Table of contents

The electrical resistivity of the high quality single crystals of one-dimensional bis(1,2-benzoquinonedioximato)platinum(II), Pt(bqd)₂, has been studied at low temperatures and high pressures under hydrostatic conditions. The resistivity along the c-axis abruptly decreases with increasing pressure up to 0.9 GPa at room temperature. Pt(bqd)₂ with an energy gap of about 0.3 eV at ambient pressure indicates the insulator-to-metal (IM) transition at around 0.8 GPa. The x-ray diffraction and the electronic spectrum of Pt(bqd)₂ have been studied at room temperature and high pressures. The mechanism of the IM transition of Pt(bqd)₂ is discussed. Below 235 K the resistivity of Pt(bqd)₂ increases with decreasing temperature at around 0.8 GPa. The metal-to-insulator (MI) transition for Pt(bqd)₂ is found at around 235 K under 0.8 GPa. This is the first example that the one-dimensional metal formed from single molecules shows the MI transition at low temperatures.

The screening of sub-surface Si impurities in an accumulation layer at the GaAs(110) surface is calculated. Such an accumulation layer can be induced by a scanning tunnelling microscope tip, and surface Friedel oscillations have been imaged around the Si dopants. This study uses the effective mass approximation to describe the electrons in the GaAs conduction band, and a fitted model potential for the impurity. Two-dimensional effects dominate, with a doubly occupied bound state pulled off the lowest sub-band by the impurity, and a depletion of one electron in the conduction states. The bound state gives a large central peak in the surface induced charge, with the Friedel oscillations coming from the change in the conduction states. To explain the amplitude of the observed oscillations, it is necessary to reduce the tunnelling contribution from the bound state.

Magnetization processes in Ni nanowire arrays are investigated. The wires are produced by electrodepositing Ni into porous anodic alumina and exhibit coercivities of the order of 0.05 T (500 Oe) along the wire axis. Transmission-electron microscopy of freed wires shows that the wires are polycrystalline and resemble a chain of nanocrystallites. To model the hysteresis loops, the wires are treated as one-dimensional random-anisotropy magnets where the magnetocrystalline bulk anisotropy is a weak perturbation to the leading anisotropy contribution. The calculation yields an analytical equation for the magnetization as a function of the applied magnetic field. For small and moderate reversed fields, the agreement between theory and experiment is very good, but the applicability of the model breaks down close to the coercive field. This failure is explained by the neglect of higher-order perturbation terms describing, for example, magnetic localization effects.

A more general formulation of the collective-mode approach is proposed to describe the processes of dynamical polarization in liquids. It is based on a consistent inclusion of non-Markovian terms into the kinetic memory kernels. This has allowed us to reproduce quantitatively the entire frequency dependence of the dielectric functions and identify the primary polarization modes for an interaction site model of water over the whole wavelength range.

The effect of thermal/electrical poling on surface structure and poling-induced second-harmonic generation have been examined for 15Nb₂O₅·85TeO₂ glass. Since the poling was carried out with the 15Nb₂O₅·85TeO₂ glass sandwiched in between two commercial borosilicate glasses containing Na⁺, penetration of Na⁺ into the anode-side surface of 15Nb₂O₅·85TeO₂ glass took place, as revealed by x-ray photoelectron spectroscopy (XPS). XPS studies also suggest that Nb⁵⁺ migrates from the anode-side surface into the bulk by the poling. It is thought that the migration of Na⁺ and Nb⁵⁺ forms an internal dc electric field, leading to the second-harmonic generation.

Photoluminescence, optical absorption and electron spin-resonance results are reported for diamond films homoepitaxially grown by chemical vapour deposition on ⟨100⟩-oriented natural IIa diamond. Measured dependencies of the corresponding signals on the growth temperature in the range 950-2100 °C suggest that while most of the nitrogen is present in our films as single atoms, a small part (about 10^-3 of all of the nitrogen) could be incorporated as N₂ molecules.

We have re-investigated the high pressure behaviour of berlinite AlPO₄ (α-AlPO₄) with x-ray diffraction using a powerful synchrotron x-ray source SPring-8. Our results show that it transforms to a crystalline phase beyond ~13 GPa. Our data seem to be consistent with a CrVO₄ type of structure in the Cmcm space group, similar to the high pressure phase observed in some isostructural phosphate compounds. The persistence of the diffraction pattern up to 40 GPa establishes that the previously accepted amorphization of AlPO₄ around 12-18 GPa is incorrect. Experimental results suggest that the so-called memory glass effect observed earlier may in fact be the reversibility of the α-phase⇔crystalline phase transformation. Comparisons of our experimental and theoretical results raise serious doubts about the theoretical understanding of the high pressure behaviour of α-AlPO₄.

