In sputtering experiments, the enhancement of the emission of positive metal ions in the presence of chemically active elements can result from bond-breaking processes occurring at the surface of the sample. Although the Landau-Zener-Stueckelberg model describes the dissociation of chemical bonds of ionic character, its application to metallic emission processes cannot be made without a preliminary study that takes into account the specificities of a bond involving a metallic cation. In this paper, the possible breaking of an isolated diatomic bond involving a Cu atom is studied. It is shown that the ground state does not generally participate in the dissociation of such a bond into its ionic components, and that the first excited states have to be taken into account. In fact, the states involved in the process are comprised between two limits determined by the electron affinity of the partner of the metal atom. These states can have very different probabilities to dissociate under an ionic form, according to their quantum numbers and to the energy. In an emission process a complete computation of the final ionization probability cannot be done as long as the probabilities for the diatomic system to reach its excited states remain unknown. It is not impossible that they could be obtained with the help of ‘‘local thermal equilibrium’’ theory. Also, the high values of the electron affinities of some molecules lead us to believe that more complex bond breakings could be responsible for the observed ‘‘matrix effects.’’

High-resolution Raman measurements have been performed to study the coupling of lattice modes to the F center in NaBr and NaI. In both cases, a prominent line near or in the region of the energy gap between the acoustic and optical phonon bands dominates the Raman spectrum. Its linewidth was measured and extrapolated to 0 K it turned out to be about 3 and 0.5 ${\mathrm{cm}}^{\mathrm{-}1}$, respectively, for NaBr and NaI. These values are related to the lifetime of the local vibrational excitations, showing a relatively fast vibrational relaxation. Calculating the nonradiative transition probability during the vibrational relaxation in the electronic excited state, the efficiency of reaching the relaxed excited state (RES) of the F center is analyzed. The calculations show that, if the experimental value of the vibrational lifetime is used, the model predicts a high probability of reaching the RES in the case of NaBr. In the case of NaI the prediction strongly depends on the choice of the electronic matrix element for the interstate nonradiative transition. By comparison with similar cases of vibronic transition matrix elements reported in the literature, the efficiency of reaching the RES in NaI is expected in the range 0.05 to 1.

Theoretical predictions, based on local-density calculations, of pressure-induced structural phase transitions in Cr, Mo, and W are reported. Phase changes at high compressions are found for all three metals. As a function of compression the structural sequence bcc→hcp→fcc→bcc is found for Cr and Mo. For W, the structural sequence bcc→hcp→fcc was found. Calculations for W were performed down to about 30% of the ambient volume, whereas Cr and Mo were studied at compressions down to about 10% of the experimental volume. The structural sequence bcc→hcp→fcc for Cr, Mo, and W can be understood from band-filling arguments, while the fact that the bcc crystal structure, found at ambient conditions, becomes stable at extreme compression (at volumes typically 10% of the experimental volume) for Cr and Mo is explained by the gradually increased p-d overlap and hybridization in these elements under compression.

A model of the orientation phase transition in pure ${\mathrm{C}}_{60}$ is proposed. Eight discrete states of the fullerene molecule are considered, taking into account the interactions between the pentagons, hexagons, and double bonds of the neighboring molecules. The entropy gain of disorder is in good agreement with the differential-scanning-calorimetry results. The behavior of the order parameters is also in agreement with x-ray and neutron-diffraction measurements. However, the transition temperature obtained in our model with no adjustable parameters is larger than the experimental one.

We present the results of an investigation by emission nuclear γ-resonance spectroscopy of the population of states in the grain-boundary diffusion (GBD) zone. It is shown that atomic probes in polycrystals of some fcc and bcc metals only populate two discrete states in the GBD zone: one in the core of the grain boundary and one in the lattice sites adjacent to the grain boundary. For the typical fcc metals Pd, Pt, and Au, we investigated the dependence of the impurity-state populations on the temperature of the atomic-probe diffusion. It was shown that the ‘‘intrinsic’’ temperature dependence of state populations can be distorted owing to the formation of interstitial impurity segregation. In gold, where the volume solubility of such impurities is low, there is a temperature range of diffusion annealings, where the population-state regularities are determined by the ‘‘intrinsic’’ parameters of the atomic-probe diffusion. We made a suggestion concerning the atomic-probe diffusion mechanism in the interstitial impurity segregation zone, which causes the atomic-probe ‘‘pumping’’ parameters to change at low temperatures.

