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Considering in-plane (Cu 3d(x²-y²) and O 2p_sigma ) and out-of-plane (Cu 3d(2z²-r²) and apical O 2p_z) orbitals, a new lowest-energy spin-bag is found for doped cuprates in the strong-coupling regime. While Zhang-Rice correlations are enhanced, a broad structure appears in the Fermi-level region with mainly non-planar character at E_F. Both features may be relevant in the high-T_c superconductivity scenario. This description stems from a mean-field calculation in terms of a general, two-centre, six-band Hamiltonian for 2n*2n (up to 8*8) cuprate clusters. Other previously known solutions are also found and discussed. Different apical O 2p_z levels are considered in order to study their possible effects.

We investigate the critical properties of an S= 1/2 isotropic Heisenberg ferromagnetic film with a simple cubic structure, consisting of two monatomic layers with the upper layer being incomplete due to dilution. We demonstrate the effective two-dimensional character of the critical behaviour of the longitudinal susceptibility for this system and study the dependence of an effective in-plane exchange constant on the concentration of the spins in the upper layer. Within appropriate limits, we compare our results with the two-dimensional percolation theory.

MA_xTe₂ (M=Ta, Nb and A=Si, Ge) compounds show a great variety of commensurately and incommensurately modulated structures depending on the value for x. It is shown that the theory of superspace groups can be used to standardize the structures and to predict the 3D symmetry of the commensurately modulated structures. The commensurately modulated structure of NbGe_3/7Te₂ is determined with the aid of single crystal x-ray data. A comparison with some already published structures in the series shows that all structures with ¹/₃ < or = x < or = ¹/₂ can be regarded as being built from definite numbers of units of the MA_1/3Te₂ and MA_1/2Te₂ type.

It has been found in nominally pure NaCl single-crystal samples that have been either gamma irradiated, plastically deformed or quenched into liquid nitrogen from 700 degrees C, that an additional quenching with a short residence time at the quenching temperature reveals the presence of dipolar defects by means of ionic thermocurrent (ITC) measurements. This procedure does not induce ITC signals in untreated samples. Since the occurrence of divacancies and divacancy clusters in plastically deformed or quenched samples is expected. It is pointed out that gamma irradiation may also form divacancies. The fact that all the ITC spectra of the samples treated in the above-indicated ways exhibit peaks at similar temperatures also supports this view. The parallelism observed between the stored energy spectra of these samples and the evolution of the ITC areas against the quenching temperature indicates that the two phenomena are related. The occurrence of dipolar defects is consistent with a previous proposal: that the stored energy release is due to the recombination of divacancies with alkali halide molecules.

We calculated the vibrational densities of states for three-dimensional Penrose tiling. Anomalous behaviour at low frequencies is observed. Two crossover frequencies omega _s, and omega _c, exist. The first can be attributed to crossover from surface phonon excitations to bulk phonon excitations, while the second is explained as crossover from phonon excitations to fracton-like excitations. The phonons follow the scaling law omega ^d-1, where d is the Euclidean dimension and equals 3 and 2 for the bulk and the surface phonons, respectively. The fracton-like excitations which have a non-extended character follow the scaling law omega ^ds-1 where d_s is the fracton-like dimension and is shown to equal 3 for the present case.

The ab initio pseudopotential method is used to study the effects of the cation potential on the electronic structure of two prototypical III-V nitrides, AlN and GaN. We examine both materials in the wurtzite and zinc-blende structures and compare these results with previous calculations for BN. Effects due to the s, p and d angular components of the Al and Ga potentials are studied. Because the charge densities of both AlN and GaN are dominated by N, the difference in the valence band charge density between these compounds is small. This is the reason that they have similar ground-state structural properties. Individual electronic levels at high-symmetry points, however, are altered by several electronvolts because of changes in the cation potential. This is the origin of the significant difference between the fundamental gaps of these compounds.

For part I see ibid., vol.5, p.5277 (1993). The elastic anomaly observed in the coherent Kondo state of Ce heavy-fermion compounds is analysed using the Anderson lattice model simulating the energy level scheme of the CeTe cubic crystal. The Gamma ₇ doublets and Gamma ₈ quartets of the 4f states are considered in the model. We solve the mean field equations to derive the temperature dependencies of elastic constants, using the random phase approximation for the interaction between the elastic strain and the crystalline field splitting. We compare the calculation with the (c₁₁c₁₂)/2 and c₄₄ constants of CeTe. The presence of the downward dip and the observed overall temperature variations of the two constants are well described by the present theory. The origin of the dip is the coupling between the elastic strain and the splitting of the Gamma ₈ quartets.

Previous theoretical predictions of metallization below 1000 kbar are in apparent disagreement with the experimental fact that nitrogen is observed to remain a molecular solid up to 1300 kbar. The 0 K transition pressure from molecular to monatomic nitrogen is calculated using simple models and a well tested pair potential. The Computed coexistence pressure is found to be 1940 kbar with a volume change of 20 Bohr³/atom. Although there is some uncertainty (about 18%) in the calculated pressure, it is clear that non-molecular nitrogen could be studied at experimentally accessible pressures.

The Carlsson-Gelatt-Ehrenreich inversion technique is applied to the derivation of pair potentials for the elemental bonds C-C and Si-Si, and the compound bond Si-C. Chen's Mobius function method is used to speed up the convergency. The cohesive energies are obtained from the ab initio pseudopotential total energy calculation of Chang and Cohen within the framework of the local-density approximation and fitted to the universal binding energy function of Rose et al. The pair potentials thus obtained reproduce exactly the full cohesive energy curves, lattice constants and bulk moduli.

