We analyze Mn absorption in ${\mathrm{CaF}}_2$:Mn by the employment of ab inito quantum-mechanical cluster calculations and ligand-field methods. The [${\mathrm{MnF}}_8$${]}^{6\mathrm{-}}$ ${\mathit{O}}_{\mathit{h}}$ cluster is chosen to represent the isolated ${\mathrm{Mn}}^{2+}$ substitutional impurity in an otherwise perfect crystal. The methods of unrestricted open-shell Hartree-Fock self-consistent field (SCF), Mo/ller-Plesset perturbation theory to second- and fourth-order, and singles and doubles configuration interaction are used to calculate the spin sextet and quartet ground states. With the active space consisting of the Mn 3d molecular orbitals, the spin quartet excited states are calculated by the method of multiconfiguration SCF. It was found that the presence of an external field designed to reproduce the Madelung potential difference within the cluster did not significantly affect the Mn d-to-d transitions. The crystal-field term splitting diagrams for the eight-coordinated ${\mathrm{Mn}}^{2+}$ impurity in ${\mathit{O}}_{\mathit{h}}$ symmetry are calculated. The results showed a narrowing of the multiplet terms in energy with respect to the six-coordinated ${\mathit{O}}_{\mathit{h}}$ result. This increases the crystal-field parameter Dq from the previously published value of 420–570 ${\mathrm{cm}}^{\mathrm{-}1}$.

The measurements of the positron lifetime and of the Doppler broadening of the annihilation line have been performed in the fullerene ${\mathrm{C}}_{60}$ as a function of temperature between 120 and 300 K. Lifetime components ${\mathrm{\tau }}_1$=(360.6±0.6 ps, 97.8%) and (1.747±0.010 ns, 2.2%) have been observed in a pressed ${\mathrm{C}}_{60}$ sample. A temperature relation for lifetime τ=337.2(1+αT) ps was observed with average slope α=2.3×${10}^{\mathrm{-}4}$ ${\mathrm{K}}^{\mathrm{-}1}$. However, a rapid change of τ is observed between 240 and 250 K. The S parameter calculated from the Doppler-broadened annihilation line is a constant with some indication of change at ∼245 K. These results indicate that the expanding lattice from the ${\mathrm{C}}_{60}$ molecules is undergoing a phase transition and the phases coexist over an ∼10 K range, as has been observed by x-ray- and neutron-diffraction experiments. No hysteresis in τ or S was observed.

We study the percolative properties of bidimensional systems generated by a random sequential adsorption of line segments on a square lattice. As the segment length grows, the percolation threshold decreases, goes through a minimum, and then increases slowly for large segments. We explain this nonmonotonic behavior by a structural change of the percolation clusters. However, an accurate measurement of the exponent ν and of the fractal dimension D of the incipient infinite percolating cluster for several segment lengths shows that these systems belong to the universality class of random site percolation.

Crystal structure factors have been measured for eight reflections in Ge using γ radiation at 0.342 and 1.382 MeV. The uncertainty of the measured structure factors is approximately 0.05 e.u. Laue-case rocking curves exhibiting Pendellosung oscillations were recorded using a double-crystal spectrometer located next to a high-flux reactor. The measured rocking curves were computer fit to obtain structure factors. Because the effects of anomalous scattering approach zero at high energies (1.382 MeV), anomalous scattering corrections for lower-energy measurements (8, 59, and 342 keV) have been derived by comparing the high-energy results to the lower-energy results. Best estimates of structure factors are derived for 12 Ge reflections.

The methyl-ring-substituted derivative of polyaniline shows increased electron localization compared to polyaniline, and the mechanism for this increased localization has been proposed as due to a decrease in the interchain diffusion rate caused mainly by enhanced disorder due to random positioning of the methyl groups. We have investigated this issue by studying the magnetic and transport behavior of the copolymer poly(aniline${)}_{\mathit{x}}$(p-fluoroaniline${)}_{1\mathrm{-}\mathit{x}}$ at five values of x. We find that the electron-localization length in the copolymer is nearly equal to the conjugation length, and it decreases on decreasing x. The comparison of this behavior with that for poly(o-toluidine) and other alkyl-ring-substituted derivatives of polyaniline shows that the primary cause of electron localization in the polyaniline derivatives is enhanced interchain disorder. Our results also seem to provide some support for a model of polyaniline comprised of metallic islands consisting of single, isolated strands of the polymer.

