Table of contents

We study both theoretically and experimentally the propagation of liquid surface waves over a periodically drilled bottom. A slab of a plate drilled with a triangular array of holes is placed in the middle of a liquid vessel. With the presence of point source waves generated on the one side of the plate, we can obtain propagative information of liquid surface waves through the observations of the outgoing waves on the other side of the plate. Our experiment confirms our previous theoretical predictions of the existence of complete band gaps for liquid surface waves propagating over a periodically drilled bottom.

Semiconductor nanostructures in biological applications are discussed. Results are presented on the use of colloidal semiconductor quantum dots both as biological tags and as structures that interact with and influence biomolecules. Results are presented on the use of semiconducting carbon nanotubes in biological applications.

A combination of classical molecular dynamics (MD) and ab initio Car–Parrinello molecular dynamics (CPMD) simulations is used to investigate the adsorption of water on a free amorphous silica surface. From the classical MD, SiO₂ configurations with a free surface are generated which are then used as starting configurations for the CPMD. We study the reaction of a water molecule with a two-membered ring at the temperature T = 300 K. We show that the result of this reaction is the formation of two silanol groups on the surface. The activation energy of the reaction is estimated and it is shown that the reaction is exothermic.

In this work we present scanning tunnelling microscopy (STM) imaging and spectroscopy of a highly p-doped wide bandgap semiconducting 4H-SiC(0001) surface. Whereas n- and p-doped 6H-SiC or n-doped 4H-SiC surfaces can be relatively easily imaged with the STM, the p-doped 4H-SiC cannot be imaged due to the absence of any surface conductivity. This is very surprising given the presence of a p-doped, degenerate epitaxial layer. The behaviour can be explained by the formation of a Schottky barrier either between the tip and the surface or between the surface and the sample holder, depending on the polarity of the applied voltage. We found that prolonged and repeated exposures of the SiC surface to a Si atomic flux followed by thermal annealing are required before the surface conductivity is sufficient to allow STM images to be recorded. The result is the deposition of overlayers of Si, with structures similar to Si(111) 7 × 7, Si(113) 3 × 2, and Si(110) 16 × 2 rather than the expected stable SiC(0001) 3 × 3 reconstruction. We have further demonstrated the ability of scanning tunnelling spectroscopy to distinguish between the Si and the SiC phases based on the difference in their bandgaps.

We performed ¹¹⁹Sn Mössbauer spectroscopic measurements on Gd/Cu multilayers with ¹¹⁹Sn probing atoms. The Cu layers are delta-doped with Sn probes at a certain depth. Hyperfine fields are observed in the Mössbauer spectra at low temperatures below the Curie temperature of the Gd layers. From the temperature-dependent measurements, the hyperfine field is found to be proportional to the spontaneous magnetization of the Gd layer. The result indicates that the Gd magnetization induces magnetic polarization in the Cu layers. The hyperfine field per Gd magnetization is found to be ∼40 kOe /μ_B(Gd), regardless of the position of the ¹¹⁹Sn atoms and the Cu thickness (20–40 Å), suggesting that the magnetic polarization in the Cu layer is uniform at any depth from the Gd/Cu interface.

Epitaxially grown V₂O₃(0001) thin films have been prepared with different surface terminations, as evidenced by atomically resolved scanning tunnelling microscopy and high-resolution electron energy loss spectroscopy (HREELS) phonon spectra. The spectral changes observed in valence band photoemission spectra and HREELS on cooling the V₂O₃ samples from 300 to 100 K have been associated with the metal–insulator transition (MIT) in the bulk of the V₂O₃ film. The reconstructed surface regions per se do not display the MIT, but affect the MIT signature observed with surface sensitive techniques, depending on the thickness of the reconstructions. Whereas the thermodynamically stable (1 × 1) vanadyl V = O surface termination allows the observation in photoemission and HREELS of a clear signature of the MIT, the latter is screened on a surface formed by V = O defect structures. Doping of the surface with small amounts of adsorbed water restores reversibly the MIT spectral fingerprints. These observations are discussed in terms of the different geometrical and electronic structures of the different surface terminations.

Transmission and reflection of light in disordered two-dimensional photonic crystals with an incomplete photonic band-gap have been modelled for various levels of disorder. It is found that ballistic and scattered light display different behaviours as a function of the disorder parameter. For ballistic light, the dependence of the transmission coefficient on the disorder parameter exhibits a threshold-like behaviour, whereas the transmission of scattered light increases rapidly for small disorder. Unlike for photonic crystals with a complete band-gap, the minimum of the transmission for scattered light does not coincide with the centre of the photonic band-gap, and Fabry–Perot-type oscillations can be seen within the photonic band-gap.

