Table of contents

Nuclear quadrupole resonance results on the inorganic spin-Peierls compound CuGeO₃were re-examined using a single crystal and enhanced resolution. A clear splitting of ^63,65 Cu NQR lines of about 250 kHz at 4.2 K was observed. Such a splitting has not been previously seen in Cu NQR spectra measured on polycrystalline samples of CuGeO₃(Itoh M et al1995 Phys. Rev.B 523410, Kikuchi et al1994 J. Phys. Soc. Japan63872). The mean frequency of the Cu doublet is in agreement with the Cu NQR frequency observed on powder samples. The splitting does not disappear with increasing temperature above T_sPexhibiting only a slight kink at around 16 K. This points to a non-magnetic origin of the splitting which can be attributed to existence of at least two non-equivalent Cu crystal sites in CuGeO₃structure, as predicted by precise x-ray study and EPR measurements (Hidaka M et al1997 J. Phys.: Condens. Matter9809, Yamada I et al1996 J. Phys.: Condens. Matter82625).

Optical transitions at the single substitutional nitrogen centre (P1) in synthetic diamond are discussed. It is shown that the 4.6 eV absorption band is not due to P1, but due to some other centre, tentatively suggested as a nitrogen-boron complex. The 4.5 eV photocurrent threshold is assigned to the transition from the valence band to the ground state of the P1 centre.

Vibrational density of states of a classical two-dimensional electron system obtained with a molecular-dynamics simulation is shown to have a peak in both solid and liquid phases. From an exact diagonalization of the dynamical matrix, the peak is identified to be vibrational modes having wavelengths of the order of the electron spacing, and the result is interpreted as persistent vibrational modes with short wavelengths in a liquid.

The tunnel splitting in biaxial antiferromagnetic particles is studied with a magnetic field applied along the hard anisotropy axis. We observe the oscillation of tunnel splitting as a function of the magnetic field due to the quantum phase interference of two tunnelling paths of opposite windings. The oscillation is similar to the recent experimental result with Fe₈molecular clusters.

Based upon the reported crystal structure data of Gd₅ (Si_x Ge_1-x )₄compounds, a possible correlation between the crystal structure and magnetic properties of the compounds is explored. An almost linear dependence of the ferromagnetic ordering temperature on a M-M (M = Si, Ge or a mixture of Si and Ge) bond length is derived within a first approximation. It is argued that the mechanism of the ferromagnetic coupling in the Gd₅ Si₄ -based (x >0.5) and Gd₅ Ge₄ -based (x 0.2) solid solutions is the same as that in the intermediate phase Gd₅ (Si_x Ge_1-x )₄(0.24x 0.5), i.e. the ferromagnetic Gd-(Gd, M)-Gd blocks couple with each other via an M₂layer, regardless of their crystal structures at room temperature. The M-M bond length in the M₂layer is a crucial structural parameter governing the ferromagnetic ordering temperature of the compounds. The orthorhombic structure of the intermediate phase at low temperature is stabilized by the ferromagnetic exchange interaction.

We report results of tunneling studies on the charge ordering compound Nd_0.5 Sr_0.5 MnO₃in a magnetic field up to 6 T and for temperature down to 25 K.We show that a gap (2_CO 0.5 eV opens up in the density of state (DOS) at the Fermi level (E_F ) on charge ordering (T_CO= 150 K) which collapses in an applied magnetic field when the charge ordered state melts. There is a clear correspondence between the behaviour of the resistivity and the gap formation and its collapse in an applied magnetic field. We conclude that a gap in the DOS at E_Fis necessary for the stability of the charge ordered state.

