Using selectively doped GaAs-${\mathrm{Al}}_{0.37}$${\mathrm{Ga}}_{0.63}$As and GaAs-AlAs heterostructures, we provide conclusive evidence for correlations among charged donors, ${\mathit{DX}}^{\mathrm{-}}$-${\mathit{d}}^{+}$ in ${\mathrm{Al}}_{0.37}$${\mathrm{Ga}}_{0.63}$As and ${\mathit{d}}^{+\mathrm{-}}$${\mathit{d}}^0$ in AlAs, which strongly enhance the mobility of a two-dimensional electron gas (2DEG), residing 10 nm away from the charged donors. The determination was accomplished by changing, in the same structure, the extent of correlations without affecting the total number of charged donors and density of the 2DEG. An approximate theory accounts for the measured mobilities.

Submicrometer-thick ${\mathrm{C}}_{76}$ films were prepared by vacuum deposition on air-cleaved mica and NaCl substrates held at 430 K, and the crystal structure and textures of the films were studied by transmission electron microscopy (TEM). ${\mathrm{C}}_{76}$ formed the fcc lattice with a lattice parameter a=1.53±0.02 nm on both substrates. A film grown on mica exhibited a single-crystalline pattern in electron diffraction, though local variations in thickness and orientation were suggested by high-resolution TEM. On NaCl, polycrystalline films consisting of small grains (10–20 nm) were formed. Electron diffraction experiments at low temperatures showed no structural transformation down to 110 K.

The stopping powers of Havar for 0.5–2.4 MeV protons and 1.2–1.8 MeV α particles have been measured with uncertainties of a little more than 0.6% and slightly less than 0.6%, respectively. Havar foils with three different tempers—cold-rolled, annealed, and heat-treated—were used in order to learn if stopping power might be influenced by type of target foil preparation. The results indicated no such influence, since all measurements were mutually consistent. Moreover, these data were thoroughly amenable to analysis with modified Bethe-Bloch theory, which yielded generally plausible values of the parameters appearing in the modified Bethe-Bloch formula. Recommended values of these parameters are I=299.3±3.3 eV for the mean excitation energy, and b=1.33±0.04 with ξ=1.0 for the two Barkas-effect parameters.

Measurements of relative permittivity (ɛ=ɛ′-iɛ″) in the complex perovskite crystal ${\mathrm{Na}}_{1/2}$${\mathrm{Bi}}_{1/2}$${\mathrm{TiO}}_3$ were carried out as functions of frequency (0.02–300 kHz) and temperature (300–900 K) in both heating and cooling processes. We have found three anomalies separating four different temperature regions. In order of decreasing temperature, the first anomaly centered near 800 K is a smooth but rapid increase in low-frequency (<50 kHz) permittivity to very high values of order 1×${10}^5$, associated with the cubic to tetragonal transition. This high permittivity shows considerable dispersion and decreases quite rapidly as temperature drops from 860 to 640 K. We attribute this large permittivity to superparaelectric clusters, and their slow relaxation to random electric fields from randomly placed ${\mathrm{Na}}^{+}$ and ${\mathrm{Bi}}^{3+}$ ions which redistribute and allow cluster reversal with the ρ${\mathrm{\varepsilon }}_0$${\mathrm{\varepsilon }}_{\mathrm{dc}}^{\prime }$ time constant determined by the conductivity and permittivity. The other anomalies, seen previously by others, are a high-frequency permittivity (50 to 300 kHz) which shows no dispersion and increases slowly down to a peak of 3×${10}^3$ near 640 K, where a trigonal phase exhibiting antiferroelectric and/or incommensurate characteristics is reported to set in. It then drops and develops weak low-frequency dispersion below 550 K which is the upper limit reported for spontaneous polarization decay.

We have performed a thermodynamical analysis of the phase diagrams for ${\mathrm{ZrO}}_2$-CaO and ${\mathrm{ZrO}}_2$-MgO solid solutions which has demonstrated that differential heats of mixing are important parameters determining the stabilization of the cubic phase of ${\mathrm{ZrO}}_2$ by impurities. It is shown that the differential heats of mixing in the cubic phase of these systems should be lower than in the tetragonal phase. To understand this effect we have studied the electronic and geometrical structures of the pure and doped ${\mathrm{ZrO}}_2$ crystals. Three computational techniques were employed: the ab initio Hartree-Fock pseudopotential method is used to study the atomic and electronic structures of the three phases of pure ${\mathrm{ZrO}}_2$ crystals; the defect energies and the differential heats of mixing values are calculated by means of the atomistic simulation technique using the shell model and the pair-potential approximation; the self-consistent semiempirical intermediate neglect of differential overlap method is used to study changes in the electronic structure imposed by the defects. From the results of various calculations, we are able to identify the key factors contributing to the mechanism of stabilization of cubic ${\mathrm{ZrO}}_2$ by impurities. These include the lattice distortion around vacancies, the lowering of the dielectric constant in the cubic phase, the impurity-stimulated increase of ionicity, and the removal of the Zr orbital degeneracy.

