Table of contents

The compound CeNi₉Si₄ has been recently reported to be an unusual Kondo lattice with a large Ce–Ce separation, with a breakdown of the Kadowaki–Woods relationship governing the low-temperature electrical resistivity and heat capacity. Here we report the results of magnetic susceptibility, heat capacity and electrical resistivity of the solid solution Ce_1−xLa_xNi₉Si₄, to understand the Kondo behaviour of this compound. The results establish that the observed properties of Ce in this compound are single-ionic in character.

The selective composition control of the electron wavefunction in a GaAs/InGaAs double quantum well device is presented for two different gating schemes. In particular, electron-beam defined surface gates schemes allow the definition of non-ballistic quasi-one-dimensional conduction channels in each of the quantum wells and result in the ability to electrostatically move the electron wavefunction between the two materials. The use of such a device as the basis for a spin qubit, due to the differing g-factors, and the investigation of other spin-related phenomena in one-dimension are discussed.

A novel experimental approach for studying the gas response mechanism of semiconducting gas sensor materials is demonstrated using the example of water adsorption on SnO₂(101). In this approach, valence band photoemission as a chemical probe is combined with photocurrent induced surface charging as a basis for contactless sample conductivity measurement.

We review recent progress in the growth and characterization of Si_1−xGe_x islands and Ge dots on (001) Si. We discuss the evolution of the island morphology with Si_1−xGe_x coverage, and the effect of growth parameters or post-growth annealing on the shape of islands and dots. We outline some of the structural, vibrational, and optical properties of Si_1−xGe_x islands and review recent advances in the determination of their composition and strain distribution. In particular, we present an analytical electron transmission microscopy study of the Ge spatial distribution in Ge dots and Si /Si_1−xGe_x island superlattices grown by molecular beam epitaxy and ultra-high vacuum chemical vapour deposition. We describe the use of undulated Si_1−xGe_x island superlattices for infrared detection at telecommunication wavelengths. Finally, we discuss various approaches currently being investigated to engineer Si_1−xGe_x quantum dots and, in particular, control their size, density, and spatial distribution. As examples, we show how C pre-deposition on Si(001) can influence nucleation and growth of Ge islands and how low temperature Si homo-epitaxy can lead to a particular surface cusp morphology that may promote dot nucleation.

In this paper we report on a comparative ab initio study of the adsorption of ethylene, cyclopentene and 1-amino-3-cyclopentene on the silicon surface. Accurate calculations of the reaction path have been carried out using a cluster model for the surface dimer (Si₉H₁₂) and Gaussian-type basis functions. The dependence of the computed reaction path on the theoretical method is investigated; activation energies turn out to be quite independent of the method, and general trends can be found for the three systems studied: the larger difference is found between linear and cyclic alkenes. Periodic calculations with plane waves are also performed on periodic slabs, finding an adsorption energy in fair agreement with the cluster model. The surface band structure is carefully studied: a strong dispersion is found along some directions for highly covered surfaces, in good agreement with the experimental data available for ethylene. Also in this case, differences arise when passing from linear to cyclic alkenes, while the second substituent on the cycle has very limited effects.

We present a microscopic theoretical description of spatially resolved photoluminescence in GaAs quantum wells with interface roughness. The theory derives the kinetic equations using the excitonic wavefunctions obtained by solving numerically the effective Schrödinger equation for the excitonic centre of mass motion in the presence of disorder. The kinetic equations describe acoustic phonon scattering, radiative decay, and inhomogeneous sample excitation and/or light detection. The influence of disorder, temperature, and spatial resolution on the image formation is analysed with emphasis on the role of different interface textures. In particular, we consider two samples characterized by effective disorder potentials with different correlation lengths. Numerically calculated two-dimensional images agree with images from spatially resolved photoluminescence experiments and put forward the potential of the method for the understanding of near-field light emission from semiconductor quantum structures.

Submonolayers of nickel oxide were grown on the Ag(001) surface by evaporation of Ni in the presence of O₂ at a pressure of 1 × 10⁻⁶ mbar. In the early stages of deposition, two-dimensional oxide islands with a (2 × 1) periodicity grow on Ag(001). After annealing at about 500 K, the (2 × 1) single atomic layer transforms into NiO(001) double layer islands. The 1:1 stoichiometry of the film is not influenced by whether the annealing is carried out in vacuum or in the presence of oxygen. However, the oxide islands exhibit different morphologies depending on the O₂ pressure during the annealing process.