A numerical technique is presented which enables the propagation of wave functions in a three-dimensional complex potential-energy distribution, as required for modelling electron `absorption'. The technique is implemented in a transfer-matrix and Green's-function general procedure to simulate field-emission and electronic projection microscopy. In particular, simulated observations of a small carbon fibre by projection microscopy reveal the effects of absorption on the final images. In the situation considered, absorption is responsible for the reinforcement of the external fringes compared to those located well inside the geometrical projection. In agreement with experimental figures and two-dimensional simulations, it turns out that absorption has to be accounted for to explain the shape of projected images, even if the sample has a low level of opacity.

In situ and conventional room temperature ⁵⁷Fe-site Mössbauer spectroscopy (MS) were used to study the phase separation process in the Fe-Cr alloy system. It was shown that MS is well able to distinguish between (i) nucleation and growth and (ii) spinodal decomposition, the two mechanisms held responsible for the separation. The Fe-rich branch of the spinodal line was located at 80.8⩽x⩽84 at.% Fe at T = 415 °C. It was also shown that the saturation of the average hyperfine field cannot be taken as an indicator of the termination of the separation process. The kinetics of the process dramatically depends on the aging temperature, being faster by a factor of 1.4 at T = 440 °C than at T = 415 °C.

We have measured the absorption spectra of a cleaned MoS₂ natural crystal in high magnetic fields up to 150 T in a Faraday configuration, and we have found Zeeman splitting and a diamagnetic shift of the n = 1 A exciton band. The g-factor and the diamagnetic parameter are calculated to be -4.6 and 1.8×10^-4 meV T^-2, respectively.

Using the full-potential linearized augmented-plane-wave (FLAPW) method, the mechanism of the rutile-CaCl₂ phase transition of RuO₂ and the phase stability of β-PtO₂ are investigated. The local density functional calculations predict quantities such as lattice constants, bulk moduli, and phonon frequencies in good agreement with experiment. The pressure-induced phase transition in RuO₂ appears to be driven by the strong repulsion between O-O and Ru-O along the diagonal direction in the xy-plane. By contrast, a strong hybridization between the Pt 5d and O 2p states is responsible for the stability of the β-PtO₂ structure, for which a pseudo-gap (instead of the strong peak for the rutile-PtO₂) is opened at the Fermi level due to the rotation of the PtO₆ octahedra.

Structural and thermodynamic properties of elemental and bimetallic nanoclusters are studied at the atomic scale. The modelling is achieved by means of molecular dynamics (MD) and Metropolis Monte Carlo (MC) sampling in the so-called transmutational ensemble. The cohesion model used is based on the second moment approximation of the tight binding model. Copper elemental and Ni_xAl_1-x binary alloy clusters are selected as case studies. Particles containing less than n = 201 atoms are predicted to be structureless, except when elemental, formed by n = 13, 55, 135 and 147 atoms. These so-called magic numbers allow icosahedral geometry. Binding energies are not found to be significantly dependent on morphology, suggesting the coexistence of several isomers. As far as Ni_xAl_1-x clusters are concerned, phase stability is systematically studied as a function of x, ranging from 0 to 1 and discussed with reference to the bulk ordered alloy. Except in one special case, and in contrast to elemental clusters, no stable phase at all is found in the smallest clusters (n < 201) as they are structureless. In the larger ones, consistently with a recent study with another cohesion model (Campillo J M, Ramos de Dibiaggi S and Caro A 1999 J. Mater. Res.14 2849), a partition shows up between a core where the bulk stable L1₂ and B2 phases are retrieved and a mantle which may be subjected to aluminium segregation. In the range of cluster sizes considered (n = 13-10 000), the results suggest that, because of the easy surface segregation, the martensitic metastable phase occurring in bulk Ni-Al systems does not take place in free clusters. The segregation efficiency is found to decrease with increasing cluster size while the relative mantle thickness is size independent. This may be the reason why the martensitic phase only occurs in systems larger than currently investigated.

