Thermally induced spectral diffusion of a spectral hole was investigated using photoautomerization systems in various polymers. The broadening rates were correlated with the packing densities of the matrix polymers calculated by Bondi in the form of a Doolittle-type equation. The result was interpreted as the result of redistribution of entropically oriented free volume. The origin of the two-level system in amorphous polymers was discussed. The free volume is thought to be the origin of the intrinsic two-level system.

The structure of liquid and amorphous selenium has been investigated by a Monte Carlo computer simulation. The semiempirical model relies upon a tight-binding approximation of the cohesive energy. The excellent agreement of the pair correlation functions and structure factors computed at different temperatures with the experiments allows a detailed analysis of the structures. The fraction of twofold-coordinated atoms never exceeds 70%. This implies that, contrary to commonly reported ideas, the chains have to be quite short, of the order of five bonds per chain, forming a highly connected network.

We study the electronic properties of a one-dimensional deterministic aperiodic nonlinear lattice using a tight-binding form of the nonlinear Schrödinger equation. A nonlinear eigenvalue problem is solved for self-consistent solutions, and the corresponding transmission problem is studied using a two-dimensional nonlinear mapping. When the aperiodicity is chosen according to the Thue-Morse sequence and the nonlinearity is small, we find solitonlike solutions in a similar way as for a periodic lattice. The coupling of an incoming plane wave to these structures results in peaks of perfect transmission in the transmission gaps of the corresponding linear model. Transmission peaks due to dark solitons, and to combinations of several weakly interacting solitons, so called ‘‘multisolitons’’ or ‘‘soliton trains’’ are also found. For larger nonlinearity we find that bounded and diverging solutions are mixed in an intricate pattern; the set of energy values yielding bounded orbits for a given nonlinearity strength showing a Cantor-like structure reminiscent of the linear Thue-Morse system.

Amorphous polycarbonate has been investigated from the point of view of free-volume microstructure in the temperature range 50–350 K by positron-annihilation-lifetime spectroscopy. Lifetime ${\mathrm{\tau }}_3$ and relative intensity ${\mathit{I}}_3$ of o-Ps show two regions of different behavior with the transition at ${\mathit{T}}_{\mathit{b}}$=130 K. The temperature dependence of ${\mathrm{\tau }}_3$ has been analyzed using the model of a particle in a spherical potential well. The correlations between the dilatometric measurements and the ${\mathrm{\tau }}_3$(T) and ${\mathit{I}}_3$(T) dependences have been interpreted in the framework of the free-volume model, which allowed us to estimate the fraction of free volume accessible to o-Ps as well as the concentration of free-volume entities. We have determined that the polycarbonate is a relatively defect-full material. On the basis of a simple thermodynamical approach allowing for simultaneous change of the size and number of free-volume entities, an attempt has been made to specify the molecular mechanism responsible for the free-volume changes. The comparison of the results of such a thermodynamical model with the simulation of the structure of amorphous polycarbonate and the dynamics of the motion of phenyl groups suggests that partial flips of this group might be the potential generators of the increased free volume at T>${\mathit{T}}_{\mathit{b}}$.

Using integral equations which adopt the hypernetted-chain equation for the interionic correlation, the convolution approximation for the interelectronic correlation, and the density-functional scheme for the ion-electron correlation, we calculate the correlation functions of liquid alkali metals (Li,Na,K) near freezing. Through a comparison with the experimental results, it is shown that the present theory is in fairly good agreement with experiment.

An inner-space approach to the structure-factor reconstruction in quasiperiodic crystals is presented. This approach is based on the representation of a quasiperiodic crystal as a higher dimensional periodic crystal (hypercrystal) whose hyperatom form factors can often be expressed as continuous functions of the inner-space components of the scattering wave vector. Thus, the reconstruction of the structure factors is reduced to a convenient parametrization of these functions (exemplified here by a Taylor series expansion) with the parameters to be determined through a fit of the experimentally measured diffraction intensities. The fitting parameters may also include locations of the hyperatoms, their chemical composition, and Debye-Waller factors. As an illustration, we applied this technique to reconstruct neutron scattering structure factors of icosahedral quasicrystal $i-{\mathrm{Al}}_{0.570}{\mathrm{Cu}}_{0.180}{\mathrm{Li}}_{0.322}$. We found consistent results with these obtained earlier using a method based on periodic approximants of quasiperiodic crystals.