The electron paramagnetic resonance (EPR) spectrum as a function of Tis experimentally studied for (La,Y)_2/3 (Ca,Sr,Ba)_1/3 MnO₃with a wide span of T_C . In the real paramagnetic regime (TT_min ), the spectrum consists of a single line with the resonance field being independent of T . The resonance linewidth is given by H_pp= H_{pp ,min} +b (T -T_min ). The exchange-narrowing spin-spin interaction and the spin-lattice interaction are explained to be responsible for the terms of H_{pp ,min}and b (T -T_min ), respectively. Based on the exchange-narrowing mechanism, the values of H_{pp ,min}are directly deduced from the T_C -values, which are well consistent with the experimental H_{pp ,min} -values. This confirms the possibility that the exchange-narrowing spin-spin interaction exists in the ABO₃ -type perovskite manganites.

SrTiO₃and KTaO₃are incipient ferroelectrics whose phase transition temperature is suppressed by quantum fluctuations. The recent observation of induced ferroelectricity due to exchange of ¹⁶ O by ¹⁸ O (1999 Phys. Rev. Lett.823540) is reproduced for the dielectric data by an anharmonic electron-phonon interaction model, and a first order phase transition is predicted. Furthermore it is shown that this isotope effect does not occur in KTaO₃since its local double-well potential is much shallower than that in SrTiO₃ .

Highly transparent carbon nitride thin films have been prepared on transparent plastic substrates at low temperatures by radio frequency reactive magnetron sputtering of a graphite target in the plasmas of pure nitrogen and nitrogen-hydrogen gas mixtures. The amorphous films thus obtained have been characterized for their optical transmittance by spectrophotometry, revealing a very high transparency in the visible region of the spectrum. The chemical composition and bonding states of a sample studied by x-ray photoelectron spectroscopy (XPS) showed that nitrogen has been predominately bonded to sp³ -hybridized carbon, in addition to a small amount of sp² -hybridization. The elemental concentration was measured to be C = 51.1 at.%, N = 43.2 at.%, and O = 5.7 at.%. The N/C ratio for the N-sp³ C component is calculated to be 1.29, close to the expected stoichiometric value of 1.33 for -C₃ N₄ . Compared to the unhydrogenated C-N thin films, the hydrogenated samples exhibited increased optical transmittance, about 90% in the whole of the visible spectrum. Such transmittance characteristics of the hydrogenated C-N films closely matched those exhibited by the plastic substrate.

The grain size effect of lead zirconate titanate PbZr_1-x Ti_x O₃(PZT, x 0.6) caused by surface bond contraction has been investigated by using the Landau-Ginsburg-Devonshire (LGD) phenomenological theory. It has been shown that, due to the surface bond contraction, both the Curie temperature and the spontaneous polarization of tetragonal PZT decrease with decreasing grain size. These effects become more significant when the grain size is in the nanometre range. A dielectric anomaly appears with decreasing grain size, which corresponds to a size dependent phase transformation. The ferroelectric critical size below which a loss of ferroelectricity will happen is estimated from the results obtained.

The resistivity of Pr_0.65 Ca_0.35 MnO₃films prepared by pulsed laser deposition has been measured in the temperature range 4.2 - 300 K. The formation of metallic phase is suggested by the analysis of the temperature dependence of resistivity at low temperatures under zero-magnetic field. It is shown that the appearance of the charge-ordered state at 205 K can be controlled by the lattice strains accumulated during the film growth. The charge-ordering energy gap was estimated to be 77.5 meV from the experimental data. Experimental results are interpreted on the basis of the phase-separation model.

This article gathers together a collection of recent experimental studies of the adsorption of oxygen on (001), (110) and (111) crystal surfaces of silver with special emphasis on the negative ion states of this model system for oxygen adsorption. These investigations were performed in a network entitled `Negative ion resonances of adsorbed molecules' supported financially by the European Union within the `Human capital and mobility programme'. The kinetics and thermodynamics of adsorption are investigated by measuring the sticking coefficient and by thermal desorption spectroscopy (TDS). The vibrational spectra provided by high-resolution electron energy loss spectroscopy (HREELS) are used to analyse the adsorbed species (physisorbed and chemisorbed) in the case of O₂on Ag(110) and on Ag(111). The mechanisms of inelastic electron scattering by adsorbed O₂are further investigated with special reference to the negative ion resonances (NIRs), formed by electron capture, which are involved in the electron-molecule collision process.

