Table of contents

Thermal (specific heat) and optical (linear birefringence) studies were performed for a new ferroelectric crystal [4-NH₂C₅H₄NH][SbCl₄]. Four phase transitions were confirmed and described. Thermal parameters (ΔH,ΔS) of the normal–incommensurate, N–IC, and incommensurate–ferroelectric commensurate, IC–F(C), phase transitions are given. The order–disorder mechanism of the N–IC phase transition is proved but the displacive one for the IC–F(C) transition can be proposed. The critical index value (2β = 0.68 ± 0.02) estimated from the linear birefringence measurements indicates that the N–IC phase transition can be described in terms of the 3d XY model.

We present a simple multiplexing device made of two atomic chains coupled by two other transition metal atoms. We show that this simple atomic device can transfer electrons at a given energy from one wire to the other, leaving all other electron states unaffected. Closed-form relations between the transmission coefficients and the inter-atomic distances are given to optimize the desired directional electron ejection. Such devices can be adsorbed on insulating substrates and characterized by current surface technologies.

In order to study the mechanism of the liquid phase hetero-epitaxial growth, the melting process of YBa₂Cu₃O_7−δ (YBCO) thin films was observed by high-temperature optical microscopy. During the heating from room temperature to a temperature above the YBCO peritectic temperature (T_p), we surprisingly find that the YBCO thin film with a MgO substrate can be substantially superheated above the T_p of the YBCO oxide (at least 50 °C) at a heating rate of 5 °C min⁻¹. This is a novel superheating phenomenon involved in a peritectic reaction and an oxide material, which is different from one reported in systems of metals and their alloys. After the melting process, x-ray diffraction analysis was performed, which shows that Y₂BaCuO₅ (Y211) grains are in good alignment on the MgO substrate. The superheating mechanism of the YBCO oxide is discussed.

Single-crystal holmium and gadolinium layers have been grown on yttrium substrates using the molecular beam epitaxy technique and their structures investigated using high resolution x-ray scattering. The experiments were performed using a Philips MRD diffractometer in Oxford, and with the XMaS facility at the ESRF. Holmium layers with a thickness below give scattering that is characteristic of a pseudomorphic film structure with the same in-plane lattice parameter as the yttrium substrate to within 0.05%. For layers thicker than , there is a sharp reduction in misfit strain due to the creation of edge dislocations. The transverse lineshape of the holmium peaks exhibits a two-component lineshape for thicknesses above , but below about 500 Å. Above 500 Å the lineshape of the transverse scans becomes Gaussian and is characteristic of a mosaic crystal.

The gadolinium layers show no sharp change of strain for layers as thick as 2920 Å and the transverse peak shape remained similar for all films. This is characteristic of pseudomorphic film growth and a failure to nucleate dislocations.

Ultrathin ferromagnetic trilayers with perpendicularly oriented easy axes—one film in-plane the other out-of-plane—attracted recent interest in the study of interlayer exchange coupling, domain formation, and canted magnetization orientation. To investigate and understand the thermodynamic ground state of such a prototype trilayer we employed in situ ferromagnetic resonance (FMR) for a Ni/Cu/Co/Cu(001) trilayer. The FMR polar angular dependence has been measured and analysed in the framework of the Landau–Lifshitz equation. In addition the thickness of the Cu spacer layer has been changed to manipulate the strength of the interlayer exchange coupling and select between ferromagnetic and antiferromagnetic coupling. The FMR experiment yields—unlike other techniques—the coupling strength and anisotropy energy in absolute energy units. This gives also a full insight into all the parameters governing the ground state free energy, the equilibrium angles of the canted magnetization, as well as the optical and acoustical mode formed by the two uniform spin-wave excitations.

We present a systematic study of the local hydrogen bonding in hydrogenated polymorphous silicon thin films (pm-Si:H), a heterogeneous material deposited on the edge of crystallinity, by means of Fourier transform infrared spectroscopy. A vibrational mode at is reported and attributed to hydrogen atoms bonded in hydrogen-rich regions present at the interface between ordered regions and the amorphous matrix. This assignment is found to be in good agreement with previous spectroscopic ellipsometry and Raman spectroscopy studies. Combined with stress measurements we draw a picture of the nanostructure of pm-Si:H films, namely a two-domain material exhibiting: (i) large fluctuations of H content, (ii) a high mass density and (iii) the coexistence of highly strained crystalline phases with a relaxed amorphous matrix. Unexplained optoelectronic properties and metastability phenomena can also be accounted for by this picture.

