Table of contents

This letter investigates ring-rotational vibration of a one-band model with electron-libron coupling and a steric potential suggested by Ginders and Epstein for the leucomeraldine-base (LB) form of polyaniline. Five local modes around a hole soliton are found, and this number if consistent with infrared and Raman lines.

The vibrational excitations and the position of hydrogen in nanocrystalline palladium were investigated by means of inelastic neutron-scattering and H solubility measurements. The study focuses on the H concentration regime (<or=4.8 at.% H) where, at room temperature, no precipitation of the H in a hydride phase ( beta -phase) was observed. In this concentration regime, the solubility measurements show an enhanced H solubility relative to coarse-grained Pd. The neutron-scattering experiments show that this additional H is incorporated in the grain boundaries and at the surface of the grains. Surface modes of H at approximately 90 and 120 meV were identified. Compared to coarse-grained Pd, no change in the H solubility was found for the crystal lattice of the nanosized grains.

The complementary variational principle is used to derive suitable expressions for the first modified bounds on the phonon thermal conductivity. The variational trial function is taken to depend on a series of variational parameters. It is shown that the relations needed to select one decomposition of the collision operator in preference to the others are independent of the number of variational parameters. The convergence of the method is investigated both analytically and numerically when the number of variational parameters is increased. All types of phonon interactions are considered and the results agree quantitatively with the experimental data for Ge and LiF over a wide temperature range.

A topological soliton of a new type which may exist in bistable energetically nondegenerate systems (BENDSS) is revealed. Such a soliton realizes the transition from the initial equilibrium state to the intermediate dynamic state, the latter relaxing to the final equilibrium state in the post-frontal region. Results of computer simulations lead to the conclusion that all the characteristics of the process just beyond the front are well described by the analytical solution for an infinite lattice. On the basis of analytical and numerical studies the cooperative mechanism of structural transitions in BENDSS is proposed.

In a hydrogen-bonded chain with a symmetric double-well potential there are slow-mode kink-antikink pairs and fast-mode kink-kink pairs. However, in a hydrogen-bonded chain with an asymmetric double-well potential, there are slow-mode bell-shaped soliton-antisoliton pairs corresponding to two parallel electric dipoles and fast-mode bell-shaped soliton-soliton pairs corresponding to two antiparallel electric dipoles. Propagating in a spatial inhomogeneous chain, soliton pairs may transform from slow-mode soliton-antisoliton pairs into fast-mode soliton-soliton pairs or the converse process. For bell-shaped soliton pairs, this transformation is accompanied by a reversion of the electric dipoles in one of the two sublattices. In the case of repulsion-type impurities there is a critical velocity v_cr for the incident soliton pair. If the velocity v of the incident soliton pair equals v_cr, its trajectory of motion in phase space will branch while, for v<v_cr, forbidden regions and allowed regions for its motion will exist.

Energy-resolved electron momentum densities are determined for a thin Si film evaporated onto a carbon foil. This is done by transmission (e,2e) spectroscopy, a technique that does not rely on crystal momentum and is therefore ideally suited for the study of amorphous materials. Spectra were collected with an energy resolution of 2 eV and a momentum resolution of 0.15 au (0.3 AA^-1). The main feature disperses in a strikingly similar way to the crystalline ones. In addition to the dispersion the intensities of the peaks are obtained. In spite of having only a qualitative understanding of the shape of the spectra, the results of the comparison of measured amorphous momentum densities with calculated crystalline ones are reasonable. The basis of this agreement between amorphous solid and crystalline theory is discussed.