The atomic structure of nanocomposite Fe/Co films has been studied using synchrotron radiation. Films in which Co nanoclusters are embedded in an Fe matrix, and films where Fe nanoclusters are embedded in Co, are both investigated. The samples were prepared by co-deposition using a gas aggregation cluster source. Co and Fe K edge EXAFS measurements were used to probe the atomic structure within the Co and Fe nanoclusters respectively as a function of cluster volume filling fraction. In an Fe matrix, the Co clusters are found to adopt a bcc structure across the composition range investigated; at the highest cluster filling fraction of nearly 40%, the atomic structure still does not revert to hcp, the crystal structure in bulk Co. Fe nanoclusters embedded in Co retain the bcc structure of bulk Fe.

An electron diffraction study performed on thin sections of Ca_xSr_1−xTiO₃ ceramics with compositions x = 0.2 and 0.5 has revealed diffraction patterns that are inconsistent with currently accepted space group symmetries. Here, the data are presented and alternative models suggested. It is proposed that Ca_xSr_1−xTiO₃ has the space group C 2/m at x = 0.2 and the space group P 2₁/m across the range 0.2<x<0.6. The sequence of phases across the solid solution is therefore proposed to be

α-GaO(OH) thin films obtained by the sol–gel process have been characterized. An x-ray diffraction study confirmed that the films were crystalline with orthorhombic structure. The average crystallite size of the thin films was about ∼2–3 nm as revealed by transmission electron microscope and x-ray diffraction studies. The optical transparency of the film was about 95% in the visible region. The films exhibited an optical semiconducting band gap of 5.27 eV, which was much higher than those of α-Ga₂O₃ and β-Ga₂O₃. The type of the optical transition was determined: it was found to be allowed direct in nature. No degradation of the films was observed after more than a year of exposure to an ambient atmosphere.

Copper oxide (CuO) thin films were deposited using a reactive DC sputtering method for gas sensor applications. The structure of the films determined by means of an x-ray diffraction method indicates that the phase of copper oxide can be synthesized in the total pressure and temperature ranges of 6–8.5 mbar and 151–192 °C, respectively. The resistivity of the film synthesized at a substrate temperature of 192 °C increases from 0.104 to 0.51 Ω m after absorbing CO₂ gas at 135 °C. The gas sensitivity of the film synthesized at the substrate temperature of 192 °C increases up to 5.1 in the presence of CO₂ gas at 160 °C. The gas sensitivity in the presence of N₂ gas reaches only 1.43 even at 200 °C.

Pressure-related structural properties of CaCrO₄ with zircon-type structure (I4₁/amd) at ambient pressure were investigated using synchrotron radiation x-ray diffraction in energy-dispersive mode in a diamond anvil cell (DAC) up to 29.1 GPa at room temperature. A sluggish first-order structural phase transition to a scheelite-type phase (I4₁/a) was observed at 5.7 GPa. The lattice parameters of both phases at different pressures were calculated. The isothermal Birch–Murnaghan equation of state (EOS) was used to fit the data of pressure versus unit cell volume, and the bulk moduli were simulated to be 103.7 ± 0.3 and 125.1 ± 6.2 GPa for the zircon and scheelite phase, respectively.

An investigation of the pressure induced phase transition from the scheelite phase (I4₁/a,Z = 4) to the fergusonite-like phase (I 2/a,Z = 4) and to the LaTaO₄ phase (P 2₁/c,Z = 4) of LiYF₄ is presented. Employing density functional theory (DFT) within the generalized gradient approximation, the structures were relaxed for a pressure range of 0–20 GPa without imposed symmetry. The influence of pressure on the lattice vibrational spectrum of the scheelite phase (I4₁/a,Z = 4) was evaluated using the direct approach, i.e. using force constants calculated from atomic displacements. This work tends to confirm the transformations . At 20 GPa, a P 2₁/c structure with a pentacoordinated lithium cation is found to be the most stable phase. This structure is compatible with a transition driven by a B_g zone-centre soft optic mode linked to a soft acoustic mode along the [] direction as observed from the evolution of the phonon dispersion curves as a function of pressure.

Quantum dynamics of in-plane Josephson junctions between two d-wave superconducting films is described by the anisotropic XY-model, where both quasiparticle and Cooper pair tunnelling terms appear to be equally non-local. Applying a combination of the weak and strong coupling analyses to this model, we find compelling evidence of a dissipative phase transition. The corresponding critical behaviour is studied and contrasted with that found previously in the conventional (s-wave) Josephson junctions.

