Table of contents

The absorption spectrum and a cyclic voltammogram for poly(3-hexylthiophene) changed upon C₆₀ doping but those for poly(3-octadecylthiophene) did not. This suggests that C₆₀ is an effective dopant for poly(3-hexylthiophene) in the ground state but not for poly(3-octadecylthiophene). The threshold of cyclic voltammetry has shifted to a higher potential in C₆₀-doped poly(3-hexylthiophene). This is explained by the lowering of the valence band top due to the electron transfer from poly(3-hexylthiophene) to C₆₀. The shift depends on alkyl chain length and decreases with alkyl chain length in accordance with the results of photoemission spectroscopy.

The magnetization of the easy-axis disordered antiferromagnets Mn_1-xZn_xF₂, K₂Fe_1-xIn_xCl₅.H₂O and K₂Fe(Cl_1-xBr_x)₅.H₂O was studied at very low magnetic fields, starting from H_axial<or approximately=0.01 Oe along the easy axis. The existence of a remanent moment M_r is confirmed in the first two materials, but contrary to a recent report the sign of M_r is not random; it always agrees with that of H_axial. A small remanence is also found in K₂Fe(Cl_1-xBr_x)₅.H₂O. The magnitude of M_r increases rapidly with H_axial when H_axial<or approximately=0.1 Oe, but approaches saturation at higher fields. The data strongly suggest that M_r would vanish if H_axial were exactly zero. For fields up to at least approximately 1 Oe, the remanent moment at all temperatures is governed by the axial field which is present as the sample cools through the Neel point.

The applicability of sum rules of non-linear optical constants is shown using the coherent anti-Stokes Raman spectrum of nitrogen Q-branch and the associated complex third-order non-linear susceptibility obtained by a procedure utilizing the maximum-entropy model.

In amorphous metallic alloys irradiated with swift heavy ions, electronic excitation induces atomic displacements at the beginning of the irradiation and anisotropic growth above an incubation fluence. The authors provide here an extensive review of their data on amorphous Fe₈₅B₁₅ (including work that is already published), as well as a general description of the topic. Samples were irradiated at various temperatures (20 K, 90 K, 223 K) with a large variety of high-energy (GeV) heavy ions (Ar to U). The atomic rearrangements occurring during irradiation were studied by in situ electrical resistance experiments. The influence of the irradiation geometry as well as of a uniaxial stress applied to the sample was investigated. The whole set of data is accounted for with a two-hit phenomenological model allowing one to extract physical parameters. A description of the ion-target interaction on the basis of the Coulomb explosion mechanism is also provided.

The authors have developed a new code for performing molecular dynamics simulations of ionic materials, in which electronic polarization is included via the shell model. Key features of the method are the use of the conjugate-gradients technique for relaxing the shells at each step, and a new device for ensuring energy conservation. The new code is used to study superionic conduction in CaF₂, which has previously been studied by conventional rigid-ion molecular dynamics. For both static and dynamic quantities, the modifications caused by explicit inclusion of electronic polarizability are remarkably small.

A method is introduced by which molecular dynamics simulations of ionic materials can incorporate the shell model of ionic polarizability. The shells are given a small mass and their motion integrated in the same way as that of the cores, by numerical integration of the classical equations of motion. The authors argue that results should be in agreement with the conventional massless shell model providing that the frequency of the spring linking the core and the shell is much higher than the lattice vibrational frequencies. It is always possible to satisfy this condition by making an appropriate choice of the shell mass. The method is tested in simulations of diffusion in molten NaCl and superionic CaF₂, and of phonon frequencies in MgO. The method works well, is easy to implement, and is computationally efficient.

The one-particle model on a lattice is proposed to account for the formation of a spanning cluster in the aggregation process. In the framework of that model, effective-medium theory is described to calculate the effective diffusion rate, sigma , and effective adsorption rate, v, within the aggregation front. A sharp increase in v is found at a very small concentration of adsorbed particles if the elementary rate of attachment, Gamma , measured in units of the elementary diffusion rate, is above a critical value Gamma * approximately=2. This peculiarity in behaviour of v signals the occurrence of rare and ramified structure. The model can also be applied to explore the concentration dependence of absorption rate. A unit rate of attachment, Gamma , corresponds to instantaneous local absorption. Then, the Smoluchowski result is reproduced for a small concentration of traps. Values Gamma <1 account for the reaction-limited mode. The concentration dependences of the rates v and sigma are found for any value Gamma in the steady-state regime.

The combination of molecular dynamics and density-functional theory introduced by Car and Parrinello (CP) has broadened very markedly the range of systems and properties that can be treated on a first-principles basis. The CP formulation involves plane-wave expansions for wavefunctions and potentials and is best suited when the atoms can be represented by weak pseudopotentials. An alternative approach employs expansions in localized orbitals and may prove advantageous where strong potentials are encountered. The approach derives forces from a density functional closely related to the Kohn-Sham functional but defined rigorously on function space, in particular for a sum of site densities. This simplifies force calculation substantially. The site densities are represented via a density basis and the orbitals by an orbital basis of exponentially localized Lambda -functions whose exponents are optimized dynamically. The potential of the approach is demonstrated via explicit calculations, including molecular-dynamics simulations for hydrogen clusters.

On the basis of the analysis of the optical absorption spectra and conductivity data the model of the electronic structure of the regular three-dimensional arrays of minimum-diameter (0.6 nm) quantum wires of Tl, Pb and Bi has been proposed. It is characterized by a dielectric type of energy spectrum, an atomic-like energy band structure near the gap, anisotropy of this spectrum and localization of the electronic states near the gap.