We have investigated CO adsorption at 300 K on ~1.5 atomic layer thick Pd films on a Mo(110) surface by high-resolution core level photoemission. We describe how high-resolution core level spectroscopy may be utilized to study the influence of laterally mobile states on the sticking probability of molecules on such a laterally heterogeneous surface. The present Pd films are laterally heterogeneous in the sense that the additional ~0.5 atomic Pd layer forms mesoscopic one-layer thick islands on top of the first Pd layer. At 300 K, CO chemisorbs on these two-layer thick islands but not on the one-layer parts of the film. The rate at which these two-layer islands are filled by CO molecules as the surface is exposed to CO is found to be consistent with a picture where CO molecules that initially impinge on the one-layer parts of the surface enter a laterally mobile state and diffuse to the two-layer islands and adsorb there. This mobile state is in many respects similar to a classical precursor state.

We use scanning tunnelling microscopy, Auger electron spectroscopy and low energy electron diffraction to study different tungsten and carbon terminated surface reconstructions on the sputtered/annealed WC(0001) surface. The tungsten terminated surface encompasses a ( 7 × 7)R19° W-trimer structure, a ( 3 × 7) reconstruction representing a transition structure from the ( 7 × 7)R19° reconstruction to a (6 × 1) phase which consists of a quadratic W overlayer on the first close-packed carbidic carbon layer. The carbon terminated WC(0001) surface consists of a single graphitic carbon layer on top of the (6 × 1) structure.

Deposition of Co on Cu(111) substrate by laser ablation changes the morphology, structure and magnetic properties of Co ultrathin films. The pulsed laser deposition method induces a good layer-by-layer growth of Co films and delays the face centred cubic to hexagonal close packed structural phase transition in the films. As a result, the pulsed laser deposited films retain an in-plane easy axis of magnetization at all thicknesses investigated (0-15 monolayers Co) while the thermally deposited films also exhibit a hysteresis loop in the perpendicular direction as soon as the hexagonal close packed structure is dominant in the film. In addition, the suppression of hexagonal close packed stacking in the pulsed laser deposited films leads to a complete antiferromagnetic coupling in Co/Cu/Co/Cu(111) trilayers for both in-plane and out-of-plane magnetization directions.

We discuss the correct expression for the classical electrostatic energy used while analysing scanning probe microscopy (SPM) experiments if either a conducting tip or a substrate or both are used in the experiment. For this purpose a general system consisting of an arbitrary arrangement of finite metallic conductors at fixed potentials (maintained by external sources) and a distribution of point charges in free space are considered using classical electrostatics. We stress the crucial importance of incorporating into the energy the contribution coming from the external sources (the `battery'). Using the Green function of the Laplace equation, we show in a very general case that the potential energy of point charges which are far away from metals is equally shared by their direct interaction and the polarization interaction due to charge induced in metals by the remote charges (the image interaction). When the charges are located close to the metals, there is an additional negative term in the energy entirely due to image interaction. The exact Hamiltonian of a quantum system interacting classically with polarized metal conductors is derived and its application in the Hartree-Fock and the density functional theories is briefly discussed. As an illustration of the theory, we consider an interaction of several point charges with a metal plane and a spherical tip, based on the set-up of a real SPM experiment. We show the significance of the image interaction for the force imposed on the tip.

We have determined the normal modes of ice VIII, a 16-molecule supercell, using ab initiocalculations. The dynamical matrices so obtained are diagonalized and the eigenvectors projected on to the stretching, bending and libration modes of each water molecule in turn. We can therefore accurately correlate the symmetry assignments of the zone centre modes and the corresponding frequencies of these dynamical results with Raman, infrared and neutron scattering data. In particular, using neutron scattering data, we obtain excellent agreement with the frequencies of the _Ry B_1uphonon mode which is inactive in Raman or infrared spectroscopy.

