Table of contents

Terahertz (THz) radiation was generated via optical rectification from organic single crystals, i.e. N-substituted (N-benzyl, N-diphenylmethyl, and N-2-naphthylmethyl) derivatives of 2-methyl-4-nitroaniline (MNA), and it was detected by means of the electro-optic sampling method. The intensity and the spectrum of the THz radiation were compared with those of ZnTe and DAST (N, N-dimethylamino-N'-methylstilbazolium p-toluenesulphonate) crystals. The integrated intensity of the THz radiation from the N-benzyl-MNA crystal was two thirds of that for the ZnTe crystal, the best-known practical THz emitter, and was as intense as that for the DAST crystal, the best organic THz emitter ever studied. The spectrum of the ZnTe crystal extended to 3 THz, while that of the N-benzyl MNA and DAST crystals dropped off around 2.5 THz. The THz radiation intensities from all of the organic crystals were compared quantitatively, on the basis of the consideration of molecular arrangements in the crystals. It was suggested that the absorption due to low-frequency phonon modes affected both the intensity and the spectral band shape of the THz radiation.

Optical internal transitions relevant to experiments based on the optical detection of far-infrared resonances are computed for the biexciton in a quantum dot in the strong-confinement regime. Valence sub-band-mixing effects are taken into account in second-order perturbation theory. The probability of transition from the ground state is concentrated in a few states with relatively high excitation energies. Level collisions with oscillator strength transfer are observed as the magnetic field is raised.

The phase diagram of superconducting copper oxides is calculated as a function of doping on the basis of a theory of dynamic stripe-induced superconductivity. The two major conclusions from the theory and the numerical analysis are that T* (the pseudogap onset temperature) and T_c (the superconductivity onset temperature) are correlated through the pseudogap, which induces a gap in the single-particle energies that persists into the superconducting state. On decreasing the doping the pseudogap Δ* increases and T* increases, but when Δ* exceeds a certain critical value the superconducting transition temperature T_c is lowered. A mixed s- and d-wave pairing symmetry is also examined as a function of doping.

We imaged at single-particle resolution the birth, life and death of a sub-critical nucleus containing ~10³ particles in a crystallizing hard-sphere colloidal fluid at volume fraction ϕ = 0.51. This gives a lower bound to the critical nuclear size for hard-sphere crystallization at this volume fraction.

Theoretical methods to calculate the optical response of crystals are reviewed. The second-harmonic generation coefficients of borate nonlinear optical crystals are calculated using the anionic group theory combined with the Gaussian'92 ab initio calculation method. Without adjusting the parameters, the calculated values are in good agreement with the experimental values. Meanwhile, a plane-wave pseudopotential total energy package is also used to calculate and discuss the theoretical background and accuracy of the theory. The results indicate that the anionic group theory can predict and calculate the nonlinear optical coefficients of borate crystals reliably, and the energy band calculations can reveal the relationship between nonlinear optical effects and microstructure.

The large variety of magnetic phenomena observed in the Co based Laves phases are reviewed. Following the band structure calculations it is argued that the outstanding magnetic features of the RCo₂ intermetallics are intimately related to the position of the Fermi level, which is near to a local peak in N(ε). This is why the Co 3d-electron system reacts sensitively either to the molecular field of the R partner element or to the changes of external parameters such as a magnetic field or pressure. Magnetic, magnetoelastic and transport measurements of RCo₂ compounds and related pseudobinaries such as R(Co_1-xAl_x)₂ with R either magnetic or nonmagnetic rare earth element are shown and discussed. The conditions for the appearance of itinerant electron metamagnetism and spin fluctuations are outlined. In particular, the influences of spin fluctuations on physical properties, e.g. the susceptibility, thermal expansion and transport phenomena, are demonstrated.

The most important symmetry properties of the incommensurately modulated crystal structures are investigated by use of exact symmetry theory of quasi-one-dimensional systems in the framework of group theory. It is shown that typical characteristic formulae developed for the description of scattering cross sections of one-dimensionally modulated crystals can be directly derived by the line-group technique. A symmetry analysis of static soliton structures is performed, representing a new method for the investigation of elementary excitations of crystals modulated incommensurately. It leads to the description of symmetry breaking, to the selection rules and hints at the similarity of symmetry behaviour of static and dynamic solitons. The actual formulae for Debye-Waller factors in the case of incommensurately modulated crystals are calculated and tabulated by using generating elements of the line groups concerned.

