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For the perovskites that show colossal-magnetoresistance (CMR) behaviour, we use the global instability index, R1, as a measure of the influence of the static lattice effects on the magnetic and electrical properties. These effects arise from the size mismatch between the ions at A and Mn sites as well as the size distribution of ions at A sites. A magnetic and electronic phase diagram as a function of R1 for (R = trivalent rare-earth ions, alkaline-earth ions) reveals four well-defined regions: paramagnetic insulator (PMI), ferromagnetic metal (FMM), spin glass or ferromagnetic insulator (FMI), and a transition region (TMI), in which the compounds exhibit a variety of behaviours.

thin films prepared by pulsed-laser ablation have been investigated by high-resolution x-ray diffraction, x-ray pole-figure, Rutherford backscattering/channelling and electrical measurements. The results show that films deposited on (0001) sapphire substrates grow with well determined orientations in the plane: and out of the plane: . The aligned-to-random ratio of backscattered yield of the spectra can be as low as 5%, and the electrical resistivity changes by a factor of during the phase transformation with a hysteresis loop width of , which implies that both the structure and the properties of the film are very close to those of bulk single-crystal . The (010)-oriented film `prefers' to form twinned structure with domain walls. Molybdenum substitutional doping up to the level of 1.5 at.% does not degrade the crystal quality.

Scanning tunnelling microscopic (STM) and spectroscopic (STS) investigations have been carried out on the grain boundaries (GBs) of sintered pellets of giant magnetoresistive perovskite manganites (LCMO), (LYMCO) and (LPMCO). Based on spectroscopic data obtained and estimation of band gap, it has been concluded that these materials possess some sort of semiconducting intergranular layer (IGL) whose thicknesses are in the range of a few nm to about 100 nm and the band gap is in the range of 0.3-0.45 eV. IGLs are usually more resistive than the grains. For semiconducting samples like LCMO and LYCMO (room temperature band gap = 0.23 and 0.27 eV respectively), IGLs bend the energy band near GBs. This bending has been estimated to be about 40-50 meV with the depletion depth of few tens of nm extending on both sides of the IGL. The decrease in conductivity near the GB is due to the disorder induced carrier scattering and the bending of the band. LPCMO is almost conducting at room temperature. The GBs in this material sometimes exhibit conducting behaviour which may be due to the accumulation of some conducting material or the trapping centres in the IGL. Scanning electron microscopic and electrical measurements also justify the STM/STS results.

In the Fe-Nb system, amorphous alloy, metastable crystalline fcc and hexagonal phases were formed in Nb-rich multilayered films by room temperature 200 keV xenon ion mixing. A heat of formation diagram of the system was constructed based on Miedema's model and Alonso's calculation method, and the diagram gave a relevant interpretation for the observed phase formation. The formation of fcc metastable phase can be interpreted by a reverse martensitic transition of bcc-fcc. The amorphous and hexagonal phases are thought to form through a traditional nucleation and growth mechanism with an additional consideration of the irradiation effect, which kinetically hindered the formation of a complicated intermediate compound.

On the basis of Brodie's definition of the work function and the length of spontaneous polarization of plasma, the following new formula for calculation of the work functions of elements has been derived: , where is the electron density parameter expressed in units of the Bohr radius, is the Fermi energy and is an empirical constant ( for the alkali metals, Ca, Sr, Ba, Ra and Tl, whereas for the remaining elements). The density parameter was calculated from the atomic mass, the bulk density of the element and the assessed number of free electrons per atom which is equal to the nominal valence of the element or, in the case of transition metals, close to this number (within ).

The values obtained by using the above formula are in excellent agreement with experimental data for pure-metal polycrystalline surfaces, within 5% in most cases. A table with the work functions and with complete input data for most of the elements is presented.

We present a diffusion-transition model of the trapping and annihilation of positrons in the grain boundaries in the form of a plane, cylinder and sphere. This model provides the closed-form expressions for the positron lifetime spectrum, the mean positron lifetime and the value of the S-parameter evaluated from the annihilation line. The comparison of this model with the standard trapping model has been performed. In our theoretical consideration we have extended this model for the case when trapping centres for positrons are distributed inside the grain. The validity of the model has been tested with positive results for study of the positron annihilation in the multilayer system which was produced by a sequential magnetron dc sputtering of copper and tin layers.

