Table of contents

Within an extended Su-Schrieffer-Heeger model, we investigate the localized phonons in a pristine polyacene chain, in which there is an interchain-coupled neutral soliton of D_2h symmetry. In addition to three pairs of twofold-degenerate edge modes (three infrared active and three Raman active), we found in total nine localized modes, among which four (three B_2u and one B_3u) modes are infrared active and the others (two A_g and three B_1g) modes are Raman active.

Positron-induced ion-desorption spectroscopy has been constructed and used for measurements of positron-induced hydrogen ion desorption from Ni surfaces. The cross-section for proton desorption at a positron incident energy of 1.9 keV was larger than that at a positron incident energy of 2.9 keV. This value is larger than that for electron-stimulated desorption with similar incident energies, suggesting that ion desorption is caused by the thermalized positrons in the bulk.

The conductance of disordered nano-wires at T = 0 is calculated in the one-particle approximation by reducing the original multi-dimensional problem for an open bounded system to a set of exactly one-dimensional non-Hermitian problems for mode propagators. Regarding the two-dimensional conductor as a limiting case of a three-dimensional disordered quantum waveguide, the metallic ground state is shown to result from its multi-modeness. On thinning the waveguide (in practice, e.g., by means of the `pressing' external electric field) the electron system undergoes a continuous phase transition from metallic to dielectric state. The results predicted conform qualitatively to the observed anomalies of the resistance of different planar electron and hole systems.

The hydride CeNiInH_1.8(1) has been studied by magnetization measurements. It exhibits a ferromagnetic behaviour below T_C = 6.8(2) K. In other words, the insertion of hydrogen in CeNiIn induces a cerium valence transition from intermediate valence to the trivalent state.

At variance with what happens in metals, the electronic charge in insulators cannot flow freely under an applied dc field, and undergoes instead static polarization. These two features arise from the difference in nature of the excitation spectra, but also from the difference in organization of the electrons in their ground state: electrons are localized in insulators and delocalized in metals. Such localization, however, is hidden in a rather subtle way in the many-body wavefunction. We review the theory of the insulating state, on the basis of electron localization, addressing on the same basis all insulators: either independent electron or correlated, either crystalline or disordered. The starting point is a 1964 milestone paper by Kohn. Significant advances occurred from 1999 onwards. These advances are deeply rooted in the modern theory of polarization: localization and polarization can be regarded as two aspects of the same phenomenon, and stem from essentially the same formalism. Starting from the many-body ground wavefunction, one defines a dimensionless complex number which vanishes in metals and is finite in insulators; in the latter case, its phase (the Berry phase) yields the macroscopic polarization, while its modulus measures localization. Conductivity features are addressed within the same theoretical scheme.

The interdiffusion process in Gd_1-xCo_x/Co multilayered systems has been investigated, in several series of samples made by sputtering, with different values of the Co content in the alloy (x). Grazing-incidence x-ray profiles and the electrical resistivity, together with magnetic measurements at room temperature, allow us to monitor and quantify the interdiffusion between layers as well as their magnetic properties. It is shown that this interdiffusion occurs on increasing the Co content of the alloy layer up to around 60% Co. In the x = 0.60 series the interdiffusion was found to be negligible and therefore a very good structure was obtained, while the ferrimagnetic structure between layers is preserved. The improved multilayered structure obtained in this system could lead to the development of new technological applications.

Er₅Ir₄Si₁₀ exhibits three phase transitions upon cooling below room temperature. At T_CDW = 151 K a combined commensurate and incommensurate superstructure develops, that has been attributed to the formation of charge density waves (CDWs). At T_LI = 60 K (LI = lock-in) the superstructure becomes commensurate, and at T_N = 2.8 K a state with long-range antiferromagnetic order develops. In this contribution we report the results of high-intensity, high-resolution x-ray diffraction for the temperature region encompassing all four phases. We have found that above T_CDW the critical scattering of the commensurate superlattice reflections persists up to much higher temperatures than the critical scattering of the incommensurate satellites. It is argued that this finding substantiates the hypothesis in which the mechanism of the CDW transition involves a structural transition towards a twofold superstructure. The superlattice reflections are found to be broader in the lock-in phase than above T_LI. This suggests that the lock-in transition results in relatively small domains, that are responsible for the broadening of the reflections. Finally, the antiferromagnetic order is observed by resonant x-ray scattering. The commensurate superlattice reflections persist down to 1.87 K, and no effect of the magnetic transition on their positions or intensities is found. Thus the magnetic order and the CDW coexist below T_N in this compound.