We present an analytic approach to the electrical response of heterogeneous systems with inclusions of arbitrary structure. The exact local field acting on the inclusions, the multipole moments, and the effective dielectric function are obtained in terms of the polarization coefficients of the inclusions, the system configuration, and the applied electric field. We then provide the results for disordered systems in the mean-field approximation. The Clausius-Mossotti relation follows in the particular case of spherically symmetric pair-distributions, labeled by L=0 in this paper. The explicit results corresponding to L=1 and L=2 pair distributions are then given. We discuss in detail the relation between the contributions of all multipole moments and the form of the pair distributions. We study in particular a system of ellipsoidal inclusions and resolve an apparent discrepancy between two previous results, showing that they correspond to different pair distributions.

Envelope solitons formed by anharmonic intramolecular vibrations in gyrotropic molecular chains are investigated. The amplitudes of the circularly polarized modes obey a nonlinear Schrödinger equation which possesses bright- and dark-soliton solutions. The dependence of the solutions on the carrier wavelength and the gyration coefficient is analyzed. Linearly polarized solitons are described by a pair of conjugated circularly polarized pulses whose dynamics is governed by a system of coupled nonlinear Schrödinger equations. The system is studied analytically and numerically and different evolutionary patterns are obtained, corresponding to a coupled or an uncoupled single-soliton solution or a coupled two-soliton solution. The process of collision of circularly polarized solitons is also investigated.

Starting from the extensional Su-Schrieffer-Heeger model Hamiltonian containing intrachain e-e interactions, a bond-bending term, and an external Coulomb potential arising from the interchain-charged solitons and counterions, all the two-dimensional vibrational modes around the soliton lattice are calculated for doping levels from y=3.33 at. % to y=16.67 at. %. Thirteen localized vibrational modes are found for dopant concentrations from 3.33 to 13.33 %, in which seven modes are infrared active and correspond with the infrared absorption peaks. No localized modes have been found for doping levels above y=16.67 at. %. In addition, the frequencies of the modes are decreased and their localizations are weakened when the dopant concentrations increase.

Using ab initio total-energy and molecular dynamics based on local orbitals and density-functional theory, we present results on the nonequilibrium Al-Si solid solution. We report on relaxations, vibrational spectrum, and superconducting transition temperature. Our results are in excellent agreement with experimental measurements on rapidly quenched samples which show evidence of lattice instabilities induced by the solute. We perform a similar analysis on the vibrational properties of the Al vacancy and discuss the results in terms of the mechanical instability leading to the crystalline-to-amorphous transition.

Tracer-diffusion coefficients D of the impurities B, Be, Fe, and Si in the amorphous alloy ${\mathrm{Ti}}_{60}$${\mathrm{Ni}}_{40}$ have been measured in the temperature range 598–690 K using the technique of secondary-ion-mass spectrometry for concentration depth profiling. The temperature dependence of the measured diffusion coefficients in each case exhibited an Arrhenius behavior and yielded the values of the activation energy Q in units of eV as (2.05±0.14), (2.20±0.15), (2.33±0.14), and (2.35±0.15) for the diffusion of B, Be, Fe, and Si, respectively. The corresponding values of the preexponential factor of the diffusion coefficient ${\mathit{D}}_0$ in units of ${\mathrm{m}}^2$ ${\mathrm{s}}^{\mathrm{-}1}$ were obtained as 7.4×${10}^{(\mathrm{-}4.0\pm 1.1)}$, 1.7×${10}^{(\mathrm{-}3.0\pm 1.2)}$, 2.5×${10}^{(\mathrm{-}3.0\pm 1.1)}$, and 5.8×${10}^{(\mathrm{-}4.0\pm 1.8)}$. The results show the size dependence of D in this alloy according to which the small boron atoms diffuse about 2 orders of magnitude faster than the big silicon atoms while the diffusivities of Be and Fe had intermediate values following the trend ${\mathit{D}}_{\mathrm{B}}$>${\mathit{D}}_{\mathrm{Be}}$>${\mathit{D}}_{\mathrm{Fe}}$>${\mathit{D}}_{\mathrm{Si}}$ which is opposite to that of atomic radii r of the diffusing species, i.e., ${\mathit{r}}_{\mathrm{Si}}$>${\mathit{r}}_{\mathrm{Fe}}$>${\mathit{r}}_{\mathrm{Be}}$>${\mathit{r}}_{\mathrm{B}}$. The present data have been compared with those available in other amorphous alloys and in crystalline α-Zr and α-Ti. The notable distinct differences in the diffusion behavior in the amorphous and the crystalline cases are highlighted. On the basis of an observed correlation between the activation energy Q and the prefactor ${\mathit{D}}_0$ the possible diffusion mechanism in amorphous alloys is discussed.