Using a nuclear γ-resonance spectroscopy method we have investigated dynamic, electronic, and magnetic properties of one of the states localized in crystallites near the grain boundary. It has been shown that at high temperatures, at which the ‘‘intrinsic’’ properties of these states are conserved, isomeric shifts and dynamic properties of these states are identical to the corresponding values for the lattice sites of corresponding metals. At low temperatures, where ‘‘extrinsic’’ effects can manifest themselves, the properties of these states in gold do not change. But in other metals, in which rather high concentrations of interstitial impurities are possible, there is segregation of these impurities in the localization region of corresponding states, and, accordingly, there is a change of physical properties of these states. There are magnetic phase transitions in two-dimensional localization regions.

This paper presents the results of an investigation of the structure and physical properties of the grain boundary (GB) core in polycrystals of bcc and fcc metals by nuclear γ-resonance spectroscopy. The atomic probes, introduced by diffusion in the core of the GB, populate, in the polycrystals investigated here, only one state of the interstitial type. In metals, the atomic probe ${}_{}{}^{57}{}_{}{}^{}\mathrm{Co}$ is a small-radius impurity, which in the GB core forms a mixed split interstitial. The average density of electrons in the GB is lower than in the lattice, which causes the Curie temperature to decrease in the GB core as compared to adjacent crystallite regions. The ‘‘intrinsic’’ properties of the GB core are modified during interstitial impurity segregation, which allows us to investigate the regularities of this important phenomenon.

It is shown that the interaction between soft atomic potentials in glasses leads to a density of the low-energy tunneling states, P¯, which does not depend on the concentration of defects. The dimensionless parameter P¯${\gamma }^2$/ρ${\mathit{v}}^2$ which characterizes the interaction between two-level systems is estimated. It appears to be small in accordance with available experimental data. The theory developed allows us to understand both the insensitivity of low-temperature properties of glasses to the concentration of the defects (e.g., created by neutron irradiation) and the relatively small interaction between two-level systems. Making use of only three macroscopic material parameters, namely the mass density ρ, the sound velocity v, and the average value of the atomic mass M¯, nearly correct values are obtained for the two-level systems’ density of states P¯, for the deformation potential γ and for other parameters describing the low-temperature properties of glasses. All of them appeared to be nearly the same for glassy systems. Thereby, the explanation of universality of the low-temperature properties of glasses is given.

We observe stimulated exciton emission and Raman gain in stretch oriented poly(paraphenylenevinylene) (PPV) films with 200 fs resolution. The kinetics of stimulated emission of excitons show that excitons form and equilibrate within 200 fs after 500-nm photoexcitation with 100-fs laser pulses. Stimulated emission is not observed when higher energy photons are used to excite the polymer. The measured polarization and excitation wavelength dependence provide insight into the probability of exciton formation in PPV.

A discussion is given of all possible two-dimensional (2D) quasicrystal structures with a six-dimensional (6D) embedding space. Such quasicrystals have some properties different from the properties of known 2D quasicrystals. It is predicted that the diffraction pattern of the quasicrystal structures considered here can be indexed with integers using six rationally independent reciprocal basis vectors. They have seven-, nine-, fourteen-, and eighteenfold symmetries. The six hydrodynamic degrees of freedom in phases can be parametrized by three two-dimensional vector fields, and hence there are two types of phason fields in each case mentioned above. All quadratic invariants and elastic energy densities are also derived for such quasicrystals with use of group-representation theory.