Ion-beam mixing and amorphization were simultaneously studied via in situ Rutherford backscattering and channeling experiments on a Zr/Ni sample irradiated at low temperature with various medium-energy noble-gas ions. The number of mixed atoms and the spatial extension of the mixed layer were found to increase linearly with the nuclear energy density deposited by the irradiating ions. The existence of a threshold at 20 eV/Å provides an indication that a different mixing mechanism takes place in the case of very light ion irradiation. A good agreement is obtained between the experimental data and the predictions of a model based on a binary collision description. The channeling experiments devoted to the study of the amorphization process show that the concentration of foreign atoms introduced by mixing controls the amorphization kinetics. A possible mechanism for the amorphization process is presented. This mechanism is compared to that occurring in direct ion-implantation experiments.

The impurity-concentration dependence of the ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ NMR line shapes and phason-induced spin-lattice relaxation rates has been studied over a wide temperature range in substitutionally disordered incommensurate (${\mathrm{Rb}}_{1\mathrm{-}\mathit{x}}$${\mathrm{K}}_{\mathit{x}}$${)}_2$${\mathrm{ZnCl}}_4$ for x=0.00, 0.02, and 0.06. The results yield information on the impurity-concentration dependence of the phason gap and phase fluctuations of the modulation wave as well as on the nature of the coupling of the impurities to the incommensurate modulation wave.

Solid-state reaction in Fe-Si multilayers occurring at 573 and 658 K has been studied. Iron is the predominantly diffusing species at lower temperatures. It is found that a large asymmetry in the mobilities of the two constituents is a basic requirement for amorphization to occur. A detailed study of transformation kinetics suggests that amorphization occurs via one-dimensional diffusion-controlled growth. Information about the possible mechanism of amorphization has been obtained.

We calculate the vertical-transition energies from the ground state to the lowest singlet excited states of pyrrole oligomers containing up to seven rings, by means of different configuration interaction techniques. Special attention is paid to the evolution with chain length of the ordering of the lowest two excited states. The geometry-relaxation phenomena in the 1B and 2A states are modeled on the basis of the formation of bipolaron-type defects. The chain-length dependence of the third-order polarizability γ as well as the two-photon absorption and third-harmonic generation spectra are evaluated from a sum-over-states approach.

We have simulated the response of an interfacial amorphous region formed between nickel and zirconium lattices upon temperature increase or the application of uniaxial load by means of molecular-dynamics simulation of a model system based on an n-body potential. The behavior of the amorphous region has been investigated as a function of the load intensity and duration. The system reacts upon uniaxial load application with the growth of the amorphous interface. This has been related to the structural change occurring in the glassy region in the form of a density variation consequent to the introduction of excess free volume. These findings are qualitatively consistent with the current hypothesis invoked to explain diffusion and growth of interfacial amorphous regions formed upon load application on bulk diffusion couples.

The attenuation and velocity of ultrasonic waves of frequencies in the range of 10 to 90 MHz have been measured in ${\mathrm{La}}_2$${\mathrm{O}}_{3\mathrm{-}}$${\mathrm{P}}_2$${\mathrm{O}}_5$ and ${\mathrm{Sm}}_2$${\mathrm{O}}_{3\mathrm{-}}$${\mathrm{P}}_2$${\mathrm{O}}_5$ glasses with high lanthanide concentrations as a function of temperature between 1.5 and 400 K. Two distinct features characterize the attenuation behavior: (i) a plateau at temperatures below 15 K and (ii) a broad high-temperature peak. The former feature is interpreted in terms of the phonon-assisted relaxation of two-level systems and the latter by assuming the existence of a distribution of thermally activated relaxing centers. For both these mechanisms the product of the deformation potential squared and the density of relaxing particles decreases with increasing lanthanide-ion concentration. This result, taken together with previous observations of the properties of oxide glasses, provides physical insight into the microscopic origin of the relaxation effects and suggests that the source of the low- and high-temperature attenuation mechanisms is the same. At temperatures below 100 K, the sound velocity, after the subtraction of the relaxation and anharmonic contributions, follows a linear law as predicted by the soft-potential model for the relaxation of soft harmonic oscillators. An encouraging agreement is obtained between the parameters regulating this mechanism and those determined from the acoustic attenuation plateau.

In recent experiments it was found that the Kapitza conductance between diamond and Pb at room temperature is as much as 100 times larger than predicted by the acoustic mismatch model. Because the diamond and Pb are in intimate contact, the phonon modes near the interface are joint vibrational modes, extending a phonon mean free path on each side of the interface. Since the phonon mean-free path at room temperature is of the order of 4000 Å in diamond but only of the order of 10 Å in Pb, the energy of a joint mode resides primarily in the diamond. Consequently, the thermal population of a joint mode is determined by the temperature of the diamond. It is shown here that the direct energy transfer between metallic electrons at the Pb temperature and joint vibrational modes at the diamond temperature explains the observed heat conduction across the interface.