Brillouin scattering is used to determine the elastic constants of the incommensurate inclusion compound of dioctanoyl peroxide and urea. The results are interpreted in terms of a simple microscopic model, based on the crystal structure of this compound. A detailed discussion is given concerning the absence of effects in Brillouin scattering due to the sliding (or Goldstone) mode. This kind of mode is expected to be present in incommensurate intergrowth compounds.

In order to derive an atomistic expression for the transfer coefficient across an interface, we extend the Gibbs dividing-surface scheme to kinetic problems. In equilibrium thermodynamics, this scheme consists in replacing the continuous concentration profile between two coherent phases by a stepped profile with a discontinuity at the dividing surface: the Gibbsian excess free energy (interfacial energy) is the difference between the free energies associated with the true continuous profile and with the artificial stepped one. Close to equilibrium, the diffusion flux along the actual continuous concentration profile is equal to minus the gradient of the chemical potential multiplied by a mobility: the latter is a continuous function of the local equilibrium concentration, which can be evaluated in a mean-field approximation. Gibbs’ dividing-surface scheme introduces a transfer coefficient across the (artificial) dividing interface. Equating the exact expression of the flux along the actual concentration profile to that predicted in Gibbs’ scheme for the same difference in chemical potential across the system yields the expression for the transfer coefficient. In the simplest mean-field description of chemical diffusion, the transfer coefficient is found to be negative. The reason for that is that the mobility increases as the concentration goes to 1/2, at least in the simplest case. Assuming the concentration to be uniform up to the interface underestimates the flux and must be compensated by a negative ‘‘contact resistance’’ between the two phases. Neglecting the transfer coefficient results in underestimating the flux: the error can be large for small samples, in particular in the case of the nucleation and growth of a phase with low diffusivity, inside a high-diffusivity matrix. The range of validity of the model is shown to coincide with that of linear diffusion theory. In this range, the transfer coefficient at the interface does not depend on the velocity of the interface.

The trends in the elastic constants of the simple and transition elemental cubic metals are explained in terms of a uniform electron-gas theory. Recently, the bonding energetics of elemental metals were systematized using a new uniform electron-gas model. Dubbed the ‘‘ideal metal,’’ this model stems from a physical interpretation of the process of cleaving the positive background, and posits that no forces act on the background in the uniform state. Previously, this new model was used to show that the cohesive energies of the elemental transition metals depend primarily on the bonding valence and the average electron density at the boundary of the unit cell. Here, we use a slight modification of the same model to show that the elastic constants of the elemental cubic metals depend primarily on the bonding valence, the density at the cell boundary, and the symmetry of the lattice.

Coherent neutron scattering and Raman scattering with IR sources were used to make a dynamical study of the structural transition of the disubstituted and fully deuterated diacetylene 2,4-hexadiynylene bis(p-toluenesulfonate) (pTS-D) in the monocrystalline polymer state. This compound may undergo a progressive polymerization in the crystalline state, which gives a quasiunique opportunity to obtain large size monocrystals of conjugated polymer. A second-order-like transition takes place at ∼${\mathit{T}}_{\mathit{c}}$=182 K; it is associated to motions of the polar side groups. We achieved a dynamical study of a soft mode which is clearly observed in the high-temperature phase (neutron) as in the low-temperature phase (Raman) and is characterized by overdamping on approaching ${\mathit{T}}_{\mathit{c}}$. In addition the growth of a central peak is observed above ${\mathit{T}}_{\mathit{c}}$. We show that this structural instability presents an intermediate character between order disorder and displacive, with a more pronounced displacive character than in the monomer crystal.

A cleaved crystal of GaSe was analyzed by high-resolution electron-energy-loss spectroscopy (HREELS) and its optical parameters were deduced by using the dielectric theory. The angular and intensity distributions of the multiple energy losses and gains of low-energy electrons scattered on GaSe(0001) were examined in detail, in order to assess the relevance of the dielectric theory for describing higher-order excitations than the first Born approximation. We find a high linear correlation between the angular shift of the successive peaks recorded on the spectrum, and we demonstrate that peak intensities are underestimated in specular measurements. Experimental as well as theoretical implications for HREELS are discussed.