We have developed a realistic model for silica aerogels based on a diffusion-limited cluster-cluster aggregation model in three dimensions with a primary particle represented by a sample of vitreous silica given by molecular dynamics simulations. Using the spectral moments method, with more than 30 000 computed moments, we calculated the densities of states and the dynamic structure factors of a simulated sample of silica aerogels of different macroscopic densities. We reproduce the vibration spectra experimentally obtained on basic silica, with a fracton region composed of two contributions associated with stretching and bond-bending modes respectively. The spectral dimensions describing these contributions are close to the measured values. The analysis of our numerical results in the light of the scaling argument is reported.

Crystal-field effects are very important as far as laser performances of Yb-doped materials are concerned. In order to simplify the interpretation of low-temperature spectra, two tools derived from a careful examination of crystal-field interaction are presented. Both approaches are successfully applied in the case of new Yb-doped materials, namely Ca₃Y₂(BO₃)₄ (CYB), Ca₃Gd₂(BO₃)₄ (CaGB), Sr₃Y(BO₃)₃ (SrYBO), Ba₃Lu(BO₃)₃ (BLuB), Y₂SiO₅ (YSO), Ca₂Al₂SiO₇ (CAS) and SrY₄(SiO₄)₃O (SYS). The ²F_7/2 splitting is particularly large in these materials and favourable to a quasi-three-level laser operating scheme. Calculations performed using the point charge electrostatic model for these compounds and using a consistent set of effective atomic charges confirm the experimental results. This should permit to use this model in a predictive approach.

The linear and nonlinear complex dielectric constants of the disordered lead scandium tantalate (Pb(Sc_1/2Ta_1/2)O₃) relaxor system, which is characterized by B-site disorder, were measured. The linear dielectric susceptibility showed a typical relaxor behaviour which can be described by the Vogel-Fulcher law and an additional weak dispersion which can be observed only at very low frequencies. The temperature dependence of the third-harmonic response also showed a frequency dispersion, and the peak temperatures of the real part obeyed the Vogel-Fulcher law with a freezing temperature close to that of the linear response. The observed nonlinear susceptibility showed a quite similar behaviour to that of lead magnesium niobate (Pb(Mg_1/3Nb_2/3)O₃). The temperature variation is discussed on the basis of the theoretical predictions of the spherical random-bond-random-field model (Pirc R and Blinc R 1999 Phys. Rev. B 60 13 470) and a recent phenomenological model (Glazounov A E and Tagantsev A K 2000 Phys. Rev. Lett. 85 2192).

We investigated the temperature-dependent infrared properties of the compound θ-(BEDT-TTF)₂RbZn(SCN)₄. At temperatures above the metal-insulator transition, the optical conductivity remains finite in the low-energy region, but shows no conventional Drude feature typical of metals. Below the metal-insulator transition temperature, the low-energy spectral weight is significantly reduced. The opening of an optical gap at about 300 cm^-1 is observed clearly in polarization parallel to the donor stacking direction. Analysis of both the electronic and vibronic spectra suggests a redistribution of charges in BEDT-TTF molecules as temperature decreases across the metal-insulator transition, providing strong support for a charge-ordering state at low temperatures.

The emission and excitation properties of a SrB₄O₇:1% Pr³⁺ powder sample and a Sr_0.7La_0.3Al_11.7Mg_0.3O₁₉:3.5% Pr³⁺ single crystal were investigated using x-ray and synchrotron radiation. The 4f5d states of Pr³⁺ in these host lattices are at a higher energy than the 4f²[¹S₀] state. In the hexa-aluminate this results in the successive emission of two photons. ¹S₀→¹I₆ emission is followed by emission from the ³P₀ state. The quantum efficiency of the total ³P₀ emission is 25%. In the tetra-borate the ³P₀ emission is quenched by multiphonon relaxation to the ¹D₂ state from which only weak emission is observed. In the hexa-aluminate as in the tetra-borate, host lattice excitation does not result in efficient emission from the ¹S₀ state. In the hexa-aluminate the excitation energy is transferred from the host lattice preferentially to the lower-energy ³P_J and ¹I₆ states, resulting in ³P₀ emission only.