The ultrasonic longitudinal and transverse sound velocities in the single-phase polycrystalline compound La_1-x Ca_x MnO₃(x= 0.33, 0.63, 0.73, 0.83) have been measured by a conventional pulse-echo-overlap technique at a frequency of 10 MHz, between 50 K and 300 K. Dramatic anomalies in sound velocity for both longitudinal and transverse modes were observed near the temperature of the ferromagnetic transition (T_C ), charge-ordering transition (T_CO ), and antiferromagnetic transition (T_N ). The detailed form of the sound velocity anomalies is somewhat different near different phase transitions. The results imply a strong spin-phonon coupling.

Polarized neutron diffraction experiments on YbAl₃have revealed the following: (i) a field-induced magnetic moment on the ytterbium site only, which is well described by a Yb³⁺free-ion form factor; (ii) a second maximum at T 15 K in the temperature dependence of the localized magnetic moment in addition to the well-known maximum at T= 130 K; and (iii) a small temperature-independent positive conduction electron polarization. The possible microscopic origin of those observations is discussed. A comparison with the isostructural intermediate-valence compound CeSn₃reveals important differences which increase our understanding of the ground state in those compounds.

Inflation symmetry is one of the peculiar features of the diffraction pattern of a quasicrystal. However, it is not an exclusive property of quasicrystalline structures and it may be present in incommensurately modulated structures, as shown recently in the Al-Mg system (Donnadieu P et al1996 J. PhysiqueI 61153-64). The conditions that a single modulation parameter of an incommensurate structure must fulfil in order to have inflation symmetry are determined. Although the number of possible distinct inflation-symmetric quasilattices is infinite, from physical/experimental arguments it can be concluded that, in practice, only a few of them can be experimentally observed, the reported phase of the Al-Mg system being one of these particular cases. A quantitative criterion to classify the modulation parameters that give rise to quasilattices with observable inflation symmetry is proposed. The generalization of the analysis of incommensurate structures with more than one single modulation parameter is also discussed. Finally, the inflation parameters of diffraction patterns with rotational point groups of finite order, C_N , are compared with the parameters of the one-dimensional case.

In silicates, generally the less dense open structures of lower pressure polymorphs have four-coordinated silicon whereas the denser more close-packed structures have six-coordinated silicon. Transformation from the fourfold coordinated crystals to the denser sixfold coordinated structures upon compression usually requires an accompanying high temperature that is much greater than ambient. We report here a first-order, reversible SiO₄ -SiO₆coordination change in a crystalline silicate (MgSiO₃ ) orthoenstatite at ambient temperature and quasi-hydrostatic conditions using Raman spectroscopic measurements in a diamond cell to 70 GPa. Annealing experiments using a CO₂laser, which controlled defect concentration, indicate that the four- to sixfold coordination change is facilitated by defects in the structure which may serve as nucleation sites for the formation of the octahedrally coordinated silicon phase.

The ultrasonic logarithmic decrement and modulus defect in high purity copper crystals was measured at 10, 30 and 50 MHz in the temperature interval 5-373 K. The samples were deformed at room temperature in the 3-20% range along the 111 crystallographic direction. The experimental data were fitted over the whole interval of temperatures, assuming the contribution of two kink mechanisms: (i) relaxation by kink pair formation with diffusion in the dislocation line and (ii) overdamped resonance of the kink chain with a temperature dependent number of kinks in the dislocation lines. With this procedure both primary and secondary properties of the high frequency Bordoni peak could be satisfactorily explained. Numerical data are reported for peak parameters and for the kink diffusion coefficient.