An analytical approach to the one-dimensional spinless Holstein model is proposed, which is valid at finite charge-carrier concentrations. Spectral functions of charge carriers are computed on the basis of self-energy calculations. A generalization of the Lang–Firsov canonical transformation method is shown to provide an interpolation scheme between the extreme weak-coupling and strong-coupling cases. The transformation depends on a variationally determined parameter that characterizes the charge distribution across the polaron volume. The relation between the spectral functions of the polaron and electron, the latter corresponding to the photoemission spectrum, is derived. Particular attention is paid to the distinction between the coherent and incoherent parts of the spectra, and their evolution as a function of band filling and model parameters. Results are discussed and compared with recent numerical calculations for the many-polaron problem.

Transition metal containing ZnO powders (Zn_1−xM_xO, 0≤x≤0.30; M = Ni, Mn, Co) have been synthesized by a sol–gel process using zinc acetate dihydrate, respective acetate and oxalic acid as precursors with ethanol as a solvent. The process essentially involves gel formation, drying at 80 °C for 24 h to provide the oxalate, and calcination at 500 °C for 2 h to undergo an exothermic reaction and yield Zn_1−xM_xO powder. Their XRD patterns correspond to a wurtzite hcp structure similar to that of pure ZnO, but with the lattice parameters varying slightly with type and extent of doping. It is shown that the dissolution of nickel and cobalt in ZnO is less than 10 at.%, whereas that of manganese lies between 10 and 15 at.%. Other phases that emerge include NiO (hexagonal, a = 2.954 Å, c = 7.236 Å), ZnCo₂O₄ (cubic, a = 8.094 Å) and ZnMnO₃ (cubic, a = 8.35 Å) in the Ni, Co and Mn containing ZnO systems, respectively. Observations of hysteresis loops both at 10 and 320 K and the nature of ESR spectra provide evidence for the ferromagnetic state in nickel containing ZnO powder. Besides, the deviation occurs in the magnetization versus temperature curves in zero field cooled (ZFC) and field cooled (FC) conditions (blocking temperature T_B being 32 K for 5 at.% Ni). The magnetic behaviour of manganese and cobalt doped zinc oxide is, however, different, namely, (i) no hysteresis loops, (ii) decrease in magnetization with increase of Mn or Co content, and (iii) identical M–T curves under ZFC and FC conditions. The inverse susceptibility versus temperature curves of Zn_1−xMn_xO compounds reveal ferrimagnetism with Néel temperature T_N of 4 K for x = 0.02, but antiferromagnetism for x = 0.15 and 0.25 with Curie–Weiss temperature of −43 and −30 K, respectively.

The coherent Hall effect of charge carriers is the quantum mechanical analogue of the classical Hall effect for the case where the charge carriers are a coherent ensemble of wavepackets. Kosevich considered this effect for the first time for semiconductor superlattices subjected both to an electric field along the growth direction and to a perpendicular magnetic field (Kosevich Y A 1999 Ann. Phys. 8 SI-145; 2001 Phys. Rev. B 63 205313). The dynamics of the charge carriers can be described successfully by means of a semiclassical model mapping the equation of motion to a simple pendulum equation with the deflection angle in the case of the pendulum being replaced by the k-vector of the wavepackets along the superlattice's growth direction. While Kosevich has presented an analytical treatment of the equation of motion, we numerically solve it and compute the real-space and k-space trajectories of the charge carriers. We thus arrive at a more illustrative and detailed analysis of the wavepacket dynamics and make predictions for optical pump–probe experiments which detect either the terahertz radiation emitted by the oscillating charge carriers or the internal electric field as a function of time after impulsive excitation of charge carriers.

Dielectric constant and ionic conductivity measurements have been performed in SrAlF₅ single crystals, along the [100] and [001] directions, as functions of frequency (10 Hz–10 MHz) and temperature (300–800 K). The real part of the ac conductivity showed the 'universal dynamic response of dielectrics', evidenced by a power-law dependency on the frequency. A first-order phase transition was observed around 715 K, with a considerable thermal hysteresis in the real part of the dielectric constant and in the electrical conductivity. The ionic conductivity varies by about four orders of magnitude in the temperature interval studied and shows various regions with Arrhenius behaviour with activation energies ranging from 0.2 to 1.40 eV.

Three metallic dodecaborides, YB₁₂, ZrB₁₂ and LuB₁₂, have been investigated by electric-field gradient (EFG) measurements at the boron sites using the ¹¹B nuclear magnetic resonance (NMR) technique and by performing first-principles calculations. The NMR powder spectra reveal patterns typical for a completely asymmetric EFG tensor, i.e., an η parameter close to unity. The absolute values of V_zz (the largest component of the EFG) are determined from the spectra and they range between 11 × 10²⁰ V m⁻² and 11.6 × 10²⁰ V m⁻² with an uncertainty of 0.8 × 10²⁰ V m⁻², being in very good agreement with the first-principles results. In addition the electronic structure and chemical bonding are analysed from partial densities of states and electron densities.

