the β band absorption in BaF₂ is predominantly due to the presence of M centres.

The results of x-ray diffraction, ⁵⁷Fe Mössbauer spectroscopy, magnetic susceptibility, and electrical conductivity studies of the metastable icosahedral alloy Al₆₀Cr_19.9Fe_0.1Ge₂₀ are reported. The observed broadening of the diffraction Bragg peaks reflects the presence of the topological/chemical disorder. The distribution of the electric quadrupole splitting derived from Mössbauer spectra indicates the existence of a multiplicity of Fe sites. The average quadrupole splitting decreases with temperature as T^3/2. The vibrations of the Fe atoms are well described by a Debye model, with the Debye temperature of 463(15) K. The temperature dependence of the magnetic susceptibility follows the Curie–Weiss law with the effective magnetic moment of 0.312(3) μ_B per Cr/Fe atom. The origin of the presence of the magnetic moment, and its absence in similar crystalline alloys, is discussed. The temperature dependence of the electrical conductivity can be successfully fitted with the use of theories of quantum interference effects, and values for spin–orbit and inelastic scattering times are extracted from the fits. The scattering process in the studied quasicrystal is dominated by inelastic electron–electron scattering.

We present a study of magnetic field induced quantum phase transitions in insulating systems. A generalized scaling theory is used to obtain the temperature dependence of several physical quantities along the quantum critical trajectory (H = H_C, ) where H is a longitudinal external magnetic field and H_C the critical value at which the transition occurs. We consider transitions from a spin liquid at a critical field H_C1 and from a fully polarized paramagnet, at H_C2, into phases with long range order in the transverse components. The transitions at H_C1 and H_C2 can be viewed as Bose–Einstein condensations of magnons which however belong to different universality classes since they have different values of the dynamic critical exponent. Finally, we use that the magnetic susceptibility is an entanglement witness to discuss how this type of correlation sets in as the system approaches the quantum critical point along the critical trajectory, H = H_C2, .