The possible formation of solitons in a Frenkel exciton system, with exciton-exciton interaction only, is studied taking into account the Pauli character of exciton operators. This is realized by choosing the trial function of the system in the form which is the particular representation of the spin coherent state for spin 1/2. Possible solutions include both `bright' and `dark' solitons. Strict conditions for the existence and stability of particular types of soliton are formulated by imposing limitations on the values of the system parameters and soliton velocity. The energy-momentum relation for both types of solution is obtained. It is concluded that neither kind of soliton can exist if the effective dynamical interaction is a repulsive one. Recent references concerning `bright' and `dark' solitons are critically assessed.

The mechanism of the formation of ionic and bonding defects and the interaction between protons and heavy ions in hydrogen-bonded molecular systems is investigated and a new model is constructed. This model considers two coupled sublattices corresponding to those of protons and heavy ions in order to study the dynamic properties resulting from deformation and local fluctuation of the heavy ion sublattice due to the protonic displacements. As compared with other models, the model emphasizes, in particular, the collective effect of the motion and the mutual correlation between the two sublattices. The equations of motion admit analytic soliton solutions of different types corresponding to two different defects. The combined effect of the two defects and the proton transfer in the system are described by means of these solutions, and the appropriate conditions associated with actual physical processes. Finally, we discuss in detail the characteristics of the proton transfer and the advantages of the model.

The self-consistent band structure and the density of states of the layer compound PtSSe were calculated using the linear muffin-tin orbital method in the atomic sphere approximation (LMTO-ASA). This calculation includes the relativistic mass-velocity and Darwin terms but excludes the spin-orbit coupling. The Barth-Hedin local exchange correlation potential was used. The result, reported for the first time, predicts that PtSSe is a semiconductor with an energy gap of about 0.64 eV. Such a behaviour is confirmed by the experimental data available. The self-consistent band structure of PtS₂and PtSe₂were calculated, by the same method, and the effect of the anion substitution in the sequence PtS₂ PtSSe PtSe₂have been studied. The results are discussed in terms of charge transfer and local coordination of the constituent atoms.

In the present paper we report theoretical calculations for the Hugoniot of shock-compressed rubidium. We use an all-electron full-potential method based on density-functional theory to obtain the total energy, pressure and electronic density of states. Thermal contributions to the equation of state (EOS) were introduced through temperature dependent occupation of the electronic states and a Grüneisen model was used for determining the nuclear thermal motion. The calculated Hugoniot is in very good agreement with the shock experiments. A temperature of about 10 000 K is reached at the pressure of about 300 kbar, the highest reached experimentally. Rubidium was chosen for this study because it undergoes a sequence of unusual pressure-induced structural transitions which have been attributed to s-d electron transfer. Since most studies of s-d transfer have been carried out at temperatures below 500 K, little is known of how it effects the equation of state at very high temperature. We found that the predicted Rb Hugoniot is very sensitive to the thermal s-d electron transfer, which leads to a considerable lowering of the predicted Hugoniot pressure and temperature.

We have synthesized perovskite oxide BaPb_1-x Zr_x O₃by solid-state reaction in different atmospheres and performed x-ray diffraction (XRD) analyses, electrical resistivity and thermoelectric power measurements on them. Nearly single-phase samples with cubic symmetry were obtained in the range 0.0<x 0.25. As xincreased, a metal-insulator (MI) transition occurred at a critical doping level x_c , which depended on the sintering atmosphere of the samples. Meanwhile, a jump in resistivity was found. At low temperatures (13 KT 100 K), the temperature dependence of resistivity followed the power law when 0<x <x_cand the hopping law when xx_c . When prepared in oxygen and in air, the resistivity decreased linearly with the rising temperature in the range between 100 and 300 K. However, the unusual linear behaviour was absent when the doped oxides were sintered in N₂ . An anomalous broad peak was also observed in the thermoelectric power of the air-sintered samples. The results are discussed in terms of the effects of disorder and electron-electron interactions, oxygen vacancies and the crystal-field splitting of the 4d levels of transition metal Zr.