We report on a novel method of charge particle transport. It is based on the application of a moving electrostatic potential and an oscillating friction force to the particle which occur due to acoustic waves travelling in a piezoelectric medium. ZnS grains placed onto the surface of a Y-cut, Z-propagating LiNbO₃ plate experience a sequence of jumps in the forward and in the backward directions with respect to the phase velocity of the plate wave. The jump probability appears to change with the Lamb mode supported by the plate and the sign of the grain charge. We present computed spatial profiles of the piezoelectric potential and the displacement components on the surface of the plate for the two lowest resonant modes of Lamb waves. The occurrence of the grain motions is then explained by the presence of the electrical and friction forces, and good qualitative correspondence of the theory and experiment is found.

In this paper, valuable formulae describing the behaviour of a number of physical properties of moderately compressed expanded graphite (CEG) are given. Many of these relationships provide confirmations of empirical power law equations which had remained unexplained. Other physical properties are also described for the first time with power laws whose exponents are derived rigorously. On the basis of early works by the authors, the origins of the observed behaviours are explained, hence providing a better understanding of the material. Thus, the variations with the density of CEG of the thermal and electrical conductivities, elastic moduli, mosaicities, permeabilities, formation factors, characteristic pore radii and diameters, surface areas and open pore volumes are described. Power laws are shown to apply, and the values of the corresponding exponents are compared both with the results of the authors and with those from the literature.

The structural states of Cu₂O at pressures between ambient and 11 GPa were re-investigated, using angle-dispersive diffraction, an area detector and synchrotron radiation. Thanks to the latter's naturally high collimation, a hitherto suspected transition from the parent cubic phase to a new tetragonal phase was experimentally confirmed at pressures between 0.7 and 2.2 GPa, depending on the hydrostaticity of the pressure medium used. The other transformation, into a pseudocubic phase, was detected at about 8.5 GPa. In the same pressure range, the application of pressure also resulted in a general and strong broadening of the diffraction peaks. This broadening is attributed to the presence of microscopic strains and a reduction in grain size down to the nanocrystalline scale.

X-ray absorption spectroscopy (XAS) is used to probe the evolution of the local order around the cobalt atoms in Co/ZrO₂ discontinuous multilayers as a function of the nominal Co layer thickness. A decrease of the mean number of Co nearest neighbours around Co atoms is observed as the Co layer nominal thickness decreases. XAS analysis is made according to a justified simple model based on two possible Co sites referred to as metallic (cobalt coordinated) and oxide (oxygen coordinated) sites. XAS results are in agreement with the results obtained by other structural characterizations using transmission electron microscopy and by magnetic and transport measurements showing the morphological evolution of the Co layers from the discontinuous state (multilayer of Co nanoparticles embedded in the oxide) to the continuous one by increasing the Co nominal thickness.

Structures and dynamics of n-dodecyltrimethylammonium ions intercalated into sodium octosilicate and tetrasilicicfluoromica were studied by powder x-ray diffraction, and ¹³C CP/MAS and ¹H NMR measurements. In the octosilicate, cations form a 2D-crystalline state in which long axes of intercalated alkylammonium ions were shown to be inclined to silicate layers and polymethylene chains (–CH₂–)_n take the all-trans conformation. On the other hand, in the tetrasilicicfluoromica, cations form disordered arrangements with random conformations of alkyl-chains, considered to be in a 2D-amorphous state. Based on the NMR results, the dynamic behaviour of these ions in 2D space is discussed.

The high-pressure room-temperature behaviour of scheelite CaWO₄ (I4₁/a,Z = 4) is studied using high-resolution synchrotron angle-dispersive x-ray powder diffraction in diamond anvil cells loaded with helium or a mixture of methanol and ethanol as the pressure-transmitting media. At about 10 GPa, there occurs a phase transition to the fergusonite type (I 2/a,Z = 4) without any discontinuity in the pressure dependence of the unit cell volumes. These observations are discussed in relation to the high-pressure–high-temperature systematics of the AMX₄ and AX₂ type compounds.