Epitaxial Co/Cu superlattices were grown by molecular beam epitaxy on sapphire substrates and investigated by high-resolution x-ray diffraction at room temperature. Detailed analysis of x-ray scattering along the [10.L] reciprocal-lattice rows revealed that about 80% of Co was hexagonal close packed (h.c.p.) and only 20% was face-centred cubic (f.c.c.). A combination of modelling of x-ray scattering from superlattices with measurements of coherence lengths of individual components of the scattering suggests that the f.c.c. Co grows as a single block at the beginning of Co layers and the growth presumably starts from islands rather than from continuous Co layers. The h.c.p. structure is one-dimensionally disordered by stacking faults which appear after approximately every 12 layers of Co on average. Upon heating in ultrahigh vacuum to 700 °C the coherent superlattice structure was completely destroyed and the resulting structure consisted of relaxed layers of Co and Cu.

Polynomial basis function tails are proposed for band structure calculations. The tails provide only the nearest- and the next-nearest neighbour interactions. The efficiency of the new tails is tested on the models with known exact solutions: the Krönig-Penny potential and 3D empty crystal. The results obtained with the new and exponential tails are compared.

We present a rigorous approach for constructing norm-conserving pseudopotentials within the exact-exchange (EXX) Kohn-Sham density functional formalism. The approach is based on the relativistic EXX scheme within the framework of the optimized potential method. We derive an integral equation that yields the exact local Kohn-Sham exchange potential due to valence electrons. This valence exchange potential is used for constructing ionic pseudopotentials and replaces the standard procedure of unscreening atomic pseudopotentials that is shown to be not applicable in the EXX formalism. The EXX pseudopotentials excellently reproduce the experimental atomic ionization energies and they show a better transferability than the standard pseudopotentials based on the local density approximation (LDA). The relativistic EXX pseudopotentials are shown to lead to the correct ordering of conduction band minima in diamond-structure Ge and reduce the LDA errors in the energetic positions of d bands in solids.

Hydrogen storage in a Pd cluster is studied theoretically using a self-consistent density-functional scheme with a norm-conserving pseudopotential. We mainly focus on the cuboctahedral Pd₁₃H_n clusters with n = 1, 6, 8, and 14. Two stable sites for hydrogen adsorption are found; one is slightly inside the square face of the cluster and the other is outside of the triangular face. Electronic states relevant to hydrogen adsorption are well described by the cluster-centre electronic states (CCSs); this term refers to the states produced by the centre cluster (hydrogen atoms and the centre Pd atom) interacting with peripheral Pd atoms. When the hydrogen atoms are adsorbed inside the tetrahedral face, the cluster is stabilized by expanding the Pd-Pd distance by 3.7%, which is close to the lattice expansion in the β-phase of bulk PdH. As the face-centred-cubic bulk can be fully `clusterized' by these clusters, we estimate the entropy change of the bulk upon transition as the change in configuration entropy between these clusters.

Non-linear optical properties of randomly oriented non-linear spheroidal metal particles in a dielectric host are investigated. Two different Maxwell-Garnett-type approximations are derived based on whether a self-consistency condition on the net polarization is invoked. These two methods lead to quite different spectral density functions. Moreover, we show that the shape of particles has a large influence on the spectral function through the depolarization effects and thereby has a pronounced effect on the optical absorption and non-linear optical susceptibility. We suggest that the self-consistent formalism can be used for large volume fractions, as mutual interaction between different polarizations is included in the theory.

The problem of detection of few-electron states has been studied for spherical quantum dots both with and without a donor centre. The confinement potential is assumed to be a three-dimensional spherical potential well of finite depth and range. The variational method has been applied to calculate bound states of one-, two-, and three-electron systems confined in donor-doped and undoped quantum dots. We have shown that the chemical potentials of the systems considered are significantly different, in contrast to the intraband optical dipole transition energies, which are almost indistinguishable. Therefore, the transport and capacitance spectroscopy techniques should yield clear evidence for the presence of a donor impurity in the quantum dot.

Quantum corrections to the conductivity of a mesoscopic ring in a nonstationary magnetic field with the flux Φ(t) = Vt are investigated. The time dependence of the quantum corrections shows periodic cusps with a period corresponding to the magnetic flux quantum Φ₀ = hc/2e. If the characteristic time of the magnetic flux variation τ₀ = Φ₀/V is much smaller than the phase relaxation time τ_φ, the cusps become asymmetrical.