We study the phase-space geometry and the low-temperature relaxation of short-range Gaussian Ising spin-glasses, by finding all the configurations and their interconnection within a ‘‘pocket’’ of states surrounding a low-lying energy minimum. Thermalization within the pocket is modeled by a master equation, which is solved numerically, yielding the time-dependent propagator. The findings show that the relaxation proceeds by a series of local equilibrations in nested ‘‘valleys,’’ and support the dynamical predictions of hierarchical relaxation models.

The infrared spectrum of the rotovibrational band v=0→2 of parahydrogen has been observed in the condensed phases down to T=2 K. In the solid, phonon and roton sidebands exhibit peaks corresponding to those observed in the fundamental. Contributions arising from the reorientation of ortho-${\mathrm{H}}_2$ impurities have been detected. The ${\mathit{Q}}_1$(0)+${\mathit{S}}_1$(0) band has been resolved into an asymmetric doublet, and the origin of this latter is discussed. In the liquid phase the observed line shapes include strong translational contributions, and are accounted for by assuming that at short times the excited molecule is encapsulated in the cage of nearest neighbors.

Anisotropic-dark-field optical imaging is shown to reveal the presence of horizontal Bloch lines (HBL’s) in the moving walls of bubble garnets. It relies on the deformation of the wall shape under dynamic conditions due to the presence of a HBL. The wall tilt is detected because it affects the intensity of the wall images much in the same way as localized wall tilts reveal the location of vertical Bloch lines (VBL’s) in static conditions. Quantitative time-resolved measurements of wall intensity are presented and shown to compare very favorably with numerical computations. These use a simplified spin dynamics equation (Slonczewski’s equations) describing the wall position and wall-core magnetization in-plane angle, as a function of position across the thickness, and time. Dynamic reversals of VBL’s contrast have also been observed. The fact that they occur for one half of the possible configurations is puzzling. They are tentatively explained on the basis of a dynamically induced charge distribution at VBL location, which arises from a wall wave developing around the moving VBL.

The three-state antiferromagnetic Potts model on a family of bipartite diamond hierarchical lattices is investigated. This exactly solvable family of models exhibits a distinctive low-temperature phase of the type predicted by Berker and Kadanoff for complex systems with a macroscopically degenerate ground state. An analytic expression for the residual entropy is derived. We prove that there are some similarities between this phase and that of Kosterlitz-Thouless, namely, the power-law decay of correlations, the vanishing of the order parameter, and the absence of divergence in the specific heat. It is also shown that the local order parameter has two distinct multifractal structures: one at the critical point and another at the unusual phase.

Hyperfine splitting arising from ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ for the ${\mathrm{Ge}}^{3+}$ (S=1/2) uncompensated [${\mathrm{GeO}}_4$${]}_{\mathrm{I},\mathrm{I}\mathrm{I}}^{\mathrm{-}}$ as well as charge-compensated [${\mathrm{GeO}}_4$/Li${]}_{\mathrm{A},\mathrm{C}}^0$ centers has been measured in an isotopically enriched α-quartz crystal. The EPR results indicate that the ${\mathrm{GeO}}_4$ tetrahedron is distorted for [${\mathrm{GeO}}_4$${]}_{\mathrm{I}}^{\mathrm{-}}$ and [${\mathrm{GeO}}_4$/Li${]}_{\mathrm{C}}^0$ so that no two oxygen ions are symmetry equivalent. In contrast, for [${\mathrm{GeO}}_4$${]}_{\mathrm{II}}^{\mathrm{-}}$ and [${\mathrm{GeO}}_4$/Li${]}_{\mathrm{A}}^0$ the expected local symmetry (${\mathit{C}}_2$) is present and only two types of oxygen atoms are observed. Inclusion of the oxygenic contribution brings the total measured spin population to 100% (within the margin of probable errors). Analysis of the ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ hyperfine matrices revealed that the majority of the oxygenic spin density is found on only two of the four oxygen atoms, in 2p orbitals. For each center studied, the unique principal direction of the ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ hyperfine matrix of the high-spin-density oxygen atoms lies along the O-O interatomic direction of these two oxygen atoms.