In this paper we compute the mean field phase diagram of a doped antiferromagnet, in a magnetic field and with anisotropic exchange. We show that at zero temperature there is a metamagnetic transition from the antiferromagnetic configuration along the z direction to a spin flop (SF) state. In the spin flop phase the system prefers a commensurate magnetic order, at low doping, whereas at larger doping the incommensurate phase is favourable. Contrary to the pure Heisenberg case, the spin flop region does not span an infinite area in the (Δ,h) plane, where Δ is the exchange anisotropy and h is the external magnetic field. We characterize the magnetic and charge-transport properties of the SF phase, computing the magnetic susceptibility and the Drude weight. This latter quantity presents a sudden variation as the SF to paramagnet phase transition line is crossed. This effect could be used as a possible source of large magneto-resistance. Our findings may have some relevance for doped La_2−δSr_δCuO₄ in a magnetic field.

The effects of a positively charged impurity on the energy levels and far-infrared spectra of one and two electrons in semiconductor nanorings under magnetic fields are studied. The effects of the nanoring size and the impurity position are also discussed. It is shown that the electron–electron interaction and electron–impurity one in nanorings are strongly dependent on the nanoring size and the impurity position. Based on the studies of the impurity and field effects, the impurity-induced Aharonov–Bohm oscillations of the far-infrared spectra are found. The results predict a possibility of observing phenomena related to electron–impurity interaction in a nanoring in the future.

The longitudinal magnetoresistance of the unusual Bechgaard salt, (TMTSF)₂FSO₃, has been studied up to 33 T under various pressures. In this compound, a single series of the Shubnikov–de Haas (SdH) oscillation is very pronounced for pressures between 5.2 and 11.8 kbar where the zero-field ground state is superconducting. Unlike the case of the rapid oscillations in most Bechgaard salts, the temperature and magnetic field dependence of the oscillations is in good agreement with the Lifshitz–Kosevich formula, implying the two-dimensional closed orbital motion of the electrons. The effective mass of the electrons and Dingle temperature are estimated as 1.4 ± 0.05 m₀ and 1.6–2.4 K, respectively. For an origin of the closed orbits, it is suggested that incomplete nesting between open Fermi surfaces is induced by pressure. Discontinuous change of the frequency, amplitude, and Dingle temperature of the oscillations around 9 kbar indicates that the electronic state below and above this pressure is different. According to the features of the SdH oscillations along with the phase diagram of (TMTSF)₂FSO₃, the pressure dependence is divided into three regions.

In this paper we have applied the full-potential linearized muffin tin orbital method and the tight-binding linearized muffin tin orbital method to investigate in detail the electronic structure and magnetism of a series of half-Heusler compounds XMZ with X = Fe,Co,Ni, M = Ti,V,Nb,Zr,Cr,Mo,Mn and Z = Sb,Sn. Our detailed analysis of the electronic structure using various indicators of chemical bonding suggests that covalent hybridization of the higher-valent transition element X with the lower-valent transition element M is the key interaction responsible for the formation of the d–d gap in these systems. However, the presence of the sp-valent element is crucial to provide stability to these systems. The influence of the relative ordering of the atoms in the unit cell on the d–d gap is also investigated. We have also studied in detail some of these systems with more than 18 valence electrons which exhibit novel magnetic properties, namely half-metallic ferro- and ferrimagnetism. We show that the d–d gap in the paramagnetic state, the relatively large X–Sb hybridization and the large exchange splitting of the M atoms are responsible for the half-metallic property of some of these systems.

We present an experimental investigation on temperature, excitation intensity and spectral dependence of persistent photoconductivity (PPC) in GaN. A grain boundary induced potential barrier is predicted to be responsible for PPC. The non-exponential nature of the PPC decay has been explained by a Gaussian distribution of capture barriers, arising from the trapped charges at the grain boundary interface. The spectral dependence of the PPC suggests the origin of PPC and the yellow luminescence band may arise from the same intrinsic defect.

