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The structural and magnetic properties of compounds in the series Sm₂Fe₁₅Al₂C_x have been investigated by means of x-ray diffraction (XRD) and magnetization measurements. Samples with x=0, 0.5, 1.0, 1.5, and 2.0 were prepared by the arc-melting method. XRD patterns indicate that these carbides are single phase with a rhombohedral Th₂Zn₁₇-type structure, except for Sm₂Fe₁₅Al₂, which contains a few per cent of BCC alpha -Fe. The saturation magnetization for all samples is about 110 emu g^-1 and has a small dependence on C concentration. The Curie temperature T_c enhancement is about 190 K for x=1.5 compared with that of Sm₂Fe₁₇. All compounds exhibit an easy c-axis anisotropy at room temperature and the anisotropy field increases to about 110 kOe when x<or=1.5. The substitution of Al, like Ga and Si, is found to not only help the formation of high-C-concentration rare-earth-Fe compounds with 2:17-type structure, but also increase the anisotropy field.

Mossbauer effect measurements at room temperature and at temperatures from 80 K down to 10 K were performed for the polycrystalline La_1.85Ba_0.15Cu_0.995Fe_0.005O_3.97 superconductor with T_c(onset)=25 K. A lattice softening was found to occur below T_c(onset) and was ascribed to a structural instability, which may be associated with the appearance of the superconductivity. In addition, asymmetry and line broadening of the Mossbauer quadrupole splitting spectrum at room temperature were observed and can be attributed to a distortion of the octahedral O coordination around the Cu atom in the sample.

The hard magnetic properties of a new interstitial nitride Sm₃(Fe_0.933Ti_0.067)₂₉N_y (y approximately 5) have been investigated by ball milling. The saturation magnetization linearly decreases with ball-milling time. By ball milling, maxima of remanence B_r=1.04 T, intrinsic coercivity mu _0iH_c=0.83 T and energy product (BH)_max=105 W m^-3 are achieved at 293 K.

The growth mode, electronic structure and structural properties of the Ce/Fe(100) interface have been investigated by Auger spectroscopy, XPS and RHEED experiments. At 650 degrees C, an epitaxial Ce_xFe_y compound is formed. The surface lattice is of square symmetry, within the limit of experimental error, with a parameter a_s=4.05+or-0.15 AA. This compound is found to be in a mixed-valence state, with a stoichiometry close to Ce₂Fe₁₇. Modifying the detection angle in order to increase surface sensitivity, XPS spectrum analysis shows a gamma -like Ce-enriched surface. At room temperature, Ce grows in a disordered phase without interdiffusion.

The fluctuations of a train of steps on a vicinal surface at equilibrium and during evaporation are studied in the framework of Burton, Cabrera and Frank's theory. Step fluctuations are treated as perturbations of the straight step's shape and within a linear analysis the step morphological stability is investigated in a general way. Previous results of Bales and Zangwill and of Uwaha and Saito are obtained as special cases when asymmetry between kinetic coefficients for adatom attachment at step edges (the 'Schwoebel effect') is included. A strong Schwoebel effect is known to lead to step bunching and to smoothing of an isolated step during evaporation. Here we show that a strong Schwoebel effect leads instead to roughening of steps in a train at evaporation temperatures. Furthermore, we show that the Schwoebel effect is negligible on vicinal surfaces with widely spaced steps at evaporation temperatures. For both reasons, we conclude, in contrast with Uwaha and Saito, that the observed phenomenon of kinetic step smoothing during silicon evaporation cannot be justified by the presence of a strong Schwoebel effect. The experimental situation is discussed and a scenario is proposed that does not invoke a strong Schwoebel effect.

Measurements are reported of Auger (autoionization) spectra of Ar and Kr produced by bombarding Al and Si substrates with Ar⁺ and Kr⁺ ions in the 110 eV-5 keV energy range. These are shown to be consistent with the simple Doppler model suggested, for Ne on Al and Si, in a previous paper. Once corrected using the model, the observed Auger energies are shown to correspond to theoretical predictions produced using Dirac-Fock calculations.