The structural and electronic properties of the Li_1+xV₃O₈ insertion electrode system with nominal range of have been explored through measurements of x-ray diffraction, magnetization and nuclear magnetic resonance (NMR) for ⁷Li. The system prepared electrochemically exhibits two kinds of phase depending on the composition: the γ1-phase with has the octahedral (Li1) and tetrahedral (Li2) coordinations for the Li sites, and the γ2-phase with has the octahedral coordination alone. These phases coexist for 1.5<x<3.5. Application of the one-dimensional chain model to the susceptibility data for the γ2-phase provides reasonable results for the Li concentration and the exchange constant. For x = 0, the quadrupole parameters are determined precisely. Both the γ1-phase and the γ2-phase indicate the activation energies for the long-range motion of Li ions to be of 3 × 10³ K at temperatures above 300 K, and the γ2-phase has another narrowing process below 100 K. Magic-angle spinning (MAS) NMR spectra for the γ1-phase reflect the site differentiation for Li; the Li1 shift originates from a negative spin transfer, while the effect for Li2 is less significant. The spin-lattice relaxation for x = 0 is explained with the quadrupole interaction mechanism. For Li_1.25V₃O₈, which exhibits variable-range hopping-type transport anomalies at  K, the phase transition for the Li local environments takes place at around T_c. The spectra for the γ2-phase are different from those for the γ1-phase, and the complicated relaxations likely originating from Li ions located in the cathode and the acetylene black, and the boundary between these substances, are observed.

The microstructure evolution in PtMn-based bottom spin-filter spin valves was investigated to clarify the alteration of the sensor performance caused by different annealing treatments during manufacturing. Neither unidirectional field annealing (UDA) nor stress annealing (SA) for 38 h, which simulates the final quality test of the finished read-write head, have any significant influence on the texture of the sensor stack. UDA causes the appearance of domain boundaries in the PtMn layer as a consequence of the fcc–fct transformation together with some disturbance in the functional layers of the sensor, neither of which affect the giant magneto-resistive performance. The SA treatment causes an inter-diffusion in the NiFeCr/NiFe seed layers, some grain growth in PtMn, more serious disturbance of the functional layers, and complete amorphization of the Ta cap layer. In comparison to other studies of spin valves, the intermixing of seed layers in the present study does not appear to influence the PtMn/CoFe interface. The alteration of the sensor performance seems to be mainly caused by the inter-diffusion of the functional layers.

The structural and electronic properties of calcium chalcogenides CaX (X = S,Se,Te) under high pressure have been investigated using the full potential linearized augmented plane wave method within density functional theory. We used both the local density approximation and the generalized gradient approximation (GGA) that is based on exchange–correlation energy optimization for calculating the total energy. Moreover, the Engel–Vosko GGA formalism is applied so as to optimize the corresponding potential for band structure calculations. The equilibrium lattice constant for CaX compounds agrees well with the experimental results. The pressures at which these compounds undergo a structural phase transition from NaCl-type to CsCl-type were calculated. A numerical first-principles calculation of the elastic constants was used to calculate C₁₁, C₁₂ and C₄₄. The energy band gaps at ambient conditions in the NaCl-type structure and the volume dependence of band gaps in the CsCl-type structure up to the band overlap metallization were investigated. Besides this, the nature of the chemical bond in these compounds was analysed in terms of electronic charge density.

The density-matrix renormalization group (DMRG) is employed to calculate optical properties of the half-filled Hubbard model with nearest-neighbour interactions. In order to model the optical excitations of oligoenes, a Peierls dimerization is included whose strength for the single bonds may fluctuate. Systems with up to 100 electrons are investigated, their wavefunctions are analysed, and relevant length scales for the low-lying optical excitations are identified. The approach presented provides a concise picture for the size dependence of the optical absorption in oligoenes.

The electronic structure as a function of the chirality and deformations for various carbon nanotori is theoretically investigated by the tight-binding method. It has been found that for the various metallic tori the deformation-dependent energy gap displays almost the same changing features versus the deformations, whereas for the various semiconducting tori the deformation-dependent energy gap does not show the distinctive regularity associated with the deformed parameters and geometric parameters, but if the deformed parameters have some particular values the energy gap would be narrowed and even approach zero, causing the semiconducting torus to be quasi-metallic. Under the circumstances of tanα = 0 and ε_L = ε_J, the electronic structure seems to be very insensitive to the existence of deformations.