The paramagnetic response of mixed-valence samarium hexaboride has been studied by inelastic neutron scattering using a low-absorption double-isotope single crystal of ¹⁵⁴Sm¹¹ B₆. Measurements were performed for energy transfers 0<or=h(cross) omega <or=50 meV at temperatures ranging from 2 to 100 K. Two main contributions were observed in the spectra: a broad intermultiplet transition of single-ion type at h(cross) omega approximately=36 meV, and a narrow low-energy excitation centred at h(cross) omega approximately=14 meV, which is strongly anisotropic and temperature dependent and exhibits a weak but visible dispersion. The latter excitation appears to be an inherent feature of the mixed-valence state due to f-electron hybridization effects, which denotes the formation of a local bound state around the Sm sites.

An explanation of the origin of inelastic peaks in magnetic neutron-scattering spectra of the mixed-valence semiconductor SmB₆ is proposed. It is shown that the excitonic theory of the intermediate valence state not only gives the value of the peak frequency but also explains the unusual angular dependence of the intensity of inelastic magnetic scattering and describes the dispersion of magnetic excitations in good agreement with experiment.

The expression for the effective-medium dielectric tensor of a superlattice in which the dielectric tensors of the constituent media are taken in general form (all elements non-zero) is employed to describe a plasma/non-plasma, e.g. doped/undoped semiconductor superlattice in a magnetic field B₀ at an arbitrary angle to the interface. The properties of surface polaritons are discussed in detail for the perpendicular (B₀ normal to interfaces) and Faraday (B₀ parallel to interfaces, propagation vector along B₀) configurations.

Classical shell-model- and embedded-cluster-type calculations are employed in order to supply theoretical arguments in favour of hole bipolarons in BaTiO₃ which have recently been speculated to exist in this photorefractive material. Our investigations concern the geometrical structure of hole bipolarons trapped at acceptor defects, their spin state and hole ionization energies. In particular the embedded-cluster modelling studies, which explicitly include the local electronic defect structure, suggest the importance of lattice relaxation and electron correlation terms in order to stabilize diamagnetic O₂^2- molecules (bipolarons) in BaTiO₃. Our simulations show that hole bipolarons are predominantly bound at Ti-site acceptor defects. A trapping of bipolarons at Ba-site acceptors is in most cases unfavourable. Finally, by extrapolating our present results to the high-T_c superconducting oxides we qualitatively discuss the possible role of small hole (peroxy) bipolarons in these materials.

The anomalies in the conductivity and thermopower associated with the charge-density-wave instabilities of 2D Bloch electrons in magnetic fields are studied. It is assumed that the electrons are scattered elastically by randomly distributed impurities. The minimum of the conductivity sigma _xx and the maximum of the thermopower Q_xx are split into several minima and maxima in the same way as the Van Hove singularities after the lattice distortion takes place. If the electron scattering is anisotropic (low angle) the results are essentially the same as in the case of isotropic scattering, with the only difference being that sigma _xx is multiplied by a large factor.

We study theoretically the magnetic coupling between a superconducting film and a 2DEG by generalizing the theory of Meincke and that of Rammer and Shelankov, correctly taking into account the Hall effect in the 2DEG. The induced voltages are discussed in the magnetic-field regime in which the magnetoresistance is dominated by the SdH oscillations. The results obtained are found to be in good agreement with the experiment of Kruithof et al. We find the modulation of the magnetic field in the 2DEG is larger than previously expected.

The motion of fluxons in a long Josephson junction with point-like inhomogeneities (microresistors) and a dissipative loss is studied in a semi-analytical and fully numerical manner. It is found that the dynamic behaviour of fluxons is markedly changed by the combined effect of microresistors and an AC drive, i.e. the AC drive allows, via the microresistors, the fluxons to overcome a dissipative resistance and to move forward linearly. A physical interpretation of the significant novel phenomenon is given. First, the microresistors generate a new type of inherent localized structure, the so-called impurity mode. Then, the impurity modes are excited and amplified by the AC drive. Finally, the fluxon gains, through the strong fluxon-impurity mode (microresistor) interactions, kinetic energy to overcome the attractive potentials of the microresistors and the dissipative resistance, and it escapes from the microresistors eventually.

