Table of contents

The authors derive an implicit form for topological solitons in a sine-Gordon system with a long-range interaction potential of the Kac-Baker type. The soliton width and energy are found to go to infinity as the long-range interaction is increased. The results recover those for the sine-Gordon system with nearest-neighbour interactions.

The authors study electron-SO-phonon coupling in the case of Frohlich interaction in cylindrical quantum wires. The polaron self-energy and renormalized effective mass due to surface optical phonons in cylindrical quantum wires have been calculated as functions of the radius of the wire by the standard LLP intermediate-coupling method. It is found that the strength of electron-SO-phonon coupling is strongly dependent on not only the size of the wires but also the dielectric constant of the surrounding medium. The authors numerical results show that the effects of surface optical phonons are very significant and cannot be neglected in thin wires.

The ground-state properties of generalized S=¹/₂ Heisenberg antiferromagnets of a triangular lattice are studied by means of the Schwinger boson approach. The generalization considered include second-neighbour interactions and nearest-neighbour couplings breaking the rotational symmetry of the lattice. Unlike in previous work using the same representation of spin operators, good quantitative agreement with exact numerical results and with other approximate methods is obtained. The results point to the existence of magnetic long-range order, with a local magnetization M=0.275 for the standard nearest-neighbour model.

The angular dependences of the free energy, thickness and stresses are calculated for the antiphase boundaries (APBS) parallel to the tetragonal z axis but otherwise arbitrarily oriented. The general results are applied to Hg₂Br₂ and KSCN. The APB in the former case is shown to decay into two ferroelastic domain walls, while it is stable in KSCN. High stresses occur around the APB in KSCN and more than 50% of the free energy originates from the inhomogeneous strain in the wall. The observed etched patterns in KSCN are discussed.

By employing the Debye-Gruneisen model, the lattice vibration effects are incorporated into the first-principles scheme to calculate the heats of formation of disordered phases for Au-based alloys. Significant improvement over the previous results without lattice vibration effects is achieved for the Au-Cu system; this can be attributed to the fairly strong temperature dependence of the bulk modulus of constituent elements.

The 'density-density' correlation function (CF) of conduction electrons in a metal is investigated. It is shown that the asymptotic behaviour of the CF depends on the shape and the local geometry of the Fermi surface (FS). In particular, the exponent of the power law which describes the damping of Friedel oscillations at r to infinity (-4 for an isotropic Fermi gas) is determined by the local geometry of the FS. The applications of the results obtained to calculations of the CF in a metal near the electron topological transition and of the RKKY exchange integral are considered as well.

Projection operator techniques are used to construct a complete set of symmetry-adapted excited states for the strongly coupled E (X) e Jahn-Teller (JT) system from the exact infinite coupling states in the presence of warping described by a quadratic coupling term. All possible phonon excitations are considered. The associated ion is assumed to occupy a site of tetrahedral T_d symmetry such as that occurring at substitutional sites in III-V semiconductors. The purpose of the calculation is to obtain an appropriate set of basis states from which first- and, more particularly, second-order JT reduction factors can be calculated by analytical methods as described in the companion paper. The states are used in this paper to obtain the energy level diagram for the first few excited states for a chosen value of the warping parameter L.

Analytical expressions for both first- and second-order reduction factors are obtained for orbital doublet systems which are strongly coupled to their surroundings using a recently derived transformation method. The problem is often referred to as the E (X) e Jahn-Teller system. The reduction factors are found from the symmetry-adapted cubic vibronic states, which have been derived in the preceding paper, in the presence of some warping in the potential energy trough. The results for the first-order reduction factors are shown to compare favourably with earlier numerical results, although the configurations are necessarily different. The calculations of the second-order reduction factors appear to be the first of their type for this system. It is shown how they may be used in modelling experimental data involving orbital doublets for E-type uniaxial stresses.

Large arrays of quantum dots have been defined in a GaAs/AlGaAs heterostructure using a Schottky gate that allows simultaneous measurement of the capacitance and the far-infrared (FIR) transmission. With zero applied gate voltage (V_g), the FIR magnetotransmission shows the characteristic cyclotron resonance (CR) and a 2D magnetoplasmon. By applying a negative V_g, quantum dots are defined and a new response is observed, which is described using an edge magnetoplasmon model. Furthermore, the measurements allows the dot radius to be extracted and show that the radius can be varied by up to 50% by altering V_g.