In this paper, we present a spintronic tunnelling theory for ferromagnet/insulator/ferromagnet (FM/I/FM) junctions. With the use of Airy functions, it can analytically account for both the low-bias and the high-bias tunnelling magnetoresistances (TMRs). We find that the sign-change behaviour of TMR can only occur in the low-bias region, due to the quantum coherence in FM/I/FM junctions. In the high-bias region, the TMR will oscillate between positive and negative with increasing bias voltage. Physically, this oscillation arises from the interference between the incident and reflected electron waves in the barrier region. The effects of the barrier height, the barrier width and the electron effective mass in the barrier are studied systematically. The theoretical results obtained from the exact Airy functions agree well with TMR experiments on Ta₂O₅- and Al₂O₃-barrier junctions, within the whole measurable range of bias voltage.

The evolution of the specific-heat anomaly in the overdoped range of a single crystal of the high-temperature superconductor YBa₂Cu₃O₇ has been studied under the influence of pressure up to 10 GPa, using AC calorimetry in a Bridgman-type pressure cell. We show that the specific-heat jump as well as the bulk T_c are reduced with increasing pressure in accordance with a simple charge-transfer model. This new method enables us through pressure-induced charge transfer to study the doping dependence of the superconducting transition, as well as the evolution of the superconducting condensation energy on a single stoichiometric sample without adding atomic disorder.

In this paper we would like to show the applicability of phonon-assisted tunnelling theories for explanation of temperature-dependent current–voltage (I–V) characteristics of diodes based on organic thin films of conjugated polymers, such as poly[2-methoxy, 5-(2'-ethyl-hexyloxy)-1,4-phenylene-vinylene] (MEH-PPV). For this purpose the I–V characteristics measured for MEH-PPV by Campbell et al (1997 J. Appl. Phys.82 6326), Lupton and Samuel (1999 J. Phys. D: Appl. Phys.32 2973), and van Woudenbergh et al (2001 Appl. Phys. Lett.79 1697) are compared with the free carrier generation rate dependence on field strength using the phonon-assisted tunnelling theories. A strong dependence of charge carrier mobility on temperature is also explained by this model. The presented model allows us not only to explain the temperature variation of I–V and mobility data, but also to estimate the density of traps taking part in the current flow.

The magnetic and structural properties of Fe–Pt nanocomposites and related idealized structures have been investigated by a combination of experimental and theoretical techniques. The dependence of magnetocrystalline anisotropy (MCA) of L1₀ FePt on the ratio of the tetragonal lattice parameters, c/a, has been calculated with a relativistic version of the full potential local orbital method, assuming complete chemical order and fixed unit-cell volume. It has been found that the well known tetragonal lattice distortion in this phase has a relatively small influence on the MCA (compared to the influence of chemical ordering) and even reduces the MCA. The calculated in-plane anisotropy is negligible. The structure, magnetic properties and magnetization reversal processes of Fe_100−xPt_x (x = 40, 45, and 50) powders produced by mechanical milling and subsequent annealing have been investigated. Structural studies reveal that upon annealing of the as-milled powders consisting of fine Fe/FePt(A1)/Pt lamellae, chemically highly ordered L1₀ FePt and, in the case of the Fe-rich compositions, L1₂ Fe₃Pt are formed. The nanometre scale multilayer structure preserved after annealing gives rise to large effects of exchange interactions between the crystallites of the phases. With decreasing Pt concentration x, the remanence enhancement increases, due to the increase of the Fe₃Pt fraction, whereas the coercivity and the switching fields for irreversible magnetization reversal are reduced.