Using the Wang-Ting model and including the influence of the thermal fluctuation on the flux motions, we have performed numerical simulation of the scaling behaviour in the negative Hall resistivity region of the high-T_c superconductors for the first time. A scaling law with an exponent beta approximately 1.7 at low magnetic fields has been found in our simulation for the YBCO system, which is in good agreement with the experimental observation.

We studied the pressure dependence of the Neel temperature and the superconducting critical temperature of the heavy-fermion superconductor UPd₂Al₃ at pressures up to 13.6 GPa by means of resistivity measurements. The pressure variation of the normal state resistivity is dominated by a large shift of the resistivity maximum to higher temperatures. The Neel temperature decreases with increasing pressure, while the onset of the superconducting state is almost invariant with pressure up to 6.5 GPa. At higher pressures a decrease of T_c has been observed.

A conceptually new version of the irreducible Green function (IRG) formalism is applied to study a Heisenberg ferromagnet with first-neighbour exchange (J₁) and second-neighbour exchange (J₂). The most important quantity which appears in IRG formalism is the commutator average lambda =((I_kk',S_q'^-)) (where k, q' and k' refer to the incoming, outgoing and internal momentum lines, respectively), which is found to be not only non-zero but also independent of k' in a natural way. This replaces the old irreducibility condition lambda =0 and enables one to recast the equation of motion into the exact Dyson form. For the estimation of self-energy the reducible operators are mapped onto irreducible observables, which introduces a parameter xi into the self-energy operator. The values of xi for various ratios J₂/J₁ and for several spins have been found from the results of exact high-temperature series. From the least-squares fit of these values of Z and 1/S, we obtain an estimate of Z for S= infinity . This is then used to evaluate the Curie temperature for S= infinity and a FCC lattice. The result is found to be in good agreement with the series result. The theory is then applied to EuS which corresponds to a S=⁷/₂ FCC lattice. Using the experimental values of T_C the exchange parameters J₁ and J₂ have been computed. The results agree very well with the series results and with those obtained from spin-wave analysis.

The spin moments of DyFe₂ and ErFe₂ samples have been determined from magnetic Compton scattering experiments performed at the KEK Accumulation Ring with 47.1 keV circularly polarized synchrotron radiation at temperatures 50-355 K. The polycrystalline samples were mounted in a cryostat and magnetized by an electromagnet producing a magnetic field of 0.5 T. The magnetic field was aligned parallel to the X-ray scattering vector and the scattering angle was fixed at 160 degrees . The good signal averaging associated with the use of an electromagnet facilitated the analysis of the magnetic Compton profiles of DeFe₂ and ErFe₂ in terms of a combination of rare-earth 4f, diffuse and iron 3d free-atom Compton profiles. The temperature dependence of the total spin moment mirrors the corresponding dependence of the rare-earth 4f moment. The rare-earth orbital moments have been deduced by combining the Compton data with bulk magnetization measurements.

Thermal properties of spin fluctuations have been investigated for amorphous La(Ni_xAl_1-x)₁₃ (0.90<or=x<or=0.975) alloys. The temperature T dependence of magnetization M for the present alloys exhibits an M²-T^4/3 relation below the Curie temperature T_C in the La(Ni_0.90Al_0.10)₁₃ amorphous alloy and its relation becomes poor with increasing Ni concentration. The inverse magnetic susceptibility shows a peculiar temperature dependence above T_c owing to the saturation of the thermal growth of spin fluctuations. The temperature dependence of the inverse magnetic susceptibility is influenced by the external magnetic field. The distribution of spin fluctuations is localized in both wavenumber and frequency space in the low-Ni-concentration range, and it becomes broader with increasing Ni concentration. The generalized Rhodes-Wohlfarth plots for the present alloy system indicate that the magnetic susceptibility is dominated by the thermal variation in spin fluctuations.