The unique principal direction of the ${}_{}{}^{73}{}_{}{}^{}\mathrm{Ge}$ hyperfine matrix lies along the bisector of angle O-Ge-O for these same oxygen atoms. Comparison of the various ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ hyperfine splitting patterns furnishes conclusive proof of an isomorphic relationship between centers [${\mathrm{GeO}}_4$${]}_{\mathrm{I}}^{\mathrm{-}}$ and [${\mathrm{GeO}}_4$/Li${]}_{\mathrm{C}}^0$ and also between [${\mathrm{GeO}}_4$${]}_{\mathrm{II}}^{\mathrm{-}}$ and [${\mathrm{GeO}}_4$/Li${]}_{\mathrm{A}}^0$. We conclude that the compensated centers basically have the same electronic structure as the corresponding uncompensated centers, with the compensating ${\mathrm{Li}}^{+}$ causing only a minor perturbation. The EPR data are consistent with a qualitative model which suggests that the MO of the unpaired electrons in electron-rich paramagnetic defects resembles the LUMO of a Si(${\mathrm{OSi}}_{1/4}$${)}_4$ cluster model for quartz.

Various bounds for the energy of collective excitations in the Heisenberg antiferromagnet are presented and discussed using the formalism of sum rules. We show that the Feynman approximation significantly overestimates (by about 30% in the S=1/2 square lattice) the spin velocity due to the non-negligible contribution of multiple magnons to the energy-weighted sum rule. We also discuss a different, Goldstone-type bound depending explicitly on the order parameter (staggered magnetization). This bound is shown to be proportional to the dispersion of classical spin-wave theory with a q-independent normalization factor. Rigorous bounds for the excitation energies in the anisotropic Heisenberg model are also presented.

We consider theoretically standing spin-wave resonance spectra from a ferromagnetic bilayer film consisting of two identical homogeneous sublayers exchange-coupled at the interface. The film (Heisenberg) Hamiltonian includes both surface and interface pinning anisotropy of a uniaxial type. The external field is applied perpendicularly to the film surface. For this model we establish conditions for the coexistence of surface and interface modes, and moreover predict the possibility of creation of bilocalized symmetrical or antisymmetrical hybrids of these modes. The calculated resonance spectrum shows that when full hybridization is achieved one of the bilocalized modes (even if symmetric) is not excited, and the remaining mode is predominant in the spectrum. The general bilocalization conditions involving the interfacial coupling and surface-interface anisotropies are tabulated separately for symmetrical and antisymmetrical modes.

A variational wave function with spin zero is presented for the infinite-U Hubbard model on a square lattice with the hole concentration δ=0.5. We find that the Nagaoka ferromagnetic state is unstable for this case in the thermodynamic limit. Some discussion about this method for other hole concentrations is given.

Magnetic behavior of Rh in paramagnetic ${\mathrm{Ni}}_{\mathit{x}}$${\mathrm{Rh}}_{1\mathrm{-}\mathit{x}}$ for x≤0.63 is studied by measuring the ${}_{}{}^{100}{}_{}{}^{}\mathrm{Rh}$ local susceptibility using the time-differential perturbed-angular-correlations method, the bulk magnetization, and through local-spin-density calculations. For x∼0.6 the measured Curie-Weiss local susceptibility yields a high Rh magnetic moment with a small spin-fluctuation temperature ≤100 K, while for x≲0.40 the observed results are consistent with an unstable moment on Rh characterized by large spin-fluctuation temperatures ∼${10}^3$ K. The magnetism of Rh in the Ni-Rh system is found to depend on the number of Ni atoms in its immediate surrounding. The calculations show that both Ni and Rh atoms acquire moments in the presence of at least two Ni atoms in the nearest-neighbor (NN) coordination shell. Further, Ni and Rh moments are found to depend on the geometrical arrangement of the NN Ni atoms. The increase in the moment value on individual atoms and the enhanced susceptibility comes from the growing proportion of three-dimensional or isotropic NN Ni configurations over the linear or planar configurations, which occur for 5–6 or more Ni atoms. Our experimental results, as well as theoretical calculations, show that Rh magnetic behavior is strongly influenced by the sign of the interatomic exchange interaction. In particular, the observed enhanced Rh magnetic response near x∼0.6 is attributed to the ferromagnetic d-d interaction between Rh and Ni atoms.