Photoemission spectra obtained from the (110) surface of the ordered CoAl alloy, calculated in the framework of a fully relativistic one-step model of photoemission, are presented. The comparison of the theoretical spectra with experimental results shows good agreement over a wide range of photon energies and emission angles. Additionally we simulate partial disorder by taking into consideration Co antistructure atoms.

The thermal stabilities and dynamics of small nickel clusters consisting of 7 to 23 atoms have been studied extensively by a molecular dynamics method based on a tight-binding many-body potential. Physical properties such as the caloric curve, melting temperature, and bond-length fluctuations are computed. The simulation indicates that the clusters undergo solid-liquid phase change with the increase in internal energy. The melting temperature is found to be a non-monotonic function of cluster size with some clusters exhibiting pre-melting behaviour. Considerable depression in the melting point of small clusters has been observed - the highest melting temperature for small clusters is found to be almost half the value for the bulk nickel. The results are discussed in the light of recent nanocalorimetric experiments on small finite particles.

The equilibrium geometries, electronic structure and magnetic properties of small Mn clusters consisting of up to five atoms have been calculated self-consistently using first principles molecular orbital theory. The electron-electron interaction has been accounted for using the local spin density and generalized gradient approximation to the density functional theory. The atomic orbitals forming the molecular orbital have been represented separately by Gaussian and numerical basis sets. Two different computer codes (Gaussian 94 and DMOL) were used to check the numerical consistency of our calculations. is found to be a weakly bound van der Waals molecule and its binding energy depends sensitively on the choice of basis set as well as the form of the exchange-correlation potential. The binding energies are less sensitive to these approximations in larger clusters. The binding improves with cluster size, but remains significantly lower than those in other transition metal clusters. The equilibrium geometries are fairly compact and symmetric although other isomers with distorted geometries and with nearly the same energy as that of the ground state do exist for . The clusters also exhibit a variety of low-lying spin multiplicities, but the ground state spin configuration is ferromagnetic with a magnetic moment of . This not only contrasts with its bulk behaviour which is antiferromagnetic, but also differs from the behaviour in other transition-metal clusters where the magnetic moments/atom are always less than the free-atom value. The results are compared with available experiments on matrix isolated Mn clusters.

2,3-dimethylanthracene (2,3-DMA, ) crystallizes in a pseudo-centrosymmetric triclinic lattice with two molecules per unit cell showing dipolar structural disorder of the acentric molecules. Using powder samples the amplitude weighted phonon density of states was measured by inelastic incoherent neutron scattering for different degrees of deuteration. The differences in the resulting spectra can be explained by changes of the dynamics of the molecular bodies or the methyl side groups. By the method of inelastic coherent neutron scattering phonon dispersion curves were determined. The phonon lines are broad even at low temperatures; this fact is supposed to be related to dipolar disorder. Both the incoherent and coherent scattering results can be described satisfactorily by lattice dynamical calculations using a `6-exp' potential for the atom-atom interactions of different molecules, including internal degrees of freedom.

The vibrational behaviour of vacancies in bcc metals is discussed with the use of the Green function method. In particular the local densities of states of the first and second neighbours of the vacancy, the atoms most affected by the defect, in , Mo and W have been calculated. The local density of states of the first neighbours is shifted to lower frequencies as compared to host spectra whereas the local spectra of second neighbours in Mo and W are little affected by the defect; in the second neighbour spectrum shifts to higher frequencies. However, in all three metals the local spectra of neighbours of the vacancy do not show any resonance or localized modes. The local spectra of neighbours have been utilized to calculate formation entropy and thermal mean-square displacements. The obtained values for formation entropy using the local density of states approach are similar to those found by other methods.

Recent experiments have shown that first-order spin transitions can occur in polymeric compounds. In such materials the metal ions are linked by ligand groups to form polymeric chains. Existing theories of the spin transition in crystalline materials are discussed, but we argue that the physical justification of these theories may be inappropriate for polymeric compounds. We describe a possible mechanism in which local strain in the ligand network can drive the transition. A model Hamiltonian is presented and solved, and the effects of pressure and dopant concentration on the spin transition are investigated. We also point out that the model describes many of the effects observed in experiments on crystalline materials.

CdS microcrystals of various sizes embedded in a germanium dioxide glass matrix have been studied by Raman scattering and optical absorption under high pressure up to about 9 GPa. Structural phase transition of the CdS microcrystals from wurtzite to rock salt phase was observed at a pressure higher than 6 GPa, which is far above the bulk CdS transition pressure of 2.7 GPa. Under a pressure higher than 6 GPa, the 1-LO Raman intensity of the wurtzite phase of CdS microcrystals decreases gradually with time. After the pressure is released from about 9 GPa to atmospheric pressure, the high pressure rock salt phase is preserved at room temperature. We found that CdS microcrystals of the rock salt phase recovered to the wurtzite phase by temperature annealing at atmospheric pressure. Our observations suggest that the matrix plays an important role.

