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The occupied and unoccupied valence band states of yttrium (Y) metal, Y dihydride and Y trihydride were measured by soft x-ray emission and absorption spectroscopy. The experiments were complemented by first principles calculations. Both experiments and theory reveal the metallic character of Y and whereas exhibits a band gap, which was experimentally determined to be 2.5 eV. The results reveal strong hybridization between H and Y states, causing a breakdown of the rigid band model description.

We consider the problem of Bose - Einstein condensation of excitons in a single quantum well with infinitely high potential barriers. A formal treatment similar to BCS theory of superconductivity is used to calculate the chemical potential of excitons in the Bose-condensed ground state as a function of quantum-well thickness in a low-density limit.

The spin-wave spectrum of the antiferromagnet with the garnet structure has been studied by inelastic neutron scattering on the triple-axis spectrometers IN12 and IN14 at ILL. Magnon dispersion curves were measured along and directions up to energies of 0.3 THz. The spin-wave symmetry analysis, which reduced the number of independent exchange parameters to six, was performed to ensure correct description of the spin-wave branches. As determined by the fitting, five of the parameters are statistically significant and have been found to be , and . This quite unusual sequence of exchange parameters clearly demonstrates the importance of the superexchange chain geometry.

An exact model which partially takes into account interactions of fluctuations is used to study the critical behaviour at phase transitions of coupled systems with cubic interactions. On the basis of the relationship between the coupling potentials, it is shown that the fluctuation-induced first-order phase transition which occurs in uncoupled cubic anisotropic systems may now be replaced by a new second-order one.

The electronic properties of the incommensurate misfit compound are investigated by different methods. Linear muffin-tin orbital atomic-sphere approximation band-structure calculation shows that this material should be a metal with a Fermi level located near a minimum of the density of states. Experimentally, the electrical resistivity is hopping-like while the magnetic susceptibility is metallic-like with a strong enhancement. We suggest that these paradoxical properties can be reconciled by taking into account the effects of incommensurability and electronic correlations. A comparison with commensurate and with Ti or Cr incommensurate misfit derivatives is discussed.

We report a study of the magnetoresistance (MR) of the metallic perovskite oxide as a function of the oxygen stoichiometry , magnetic field and temperature . We find a strong dependence of the nature of the MR on the oxygen stoichiometry. The MR at low temperatures changes from positive to negative as the sample becomes more oxygen deficient (i.e. increases). Some of the samples, which are more resistive, show resistivity minima at . We find that in these samples the MR is positive for and negative for . We conclude that in the absence of strong magnetic interaction, the negative MR in these oxides can arise from weak-localization effects.

The electronic structures of thin-layer superlattices (SLs) are investigated versus the SL layer thicknesses (m, n) and the band offsets. The calculations are based on the empirical tight-binding model, which includes only nearest-neighbour interactions. Particular attention is given to the effect of the interface parametrization on the SL electronic properties. This is done, mainly, by varying the band offsets over a sufficiently broad range. The results show that the existence of type-II behaviour in the ultrathin-layer SLs necessitates a large valence band offset and small conduction band offset . Providing that these offset values are achieved, it is found that the highest state of the valence band is always confined to the GaAs slabs whereas the bottom state of the conduction band shows different behaviours as it is sensitive to band-mixing effects. It is due to these mixing effects that most of the ultrathin-layer SLs (with ) behave as type-II heterostructures, where the electrons are localized in the AlAs valley. The rest of the ultrathin-layer SLs behave as type-I heterostructures with a direct bandgap at the point, whenever the GaAs slabs are thick enough to make the electron confinement energy small in the GaAs wells. For thick-layer SLs, our results suggest the existence of a critical barrier thickness, beyond which the GaAs wells become completely decoupled and the SL behaves as a type I heterostructure. The estimated critical layer thickness, for the crossover from type-I to type-II behaviour, is for the SLs with m = n when using VBO = 0.56 eV. This -value is consistent with the photoreflectance experiments. The relevance of our work to photonic device applications is discussed further.

We performed high-resolution inelastic neutron scattering experiments to examine the low-energy crystalline-electric-field (CEF) excitations of single ions in . The observed magnetic response versus oxygenation exhibits a discrete character in energy which suggests the presence of different types of cluster for different doping levels. In the undoped sample (x = 6.14) we observe a splitting of the CEF level due to the exchange interaction between the ions and the antiferromagnetic subsystem of copper spins. For all the doped compounds we observe line asymmetries for which possible origins are discussed.

The chemical and magnetic structures of the intermetallic compound were studied by means of bulk magnetic measurements and neutron diffraction. The crystal structure of was confirmed to be isotypic with that of , space group , No 225, with Tb being located at the two crystallographic sites 4a and 8c. The compound undergoes two successive second-order magnetic phase transitions at the Néel temperatures and . Below , the 8c sites order in an antiferromagnetic sequence with a magnetic propagation vector , leaving the 4a sites disordered down to , where additional antiferromagnetic ordering of the 4a sites sets in according to .