Infrared-reflectance spectra of the solid solutions ${\mathrm{Zr}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Hf}}_{\mathit{x}}$${\mathrm{O}}_2$ (0≤x≤1) have been measured at room temperature in the frequency region 50–4000 ${\mathrm{cm}}^{\mathrm{-}1}$ by Fourier-transform infrared spectroscopy in order to study the composition effect on the vibrational properties of ${\mathrm{Zr}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Hf}}_{\mathit{x}}$${\mathrm{O}}_2$. At most, twelve infrared-active modes of 5${\mathit{A}}_{\mathit{u}}$ and 7${\mathit{B}}_{\mathit{u}}$ symmetry were identified in the reflectance spectra of ${\mathrm{Zr}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Hf}}_{\mathit{x}}$${\mathrm{O}}_2$. The spectra of ${\mathrm{Zr}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Hf}}_{\mathit{x}}$${\mathrm{O}}_2$ varied systematically with x, with changes in the frequency, linewidth, and intensity of these modes. Particularly, the ${\mathit{B}}_{\mathit{u}}$ modes at 585 and 606 ${\mathrm{cm}}^{\mathrm{-}1}$ for ${\mathrm{ZrO}}_2$ considerably stiffened in a nonlinear way, indicating a pronounced stiffening around x=0.8 as x increases. It was found that the lattice parameters a,b, and c of ${\mathrm{Zr}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Hf}}_{\mathit{x}}$${\mathrm{O}}_2$ decrease linearly with x whereas β initially increases up to x=0.6 and then decreases towards that of the end member ${\mathrm{HfO}}_2$. Hence, it was concluded that the stiffening of all other modes except a ${\mathit{B}}_{\mathit{u}}$ mode is associated with the bond reinforcement evidenced by the reduced cell volume of ${\mathrm{Zr}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Hf}}_{\mathit{x}}$${\mathrm{O}}_2$ as the ${\mathrm{Hf}}^{4+}$ ions of smaller radius are substituted for the ${\mathrm{Zr}}^{4+}$ ions. It was also concluded that the anomalous feature around x=0.8 in the frequency, linewidth, and intensity of the modes as a function of x can be reconciled with two-mode behavior, which has been characterized by discontinuities at either x=0.6 or 0.3 in the x dependence of seven Raman mode frequencies for ${\mathrm{Zr}}_{\mathit{x}}$${\mathrm{Hf}}_{1\mathrm{-}\mathit{x}}$${\mathrm{O}}_2$ [C. Carlone, Phys. Rev. B 45, 2079 (1992)].

Assuming that the interionic forces can be modeled by the effective pair potential u(r) arising in second-order pseudopotential theory, we have calculated the structure factor S(q) and some thermodynamic properties of the alkali metals. The pair potential is derived from a local empty-core pseudopotential including a core-valence exchange correlation. The calculations of the structure and thermodynamics are performed with the self-consistent integral equation called the hybridized mean spherical approximation (HMSA). The results presented in detail are in very good agreement with experiments for all the alkali metals near their melting points as well as for Rb and Cs along the liquid-vapor coexistence curve.

We present a study of the nucleation mechanism that allows the decay of the metastable phase (trans-cisoid) to the stable phase (cis-transoid) in quasi-one-dimensional nondegenerate polymers within the continuum electron-phonon model. The electron-phonon configurations that lead to the decay, i.e., the critical droplets (or transition state), are identified as polarons of the metastable phase. We obtain an estimate for the decay rate via thermal activation within a range of parameters consistent with experimental values for the gap of the cis configuration. It is pointed out that, upon doping, the activation barriers of the excited states are quite smaller and the decay rate is greatly enhanced. Typical activation energies for electron or hole polarons are ≊0.1 eV and the typical size for a critical droplet (polaron) is about 20 Å. Decay via quantum nucleation is also studied and it is found that the crossover temperature between quantum nucleation and thermal activation is of order ${\mathit{T}}_{\mathit{c}}$≤40 K. Metastable configurations of nondegenerate polymers may provide examples for mesoscopic quantum tunneling.

A semiclassical method is presented for tunneling of self-trapped states in many-body systems with electrons and phonons. An overcomplete set of Slater determinants, lattice coordinates, and lattice momenta is used to represent a functional integral. Stationary phase equations are solved numerically without any constraint on the dynamics of electrons and phonons, i.e., without the use of the adiabatic approximation. To evaluate transition amplitudes, we integrate over small fluctuations in both electronic and phonon degrees of freedom, keeping their time order correctly. This method can be applied to general electron-phonon systems is and useful when self-trapped states have complex structures in charge or spin densities and lattice displacements. The effective hopping strength is calculated for a self-trapped kink in the commensurate charge-density-wave state in one dimension. At strong coupling in the Holstein and attractive Hubbard models, where tunneling involves effectively a single tightly bound bipolaron, this method reproduces previous analytic results. New results are obtained at intermediate coupling where the kink is extended over a couple of lattice sites and for models with both electron-electron and electron-phonon interactions.