In this paper we present a computer simulation of a random walk (RW) for diffusion on a rearranging lattice. The lattice consists of two types of sites - one highly conducting (type 1) and the other poorly conducting (type 2), distributed at random. The two types of site are assigned different waiting times ( for type 1 and for type 2). We assume that at intervals of time the site distribution changes. The effect of this rearrangement on the diffusion coefficient is studied with varying . We study this effect for different ratios of dwell time of the two types of site (R) and also for different fractions (X) of the less conducting sites. An empirical relation for is suggested. We have employed the well model and considered diffusion controlled by sties, rather than bonds. So our approach is different from the dynamic bond percolation model, which studies these aspects. Our results show that the diffusion coefficient D may change by a factor of up to 3 (approximately) for rapid rearrangement, and there is a considerable effect of varying X and R on the range of variation of D, where X is the fraction of poorly conducting sites, and R is the ratio of the dwell times for types of site. Further for ( is the time unit for the random walk) the effect of rearrangement becomes negligible. The results may be useful for studying diffusion and conduction of ion conducting polymers.

The structural properties of copper (I) iodide have been investigated at elevated pressures and temperatures using the neutron powder diffraction technique, to probe the effects of pressure on the superionic properties of this compound. On increasing temperature at a pressure of p = 1.30(8) GPa, three structural phase transitions are observed. The first is from the ambient temperature zincblende structured phase CuI-III to rhombohedral CuI-IV at T = 444(6) K. There is only limited cation disorder in CuI-IV which increases gradually with temperature. The preferred locations of the interstitial cations are sites between the tetrahedral and octahedral interstices within the slightly distorted face-centred cubic (f.c.c.) anion sublattice. A subsequent transition to the disordered f.c.c. structured phase CuI-I occurs at T = 694(5) K. This phase shows complete cation disorder at all measured pressures and temperatures. Finally, CuI undergoes a further phase transition at a temperature of T = 920(15) K. The first diffraction studies of this high pressure phase (labelled CuI-VII) are presented, which indicate that this phase is a body-centred cubic (b.c.c.) superionic with complete disorder of the cation sublattice. The cations are found to preferentially occupy the tetrahedral sites, in a manner similar to that in isostructural (ambient pressure) superionic phases such as and . The structural systematics of the superionic binary halide compounds and their thermally induced disorder are briefly summarized.

We demonstrate how the screening procedure works in a one-dimensional application of the KKR method by constructing an explicit analytic expression for the full scattering path matrix for the reference system. As an example of where screening renders novel calculations possible, we discuss the Friedel oscillations in a one-dimensional chain of potential wells with broken crystal symmetry.

The properties of hcp magnesium are investigated using the density functional method with the linear combination of atomic orbitals as implemented in the CRYSTAL95 code. The lattice equilibrium parameters and the binding energy have been calculated at the Hartree-Fock level, at the hybrid Hartree-Fock density functional level, and at the Kohn-Sham density functional level using local and non-local exchange and correlation potentials. The electronic properties (band structures, topologies of the Fermi surface, and densities of states) and the elastic constants are computed for each type of functional, and compared to experimental data.

We have evaluated interatomic potentials of Cu, Au and Cu-Au ordered alloys in the framework of the second-moment approximation to the tight-binding theory by fitting to the volume dependence of the total energy of these materials computed by first-principles augmented-plane-wave calculations. We have applied this scheme to calculate the bulk modulus and elastic constants of the pure elements and alloys and we have obtained a good agreement with experiment. We also have performed molecular-dynamics simulations at various temperatures, deducing the temperature dependence of the lattice constants and the atomic mean square displacements, as well as the phonon density of states and the phonon-dispersion curves of the ordered alloys. A satisfactory accuracy was obtained, comparable to previous works based on the same approximation, but resulting from fitting to various experimental quantities.

A minimal-parameter tight-binding theory incorporating explicit use of nonorthogonality of the basis is used to generate a transferable scheme for germanium. Good results are obtained for high-pressure bulk phases and vibrational frequencies. Diamond structure is found to be the ground state even when compared with the clathrate structure. The results for clusters show good agreement with ab initio predictions.