We investigate analytically the quantum phase transition in a system of two chains of ultrasmall Josephson junctions, coupled capacitively with each other. Two different schemes of the coupling, straight and slanted, between the two chains are considered. In both coupling schemes, as the coupling capacitance is increased, the transport of particle-hole pairs is found to drive a quantum phase transition of the Berezinskii-Kosterlitz-Thouless type from insulator to superconductor. A substantial discrepancy between the transition points in the two coupling schemes is observed, which reflects the difference between the transport mechanisms.

A model of a two-leg spin-Sladder with two additional frustrating diagonal exchange couplings J_D , J_D ´ is studied within the framework of the nonlinear sigma model approach. The phase diagram has a rich structure and contains 2Sgapless phase boundaries which split off the boundary to the fully saturated ferromagnetic phase when J_Dand J_D ´ become different. For the S= 1/2 case, the phase boundary is identified as separating two topologically distinct Haldane-type phases as discussed recently by Kim and co-workers (Kim E H, Fáth G, Sólyom J and Scalapino D J 1999 e-printcond-mat/9910023).

Ceramics of A₂ FeReO₆double perovskites have been prepared and studied for A = Ba and Ca. Ba₂ FeReO₆has a cubic structure (Fm3m)with a 8.0854(1) Å whereas Ca₂ FeReO₆has a monoclinic symmetry with a 5.396(1) Å, b 5.522(1) Å, c 7.688(2) Å and = 90.4° (P21/n) . The barium compound is metallic from 5 K to 385 K, i.e. no metal - insulator transition has been seen up to 385 K, and the calcium compound is semiconducting from 5 K to 385 K. At 5 K, we observed a negative magnetoresistance of 10% in a magnetic field of 50 kOe for Ba₂ FeReO₆ . Magnetization measurements show a ferrimagnetic behaviour for both materials, with T_c 315 K for Ba₂ FeReO₆and above 385 K for Ca₂ FeReO₆ . A specific heat measurement on the barium compound gave an electron density of states at the Fermi level, N( E_F) , equal to 5.9 × 10²⁴eV^-1mol^-1 . Electrical, magnetic and thermal properties are discussed and compared to those of the analogous compounds Sr₂ Fe(Mo,Re)O₆ .

The magnetization and magnetic torque of the triangular antiferromagnetic system CsCu_1-x Co_x Cl₃with x 0.032 have been measured in magnetic fields up to 7 T. CsCuCl₃is a spin-½ triangular antiferromagnet with planar anisotropy, and has a 120° spin structure in the c -plane below T_N= 10.5 K. It is observed that the macroscopic anisotropy changes from the planar type to the axial one with increasing Co²⁺concentration x . It is found that the small amount of Co²⁺doping produces a new ordered phase in the low-temperature and low-field region, i.e., the present system can undergo two phase transitions at zero field. The transition field at which the new phase becomes unstable is minimum for Hcand maximum for Hc . With increasing Co²⁺concentration x , the area of the new phase is enlarged in the phase diagram for temperature versus magnetic field. The new phase has a weak spontaneous magnetization in the c -direction.