The application of equilibrium thermodynamics to a nanosystem changes Gibbs's rule of geometrical thermodynamics. This fact leads to the necessity to reconsider the phase diagram and solubility curve concepts. The notions of 'solubility diagram', 'solidus', 'liquidus' are used to discuss the case of phase transition in Cu–Ni nanoparticles. It is shown that, in the limit where thermodynamic arguments remain valid, the solubility diagrams of nanoparticles are functions of their size and nucleation mode. This is demonstrated for different sizes.

Near normal incident infrared reflectivity spectra of (001) SrTi_1−xNb_xO₃ single crystal have been measured at different temperatures in the frequency region between 100 and 6000 cm⁻¹. The optical parameters of phonons and plasmons are obtained on the basis of the double-damping extended Drude model. The lowest frequency phonon mode at 175 cm⁻¹ softens with decreasing temperature, and the plasmon frequency triples when the temperature decreases from 300 to 10 K. Extra absorption near the highest frequency longitudinal phonon mode shows that the effect of the polarons should be taken into account in addition to the plasmon–phonon coupling. The splitting of the last high reflectivity band, the temperature dependence of the 615 cm⁻¹ structure and the appearance of a new weak peak-like defect mode provide evidence for the existence of small polarons.

The magnetic susceptibility, M–H plot, magnetoresistance and thermoelectric power of the RuSr₂Eu_1.5Ce_0.5Cu₂O_10−δ superconductor are measured. Values of the magnetic transition temperature T_mag, superconductivity transition temperature T_c, upper critical field H_{c 2}, chemical potential μ and energy width for electric conduction W_σ are obtained from these measurements. It has been found that T_mag = 140 K, T_c = 25 K (33 K) from susceptibility (magnetoresistance) measurements, H_{c 2}(0)> 32 T, μ = 8 meV and W_σ = 58.5 meV. These values are compared with other ruthenate superconductors, and the resulting physical information is discussed.

We carry out a theoretical analysis of quantum well electron dynamics in a parallel magnetic field of arbitrary strength, for a narrow quantum well. An explicit analytical closed-form solution is obtained for the retarded Green's function for Landau-quantized electrons in skipping states of motion between the narrow well walls, effectively involving in-plane translational motion, and hybridized with the zero-field lowest subband energy eigenstate. The dispersion relation for electron eigenstates is examined, and we find a plethora of such discrete Landau-quantized modes coupled to the subband state. In the weak field limit, we determine low magnetic field corrections to the lowest subband state energy associated with close-packing (phase averaging) of the Landau levels in the skipping states. At higher fields the discrete energy levels of the well lie between adjacent Landau levels, but they are not equally spaced, albeit undamped. Furthermore, we also examine the associated thermodynamic Green's function for Landau-quantized electrons in a thin quantum well in a parallel magnetic field and construct the (grand) thermodynamic potential (logarithm of the grand partition function) determining the statistical thermodynamics of the system.

In this paper we study under the small-signal approximation the properties of the output electric field of second harmonic generation (SHG) for vertical transmission in Family A of the generalized Fibonacci (GF(m, 1)) quasiperiodic ferroelectric domain system. It is found that under perfect quasi-phase-matched (PQPM) conditions there exists self-similarity for the intense peaks of SHG (IPSHG) in real space and the two integers q and p indexing IPSHG make an interesting zero–odd set when m is large enough. On the other hand, self-similarity for IPSHG is broken under imperfect quasi-phase-matched (IQPM) conditions and the SHG spectra comprise a group of intense peaks and another group of satellite weak lines when m is very large. The corresponding integers q and p make an interesting odd–odd set and a successive integer set, respectively. Two kinds of effects of vacancies on SHG have also been found. The analytical results are confirmed by numerical simulations.

An approach to computing conductances of tunnel junctions within the framework of the Landauer–Büttiker theory for electronic transport is introduced and formulated for the Korringa–Kohn–Rostoker (KKR) method for electronic-structure calculations. After a general introduction to the idea behind the approach, tests and comparisons with other methods, namely a 'transmission of Bloch-waves' approach and an approach based on the Kubo–Greenwood formula for the conductivity tensor, reveal a high accuracy and robustness of the proposed method, thus proving its suitability for state-of-the-art computations of spin-dependent ballistic transport. Based on Green functions, it is flexible and can easily be implemented in present KKR computer codes.