We present a detailed structural investigation via neutron diffraction of differently heat-treated samples of Fe₂VAl and Fe_2+xV_1−xAl. The magnetic behaviour of these materials is also studied by means of μSR and Mössbauer experiments. Our structural investigation indicates that quenched Fe₂VAl, exhibiting the previously reported `Kondo insulating like' behaviour, is off-stoichiometric (6%) in its Al content. Slowly cooled Fe₂VAl is structurally better ordered and stoichiometric, and the microscopic magnetic probes establish long-range ferromagnetic order below T_C = 13 K, consistent with results from bulk experiments. The magnetic state can be modelled as being generated by diluted magnetic ions in a non-magnetic matrix. Quantitatively, the required number of magnetic ions is too large to be explained by a model of Fe/V site exchange. We discuss the implications of our findings for the ground-state properties of Fe₂VAl, in particular with respect to the role of crystallographic disorder.

A lower limit for the grain size of nanocrystalline solids obtained by crystallization of the glass and its dependency on the crystallization temperature are thermodynamically considered. It is found that the nanocrystalline materials have the smallest grain size when the crystallization temperature is roughly half of the melting temperature. At this temperature, the Gibbs free energy difference between the undercooled liquid and the crystal reaches the maximum. It is found that for polymorphous crystallization the lower bound of grain size is essentially dependent on melting entropy. The results are consistent with available experimental evidence.

First-principles calculations have been made for the 50–50 ordered alloy CuZn, β-brass, of the energy per atom in tetragonal structure as a function of the tetragonal lattice parameters a and c. A full-potential electronic-structure program was used with both the local-density approximation (LDA) and the generalized gradient approximation (GGA). Both formulations confirm the existence of a shallow energy minimum (the ground state) at c/a = 1 (β-brass has CsCl structure), and reveal the occurrence of a shallow secondary minimum at approximately c/a = 1.26. With decreasing electronic charge on the Zn atom this minimum becomes deeper, and moves toward larger c/a values, reaching the value c/a = 2^1/2 when one full electron is taken away from Zn, i.e., when Zn is replaced with Cu. The three elastic constants of β-brass have been calculated and compared with experiment. The large elastic shear anisotropy of β-brass discussed by Clarence Zener in 1947 has been confirmed in this work, although the anisotropy is not quite as large as the then available experimental data indicate.

We have investigated various Cr-based Heusler alloys using core-level x-ray photoelectron spectroscopy (XPS). Of special interest are the 2p XPS lines, which always exhibit magnetic splitting in the case of the existence of a well-defined local magnetic moment at the transition metal site. The Cr 2p XP spectra, however, do not exhibit this exchange splitting for the nonmagnetic Cu₂CrAl sample and for Fe₂CrAl or Co₂CrAl alloys with a magnetic moment at the Cr sites of about 1.6 μ_B. This behaviour is discussed here.

The electronic structure of La_2/3Sr_1/6Pb_1/6MnO₃ has been studied by x-ray photoemission spectroscopy (XPS). The valence band spectrum is compared with ab initio electronic structure calculations using a linearized muffin-tin orbital (LMTO) method. The XPS measurements and the theoretical band structure calculations for La_1/2Sr_1/4Pb_1/4MnO₃ show that the electronic structure consists mainly of Mn(3d) and O(2p) states. In addition, the Mn(3d) and O(2p) states are hybridized over the whole valence band. States of 3d character localized on Mn sites predominate near the top of the valence band. It was found that the doping, both with the Pb and the Sr ions, increases the spin polarization by up to 48%.

Magnetic susceptibility, magnetic hysteresis, Mössbauer effect, and specific heat measurements were performed for R₂Ir₂O₇ (R = Y, Sm, Eu and Lu), which show magnetic transitions at 150, 115, 120 and 120 K, respectively. Below the transition temperatures there is a large difference in the temperature dependence of the magnetization measured under zero-field-cooled conditions (ZFC) and under field-cooled conditions (FC), but no magnetic hysteresis loop has been observed. The magnetic properties of the Ir pyrochlores are similar to those of isostructural Ru pyrochlores. No sharp anomaly is observed in specific heat data for any of these compounds down to 1.8 K. These results suggest that R₂Ir₂O₇ shows spin-glass-like behaviour below the transition temperatures.

We report the estimation of diffusion constants for oxygen vacancies in MgO based on optical absorption spectra. MgO single crystals were annealed in a reducing atmosphere (H₂ + Ar) to create oxygen vacancies and the samples were annealed in an oxidizing atmosphere (O₂ + Ar) to annihilate them. Fe³⁺ impurities in the MgO samples were utilized to monitor the degree of reduction or oxidation of the samples during the annealing cycle. An activation energy of 3.08 eV for the diffusion of oxygen vacancies (F-centres) in MgO was obtained, based on the absorption measurements of Fe³⁺ band.