A local moment approach is developed for single-particle excitations of a symmetric Anderson impurity model (AIM) with a soft-gap hybridization vanishing at the Fermi level: _I | |^r , with r >0. Local moments are introduced explicitly from the outset, and a two-self-energy description is employed in which single-particle excitations are coupled dynamically to low-energy transverse spin fluctuations. The resultant theory is applicable on all energy scales, and captures both the spin-fluctuation regime of strong coupling (large U ), as well as the weak-coupling regime where it is perturbatively exact for those r -domains in which perturbation theory in Uis non-singular. While the primary emphasis is on single-particle dynamics, the quantum phase transition between strong-coupling (SC) and local moment (LM) phases can also be addressed directly; for the spin-fluctuation regime in particular a number of asymptotically exact results are thereby obtained, notably for the behaviour of the critical U_c (r ) separating SC/LM states and the Kondo scale _m (r ) characteristic of the SC phase. Results for both single-particle spectra and SC/LM phase boundaries are found to agree well with recent numerical renormalization group (NRG) studies; and a number of further testable predictions are made. Single-particle spectra are examined systematically for both SC and LM states; in particular, for all0r <½, spectra characteristic of the SC state are predicted to exhibit an r -dependent universal scaling form as the SC/LM phase boundary is approached and the Kondo scale vanishes. Results for the `normal' r= 0 AIM are moreover recovered smoothly from the limit r 0, where the resultant description of single-particle dynamics includes recovery of Doniach-Sunjic tails in the wings of the Kondo resonance, as well as characteristic low-energy Fermi liquid behaviour and the exponential diminution with Uof the Kondo scale itself. The normal AIM is found to represent a particular case of more generic behaviour characteristic of the r >0 SC phase which, in agreement with conclusions drawn from recent NRG work, may be viewed as a non-trivial but natural generalization of Fermi liquid physics.

X-ray powder diffraction, magnetization, electrical resistivity and neutron diffraction experiments were performed on the dense Kondo system U₂ (Ni_1-x Pt_x )₂ In. Up to x 0.5 the alloys crystallize in a tetragonal structure with space group P 4/mbmwhile for x 0.6 they crystallize in space group P 4₂ /mnm . The observed structural change coincides with the magnetic-nonmagnetic crossover in the system. In the concentration range 0x 0.5, an antiferromagnetic ordering occurs, characterized by a noncollinear arrangement of the magnetic uranium moments in the tetragonal basal plane. In addition, for x= 0 and 0.2 a commensurate magnetic ordering is formed with a propagation vector k= (0,0,0.5) while for x= 0.5 an incommensurate one with k= (0,0,0.5± ), = 0.06 is found. In the concentration range 0.6x 1.0 no magnetic ordering down to 1.7 K is observed. The bulk and neutron scattering data exhibit features attributed to the interplay of the Kondo effect and magnetic exchange interactions. For interpretation of the magnetic behaviour of the U₂ (Ni_1-x Pt_x )₂ In alloys, a strong modification of the density of states at the Fermi surface has been invoked.

The localization and scaling behaviour of quasiperiodic structures are studied for a geometry where the magnetization is perpendicular to the interfaces of the superlattices. Numerical results for the bulk and surface spin waves in the magnetostatic regime are presented for the Fibonacci, Thue-Morse and period-doubling sequences. The results are obtained for both ferromagnetic and antiferromagnetic ordering by using the transfer-matrix method. Interesting features of the localized modes are shown for Fe, EuS and MnF₂ .

The results of theoretical study of the influence of mechanical stress, electric and magnetic fields on the conditions for magneto-elastic solitons existence and their dynamic properties in the symmetric phase of a tetragonal antiferromagnet are presented. The conditions for the occurrence of nonlinear magneto-elastic resonance of long and short waves (which is analogous to the Zakharov-Benney triad resonance) in this system are derived. The possibility of experimental detection of this new phenomenon is discussed.

The origin of the interesting magnetic properties of GdMn₆ Ge₆in the low-temperature, low-field phase is a spiral arrangement of the magnetic moments. This has been established by pulsed nuclear magnetic resonance experiments. Spin-echo spectra taken in an external magnetic field, and the observed enhancement of the radio-frequency field, suggest the presence of a spiral structure. This information is crucial for a quantitative analysis of the complicated ⁵⁵ Mn spectra. The anisotropy of the hyperfine field at the ⁵⁵ Mn site is about 10% and the enhancement has been found to be highly anisotropic, as well. Comparing the temperature dependencies of the Zeeman splitting at the individual lattice sites, Mn has been identified as the source of the transferred hyperfine fields at the ⁷³ Ge nuclei. An analysis of the ⁷³ Ge and ^155/157 Gd spectra yields the size and relative orientation of the hyperfine fields and electric field gradients at the nuclei in agreement with ¹⁵⁵ Gd Mössbauer spectroscopy results.