The absorption spectrum of U³⁺ in LiY F₄ has been well calculated using the model proposed by Reid for calculations of spectra. The relevant formulae for the matrix element calculations which were omitted in this model are now described in detail, and the values of the direct and exchange coefficients associated with the f–d Coulomb interactions within the f²d configuration are derived and listed. The amount of reduction for the f–d Coulomb interaction parameters from the free-ion values is found to be , which is much larger than the value of 26% for the isoelectronic Nd³⁺ lanthanide ion in the same host.

Transport and low-frequency optical reflection measurements (180–380 GHz) are reported for the quasi-three-dimensional conducting alloy Cu[(2,5(CH₃)₂–DCNQI)_0.70(2,5(CD₃)₂–DCNQI)_0.30]₂ between room temperature and 35 K. The optical properties of the system are strongly anisotropic. It is metallic down to 60 K where a Mott–Peierls phase transition occurs. While the transverse conductivity remains practically unchanged, the longitudinal conductivity abruptly drops at the phase transition. Comparing our latest results with previous dc data and measurements of the microwave conductivity also reported here, we find indications of an ageing effect in these samples.

Electron paramagnetic resonance (EPR) measurements on pure polycrystalline CaCu₃Ti₄O₁₂ have been performed and are discussed within a crystal-field approach. A symmetric signal centred at g = 2.15 is observed for T>25 K, with no evidence of hyperfine structure. At this temperature an antiferromagnetic transition is observed as confirmed by static magnetization data. Cu defective and 2% doped (V, Cr, Mn, La) samples were also prepared and considered, mainly to understand the nature of the observed paramagnetic centre. Substitutions in the octahedral sites, causing variations of the configuration in CuO₄–TiO₆–CuO₄ complexes, change the magnetic and EPR features. To justify the EPR response a strong copper-hole delocalization is suggested.

The higher energetic magnon at 27 cm⁻¹ was systematically investigated by far-IR-absorption and Raman scattering in α-O₂ (T = 11–24 K). The softening of the magnon frequency—described in mean field theory—can be explained by a weakening of the exchange interaction due to thermal expansion and by a loss of magnetic order due to thermal fluctuations. Bandwidth as a function of temperature can be fitted by a power law, with a zero bandwidth at T = 0 K (Γ = aT^4.4). Dephasing processes via magnon–magnon scattering are the only meaningful relaxation mechanisms of this magnon mode. The integrated band intensity, normalized to our low temperature value, directly follows the spin–spin correlation function and describes the long range magnetic order parameter in α-O₂. The spectral characteristics, band frequency and band intensity (ω and I) prove that the α–β phase transition (at T_αβ = 23.8 K) is not purely magnetic.

Structure and magnetic transitions were investigated by x-ray diffraction and Mössbauer spectroscopy in LaFe_13−xSi_x compounds with x = 1.6, 2.0 and 2.6. With increasing Si content, the La–Fe interatomic distance decreased while the average Fe–Fe distance increased. These changes affect the structural stability and the magnetic properties of the compounds. The temperature dependence of the hyperfine field for the compound with x = 2.6 can be fitted very well using a mean field model with a Brillouin function (BF) while that for the compounds with x = 1.6 and 2.0 changes more sharply than that predicted by the BF relation near the Curie temperature. The different nature of the magnetic transition with different Si content originates from the spatial distribution of the Si atoms and related variation of the La–(Fe, Si) and the Fe–Fe distances in the cubic NaZn₁₃ structure.

Magnetic and iron-57 Mössbauer spectral measurements between 4.2 and 640 K have been carried out on TbFe₁₁Ti and TbFe₁₁TiH. The insertion of hydrogen into TbFe₁₁Ti to form TbFe₁₁TiH increases its ordering temperature, magnetization, magnetic hyperfine fields, and isomer shifts as a result of lattice expansion. Further, the insertion of hydrogen reinforces the basal magnetic anisotropy of the terbium sublattice and, as is shown by ac susceptibility measurements and thermomagnetic analysis, the spin reorientation observed in TbFe₁₁Ti at 338 K disappears in TbFe₁₁TiH. The Mössbauer spectra have been analysed with a model that considers both the easy magnetization direction and the distribution of titanium atoms in the near-neighbour environment of the three crystallographically distinct iron sites. The assignment and the temperature dependencies of the hyperfine fields and isomer shifts are in complete agreement with a Wigner–Seitz cell analysis of the three iron sites in RFe₁₁Ti and RFe₁₁TiH, where R is a rare-earth element. A complete analysis of the quadrupole interactions in both magnetic phases and in the paramagnetic phase of TbFe₁₁Ti supports the Mössbauer spectral analysis, and indicates that in the basal magnetic phase the iron magnetic moments are oriented along the equivalent [100] and [010] directions of the unit cell.