Dyre has proposed that in the low-temperature limit an effective medium approximation, termed the Bryksin equation here (the BEM), predicts a universal frequency dependence for the normalized small-signal ac frequency relaxation response of nonmetallic disordered solids. This response has been claimed to be practically identical to that found for an exponential distribution of transition rates (EDTR) in the particular limiting uniform-energy-barrier-distribution case, but comparison of the two responses has been inadequate so far. Although it is shown here that they can be well differentiated, the question of which or either is universal still requires further comparisons with experiment for its answer. A generalization of the limiting low-temperature BEM equation applicable for nonzero temperatures, the GBEM, is developed and used to evaluate the temperature and frequency ranges for which the BEM is still adequate. It is found that GBEM response can be well approximated by the important EDTR solution and leads to a frequency exponent with the same temperature dependence as the latter. An expression derived herein for the dc conductivity predicted by the GBEM involves 1/3 of the maximum thermal activation energy (i.e., the effective percolation energy), however, rather than the energy itself. Further, unlike the BEM, the GBEM predicts the presence of an intrinsic temperature-independent high-frequency-limiting conductivity whose magnitude is evaluated.

The combination of conductive- and dielectric-system response, always experimentally present for a conductive system, is evaluated for the GBEM, and in the frequency range where the GBEM and BEM are indistinguishable it leads to frequency and temperature response remarkably similar to that observed for most disordered materials. Finally, it is suggested that Dyre’s macroscopic simulations of the relaxation problem do not seem fully relevant to physical situations of interest and thus should not be taken to confirm the universality of the BEM equation response. Nevertheless, the present results broaden the likely range of applicability of both the BEM and GBEM and the EDTR and suggest that one or the other may indeed be particularly appropriate for describing the frequency and temperature response of a wide variety of disordered materials.

Molecular-dynamics simulations of porous silica, in the density range 2.2–0.1 g/${\mathrm{cm}}^3$, are carried out on a 41 472-particle system using a multiple-instruction multiple-data parallel computer. An effective interatomic potential consisting of two-body and three-body covalent interactions is used. Structural correlation is analyzed using the pair-distribution function, neutron-scattering structure factor, and bond-angle distribution. The internal surface area, pore surface-to-volume ratio, pore-size distribution, fractal dimension, correlation length, and mean-particle size are determined as a function of the density. Structural transition from the condensed amorphous phase to the low-density porous phase is characterized by these quantities. Various dissimilar porous structures are prepared by controlling the preparation condition.

A simple mechanical model of planar fibrous materials with mesoscopic disorder is introduced and analyzed. In this scalar model a shear modulus controls the stress transfer in the transverse direction. The system is studied using the effective medium approximation and computer simulations; the comparison between them is quite favorable. In the disorder-controlled regime the stress-strain relation, the number of broken cells at the onset of crack propagation, and the length of the final crack scale with the system size as ${\mathit{L}}^2$, ${\mathit{L}}^{1.7}$, and L, respectively. The mechanical properties are controlled by the interplay between disorder and shear modulus, which is studied in detail.

The calculation of the temperature and concentration dependence of the quadrupolar-glass (QG) order parameter of solid hydrogen is presented. Starting from the microscopic Hamiltonian describing the electric quadrupole-quadrupole interaction, the effective Hamiltonian containing only commuting operators related to the components of the tensor of molecular quadrupolar momentum has been obtained. By means of this Hamiltonian the equation for the average QG order parameter has been formulated within the replica theory. The calculated tmeperature and concentration dependence of the order parameter is compared with the experimental data obtained by Meyer and Washburn.

A simple model for nonlinear collective transport in random media with strong disorder is presented and analyzed. It should apply to systems such as strongly pinned vortex lines in thin superconducting films or low-density fluid flow down a randomly rough inclined plane, in which the randomness is strong enough to break the flow into channels. The model exhibits a threshold force above which macroscopic flow exists. The critical behavior around this threshold is obtained analytically in mean-field theory, which should be valid in three dimensions and higher, and in one dimension. Analytical bounds and numerical simulations are used in the two-dimensional case. Correlation lengths above and below threshold scale differently. Multiple divergent length scales are seen above threshold. Possible modifications due to thermal fluctuations and other effects, and applications to physical systems, are discussed.

We derive a discrete Hamiltonian describing a Fibonacci superlattice in which the electronic potential is taken to be an array of equally spaced δ potentials, whose strengths modulate the chemical composition in the growth direction. In this model both diagonal and off-diagonal elements of the Hamiltonian matrix become mutually related through the potential strengths. The corresponding energy spectrum and related magnitudes, such as the Lyapunov coefficient, transmission coefficient, and Landauer resistance, exhibit a highly fragmented, self-similar nature. We investigate the influence of the underlying spectrum structure on the dc conductance at different temperatures obtaining analytical expressions which relate special features of the dc conductance with certain parameters that characterize the electronic spectrum of Fibonacci superlattices.