We report variations of between 10.7 and 15.8 AA in the periodicity of oscillatory interlayer exchange coupling of (111)-oriented Cu_1-xGe_x-Co and Cu_1-xNi_x-Co multilayers which depend on the germanium and nickel concentrations. These results agree with theoretical predictions that relate the exchange coupling periodicity to the diameter of the neck of the Fermi surface of Cu-Ge or Cu-Ni alloys.

We have carried out sub-monolayer sensitivity measurements of laser-induced Ga⁰ emission from the GaP(110) and GaAs(110) surfaces using a resonant ionization technique. The laser ablation threshold fluence is defined as the critical fluence above which an increase of the Ga⁰ emission yield is observed by repeated irradiation with laser pulses on the same spot but below which a decrease of the yield is observed. The photon-energy dependences of the laser ablation threshold fluence and the sub-ablation emission yield have been investigated for photons below, near and above the bulk band-gap energies in both GaP and GaAs. For photons below the band-gap energies, the sub-ablation emission and laser ablation are observed and ascribed to photon absorption by defects on the surfaces. The sub-ablation emission is found to be reduced substantially at a photon energy slightly below the band-gap energies for both GaP and GaAs; the reduction is of resonance type for GaAs and stepwise for GaP. Only a small amount of emission is observed for GaP for photons above the band-gap energy. The ablation threshold fluence for GaAs shows a resonant-type increase, corresponding to me resonance-type reduction of the sub-ablation emission yield, while the ablation threshold fluence for GaP does not change on crossing the band-gap energy. For photons above the band-gap energies, the ablation threshold fluence decreases with increasing photon energy in a similar manner for both GaP and GaAs. The decrease in the ablation threshold fluence for the GaAs(110) surface appears to be correlated to the increase in the sub-ablation emission yield as observed previously. These results of the photon energy dependence of the sub-ablation emission yield and ablation threshold fluence are explained in terms of the electronic excitation of defects on surfaces and of surface occupied states.

The mobility and thermopower of a 2DEG are calculated in order to explain measurements in two Si-MOSFETs. In one of the samples at T<2 K and for electron densities N_s>10¹⁶ m^-2, positive thermopowers have been measured. The change of sign in the thermopower is attributed to the dominance of interface roughness scattering. Simultaneous consideration of two transport properties indicates the inadequacy of the current theory of interface roughness to explain electron scattering by interface irregularities in one of the samples. At T>2 K, thermopower is explained by phonon drag and good agreement is found between theory and experiment.

The effect of formation in a neon cryocrystal of atomic-type 2p⁵3s ³P₂ centres obtained as a result of He gas discharge products being trapped in a growing neon cryocrystal is discovered. The unstable excited 2p⁵3s³P₂ states in the neon cryocrystal were detected by ESR. This effect is interpreted as a new phenomenon: quasi-resonance transfer of excitation energy from the metastable He 2³S₁ atom trapped in a growing neon cryocrystal to the exciton energy band of the neon crystal followed by the exciton self-trapping into the 2p⁵3p state and subsequent decay, ending in the 2p⁵3s ³P₂ state recorded by ESR in our experiment. A temperature study showed that the rate constant lambda of the process of excitation energy transfer from the He 2³S₁ atom to the neon cryocrystal exciton band follows the Arrhenius-like law lambda - lambda ₀ exp(-E/kT); the 'activation energy' E of the process turns out to be 0.0010(5) eV.

A perturbative approach for large (in comparison with electron overlap and onsite disorder) long-range Coulomb interaction is developed for a 1D ring model. The explicit expression for low-lying energy levels has been obtained. It is shown that in the case of strong repulsive long-range interaction persistent current does not depend exclusively on the filling factor nu =p/q but is extremely sensitive to its denominator q. It also depends on the common factor M of electrons and sites number. In the absence of a common factor (M=1) there exists only one quasiparticle that experiences no strong Coulomb interaction, thus moving freely with bare electron overlap, which provides an unexpectedly large persistent current. For the case of an arbitrary M and a large enough q a system of M weakly interacting quasiparticles has been introduced, which undergoes a metal-insulator transition at unusually small overlap even for a filling factor close to one half. Suppression of the persistent current by disorder is taken into account. Comparison of analytically obtained expressions with the results of numerical calculations shows a reasonable agreement.