The standard diagram technique for impurity scattering is extended to the case in which the impurity potential linearly depends on the intensity of the local field. A Bethe-Salpeter equation is derived which describes diffusion of light in a nonlinear random medium. In order to cope diagrammatically with the nonlinear behavior of the impurities, one has to introduce vertices at which four propagators meet. As for th case of linear impurities, configurational averages can be taken at the diagrammatic level. However, the resulting dressed diagrams are of a much more complicated structure than usual. Furthermore, two diagrams that are topologically different from each other may represent the same expression. In decomposing the dressed diagrams, certain cuts can be performed only if other cuts are carried out first. This implies that the Bethe-Salpeter equation cannot be derived by simply cutting all dressed diagrams into irreducible pieces. Instead, a more sophisticated procedure has to be followed. It is developed by performing a topological classification of the dressed diagrams. In calculating the diagrams of the Bethe-Salpeter equation those with more than two vertices are discarded. This leads to three coupled nonlinear integral equations which describe radiative transport in a dilute suspension of weakly nonlinear Kerr particles. The dimension of the particles is small as compared to the wavelength of the light. The solution of the integral equations must satisfy a constraint that follows from energy conservation. Analytical work and predictions on experiments will be presented in a forthcoming paper.

The dynamics and the switching processes of bistable nuclear polarization states in gallium oxide are studied. Conduction-electron spin resonance of oxygen-deficient β-${\mathrm{Ga}}_2$${\mathrm{O}}_3$ crystals exhibits a bistable Overhauser effect, observable from hysteresis of the resonance line when the external magnetic field is swept up and down. The dynamics of bistability are investigated by considering the electron spin system coupled to nuclear spins of gallium as a simple (S=1/2, I=1/2) pair system. In this representation, the dynamics can be visualized as the overdamped motion of a fictitious particle (the spin system) in a potential with two minima, which are the stable steady states of the bistable system. The observed phenomena are related to the rate of displacement of the fictitious particle in the potential well, which is controlled by the nuclear relaxation time, and to the rate of modification of the potential, which corresponds to the rate of variation of the control parameter. This model gives a simple physical image of bistable switching operations and accurate simulations of bistable-electron-spin-resonance spectra. Addressing of the system with rectangular pulses has been demonstrated for two control parameters: the external magnetic field and the incident microwave power.

A profound analogy exists between the modulate structures in magnetic materials and in nematic liquid crystals, especially for the behavior in an external field. Starting from this point, we study the influence of an electric field on the spatially modulated spin structure (spin-density-wave state) of the magnetic ferroelectric ${\mathrm{BiFeO}}_3$, discovering and investigating the possibility of a transition between the spin-density-wave state into a homogeneous antiferromagnetic configuration.

The single-ion spin Hamiltonian parameters have been determined for zinc fluoroborate hexahydrate doped with ${\mathrm{Ni}}^{2+}$ (3${\mathit{d}}^8$, S=1) and compared with those obtained for nickel fluoroborate hexahydrate. To account for the differences observed, EPR for several concentrations (x=0.005 to 1) was studied from room to liquid-helium temperature. Between 4.2 and 1.5 K, for low doping, they are constant with ${\mathit{g}}_{\mathrm{\parallel }}$=2.229±0.006, ${\mathit{g}}_{\mathrm{\perp }}$=2.261±0.025, and D=-(0.1291±0.0008) ${\mathrm{cm}}^{\mathrm{-}1}$. Appropriate values for nickel fluoroborate are nearly the same but they are weakly temperature dependent, attributed to a ferromagnetic interaction. At higher temperatures these parameters are remarkably concentration dependent. Two main lines arise in parallel orientation. For concentrations up to x≊0.04 and at 77 K, they show asymmetrically distributed satellite lines outside the more intense central portion. The satellite lines seem to increase with concentration but only those assigned to a ferromagnetic-coupled first-neighbor pair survive the simultaneous broadening effect. As the broadening is asymmetric, the lines’ centers shift smoothly outwards. At x≊0.7, we observe maximum broadening; for larger x no structural detail is seen and the spectra are progressively exchange narrowed. At the same time, the two lines are drawn together. The linewidth dependence on concentration indicates that the Ni ions have randomly occupied the Zn sites as expected, and that the contribution from spin-lattice relaxation practically does not change with concentration. We conclude that the differences in the spin Hamiltonian parameters are due essentially to magnetic interactions throughout the whole temperature range.