Table of contents

The authors report X-ray diffraction measurements of epitaxial crystallites in thin films of YBa₂Cu₃O₇ on (001) SrTiO₃. They are found to be orientated with either the a or c axis normal to the substrate surface, but no reflections from b-orientated crystals are observed. This means that not only are b-orientated crystals not present, but also that the a-orientated crystals are free from a-b twins. It is proposed that this is due to substrate-induced strain.

For electrons otherwise freely accelerating in a one-dimensional ballistic channel FET device, it is shown theoretically that the radiative impedance for a single band without magnetic field is quantised at the value R₁=( pi h(cross)/e²). Other implications that the quantum electrodynamic photon radiation viewpoint may have for electron processes within the channel are briefly discussed.

The polarisation properties of the luminescence of Zn_1-xMn_xTe (x=0.16) in a magnetic field are investigated and compared with those of Cd_1-xMn_xTe (x=0.17). The 'free'-exciton emission in Zn_1-xMn_xTe is almost completely polarised perpendicular to the magnetic field when measured in the Voigt configuration, whereas the corresponding Cd_1-xMn_xTe emission is unpolarised. The significance of these results for understanding recombination luminescence in these materials is discussed.

The structural phase transition of cuprous selenide has been studied by means of X-ray diffraction, electron diffraction and high-resolution transmission electron microscopy. The room-temperature phase is pseudo-monoclinic (a=c=7.14 AA approximately=¹/₂a_c(112), b=81.9 AA approximately=8a_c(111) and beta =120 degrees , where a_c is the lattice constant in the high-temperature cubic phase). The structure is characterised by a periodic array of vacant tetrahedral and nearby interstitial copper atoms. A simplified structural model is presented for the arrangement of vacant copper sites; the vacancies occupy the 3¹2/*3^1/2 units in the (111)_c basal planes, and these vacancy layers are stacked every four copper layers, forming a ladder-like structure.

The complicated structure of one of the phases of tetramethylammonium trichlorocuprate (TMACuCl₃) is discussed. The observed deformations with respect to the high-symmetry space group have been decomposed into irreducible representations of this group (P6₃/mmc). The representations that induce the deformations are tau ₅ (k=2 pi (0,0,¹/₅c)) and tau ₆ (k=2 pi (0,0,²/₅c)) (Kovalev notation). Structural resonance occurs between these two waves through anharmonic energy contributions. This resonance is such that in the resulting structure two building blocks are discernible, one block showing all characteristics of CsCuCl₃ and one some of RbCuCl₃. The elements of the gyration tensor have been calculated using single-crystal diffraction data. The chiral functions from which the tensor elements are calculated form fourth-order invariants when squared. This concept can be demonstrated on both the Jahn-Teller octahedra and the successive Cu and Cl layers of this compound. Furthermore, an explanation is given for the fact that some of the TMA groups are disordered, thus contributing to the free energy by an entropy term, and some are not. The superposition of the two waves is such that the E(X) in Jahn-Teller effect of the individual octahedra fits in precisely.

The electronic structure and some structural and thermodynamic properties of beta -PbF₂ have been theoretically investigated by means of the periodic Hartree-Fock method within a core-pseudopotential framework, and using a 3-1G (split-valence) basis set for valence electrons. Band structure, projected densities of states (DOS) and Mulliken populations are discussed; the ionic character of beta -PbF₂ is confirmed (net charge on fluorine, -0.80 electron). The calculated total DOS is compared with the available photo-emission spectrum. The unit-cell parameter, the energy of formation, the bulk modulus and its derivative with respect to pressure (related to the Gruneisen constant) are also studied. The results are in overall agreement with previous semi-empirical theoretical calculations and with experimental data.

The averaged quasi-classical Green functions are used to study the influence of impurities on the electronic structure of conjugated polymers. It is found that the electronic gap and the size of the dimerisation are decoupled in contrast to what was found in previous investigations. This leads to a situation where the polymer still can be dimerised without a gap in the electronic spectrum. Consequences for non-linear excitations (kinks and polarons) are discussed.