High-resolution neutron diffraction and ⁵⁷ Fe Mössbauer experiments have been performed on powder samples of the ternary intermetallic YbFe₆ Ge₆ . This compound crystallizes with a hexagonal structure (P 6/mmm ) which can be described as an ordered state intermediate between the HfFe₆ Ge₆ - and YCo₆ Ge₆ -type structures, with cell parameters suggesting that the Yb ion is in (or close to) a trivalent state. At room temperature, the Fe-sublattice magnetic arrangement consists of an antiferromagnetic stacking along the c -axis of the ferromagnetic (001) Fe planes with the easy direction of magnetization along [001]. Below about 85 K a spin-reorientation process occurs; a fraction (T ) of the iron moments rotate abruptly from the c -axis to a given direction in or close to the basal plane. This phenomenon allows the observation of anisotropic contributions to the total hyperfine field at the Fe site. At 4.2 K, approximately 18% of the iron moments remain along the c -axis. No long-range magnetic order of the Yb sublattice is observed, at least above 1.5 K. We also report ⁵⁷ Fe Mössbauer investigations of the HfFe₆ Ge₆ -type LuFe₆ Ge₆compound.

The influence of probing laser power density on the band-edge and the trap luminescence from CdS_x Se_1-xnanoparticles embedded in a glass matrix is reported. Both the position and the strength of the band-edge luminescence are found to be very sensitive to the laser power. It is observed for the first time that the band-edge luminescence shifts initially towards low energy and then towards high energy with increasing laser power. The results are analysed in terms of laser-induced local heating and a band-filling mechanism, respectively, which are found to explain adequately the experimental observations. The shift due to the latter shows up only beyond a threshold value of the probing laser power density. The trap luminescence shows large contributions from surface states and undergoes photodarkening at high laser power, the degree of which depends strongly on the local temperature of the nanoparticles during illumination; this is reported here for the first time. This effect is explained in terms of a permanent trapping of charge carriers due to optical processes and thermally assisted optical processes.

Requirements on the energy resolution for the detection of an exciton by the x-ray transmission, fluorescence, photocurrent and reflectivity techniques have been considered. X-ray reflectivity appears to be the most suitable technique for exciton detection. An excitonic origin of the narrow peak in the Si L_2,3reflectivity spectrum is suggested.

The method for quantitative determination of interatomic distances and coordination numbers (CN) from experimental K-XANES of light metallic atoms in crystalline compounds with distorted local structure is proposed, based on the extraction of the first shell term and the multiple-scattering term. In crystalline minerals the extraction of these terms can be obtained under uncertainties of ~0.3 - 0.4 Å in the values of the second and more distant shell radii, using the unambiguous diffraction data on cell unit parameters and the type of point symmetry. The fitting of the extracted first shell term by theoretical terms, generated within alternative models for the split of this shell under the local structure distortions in the environment of the absorbing metallic atom, permits one to obtain the interatomic distances and CN in reasonable quantitative agreement with the diffraction data for the reference compounds studied.

The transition between cubic and tetragonal phases in KMnF₃has been studied by x-ray diffraction rocking curves and calorimetry. Comparison of the excess entropy with the order parameter Qobtained from spontaneous strain shows that the mean field relationship SQ²is obeyed to within experimental error. The data are fitted to a Landau free energy expression G= ½A (T -T_C )Q² +(1/4) BQ⁴ +(1/6)CQ⁶ , with A= 2.781 J K^-1mol^-1 , B= -57.63 J mol^-1 , C= 574.2 J mol^-1 , T_C= 185.76 K. No significant excess specific heat is found at T >>T_C .