The Fermi contact magnetic hyperfine fields in compounds (x = 0, 2, 4) and in and carbides are calculated self-consistently by the TB-LMTO method using two types of approximation for the exchange-correlation potential. For , the calculated magnitude of the hyperfine field is close to the experimental one. The proportionality of the average hyperfine field to the vanadium content in observed experimentally over the narrow concentration range x = 1.5-2.8 is not reproduced for the limiting concentrations x = 0 and 4. The hyperfine fields at Fe atoms at crystallographically non-equivalent sites of the unit cells are analysed. It is proved that the assumption of proportionality of the hyperfine field to the local magnetic moments of Fe atoms is not justified in the cases of the compounds investigated. It is shown that, though the local Fe magnetic moments for all compositions satisfy the relation , the hyperfine field at the Fe at the 8( j) positions of the unit cell has the lowest magnitude for all of the compounds. Analysis of the different components of has revealed that the opposite variation in the hyperfine field with the 3d local magnetic moment is due to the hybridization interaction of the 4s valence electrons with the spin-polarized d shell of the surrounding atoms.

We analyse the two-dimensional spin-fermion model in the strong-coupling regime relevant to underdoped cuprates. We recall the set of general sum rules that relate the moments of spectral density and the imaginary part of the fermion self-energy to the static correlation functions. We show that the two-pole approximation of the projection method satisfies the sum rules for the first four moments of the spectral density and gives an exact upper bound for the quasiparticle energy near the band bottom. We prove that the non-crossing approximation that is often made in perturbative considerations of the model violates the sum rule for the third moment of the spectral density. This leads to incorrect positioning of the lowest quasiparticle band. On the other hand, the projection method is inadequate in the weak-coupling limit because of the approximate treatment of the kinetic energy term. We propose a generalization of the projection method that resolves this problem, and give a fermion self-energy that behaves correctly in both the weak- and strong-coupling limits.

The low-lying energy levels of three-electron quantum dots in a magnetic field are calculated by both the Hartree-Fock (HF) and the numerical diagonalization methods. Many-body effects on the energy level structure are investigated by comparing the results from these two approaches. It is found that many-body interactions can apparently change the relative positions of the low-lying energy levels, especially at larger angular momentum quantum numbers, and the relative changes of the levels almost have nothing to do with the external magnetic field, but linearly depend on the dot size.

We investigate the thermodynamic properties of Heisenberg ferrimagnetic mixed-spin chains both numerically and analytically with particular emphasis on the combination of ferromagnetic and antiferromagnetic features. Employing a new density-matrix renormalization-group technique as well as a quantum Monte Carlo method, we reveal the overall thermal behaviour: at very low temperatures, the specific heat and the magnetic susceptibility multiplied by the temperature behave like and , respectively, whereas at intermediate temperatures, they exhibit a Schottky-like peak and a minimum, respectively. Developing the modified spin-wave theory, we complement the numerical findings and give precise estimates for the low-temperature behaviour.

Analysis of the irreversible field-cooled (FC) and the zero-field-cooled (ZFC) magnetic susceptibilities of one ferrimagnetic and three ferromagnetic systems, measured at different applied magnetic fields, shows that the irreversibility indicated by the difference between the FC and the ZFC susceptibilities arises from magnetic anisotropy. The two susceptibilities are related to each other through the coercivity which is a measure of the anisotropy. The ZFC susceptibility can be calculated from the FC susceptibility (or vice versa) and the coercivity.

A new series of compounds with the -type structure (, Dy, Ho, Er and Lu) has been synthesized and studied by x-ray diffraction, ac susceptibility and magnetization versus temperature and field measurements. The maximum Curie temperature is for the Tb compound . Spin reorientation transitions have been observed for the compounds and . A first order magnetization process has been observed at low temperature in and . The importance of the metallic radius of M in the magnetic properties of (for the same rate of M substitution) is discussed.