The magnetization up to 50 kOe, the magnetic susceptibility under external pressure up to 5 kbar and the thermal expansion of the cubic Laves phase compounds were studied over a wide temperature range. Two well defined concentration regions were isolated in the x-T phase diagram: , in which the antiferromagnetic structure is primarily determined by the d-d interaction (-type), and , in which the f-d interaction plays a dominant role (-type). It is concluded that both the Gd and the Mn sublattices are ordered in below , the change in the magnetic characteristics at 40 K being interpreted as an antiferromagnetism-non-collinear ferrimagnetism transition. The intermediate compound shows a freezing and time-dependent behaviour at low temperatures characteristic of short-range order. The effects can also be induced by external pressure at higher Gd concentrations.

We report on the electronic properties of , which crystallizes in a hexagonal structure with space group . Anomalies in the temperature dependences of the transport, magnetic and thermal properties indicate that two magnetic transitions occur at and , the lower transition being of antiferromagnetic type. The assertion that the ground state of is antiferromagnetic is corroborated by magnetization measurements, which reveal a slight S shape in the M versus B curve with no indication of saturation in fields up to 18 T at 2.2 K. The electrical resistivity increases with lowering temperature, reaching a value of at 0.26 K. Magnetoresistance studies at low temperatures show that a large part of the zero-field resistance is due to antiferromagnetic correlations, and a resistivity drop of is observed upon the application of a field of 18 T at 2 K. Another striking feature is the enhanced Sommerfeld coefficient of , extracted from extrapolation of the specific-heat data to T = 0 K, which we report here for the first time. Therefore, can be described as a moderately enhanced heavy-fermion system.

We consider the effect of quantum spin fluctuations on the ground-state properties of the Heisenberg antiferromagnet on an anisotropic triangular lattice using linear spin-wave (LSW) theory. This model should describe the magnetic properties of the insulating phase of the family of superconducting molecular crystals. The ground-state energy, the staggered magnetization, magnon excitation spectra, and spin-wave velocities are computed as functions of the ratio of the antiferromagnetic exchange between the second and first neighbours, . We find that near , i.e., in the region where the classical spin configuration changes from a Néel-ordered phase to a spiral phase, the staggered magnetization vanishes, suggesting the possibility of a quantum disordered state. In this region, the quantum correction to the magnetization is large but finite. This is in contrast to the case for the frustrated Heisenberg model on a square lattice, for which the quantum correction diverges logarithmically at the transition from the Néel to the collinear phase. For large , the model becomes a set of chains with frustrated interchain coupling. For , the quantum correction to the magnetization, within LSW theory, becomes comparable to the classical magnetization, suggesting the possibility of a quantum disordered state. We show that, in this regime, the quantum fluctuations are much larger than for a set of weakly coupled chains with non-frustrated interchain coupling.

The proton second moment for the trimethylamine-borane complex - - has been measured and calculated by Reynhardt who assumed a rather complicated model of internal dynamics to achieve agreement between the measured and calculated values. In this paper the proton second moment for the same material has been calculated assuming a different and simpler model of internal dynamics taken from the deuterium NMR study of by Penner et al. The agreement between experimental and calculated values of the second moment, reported in our paper, is as good as in Reynhardt's study, but the model of internal dynamics consistent with the Penner results seems to be more probable as it does not require the assumption of the existence of non-equivalent molecules or even `non-equivalent' crystallographical unit cells.

Mössbauer spectra of and proper uniaxial ferroelectrics were investigated over a wide temperature region including phase transition points with the aim of studying the dynamics of ferroactive tin ions used as a Mössbauer isotope . The characteristics of the gamma-resonance lines (isomer shift, effect probability, etc) and the parameters calculated from the measured spectra in the Debye approximation (mean square displacement, Debye-Waller factor, Debye temperature, etc) were analysed in comparison with the available structural and thermodynamical data. The data obtained support the assumption of a two-well local potential of the tin atoms in addition to the displacement of their time-averaged position below the phase transition points. It was concluded that displacement and ordering occur in the same Sn sublattice, and that the variation in long-range forces leads to lattice instability.

Electrical junctions were fabricated in sandwich configuration from Langmuir-Blodgett (LB) films of two types of material, -conjugated, peripherally substituted ring systems or a -bonded polymer. The sandwich junctions consisted of four to ten monolayers between two micro-structured gold electrodes, corresponding to a nominal film thickness between about 8 and 20 nm. At liquid helium temperature, the current (I)/voltage (V) characteristics generally exhibited smooth exponential behaviour or irregular steps. However, for a small fraction of the LB sandwiches comprising a -conjugated or -bonded compound, regular staircases were observed. It was possible to fit such characteristics with curves calculated on the basis of a Coulomb blockade model. These results are accounted for by the presence of nanometre-sized gold particles formed upon evaporation of the top electrode. Single electron tunnelling is assumed to proceed through double tunnel barrier junctions consisting of a gold island asymmetrically located between the top and bottom electrode.

The photoluminescence of orthorhombic and cubic crystals has been studied with the use of a deuterium lamp or synchrotron radiation as the light source. The spectra of orthorhombic samples exhibit a single band at 2.07 eV, while the spectra of cubic samples show an intense band at 4.10 eV, accompanied by some other bands depending on the sample. These two main bands are stimulated only under exciton-band excitation. No luminescence is detected when both crystals are excited with photons in the band-to-band region. The present results are compared with earlier data for and which have the orthorhombic structure. The relaxation processes of excitons and electron-hole pairs in lead halides are discussed.