Multilayer fullerene onions present an almost perfect round shape in transmission electron micrographs. On the other hand, single-layer fullerenes seem to become polyhedral with flat faces as the number of carbon atoms grows. We study geometries of fullerenes with symmetrically arranged defects. It is shown that these structures have a rounder shape, after energy minimization with a Tersoff-Brenner potential, than fullerenes with no defects.

Carbon thin films have been deposited on Si(111) wafers through thermally evaporated C₆₀ with simultaneous bombardments of Ne⁺ ions. C₆₀ film can be prepared for Ne⁺ ion energy up to 500 eV, indicating the high stability of the cagelike structure of the C₆₀ molecule. The conversion from C₆₀ structure into amorphous carbon takes place on increasing Ne⁺ ion energies to 700 eV, in which a morphological change from a nodule-like surface to featureless structure is observed. With further increase of Ne⁺ ion energies from 1 to 5 keV, the surface roughness of the amorphous carbon films is enhanced, while a higher fraction of sp³ bonding and a larger optical bandgap are obtained at relatively low Ne⁺ ion energies. These suggest that the ion sputtering is not only responsible for the surface roughening but also has significant influence on the bonding structure in ion-assisted amorphous carbon films.

The symmetry structure of the six families of inner elastic constants (derivatives of the free energy with respect to one, two, or three components of the relative sublattice displacement) that enter the elasticity, through the third order, of the cubic and hexagonal diamond allotropes and of the hexagonal and rhombohedral graphite allotropes is analysed in detail. This is followed by derivations (i) of the forms of the linear and quadratic internal strain tensors, (ii) of the expressions for the zone-centre optic mode frequencies and eigenvectors, and (iii) of the effective inner elastic constants that determine these frequencies in arbitrarily strained crystals together with (iv) the derived pressure dependences of those frequencies.

The formal contributions of inner elastic constants to the macroscopic second- and third-order elastic constants of four diamond and graphite allotropes of carbon are analysed. Second- and third-order compliances and compressibilities, effective elastic constants, and pressure derivatives of the second-order constants are also presented. A generalized method of homogeneous deformation is developed to relate the computationally friendly infinitesimal strain approach to the thermodynamically rigorous finite-strain formalism. Computational protocols, involving up to nine distinct shape-changing deformations, are developed to facilitate the determination of all second- and third-order elastic and inner elastic constants, and hence of all derived quantities.

The universal equation of state with an arbitrary reference point presented by the author (Fang Zheng-Hua 1998 Phys. Rev. B 50 16 238) is applied successfully to the analysis of the experimental compression data of barium in different structural phases (I, II, and V). The comparison given in this paper shows that this equation suits for the isothermal compression behaviour of barium in the high-pressure phases (II and V) better than the Birch-Murnaghan equation. The applicability of equations of state for solids in high-pressure phases is also discussed.

Brillouin spectroscopy has been used to determine the temperature dependence of the velocity of longitudinal acoustic modes in a lithium-potassium sulphate single crystal in the temperature range from 20 to 150 K. Small anomalies were recorded for the modes propagating in [101] and [011] directions. The frequency of the [100] mode is practically temperature independent. The velocity of the [001] mode shows a minimum (relative change, 4.5%) at 52 K which corresponds to the known phase transition temperature in this crystal. The results of two sequential runs performed on the same sample (thermal cycling) are compared and discussed.

We present measurements of the electrical resistivity ρ(T) on high-quality single-crystalline CeNiSn under both hydrostatic pressure up to 1 GPa and uniaxial pressure up to 0.25 GPa. At ambient pressure, ρ(T) along the orthorhombic a-axis (b-axis) shows two maxima at T_L = 12 K (14 K) and T_H = 74 K (40 K), respectively, which arise from the Kondo scattering of conduction electrons by the crystal-field ground state and excited states. With increasing hydrostatic pressure, both T_L and T_H increase linearly, and for P⩾0.8 GPa, the anisotropy in ρ(T) for I∥a and I∥b almost vanishes as a result of increased hybridization between the 4f electrons and the conduction electrons. Under P∥a, both T_L and T_H in ρ(I∥b) increase similarly to under hydrostatic pressure. Under P∥c, however, the depression of T_L in ρ(I∥a) and ρ(I∥b) suggests that the c-f hybridization in the crystal-field ground state is weakened in the a-b plane of CeNiSn.