The theory of the AC Hall effect for systems which consist of randomly oriented anisotropic crystallites is developed. It is shown that the real parts of the dynamic conductivity, the Hall conductivity, the Hall mobility and the Hall coefficient have low- and high-frequency plateaux. The high-frequency plateau begins in the vicinity of the frequency omega ₀ approximately= tau _M^-1, where tau _M is the Maxwell relaxation time. The imaginary parts of these quantities have sharp peaks at the frequency omega ₀.

The dynamic effect of Bloch electrons in a spatial periodic system under the influence of a driving laser field is studied within the single-band approximation. A quasienergy band is obtained exactly for the case of long-range intersite interactions. It is found that the quasienergy band will be suppressed heavily by the laser field if the ratio of the Bloch frequency to the laser frequency is a root of the ordinary Bessel function of order zero. If only the nearest-neighbour intersite interaction is involved, the band suppression will turn into the band collapse proposed very recently. However, a numerical calculation of the band width shows that in comparison with the nearest-neighbour intersite interaction, the contribution of other intersite interactions to the quasienergy is not negligible. This may imply that the experimental observation of the dynamic effect will be that of band suppression rather than band collapse.

The infinite-U one-dimensional Hubbard model is 'solvable' in the limit that the nearest-neighbour hopping is dominant and infinitesimal longer-range hopping lifts the one-dimensional spin degeneracy. In this limit the Hubbard model maps onto an effective spin interaction: cyclic or ring exchange with a variety of lengths and strengths. For bipartite geometries we find a region of stable ferromagnetism, which originates from the mechanism intrinsic to Nagaoka's ferromagnetism. We look at connectivities that limit exactly to the square-lattice connectivity as the range of the infinitesimal hopping diverges. When we extend this hopping range, we find that the stable region of ferromagnetism shrinks, eventually vanishing as the infinitesimal hopping range diverges and we limit to the square-lattice geometry. Since the calculation involves a non-trivial density of particles in the thermodynamic limit, it suggests how the proposed region of ferromagnetism may become lost for the case of the two-dimensional square lattice, as numerical simulations predict.

We present a new class of variational wavefunctions that can be used to investigate quantum antiferromagnetic systems. The states are a natural generalization of the nearest-neighbour dimer phases which have been thoroughly studied in the literature. We describe a one-dimensional class of total-spin-singlet wavefunctions, which include as special cases the two nearest-neighbour dimer phases and the exact solution to the total-spin-two projector Hamiltonian. These different solutions are 'smoothly' interpolated by a range of singlets with short-range correlations. Although the states have only short-range order, we can obtain over 98% of the ground-state energy of the nearest-neighbour Heisenberg model. We apply our basis to the J₁-J₂ Heisenberg model and the single hole in the t₁-t₂ infinite-U Hubbard model, successfully predicting the behaviour observed in finite-size scaling calculations.

Low-frequency (1 kHz-1 MHz) dielectric permittivity studies of Cd₂Nb₂O₇ single crystals under a uniaxial pressure of 0 MPa<or=X<or approximately=4 MPa in a wide measurement AC field strength range of 1 V cm^-1< epsilon <110 V cm^-1 are reported. An analysis of the experimental data reveals that we are concerned with two Debye-type relaxation mechanisms in the ferroelectric phase: T<T_c(=196 K)<T_c (=205 K). Both of these are manifested when the frequency or the field epsilon is changed, while only the domain relation is manifested with increasing pressure. The relaxation caused by domain wall motion and by reorientation of spontaneous polarization of domains dominates. The second relaxation has its origin in the orientational polarizability of the steady Cd²⁺O^2- (seventh) dipoles in the measurement AC field. The disordered Cd²⁺-O^2- dipole system in the pyrochlore network is formed in the paraelectric phase. Their orientational polarizability in the external electric field results in an anomalously high static dielectric permittivity epsilon ₀=450 compared with epsilon _infinity =5.43 at room temperature, as well as dielectric dispersion above T_c and T_s.