The electronic structures of substitutional Ge, Sn and Sb as well as tetrahedral interstitial Sn impurities in diamond-structured Si, Ge and alpha Sn hosts are calculated self-consistency with the linear-muffin-tin-orbital Green function method. Experimental isomer shifts are discussed in terms of host and impurity characteristics. The most important parameter determining the isomer shift is the host specific volume. For the substitutional impurities the p-charge cloud is more compressed in the denser hoists. This leads to lower isomer shifts through the enhanced shielding of the s partial wave. The narrow 13.3 keV ⁷³Ge Mossbauer transition is calibrated leading to a value of the relative nuclear radius change of Delta R/R=1.7*10^-3.

A configuration interaction calculation and a covalency model derivation for Mn²⁺ are presented, giving estimates of the c_k parameters describing the spin-correlated crystal field (SCCF). Expressions for the SCCF contributions to the S ground-state splitting of Mn²⁺ are also given.

The pinning of the Fermi energy to the localised Fe²⁺-donor level degenerate with the conduction band in Hg(Fe)Se produces QHE-like modulations of the Hall resistance in both quasi-2D and 3D.

The influence of linear Jahn-Teller coupling on the spin-Hamiltonian parameters of ⁴A₂( Gamma )₈ ground-state ions is studied. In particular, it is shown that the Jahn-Teller coupling can change the sign of the Bleaney-Koster-Statz parameters from that predicted by crystal-field theory. Vibronic states are constructed using symmetry-adapted vibrational states with up to three quanta excited. The Hamiltonian matrix using these states as bases is determined and the spin-Hamiltonian parameters and reduction factors found by numerical diagonalisation. Perturbation formulae are also presented for the reduction factors correct to fourth order in the vibronic coupling constants. As an example, it is shown that the theory can provide an explanation of the electron spin resonance spectrum of Re⁴⁺ in (NH₄)₂PtCl₆ and an estimate of the vibronic coupling constants for this material is obtained.

The existence of a re-entrant ferromagnetic phase seems to be a general characteristic of systems with frustration and disorder even when no spin-glass phase exists. Using two different methods-the finite-cluster approximation and a new renormalisation technique-the authors studied a two-dimensional nearest-neighbour random-interaction Ising model in which the interactions are independent random variables distributed according to the following law: P(K_ij)=p delta (K_ij-K)+(1-p) delta (K_ij-aK) where K>0 and mod a mod <or=1. In a certain range of negative values of a, the authors found re-entrant phase diagrams but, in contradiction with a recent conjecture, threshold p₁ at which the critical temperature goes to zero depends on a.

The magnetic susceptibilities of the compounds (Dy_xY_1-x)₂Mo₂O₇ for x=1.0, 0.8, 0.6, 0.4 and 0.2 have been measured in the temperature range 600-4.2 K to check whether any magnetic ordering is found at low temperatures and to separate the contributions from Dy³⁺ and Mo⁴⁺ to the susceptibility in the paramagnetic region. The effective magnetic moment of Dy³⁺ was found to be less than that of the free-ion magnetic moment and is attributed to the crystal-field splitting of the energy levels.

The triclinic modification of DMM-TCNQ₂ is a linear-chain antiferromagnet with an incommensurate modulation of the lattice. As a consequence the antiferromagnetic exchange constant is also modulated along the chain. The effect of this modulation on the magnetic properties is calculated and is found to be in agreement with experiment. At 1.7 K a transition occurs to a low-temperature phase, which in zero field has a modulation q=0.54, a value that coincides with the fourth harmonic of the high-temperature modulation of the lattice. It is shown that this phase strongly resembles the field-induced phase of a regular spin-Peierls system.

A phenomenological theory of the dielectric susceptibility in the biasing electric field near the incommensurate-to-commensurate improper ferroelectric phase transition is presented. The incommensurate phase is treated in the soliton limit, i.e., as a periodic sequence of domains with opposite polarisation separated by domain walls of negative energies. Experimental results can be explained quantitatively on the assumption that in the AC electric field used for the measurement the number of domain walls remain constant since the processes leading to its change are much slower.

The transmission of radiation through a finite quasi-periodic multi-layered medium, constructed using two types of film arranged in the Fibonacci way, is theoretically investigated. Almost all parameter regions are found to be 'stop gaps', in which the transmissivity of the medium is exponentially small when the length N of the medium is several times larger than a defined number, which may be extremely large. For some special values of parameters, the medium shows a self-similar N-dependent phenomenon as well as an N-independent phenomenon in the transmission processes. Comparisons with periodic media and other quasi-periodic media with different components are also included.