The generalization of the tight-binding linear-muffin-tin-orbital recursion method for impurity electronic-structure calculations, which takes into account the influence of the nearest neighbour atoms in self-consistent way, is presented. The calculations for boron impurities in BCC iron have been performed and used for the investigation of magnetic behaviour of iron atoms in four nearest-neighbour shells. It is shown that the impurity significantly influences the states of first coordination shell atoms, but does not significantly change their magnetic moments. The mechanisms of magnetic interactions between the impurity and the matrix are discussed, based on the calculated results.

The ground-state energies of both heavy-hole and light-hole excitons in a GaAs/Ga_1-xAl_xAs quantum well have been studied by taking into account the interaction of different optical phonon modes with the excitons. The exciton energies and the corrections to the effective masses are obtained as functions of the well width and the Al concentration. Numerical calculation shows the strong influences of different phonon modes on the exciton behaviour in a quantum well as well as their dependence on the well width and height.

Mossbauer studies are reported on YBa₂(Cu_1-xFe_x)₃O_{7- delta} samples with x=0.02, 0.05, 0.15, and 0.25. Samples are studied in the untreated, fully oxygenated state, as O is progressively removed by vacuum annealing at 400, 500, 600, 700, and 800 degrees C, and after vacuum annealed samples are reoxygenated at low temperature. By following the sites as O is progressively removed at increasing annealing temperatures, we conclude that there is no evidence that Fe substitutes on Cu(2) sites. We interpret our results in terms of models based on neutron diffraction and recent EXAFS experiments. The general tendency at high annealing temperatures is to create sites with smaller quadrupole splittings, which are therefore more symmetric with respect to O coordination, at the expense of sites with less symmetric coordinations. We attribute this to clustering of the Fe atoms, and the increased coordination this creates. Observed magnetic sites are also attributed to clustering. Low-temperature reoxygenation of high-temperature-vacuum-annealed samples restores the original doublets, but with a higher population of symmetric sites, as has been reported in other studies. We conclude that this is evidence of the stability of the clusters to low-temperature reoxygenation. The present results and interpretation lead to consistency between Mossbauer and EXAFS. They also eliminate the problem created by the interpretation of previous Mossbauer experiments where thermal treatment resulted in a large population of Fe in Cu(2) sites without a deterioration in the superconducting transition temperature.

The temperature-dependent behaviour of the RKKY interaction in one dimension is investigated for the free electron gas model by both analytical and numerical methods. The present studies show that the range function has the form cos(2k_Fr)e^{- alpha (T}r)/r^{beta (T}), where the exponents alpha (T) and beta (T)increase and decrease monotonically, respectively, with increasing temperature.

Amorphous BaCoTiFe₁₀O₁₉ films have been prepared by the RF sputtering technique. The magnetic and magneto-optical properties, the structure and the effect of annealing have been investigated. The 4.2 K magnetization increases with increasing magnetic field and is not saturated at a high magnetic field of 65 kOe. Above 100 K the susceptibility obeys a Curie-Weiss law with a low Curie constant of 0.81 and a large negative Weiss constant of -350. The effective paramagnetic moment (2.54 mu B) for ion ions is much smaller than the theoretical moment. Amorphous BaCoTiFe₁₀O₁₉ films have a lower magnetization(1.5 emu g^-1 at 1.5 K) and higher Curie temperature (850 K) and coercivity (700 Oe at room temperature) than crystalline bulk samples do. After annealing above 750 degrees C, amorphous samples crystallize into single M-type hexaferrite with a magnetization of 320 emu cm^-3, a coercivity of 100 Oe and a magneto-optical Faraday rotation of 1 degrees mu m^-1 at room temperature.

The theoretical angular dependence of the direct-process relaxation rate T₁^-1 of the trigonal Yb³⁺ ions (T₄ centres) in a SrF₂ single crystal was determined and compared with the experimental dependence. The theoretical estimation was made according to the Van Vleck-Kronig mechanism in the formalism of the dynamical spin Hamiltonian developed by Kumar and Ray.

An electronic Raman scattering study at low frequency has been performed on YBa₂Cu₃O₇ single crystals below and above the transition temperature T_c. The intensity of the electronic continuum at low frequency shows a drastic change with temperature variation. The differential cross section calculation based on the Green's function formalism in the random-phase approximation has permitted us to isolate the low-energy electronic contribution from the phonons and point out its behaviour as a function of temperature. A decrease of the electronic contribution in the superconducting state compared with that in the normal state then appears, which it is possible to assign to the pairing of the free carriers near the Fermi energy. However, this low-energy electronic contribution persists well below T_c, suggesting the existence of electronic states near the Fermi energy even in the superconducting state.

A detailed study of the optical absorption and emission spectra of Ho³⁺ ions in LiNbO₃ crystals in the visible spectral region is presented. The positions of the crystal-field levels are given for the ⁵F₅, ⁵S₂+⁵F₄, ⁵F₃ and ⁵G₆ multiplets of Ho³⁺ ions. The type and character of the Stark transitions have been determined by polarized absorption measurements. Optical absorption measurements and site-selective spectroscopy have shown the existence of two crystal-held sites for Ho³⁺ in the LiNbO₃ matrix. The effect of co-doping with MgO on the optical spectra of Ho³⁺ ions has also been studied and a new Ho³⁺ Site perturbed by the presence of a neighbouring Mg²⁺ ion is evidenced.