The commensurate (c)-incommensurate (INC)-normal (N) sequence of phase transitions in K₂ZnCl₄ has been investigated through Raman scattering (external and internal modes), X-ray diffraction and DSC experiments. A low-frequency mode, observed in the b(ac)a orientation, is assigned to the pseudo-phase mode. It is anomalously active up to 50 K above the C-INC phase transition temperature (T_L=403 K) though with a weak intensity. The conservation of a splitting of an internal vibrational mode is also observed up to 453 K. From X-ray diffraction the lineshape of the (-4-10-1) satellite exhibits a pinning in the superstructure position which remains observable more than 100 K above T_L. This could be connected with the existence of ferroelectric domains in a wide temperature range in the INC phase. Such a hypothesis seems to be confirmed by DSC experiments.

The birefringence and optical activity of Rb₂ZnCl₄ have been measured in the normal, incommensurate and ferroelectric phases. The measurements have been carried out using a high-accuracy universal polarimeter (HAUP). In contrast to previous observations, an essentially null g₁₃ gyration coefficient (setting Pcmn) has been obtained in the incommensurate phase (g₁₃<5*10^-6), and no measurable change has been detected on entering the ferroelectric phase. Optical parameters, estimated in this phase using a classical point dipole-dipole polarizability theory, are consistent with the observed behaviour. In addition, a reprocessing of the previously reported measurements also shows that, within the experimental error, the resulting optical activity value is in agreement with ours. The conclusions deduced from the present study, together with some recently published results on gyration effects in centrosymmetric incommensurate phases, support the view that optical activity, like any other macroscopic property, is symmetry restricted by the point group associated with the structure. Possible release from this rule when considering point defects in real incommensurate structures is also discussed.

Crystalline AgCl:Cd²⁺ thin foils (Cd²⁺ concentration 62.5-5000 ppm) were studied by time-resolved photoluminescence spectroscopy in the temperature range 20-80 K using a mechanical phosphoroscope. It has been found that the emission spectrum consists of two bands, located at approximately 480 nm (blue) and approximately 590 nm ('red'). With increasing delay after excitation (3-100 ms), the emission spectrum shifts considerably to the red region. Similarly, a temperature increase at fixed delay results in a red-shifted spectrum. To explain these results, a simple kinetic model is suggested supporting the existence of two radiative channels (i) self-trapped exciton and (ii) a nearby Cd²⁺ emission centre. A simple method is presented for how to use the photoluminescence behaviour to check the suitability of AgCl:Cd²⁺ foils to record high-energy-particle tracks.

A Green function calculation for the impurity modes of single impurities and clusters of impurities in a photonic band structure is given. Specifically, a truncated two-dimensional periodic dielectric medium composed from a square lattice array of dielectric rods placed perpendicularly between two parallel perfectly conducting plates is studied. Impurities are introduced into this system substitutionally by replacing one (single impurity case) or a number (cluster impurity case) of the rods in the periodic system by impurity rods. Exact expressions for the impurity mode frequencies are obtained in terms of the Green functions of the pure system. These expressions are then evaluated for the case of impurities with small cross-sectional areas using numerically determined Green functions. Linear frequency-independent, linear frequency-dependent and non-linear impurity dielectric media are treated.

The Mo L₃M_4.5M_4.5 Auger electron spectra have been measured with tubbable synchrotron radiation in the vicinity of the Mo 2p₃2/ absorption edge. It has been found that the L₃M₄4d.₅M_4.5 Auger lines shift to a higher kinetic energy with increasing photon energy, about 4 eV for Mo and about 12 eV for Li₂MoO₄. It is also shown that the L₃M_4.5M_4.5 Auger lines of Li₂MoO₄ are split into two-peak structures just above the resonant Mo 2p_3/2 to 4d(5s) excitation, which are assigned as the spectator and normal Auger transitions. The peculiarities in the Li₂MoO₄ case are interpreted in terms of the Auger resonant Raman scattering of the incident photons and band-like structures of the unoccupied molecular orbitals in the insulating solid.