Tunnelling characteristics have been measured for Co-insulator-Py magnetic junctions with Dy- or Gd-doped Al₂O₃ barriers. The theoretically predicted enhancement of the magneto-resistance due to resonant tunnelling in the presence of paramagnetic impurities is not borne out. However, even a full monolayer of Dy or Gd has no detrimental effect on the junction magneto-resistance (JMR) at low temperature and low bias voltage. With increasing temperature and bias, the JMR of the doped junctions decreases significantly faster than the JMR of the Al₂O₃ control junctions. Junctions in which the entire barrier has been replaced by Dy₂O₃ or Gd₂O₃ show strong non-linear current-voltage characteristics, but display no JMR. It is shown that not direct tunnelling but spin-independent impurity-assisted tunnelling is the primary conductance channel in these junctions.

Neutron diffraction investigations have been performed to study the magnetization process of CsCuCl₃ with the magnetic field aligned within the ab-plane. In zero field the stacked, triangular-lattice antiferromagnet (TLA) CsCuCl₃ has a helical structure incommensurate in the chain direction normal to the ab-plane. The magnetic phase diagram was investigated from 2 K up to T_N in fields up to 17 T. The phase line for the expected incommensurate-commensurate (IC-C) phase transition could be determined throughout the whole phase diagram. At low temperature the IC-C transition is roughly at half the saturation field H_S. The neutron diffraction patterns were found to be well described by a sinusoidally modulated spiral in fields up to H_S/3. The initial increase of the scattering intensity in rising field indicates a continuous reduction of the spin frustration on the triangular lattice. Between H_S/3 and H_S/2 a new phase occurs where the spiral vector has a plateau in its field dependence. Close to the IC-C transition a growing asymmetry of magnetic satellite-peak intensities indicates domain effects which are related to the lifting of the chiral degeneracy in the ab-plane in rising field. The phase diagram obtained has some similarities with those calculated for stacked TLAs by considering the effects of quantum and thermal fluctuations.

The title compounds were prepared from the elements by reactions in sealed tantalum tubes in a water-cooled sample chamber of a high-frequency furnace. They crystallize with the ZrNiAl-type structure, space group P2m. The structures of the cerium compounds were refined from single-crystal x-ray diffraction data: a = 767.3(1) pm, c = 410.37(4) pm, wR2 = 0.0324, 521 F²-values for CePdMg; a = 755.02(7) pm, c = 413.82(4) pm, wR2 = 0.0393, 514 F²-values for CePtMg; and a = 774.1(3) pm, c = 421.6(1) pm, wR2 = 0.0355, 395 F²-values for CeAuMg, with 14 variables for each refinement. The palladium compound shows a small homogeneity range: CePd_1+xMg_1-x. The structures contain two crystallographically different transition metal sites T1 and T2 which are located in tri-capped trigonal prisms: [T1 Mg₆Ce₃] and [T2 Ce₆Mg₃]. Magnetic susceptibility and heat capacity measurements reveal long-range magnetic ordering at 2.1(2) K for CePdMg, 3.6(2) K for CePtMg, and 2.0(2) K for CeAuMg. Curie-Weiss behaviour at higher temperatures shows that the cerium ions are in the 3+ oxidation state. The isotypic LaTMg compounds are Pauli paramagnetic down to lowest temperatures (T = 0.3 K). All the compounds, RETMg (RE = La, Ce; T = Pd, Pt, Au) show metallic behaviour.

According to its relation to the dielectric response, the potential structure of CsH₂PO₄ can successfully be modelled either as a three-well or as a single-well potential. The deviation from the Curie-Weiss law of the dielectric response has an intimate relationship to the low central potential in the unit cells, while ordering P atoms are assumed to drive its ferroelectric phase transition.

The discovery of a new monoclinic phase in the PbZr_1-xTi_xO₃ (PZT) system in the vicinity of the morphotropic phase boundary (MPB), previously considered as a region where the rhombohedral and tetragonal phases of PZT coexist, was recently reported. Investigations of this new phase were reported using different techniques such as high-resolution synchrotron x-ray powder diffraction and Raman spectroscopy. The main objective has been to define a new phase diagram of PZT. In this context, infrared spectroscopic studies were performed in the vicinity of the MPB and studies were initially centred on a PZT sample with x = 0.49 mol% Ti content. Results suggested that the monoclinic → tetragonal phase transition occurs at 237 K, confirming the use of IR as a useful technique to investigate this phase transition.