Magnetotransport investigations of silicon delta -doped Ga_0.47In_0.53As layers with different electron densities were performed in tilted magnetic fields up to 12 T. The conduction electrons of the samples showed two-dimensional behaviour determined by the dependence of the Hall coefficient on the tilt angle of the magnetic field. Several populated subbands were identified by the normal and the diamagnetic Shubnikov-de Haas effect. The diamagnetic depopulation features of the samples were explained on the basis of calculations done by Reisinger and Koch for symmetrical potential wells. Whereas no quantum Hall effect could be observed at any of the samples in perpendicular magnetic fields up to 12 T, well developed quantum Hall plateaux occurred at a sample with a low doping level above a certain tilt angle. This was found to be due to the depopulation of the higher subbands by the parallel component of the magnetic field.

The influence of the electronic Coulomb repulsion on the critical temperature for a layered superconductor is studied by using the Eliashberg theory. The dependence of the critical temperature, T_c, on the interlayer distances, tunnelling integral t_{perpendicular to} and the thickness of the conducting layers is established. The Lawrence-Doniach free energy functional for quasi-two-dimensional superconductors is used to study the influence of fluctuations of the order parameter phase on T_c and the dependence of T_c on t_{perpendicular to} is also obtained. The critical temperature appears to increase with t_{perpendicular to} for T_c within the range T_c*<T_c<T_c0⁽²⁾, where T_c0⁽²⁾ is the critical temperature formally evaluated by means of BCS theory and T_c*=(1/T_c0⁽²⁾+1/2 pi in _F)^-1.

The spectrum of longitudinal spin fluctuations, observed in inelastic neutron scattering experiments, is provided for a Heisenberg model of an ordered ferromagnet including dipolar forces and a magnetic field. A numerical method is used to obtain results, from an expression derived with linear spin-wave theory, for wavevectors (k) throughout the Brillouin zone as a function of frequency ( omega ) and at various temperatures. Particular attention is given to realistic models of EuO and EuS. Results show that dipolar forces diminish the intensity in the longitudinal response function S(k omega ) by about a factor of 2, in accord with previous results for the isothermal susceptibility. The evolution of S(k, omega ) with k displays several pronounced features, and for EuS there is a strong dependence on the direction of k for k near the zone boundary.

Magnetic properties of the FCC type II antiferromagnetic CeSe have been studied by elastic, diffuse critical and inelastic neutron scattering experiments. The diffuse critical scattering above the Neel temperature corresponds to highly anisotropic longitudinal spin fluctuations with a lenslike intensity distribution. The magnetic phase diagram for fields along the (110) direction contains a first-order magnetic phase transition at H=H_crit(T)<or=5.7(1) T. Broad paramagnetic crystalline-electric-field excitations are observed. The spin-wave dispersion in the magnetically ordered phase exhibits a molecular-field splitting of 1.06(2) meV and an energy gap at 0.34(2) meV.

Based on measurements of spontaneous linear birefringence, the phase diagram of Rb_1-x(NH₄)_xH₂AsO₄ has been completed. The ferroelectric compounds (x<0.19) undergo second-order transitions, whereas the antiferroelectric transitions (x>0.46) are strongly first-order.

The second-harmonic generation in a newly developed optical superlattice is analysed. Two second harmonics are predicted: a forward-travelling harmonic and a backward-travelling harmonic. Each can be enhanced enormously by the quasi-phase-matching technique.

Absorption and luminescence were investigated in a nearly one-dimensional antiferromagnetic crystal [(CH₃)₃NH]MnCl₃.2H₂O. The luminescence decay process was studied by time-resolved spectroscopy using a streak camera. An intrinsic emission band with a peak at 661 nm was observed by the excitation in the Mn²⁺ absorption band at 15 K. This emission shifts towards a higher energy as the temperature is increased. Transfer of electronic excitation energy was found to occur in the high-temperature range above 90 K where short-range order becomes ineffective. An emission from Mn²⁺ traps, which is caused by the energy transfer from the bulk Mn²⁺ ions, was observed spectrally separately from the Mn²⁺ intrinsic emission.

The axial absorption spectra of FeF₂ have been measured at wavelengths between 200 and 3000 nm and temperatures T where 15<T<300 K, to complete previous work, which was restricted to selected wavelength regions. Thirteen absorption bands have been observed at wavelengths longer than 260 nm; these are denoted as A to M in order of increasing energy. Basically they are analysed in terms of excited Fe²⁺ states in a cubic crystal field. Most of these transitions reveal sharp, weak, pure exciton lines at low temperatures. Some of them (E, H1, H2, K) are accompanied by one-magnon cold and hot sidebands. Their moments of order zero and one have the typical temperature dependence predicted by theory. Three-magnon hot sidebands occur within the E and H1 bands. The E₁ line, formerly attributed to a two-magnon transition, is identified as a pure exciton transition. A double-exciton transition is connected with the M band at about 300 nm. A weak low-energy satellite of the A band exciton, A', refers to axial crystal field splitting of the ⁵T_2g ground state.

Using an improved variational wave function the 2s excited state energy of a Coulomb impurity bound polaron in a purely two-dimensional quantum well is calculated and the excitation energies and the phonon-induced Lamb shift corrections are obtained for several polar materials.