In this paper, temperature-induced magnetization reversal in FeCr_2−xAl_xS₄ (x = 0.8, 1.0) is reported. Low-field magnetization measurements indicate that this system displays substantial irreversibility between zero-field-cooling (ZFC) and field-cooling (FC) sequences. Upon warming, the ZFC magnetization is found to change sign twice; however, the FC magnetization changes sign only once. Magnetic hysteresis loop measurements reveal that this system displays a large coercivity at lower temperatures; with increasing temperature, the coercivity decreases abruptly. By accounting for magnetic domain freezing induced by large magnetic anisotropy and domain wall pinning of non-magnetic ions, the multiple sign changes of the magnetization are explained qualitatively.

The muon spin precession in the axial-symmetric muonium Mu_bc was studied in a magnetic field applied along the initial muon polarization which was, in turn, parallel to the [111] axis of a silicon single crystal. Hyperfine fields were measured at temperature T = 12 K. The transversal parameter is in good agreement with work by Blazey et al 1983 Phys. Rev. B 27 15, but obtained in this work is less than the value published by Blazey et al by 0.29 MHz. This discrepancy is attributed to the accuracy of determination of the angle between the [111] axis and the magnetic field direction.

In this study the upconversion of the ⁴I_13/2 and ⁴I_11/2 multiplets, and the cross-relaxation (CR) processes of ⁴S_3/2 and ⁴I_9/2 of Er³⁺ in Y AlO₃, which are related to its 3 µm laser operation, have been investigated. The dominant energy transfer (ET) pathways are discussed and the ET rates of the contributing multipolar interaction mechanisms have been estimated, based upon the Holstein–Lyo–Orbach or the Miyakawa and Dexter formalisms for phonon-assisted ET. The Kushida formalism has been employed for the investigation of migration processes, and the matrix elements of Kaminskii for the relevant electronic transitions have been used in our calculations. The major results are that at room temperature, the contributions of ²H_11/2 (particularly from electric quadrupole–electric quadrupole interactions) to the CR and migration of ⁴S_3/2, and to the upconversion of ⁴I_11/2, are important, and are even the dominant ones in some cases.

Natural alexandrite (BeAl₂O₄:Cr³⁺) crystals are investigated as regards the effects of annealing on their optical properties. Optical absorption spectra are measured from the ultraviolet (190 nm) to the near infrared (900 nm), for a sample subjected to consecutive annealing processes, where time and temperature are varied. Besides this, luminescence spectra are simultaneously obtained for this sample, excited with a Kr⁺ laser source, tuned on an ultraviolet multi-line mode (337.5, 350.7 and 356.4 nm). We observe from absorption as well as from emission data that annealing mainly influences the distribution of Cr³⁺ and Fe³⁺ ions, located on sites of a mirror plane (C_s symmetry), which are responsible for the optical properties of alexandrite. The results obtained lead to the conclusion that annealing induces a modification of the population of Cr³⁺ on C_s sites as well as on sites located on an inversion plane (C_i). Annealing could improve the optical properties of this material, as regards its application as a tunable laser.

Annealing at up to 1400 K of hydrogen plasma etched Czochralski silicon (Cz-Si:H) under enhanced hydrostatic pressure (HP) up to 1.1 GPa (HT–HP treatment) results in specific effects related both to the presence of hydrogen and to enhanced HP. The latter strongly affects the sample microstructure, precipitation of oxygen interstitials and creation of thermal donors (TDs).

The HT–HP induced effects in Cz-Si:H are related to the HP stimulated creation of nucleation sites for the creation of TDs and oxygen precipitation as well as to decreased hydrogen out-diffusion under HP.