Electron states in a magnetic quantum ring are investigated. The electron is assumed to be confined to a plane in the presence of a magnetic field which is zero within the ring a<r<b and has a constant value B₀ outside it. In this way, the perturbation caused by this field distribution is found to be quite different from that corresponding to a magnetic quantum disk, as regards electron states and circulating currents. In the case of electron states, the main effect is the splitting of the corresponding Landau levels into a sort of quasi-miniband around them and modulated by the inner radius. It is also found that probability current can change its direction depending on the missing flux quanta, a mechanism that is also governed by the size of the inner radius. Moreover, when the inner radius approaches zero, the results for a magnetic quantum disk are recovered, as expected.

We develop a linear theory of electron transport for a system of two identical quantum wires over a wide range of the wire length L, unifying both the ballistic and diffusive transport regimes. The microscopic model, involving the interaction of electrons with each other and with bulk acoustical phonons, allows a reduction of the quantum kinetic equation to a set of coupled equations for the local chemical potentials for forward- and backward-moving electrons in the wires. As an application of the general solution of these equations, we consider different kinds of electrical contacts to the double-wire system and calculate the direct resistance, the transresistance, in the presence of tunnelling and Coulomb drag, and the tunnelling resistance. If L is smaller than the backscattering length l_P, both the tunnelling and the drag lead to a negative transresistance, while in the diffusive regime (L>>l_P) the tunnelling opposes the drag and leads to a positive transresistance. If L is smaller than the phase-breaking length, the tunnelling leads to interference oscillations of the resistances that are damped exponentially with L.

The projection operators that project the potential energy of electron interactions onto the lowest Landau level of a fractional quantum Hall state are considered for both disk and spherical geometries. The study has been performed by using the Laughlin, the quasi-hole and the quasi-electron wave functions. In the case of spherical geometry, the form of the coordinate-operator transformation obtained in a recent article has been amended. The results obtained are expected to be useful in the calculation of the quasi-hole energy, the quasi-electron energy and the energy gap.

In this article, we present new results of high-order coupled cluster method (CCM) calculations, based on a Néel model state with spins aligned in the z-direction, for both the ground- and excited-state properties of the spin-half XXZ model on the linear chain, the square lattice, and the simple cubic lattice. In particular, the high-order CCM formalism is extended to treat the excited states of lattice quantum spin systems for the first time. Completely new results for the excitation energy gap of the spin-half XXZ model for these lattices are thus determined. These high-order calculations are based on a localized approximation scheme called the LSUBm scheme in which we retain all k-body correlations defined on all possible locales of m adjacent lattice sites (k⩽m). The `raw' CCM LSUBm results are seen to provide very good results for the ground-state energy, sublattice magnetization, and the value of the lowest-lying excitation energy for each of these systems. However, in order to obtain even better results, two types of scheme for extrapolating the LSUBm results to the limit m→∞ (i.e., the exact solution in the thermodynamic limit) are presented. The extrapolated results provide extremely accurate results for the ground- and excited-state properties of these systems across a wide range of values of the anisotropy parameter.

Detailed measurements of the field- and temperature-dependent ac susceptibility and magnetization of LaMnO₃ substituted with 5 at.% of the divalent cation Mg are presented. This system is a semiconducting ferromagnet at low temperature; in particular, analysis of these data yields T_c = 147.2±0.2 K, with critical exponents δ = 4.75±0.15 (from the field dependence of the susceptibility along the `crossover' line, and confirmed by measurements along the critical isotherm), γ + β = 1.75±0.05 (from the temperature dependence of the crossover line) and γ = 1.39±0.05 (from the temperature dependence of the susceptibility along the same line). Within experimental uncertainty these exponent values agree with those predicted by the isotropic, three-dimensional Heisenberg model. Nevertheless this system exhibits some unusual characteristics, specifically in the temperature dependence of the (low-) field-cooled and zero-field-cooled magnetization. The latter have been modelled by a Preisach-based approach, which helps to resolve a ubiquitous dilemma in the doped manganites, viz. the appearance of a technical hardness irreconcilable with low coercive field values. A possible origin for the large, reversible component in the response below T_c is presented.

We report the first electron paramagnetic resonance studies of single crystals and powders of Pr_0.6Ca_0.4MnO₃ in the 300-4.2 K range, covering the charge-ordering transition (T_co) at ~240 K and antiferromagnetic transition (T_N) at ~170 K. The asymmetry parameter for the Dysonian single-crystal spectra shows an anomalous increase at T_co. Below T_co the g-value increases continuously, suggesting a gradual strengthening of the orbital ordering. The linewidth undergoes a sudden increase at T_co and continues to increase down to T_N. The intensity increases as the temperature is decreased until T_co is reached, due to the renormalization of the magnetic susceptibility arising from the build-up of ferromagnetic correlations.