Complementary molecular-dynamics and Metropolis Monte Carlo algorithms for the atomistic simulation of crystals with imposed laboratory conditions of temperature and tensorial pressure are presented. Inclusion of the nonlinear expression for the elastic energy of a crystal yields simulations that conserve the Gibbs potential of the crystal despite finite deformations. The molecular-dynamics equations of motion contain an explicit expression of the virial theorem for nonlinear elastic media; the dynamical balance of the ‘‘internal’’ and ‘‘external’’ pressures includes the elastic response of the system to the applied stress. Thus the ‘‘internal’’ and ‘‘external’’ pressures remain in dynamical equilibrium even when the microscopic dynamics generate a phase transformation and the initial isotropy of the macroscopic stress field is broken. Deterministic molecular-dynamics trajectories for a simple pair-potential model of a pressure-induced martensitic transformation are presented; manifestly nonlinear behavior is observed while satisfying the tensorial virial theorem for nonlinear elastic media. Stochastic Monte Carlo trajectories yield comparable results, and independently verify the nonlinear extension of the virial theorem. This is despite the fact that the Monte Carlo algorithm contains no explicit driving terms that insure the theorem be satisfied.

Previous ultrasonic measurements of a dilute Al-Li alloy irradiated by 2.3-MeV electrons gave strong evidence that self-interstitial atoms trapped by Li atoms form mixed dumbbells with tetragonal symmetry. The dumbbell is detected through its relaxation, which anneals away by 130 K. In this work, Fe is added to the alloy to determine whether this annealing stage represents migration of the defect to a trap or its dissociation. Fe atoms are known to trap self-interstitials, forming a complex stable to stage III that gives a set of relaxations different from the Al-Li relaxation. If Al-Li complexes dissociate, then self-interstitials would be released and captured by Fe atoms, resulting in larger Al-Fe relaxations. This result is not seen for this annealing stage. Furthermore, it is unlikely that the annealing is due to a change of configuration of the defect, since the anisotropy of the diaelastic effect remains constant during the annealing. Therefore, the Al-Li complex migrates as a unit without dissociating during this annealing stage. However, Al-Li complexes are observed to dissociate at 200 K, or about 20 K below stage III.

A hexagonal two-dimensional model of particles with displacive degrees of freedom and interacting via potential energy with harmonic and anharmonic third- and fourth-order terms has been studied by the molecular-dynamics technique. The modulated phases characterized by the wave vectors lying in the interval from 1/4 to 1/3 have been specifically considered. The simulations have shown that: (i) the 1q commensurate 1/4→1q incommensurate phase transition is driven by the stripple mechanism. The stripples are built from four discommensuration planes and the serial mechanism of stripple correlated nucleation has been observed. (ii) The reverse phase transition from 1q incommensurate to 1q commensurate 1/4 phase was obtained only under extreme metastable conditions. No antistripples with four discommensurations were observed. Instead, a rearrangement of same domains gave a possibility to remove discommensuration planes by simple antistripples built up from two discommensurations only. (iii) At the 3q→1q phase transition, the columns of 3q phase merged together to form a stripe of a 1q incommensurate phase. (iv) The commensurate phase 1/3 is degenerate with respect to the 1q and 3q modulation patterns.

This work presents an x-ray-absorption study at the L edges of several rare-earth compounds. In both ${\mathit{L}}_1$-edge and ${\mathit{L}}_3$-edge spectra, anomalous features are clearly detected in the EXAFS region and have been associated with the creation of 2s4d and 2p4d double-core hole states, respectively. Comparison of theoretical calculations with the experiments are carried out using the results of explicit computation of cross sections for the allowed mainline (L edge) and double-electron (${\mathit{LN}}_{4,5}$ edges) which identify the transitions expected to be observable. In addition, the excitation energies of all of these allowed transitions are given. The influence of multielectron transitions on the EXAFS data analysis is discussed for several rare-earth-based systems. An estimate of the error introduced by neglecting the presence of double-electron transitions on coordination numbers and bond distances is given, based on these experimental data.

A detailed x-ray-absorption fine-structure (XAFS) investigation of two mixed alkali halides ${\mathrm{Rb}}_{0.76}$${\mathrm{K}}_{0.24}$Br and ${\mathrm{RbBr}}_{0.62}$${\mathrm{Cl}}_{0.38}$ was performed. The concentrations of the mixtures had been chosen to produce a single homogeneous phase for each, and it was checked by XAFS that the salts were randomly mixed on the atomic level. Detailed analysis of the data including multiple-scattering contributions revealed an rms buckling angular deviation of both mixtures from the average NaCl collinear structure of 7–9°. The angles are defined by three atomic positions determined through double- and triple-scattering paths. These angles are new parameters which should be added to characterize the buckled crystals. Adding to diffraction results the parameters determined from XAFS as input into a molecular-dynamics simulation the structures of the mixed salts with their fluctuations about the NaCL structure are solved and displayed.