The longitudinal components of the `zero-field-cooled' and `field-cooled' magnetization of amorphous ( TM= Co, Ni) and re-entrant ferromagnetic alloys have been measured in the static mode and at different thermal cycling rates varying from to in constant magnetic fields (H) ranging between 1.5 Oe and 15 kOe. The difference, , is taken to be the direct measure of irreversibility in magnetization. The onset of weak irreversibility and a crossover from weak to strong irreversibility are observed at the temperatures and , respectively, for fields ; depends on x and y. and follow the relations and , where ( stands for or ), which have the same form as those predicted by the modified versions of the Gabay-Toulouse (GT) and de Almeida-Thouless (AT) mean-field (MF) theories that include non-vanishing spontaneous magnetization, but the observed values of the coefficients and are several orders of magnitude larger than the MF estimates. is relatively insensitive while is extremely sensitive to thermal cycling rates (TCR) if they exceed a threshold value. The physical implications of extremely large magnitudes of and , and of the observed TCR-induced shifts in the GT and AT irreversibility lines in the H-T plane, have been brought out clearly while discussing these results in terms of the existing theoretical models.

The theory of the nonlinear dielectric response to an external dc electric field is developed in the random-field-theory framework. The equations describing the dependence of the order parameter and the dielectric susceptibility (both linear and nonlinear) on the temperature, dc electric field, frequency, parameters of the host lattice and random-field sources are obtained. The numerical solution of these equations for several random-field-source concentrations and other parameters has shown that in the dipole-glass phase the dc electric field always decreases the dielectric response, while in the mixed ferroelectric-glass phase the dc field can either decrease or increase this response. Approximation of the numerical results for the nonlinear part of the susceptibility leads to with ( is the dimensionless dc-field value). It was shown that, for all of the cases considered, remains finite and has a maximum ( is the dipole-glass freezing temperature). The absence of critical divergency of the nonlinear susceptibility both in theory and experiment proves that, unlike conventional spin glass, the dipole-glass state in relaxors is a metastable state with long (up to infinite) relaxation times. A comparison of the theoretical results obtained with available experimental data for PMN and PMN-10PT is carried out. The calculated temperature and dc-field dependences of the nonlinear susceptibility are in agreement with observed data.

Blue, green and red up-conversion luminescences at around 490, 545 and 650 nm, which result from the , and transitions, respectively, were observed in co-doped -based fluoride glasses under 800 nm excitation. Among these up-conversion luminescences, the green emission was extremely strong and the blue and red emission intensities were very weak. Selectively strong green up-conversion luminescences of these glasses indicate a high possibility for realizing a green up-conversion laser. Up-conversion processes for the blue, green and red emissions are two-photon processes assisted by energy transfer. It is proposed that the up-conversion mechanism for the blue and green emissions is different from that for the red emission. The respective mechanisms are discussed.

We study photoluminescence (PL) and photoluminescence excitation (PLE) spectra of Mg-doped n-GaN grown by hydride vapour-phase epitaxy. Defect-related red and blue emission bands around 1.85 eV and 2.90 eV, respectively, are observed in addition to the yellow emission band. The red and the blue emission bands are attributed to the recombinations involving the same Mg-related defects according to the result of the thermal annealing experiment. The PLE spectra of the red, the yellow, and the blue emission bands show that all of these emission bands can be interpreted in terms of a configuration coordinate (CC) diagram. The blue emission at 2.90 eV is attributed to the transition from the conduction band to the Mg-related deep acceptors. The CC diagram shows that the yellow luminescence is due to the transition from a deep donor state to a shallow acceptor state. A possible origin of the deep donor level is also discussed.

We have investigated the dependence on hydrostatic pressure of the photoluminescence of an InAs submonolayer embedded in a GaAs matrix at 15 K and for pressure up to 8 GPa. Strong InAs-related emissions are observed in all three samples at ambient pressure. The temperature dependence of the emission intensity for these peaks can be well characterized by the thermal activation of excitons from the InAs layer to the GaAs matrix. With increasing pressure, the InAs-related peaks shift to higher energies. The pressure coefficients of these peaks are very close to that of the free exciton in bulk GaAs. Some weak peaks observed at pressures above 4.2 GPa are attributed to indirect transitions involving X states in the InAs layer. These results are similar to the pressure behaviour observed in the InAs/GaAs monolayer structures. A group of new lines has been observed in the spectra when pressure is increased beyond 2.5 GPa, which is attributed to the N isoelectronic traps in the GaAs matrix.

Fourier transform infrared transmission and reflectance spectra of nanometer with particle sizes ranging from 4 to 80 nm have been recorded at room temperature. The results show that even in very small sizes (4-8 nm), the grains retain the main features of rutile structure. Two surface modes at 640 and 520 without specific symmetries have been identified. The peak at 550 manifests the lattice imperfections in bigger nanometre samples. The present results indicate that the FT-IR technique is more sensitive than Raman scattering in the characterization of the transformation from nano-grains to polycrystalline.