A Gibbs approach to the much studied problem of the thermal stability of nonlinear structures such as Davydov solitons is used with focus on spatially extended systems. The systems studied consist of a quantum-mechanical quasiparticle such as an electron or an excitation, interacting strongly with classical vibrations of the lattice, the entire assembly being in thermal equilibrium with a heat bath. The technique used for the study of thermal stability consists of the diagonalization of an N×N matrix where N is the number of sites among which the quasiparticle moves, followed by M thermal integrations where M is the number of vibrational coordinates with which the quasiparticle interacts. A basic duality emerges regarding the effect of temperature on the stability of nonlinear structures: temperature is found to help the nonlinearity in certain parameter and temperature regimes by inducing disorder and to destroy the nonlinearity in other regimes, e.g., always at large temperatures as a consequence of Boltzmann equalization. Particularly interesting features are found at low temperatures. A magnetic analogy reported earlier for smaller systems is reinforced by the present analysis for extended systems.

A photopyroelectric technique has been used to simultaneously measure the critical behavior of specific heat, thermal conductivity, and thermal diffusivity of ${\mathrm{Cr}}_2$${\mathrm{O}}_3$ at the antiferromagnetic-paramagnetic phase transition. An Ising-like behavior has been found for ‖${\mathit{t}}_{\max }$‖≤3×${10}^{\mathrm{-}3}$ with α=0.10±0.02 and A/A’=0.48±0.03. A possible crossover between Ising and Heisenberg behaviors has been found if ‖${\mathit{t}}_{\max }$‖ is increased. Thermal diffusivity data have been fitted according to the model for a uniaxial antiferromagnet with energy conservation for ‖${\mathit{t}}_{\max }$‖≤3×${10}^{\mathrm{-}3}$ with a critical exponent b=-0.11±0.02. A correction term similar to the static correction to scaling factor has been introduced in the fitting function. No anomaly has been found in thermal conductivity.

Intermolecular tunneling of polarons and bipolarons is studied in ${\mathrm{C}}_{60}$ in the limit of weak electron-electron interactions. It is shown that the effective mass of the polaron is renormalized only weakly by intramolecular electron-phonon interactions. However, using the adiabatic approximation, it is found that the Franck-Condon effect would essentially localize the bipolaron on a single molecule in the limit of narrow conduction bandwidth. Indeed, the effective mass of the bipolaron would be increased by more than a factor of ${10}^3$. Combined with the observed metallic behavior of doped ${\mathrm{C}}_{60}$, this suggests that either the electronic correlation effects are important or that the conduction band is not too narrow in order to suppress the artificially large mass renormalization or to inhibit the bipolaron formation altogether.

We have developed a variational approach to treat the nonadiabaticity, that is, the quantum-lattice fluctuations, of the electron-phonon interactions in the one-dimensional half-filled spinless Anderson lattice model including Coulomb interaction between both types of electrons. The nonadiabaticity due to finite phonon frequency is treated through a variational polaronic-type wave function, in which two variational parameters δ and ${\mathrm{\tau }}^2$ are used to take into account the dynamical distortion and the squeezing effect of phonon modes. We have found that the quantum-lattice fluctuations gradually smoothes the valence transitions when the polaronic level ${\mathrm{\varepsilon }}_{\mathit{f}}$-v changes. We have shown that conditions somewhat different from those of Hewson and Newns should be satisfied for the occurrence of a significant reduction of the effective hybridization. The effect of the Coulomb repulsion U is to suppress the quantum-lattice fluctuations, that is, to suppress the reduction of the fluctuating-valence frequency and the relaxation shift. We have also discussed the valence-density-wave ordering in the symmetric case. Our results show that the quantum-lattice fluctuations disfavor the ordering and the lattice dimerization parameter decreases with increasing phonon frequency. We have pointed out the possibility of an order-disorder transition in such systems.