We report the discovery and structural refinement of a large unit cell bcc crystalline phase in a Ti-Cr-Si alloy that is a 1/1 rational approximant to the icosahedral phase in that system. The crystal structure was determined from a Rietveld analysis of x-ray and neutron powder diffraction data. Our results demonstrate that this phase is closely related to the α(Al-Mn-Si) phase, which in turn, is closely related to the structure of the icosahedral phase in the Al-transition-metal alloys. The neutron data indicate that the structure contains a significant amount of oxygen located in the Mackay clusters. Partial ordering of this oxygen may provide an explanation for the localized diffuse scattering often observed in the i phase and related crystalline phases in Ti-transitional-metal alloys.

Acoustic properties below incommensurate phase transitions are severely influenced by phason modes. These unique massless Goldstone modes may lead to coexistence anomalies within the whole phase of broken continuous symmetry. The present theory, based on an order parameter model appropriate for a universality class containing the ${\mathit{A}}_2$${\mathit{BX}}_4$ family, employs an extended renormalization scheme. Within this scheme, critical behavior of the Goldstone modes is governed by a fixed point, permitting an exact treatment of the anomalies induced by them. With regard to the coefficient of sound attenuation, the hydrodynamic behavior is not altered by the phason modes, but they lead to a singularity of the scaling function, which describes the crossover from the critical region to the coexistence regime. Determining the universal scaling function and revealing its coexistence anomaly constitute a main result of our study, which explicitly takes fluctuation contributions of the phason and amplitudon modes into account. The sound velocity is predicted to display a minimum in its temperature dependence below the maximum sound attenuation at the same frequency. A treatment of the high-temperature phase is also included. Application to experiments on ${\mathrm{Rb}}_2$${\mathrm{ZnCl}}_4$ and ${\mathrm{K}}_2$${\mathrm{SeO}}_4$ reveals agreement with our theoretical predictions over several orders of magnitude of the scaling variable.

We present a numerical study of a simple model to describe the electronic structure of polymer fluids such as liquid Se. Using a simplified geometry and a one-electron tight-binding Hamiltonian, we investigate the density of states and the localized or extended nature of eigenstates. The metallic or nonmetallic character of the system is analyzed utilizing a scaling procedure. With increasing chain length, a transition from metallic to nonmetallic behavior can be observed, indicated by the presence of localized states in a pseudogap.

A method is presented that makes computer simulations of hopping conduction in symmetric hopping models with thermally activated jump rates possible at arbitrarily low temperatures. The method utilizes the ac Miller-Abrahams electrical equivalent circuit which is systematically reduced by the general star-mesh transformation until one ends up with an admittance matrix referring to the voltage generators. From this matrix the conductivity is easily calculated. Results from simulations of hopping in two dimensions are presented and compared to the predictions of the effective-medium approximation (EMA). Generally there is good agreement, with some deviation at the lowest temperatures. It is shown that, as the temperature goes to zero, the frequency-dependent conductivity in the EMA becomes universal, i.e., independent of the activation energy probability distribution. The computer simulations confirm the existence of universality, although there is no exact agreement between the simulations and the EMA universality prediction.

The emission spectra of poli(ethylene oxide) (PEO) and poli(propylene oxide) (PPO) modified by different concentrations of europium bromide, ${\mathrm{PEO}}_{\mathit{n}}$${\mathrm{EuBr}}_3$ (n=8,12,16,20,24,28,32,80) and ${\mathrm{PPO}}_{\mathit{n}}$${\mathrm{EuBr}}_3$ (n=32,80), were recorded and the observed lines assigned to the intra 4${\mathit{f}}^{\mathit{N}=6\mathrm{}}$ ${}_{}{}^5{}_{}{}^{}$${\mathit{D}}_{0,1}$${\to }^7$${\mathit{F}}_{0–4}$ transitions. The corresponding Stark levels were identified for the ${}_{}{}^7{}_{}{}^{}$${\mathit{F}}_{0–4}$ and ${}_{}{}^5{}_{}{}^{}$${\mathit{D}}_0$ states. The luminescence of both these polymer complexes (presenting almost identical characteristics) is sufficiently intense to suggest their use as possible luminescence devices. The local coordination of a rare-earth ion in polymer electrolytes is attempted. We used a local-field interaction model, the polarization behavior of the ${}_{}{}^5{}_{}{}^{}$${\mathit{D}}_0$${\to }^7$${\mathit{F}}_0$ transition, certain moment operators, and the allowed optical activity of the ${}_{}{}^7{}_{}{}^{}$${\mathit{F}}_{2,3,4}$ components for identifying the approximate local symmetry of ${\mathrm{Eu}}^{3+}$ to the ${\mathrm{C}}_{2\mathit{v}}$ point group.