The proton nuclear magnetic resonance (T₁, T₂, linewidth) is measured for lanthanum hydrides LaH_x (2.0⩽x⩽3.0) at temperatures between 4 K and 300 K and Larmor frequencies of 300 MHz and 50 MHz. In addition to the resonance line narrowing and the increase of the spin-lattice relaxation rate for temperatures above 200 K, originating from the well established type of thermally activated proton hopping motion in lanthanum hydrides, a low-temperature motion-induced relaxation anomaly is derived for temperatures below 40 K. Thus the Korringa relaxation contribution in metallic LaH_2.0 is most reliably separated in the 40-200 K temperature range.

Raman scattering observations were performed on samples of K_1-xLi_xTa_1-yNb_yO₃, with x in the range 0.6-1.0 mol% and y in the range 0.2-0.3 mol%. Concentrations of Li and Nb, determined by analytical methods, were below the known thresholds at which these substituents can separately induce a ferroelectric transition in the quantum paraelectric KTaO₃. The temperature evolution of TO₁ and TO₄ spectra was monitored. The results show a critical behaviour below 50 K, characterized by an abrupt increase of first-order scattering strengths and by spontaneous splitting of the non-softening TO₁ mode. From our data, we infer the occurrence of a ferroelectric phase transition of order-disorder character. This proves that small additions of Nb enhance the tendency of the Li subsystem towards cooperative ordering in the KTaO₃ matrix.

Using a pump-probe technique, absolute excited state absorption cross sections are measured in various Er³⁺ doped laser crystals in a wavelength domain extending from 0.53 to 2.9 µm. The gain cross section around 2.8 µm and the population ratio between the involved levels ⁴I_11/2 and ⁴I_13/2 are measured for different Er³⁺ concentrations and pumping wavelengths. The cross sections for the transitions involved in the upconversion pumping processes of the characteristic green emission of Er:LiYF₄ are also estimated and discussed. A comparison is made between these results and those obtained semi-theoretically by using the Judd-Ofelt formalism.

Excitation and luminescence spectra of Pr³⁺ in GdOCl were measured in the spectral range from 11 000 cm^-1 to 22 500 cm^-1. At ambient pressure a total of 54 spectral lines were observed. Some of these lines cannot be explained by transitions between crystal-field levels of the regular Pr³⁺ site in GdOCl. Possible explanations of these lines are discussed. Under pressure up to 16 GPa, all lines exhibit a red-shift, from which a part of the energy level scheme of the 4f² configuration could be derived. From the 34 levels at ambient pressure, 31 could also be obtained under pressure. The energy level scheme is described in terms of the free-ion parameters (F^k, α, β, γ, ζ, M_k, P^k) and crystal-field parameters (B²₀, B⁴₀, B⁴₄, B⁶₀, B⁶₆). The variations of these parameters under pressure were determined. Surprisingly, the crystal-field strength, calculated from the crystal-field parameters, first shows a slight decrease with increasing pressure.

We have measured high-resolution Ru K-edge x-ray absorption spectra for the intermetallic La_1-xCe_xRu₂ system. Multiple-scattering calculations are made to explore the origins of different features observed in the experimental x-ray absorption near-edge structure (XANES) and their variation with Ce doping. The experiments and theoretical analysis demonstrate that the Ce doping in La_1-xCe_xRu₂ has a direct influence on the states at the Fermi level and hence the hybridization between the f and d states induced by doping.

A comparative study of the atomic dynamics of a-Li₂Si₂O₅ and pure SiO₂ glass was performed using neutron inelastic scattering and applying the isotope substitution technique to suppress the coherent scattering from the network modifying Li atoms. The generalized vibrational density of states G(ω) determined in the full energy range up to 150 meV differs clearly in the high energy range and also at low energies. A strong broadening and a shift to lower energies of the sharp lines in the spectrum of SiO₂ is observed in the spectrum of the modified network glass. A detailed analysis of the measured dynamic structure factor S(Q,ω) is used to determine the change of the SiO₂ network force constants by the network modifier, which again is used to support a specific model of the modification in the Si-O network.