High-field magnetization and low-temperature specific heat of the nearly ferromagnetic compounds Lu(Co_1-xGa_x)₂ have been investigated. A sharp metamagnetic transition has been observed in these compounds. The critical field H_c decreases with increasing x and becomes lower than 1 T at x=0.12. However, no spontaneous magnetization is confirmed even above x=0.15. With increasing x, the electronic specific heat coefficient gamma increases and reaches 36 mJ K^-2 mol^-1 for x=0.12 in 0 T, being 1.5 times as large as that of LuCo₂. Such a mass enhancement is discussed in terms of the spin fluctuation. The gamma value of the ferromagnetic state for the specimen with x=0.12 in a magnetic field of 14.6 T is reduced about 50% compared with that of the paramagnetic state in zero field. This means that the metamagnetic transition results in a drastic reduction of the spin fluctuation in the compound.

Neutron diffraction studies of the magnetic structures of erbium metal at 4.5 K and 11.5 kbar hydrostatic pressure have revealed that the transition to a conical structure at low temperatures is suppressed and that the cycloidal structure, with modulation vector Q approximately=(²/₇2 pi /c)c persists down to 4.5 K. The existence of higher-harmonic magnetic satellites indicates that the cycloid is squared up. Preliminary data at higher temperatures show that the cycloidal structure changes to a longitudinally modulated structure at approximately 50 K and that the modulation vector remains unchanged between 4.5 K and approximately 50 K, where it tends to increase. The intensities of the magnetic satellites originating from higher-order harmonics of the c-axis moment component observed at 4.5 K decrease slowly with increasing temperature, but persist up to approximately 60 K.

Co alloys with Cr, Fe, Ru and Cu in the low concentration range have been studied by ⁵⁹Co NMR. Regularly spaced satellite lines are observed that develop with increasing impurity concentration. The satellite intensities as a function of the impurity concentration have been compared with the binomial distribution law of particular Co environments. Deviations from the exact binomial law in some cases are briefly discussed against the properties of their respective phase diagrams. From this comparison it is concluded that satellite frequencies are primarily determined by the number of impurity atoms in the first neighbour shell: each additional impurity atom in the shell introduces a constant (element-dependent) modification to the hyperfine field value on a Co site. This assignment of satellite frequency shifts to given configurations in the first coordination shell provides references for structural investigations in related binary metallic systems such as metallic multilayers. Preliminary results for CoPt alloys are also presented which do not follow the systematics of the previous alloys but rather suggest an extended oscillating perturbation around Pt impurities.

The re-emission of gamma -quanta in the forward direction by an ensemble of excited nuclei after abruptly changing the energy of the nuclear excitation is studied. A sublevel of the 14.4 keV excited state with a definite spin projection of ⁵⁷Fe nuclei in a magnetic ⁵⁷FeBO₃ crystal is selectively populated. The abrupt change of the energy of the excited nuclear state is achieved by fast (<or=5 ns) reversal of the hyperfine (HF) magnetic field direction at the nuclei. The nuclear target was a black absorber prior to HF field reversal. A short (about 10 ns) intense flash followed by beats of gamma -radiation intensity is detected with some delay after reversal. The delay of the flash and the beat period depend on the HF nuclear transition excited. The duration of the flash depends on the effective thickness of the nuclear target. The theory developed interprets the time evolution of the re-emission as a result of the interference of two main spectral components of gamma -radiation. The first one has the original frequency, which represents the primary radiation. The second one has a shifted frequency, which represents radiation coherently re-emitted in the forward direction from the nuclear sublevel with energy changed by the HF field reversal. The observations reveal the feasibility of inelastic coherent gamma -resonance scattering and demonstrate the enhancement of the radiative channel in the coherent re-emission of gamma -radiation in the forward direction.

On the basis of both the crystal-field theory and the Judd-Ofelt method, the relative intensities and polarization characteristics of the individual transitions between the manifolds ⁴F_3/2 to ⁴I_11/2 and ⁴F_3/2 to ⁴I_9/2 for Nd³⁺ ions in an NdAl₃(BO₃)₄ crystal were studied by experimental fluorescence spectroscopy and branching ratios. The theoretical results are in agreement with those of laser and fluorescence experiments. In this study, the crystal-field wavefunctions were obtained by fitting the Stark energy sublevels with a constraint condition imposed on the electrostatic ratios of the crystal-field parameter belonging to the same order of k, in a similar manner to that proposed by the present authors previously and referred to as the quasi-three-parameter method. It was shown that the minimum obtained in this way was the only solution corresponding to physical reality.

The cross-luminescence (CRL) resulting from radiative electronic transitions from the mainly anion-related valence band to the uppermost cation core band has been studied in a number of complex halides containing CsCl, RbF, KF, and BaF₂. The energy of the CRL photons is determined by the energy difference of the two bands, and the shapes of the spectra reflect the grouping of the molecular orbitals in the clusters involving cations with a hole in the core shell and nearest-neighbour halide ions. For crystals with more than one type of cation, the spectrum reveals information about the 'active' cation which contains the core hole and also about the other cations. Correlations have been established between the local symmetry of the clusters in different crystals and the number and position of the subbands in the CRL spectra.