The electronic and magnetic properties of four series of rare earth manganates of the general formula La_0.7−xLn_xBa_0.3MnO₃ (Ln = Pr, Nd, Gd or Dy) have been investigated to examine the effect of large size disorder in a system where the average radius of the A-site cations, , remains high (1.216–1.292 Å) and the Mn³⁺/Mn⁴⁺ ratio is kept constant. The size disorder, as measured by the Attfield σ² parameter, has been varied over a wide range of 0.001 and 0.03 Å². As x is increased, the materials exhibit a decrease in the ferromagnetic Curie temperature, T_c, or lose ferromagnetism entirely. This is accompanied by an insulator–metal (I–M) transition, with T_IM decreasing with increasing x, or an entirely insulating behaviour. The insulating behaviour and loss of ferromagnetism occurs when σ² is close to 0.02 Å². Thus, in the Ln = Nd, Gd and Dy series, the non-magnetic insulating behaviour occurs at x values of 0.7, 0.5 and 0.3 respectively. Where an I–M transition occurs, T_IM< T_c, indicating the presence of a ferromagnetic insulating regime. The absence of long-range ferromagnetism in some of the compositions is accompanied by a divergence between the zero-field-cooled and the field-cooled magnetization data. The ferromagnetic or non-magnetic insulating state is due to phase separation wherein ferromagnetic clusters are present in an insulating matrix. The non-magnetic insulating compositions can be rendered ferromagnetic and metallic by decreasing σ² while keeping constant. This extraordinary display of the effect of size disorder on the properties of the rare earth manganates is noteworthy.

The properties of Nd_1−xBa_xCoO₃ (0≤x≤0.24) solid solutions were studied at non-ambient temperatures by x-ray and neutron powder diffraction methods as well as magnetization and resistivity measurements. All the samples have an orthorhombically distorted perovskite structure (Pbnm space group). It was found that compositions with x≤0.06 exhibit paramagnetic behaviour down to liquid helium temperature, and those with 0.08≤x≤0.16 are spin glasses, whereas both long-range ferromagnetic order ( K) and metallicity above T_C seem to occur simultaneously at x≥0.18. According to neutron diffraction data at different temperatures (T = 3–540 K), there are no crystal structure anomalies associated with orbital ordering of the Co³⁺ ions in the intermediate-spin state for NdCoO₃. This is in agreement with gradual thermal excitation of the Co³⁺ ions from low- to intermediate- or high-spin state in a wide temperature range up to a metal–insulator transition (∼600 K). The possible mechanisms of magnetic interactions are discussed.

Mn L_3,2-, K-, O K- and Pr M_5,4-edge x-ray absorption near-edge structure (XANES) spectra of (La_1−xPr_x)_0.85Zr_0.15MnO₃ (LPZMO) with x = 0, 0.05, 0.1, 0.15 and 0.2 were measured to study the electronic structure and the effect of Pr substitution. An analysis of Mn L_3,2- and K-edge spectra indicates that LPZMO is an electron-doped system. Pr substitution is found to increase the average effective charge of Mn ions and to induce an additional unoccupied majority spin ( spin) e_g subband in the Mn L_3,2-edge XANES spectrum. Mn L_3,2-, O K- and Pr M_5,4-edge XANES results demonstrate that the substitution of La ions by smaller Pr ions changes O 2p–Mn 3d and O 2p–Pr 4f hybridized states. Extended x-ray absorption fine structure (EXAFS) analysis at the Mn K edge reveals clear changes in local atomic structure around Mn ions due to the Pr substitution. XANES and EXAFS analyses show a small distortion in the MnO₆ octahedron or a disorder in the lattice, which may be due to localization of charge carriers and/or the increased effective positive charge of Mn ions.

In this paper we present a method to control the electronic position in a quantum dot array coherently with a two-electron model. In this novel phenomenon, we find that the external ac driving field has a good manipulative effect on the tunnelling, and therefore the penetrability of the barrier can be easily adjusted. Through studying the Floquet spectrum of the system, we find that it can be divided into two minibands. The upper miniband describes the physical process of states in which the two electrons stay in the same dot and the lower miniband describes the states in which the two electrons stay in different dots. The parameter regions in which the manipulation of tunnelling is investigated exactly correspond to the collapse zones of quasi-energy levels.

The decomposition of the metastable β₃ phase and its influence on the martensitic transformation (MT) has been analysed in a Cu–14.37Al–4.2Ni (wt%) shape memory alloy processed by powder metallurgy by differential scanning calorimetry, resonant ultrasound spectroscopy and in situ neutron powder diffraction. The pro-eutectoid γ₁ precipitation, the L2₁ to B2 disordering process and the eutectoid decomposition have been observed during heating, followed by its subsequent dissolution at higher temperatures. The γ₁ precipitation limits high-temperature applications. The evolution of the kind of induced martensite, hysteresis and transformation temperatures has been analysed during the precipitation of the γ₁ phase. The elastic stresses stored during quenching and the ordering degree play an important role in the MT evolution below 300 °C. At higher temperatures, the γ₁ precipitation process induces an increase of the MT transformation temperatures and hysteresis.