We report room-temperature Raman scattering results on perovskite-type [(La_0.7Sr_0.3MnO₃)_m/(SrTiO₃)_n]₁₅ multilayers, m and n being the thicknesses of layers alternated 15 times. Distinct changes in the Raman spectra for different n and m have been analysed in the light of structural modifications and point to a tensile-strain-induced rhombohedral-to-orthorhombic phase transition in the La_0.7Sr_0.3MnO₃ (LSMO) layers. As a matter of fact, the presented results provide a new structural basis for the earlier proposed magnetic phase separation model. The present work validates tensile strain as an important variable in thin films, which not only allows us to distort the perovskite structure within a crystallographic space group but can readily induce a transition towards another space group and its related physical properties. As shown in this study, the fabrication of superlattice materials then constitutes a powerful method to stabilize such artificial crystal structures (here artificial orthorhombic LSMO at 300 K) over a greater overall thickness than would be possible in single films.

An accurate structural study on the β-phase of LiNaSO₄ was carried out. This study shows that twin crystals are usually obtained when crystallization takes place from solution, which explains the observed low spontaneous polarization. Raman scattering of Li₂SO₄, Na₂SO₄ and LiNaSO₄ is explained from the structural data. The phase diagram of the binary system Li₂SO₄-Na₂SO₄ was determined by x-ray diffraction and the DTA method. Mixed crystals of the low-temperature phase of Li_2-xNa_xSO₄ with 1⩽x⩽1.22 were observed for the first time. The temperatures of the process in this diagram are high, depending on the cooling rate. A more congruent diagram was obtained working at a lower cooling rate.

CaMoO₄ single crystals doped with Dy³⁺ were grown from sodium molybdate flux. Their absorption and visible emission spectra and decay curves were measured at different temperatures from 10 to 298 K. The complete energy level scheme has been deduced from the low-temperature measurements. The Judd-Ofelt parametrization scheme has been applied to the analysis of the room temperature absorption spectra. The calculated radiative lifetime of the ⁴F_9/2 state is 152±5 µs, and it is in reasonable agreement with the experimental data. The decay curves measured in the 10-170 K temperature range are not exponential and obey the Inokuti-Hirayama model for energy transfer for an electric quadrupole-quadrupole interaction in the absence of diffusion among the donors. All spectral features are strongly affected by an inhomogeneous broadening connected with the `disordered crystal' character of the title compound.

The room temperature excitation spectrum of luminescence of Cr³⁺ in the YAl₃(BO₃)₄ single crystal have been used to estimate the crystal-field strength Dq and the Racah parameters B and C. The Dq/B = 2.4 value indicates that dopant ions occupy strong-crystal-field sites. Emission spectra of Cr³⁺ are consistent with a strong-crystal-field case in which the ²E excited state is the lowest. The influence of temperature on the R₁ line position has been examined and it has been found that the thermal line shift is consistent with predictions derived from consideration of ion-phonon interaction. The influence of temperature on emission lifetime in the 5-300 K temperature range has been analysed. In doing this, the energetic structure of Cr³⁺, consisting of the ground-state and excited electronic manifolds mixed by the spin-orbit interaction, has been constructed using the diabatic representation. An expression for the total probability of the depopulation of luminescent states was next derived and fitted to experimental data yielding the ²E-⁴T₂ separation energy Δ = 730 cm^-1 and the ⁴T₂ radiative lifetime τ₀ = 18.9 µs.

We develop a suitable theoretical framework for studying plasmons in nanotube bundles. In the plane perpendicular to the tubes, the nanotubes form a two-dimensional lattice. We use a simple model of free electron gas confined to arrays of cylindrical surfaces, however the theoretical framework can also be applied to more sophisticated models for carbon nanotubes. Both intrasubband (classical) and intersubband (quantum) plasmons in such nanotube bundle systems are studied. Analytical solutions have been obtained for the case where only one subband is occupied, while numerical solutions have been obtained for the case of many occupied subbands. Intertube Coulomb coupling is found to play an intricate role, as it can both harden and soften plasmon modes in the same nanotube bundle. Intertube Coulomb coupling is also responsible for sizable plasmon dispersions in the transverse plane. All plasmons are found to be undamped by the corresponding single particle type electron-hole pair excitations. The classical plasmon exhibits three-dimensional character in the long wavelength limit, and one-dimensional character in the short wavelength limit. For quantum plasmons, the plasmon energy can be significantly larger than the corresponding single particle excitation energy between subbands. This feature is similar to quantum plasmons in semiconductor quantum wire systems.