The transport of femtosecond-laser-excited nonequilibrium electrons across polycrystalline and single-crystalline gold films has been investigated through time-of-flight measurements. The thicknesses of the films range from 25 to 400 nm. Ballistic electrons as well as electrons interacting with other electrons and/or with the lattice have been observed. The ballistic component dominates the transport in the thinner films, whereas the interactive transport mechanism is dominant at the upper end of the thickness range. A slower effective velocity of the interactive component is observed in the polycrystalline samples, and is assumed to arise from the presence of grain boundaries. The reflection coefficient of excited electrons at the grain boundaries is extracted from the experiment and is estimated to be r≃0.12. The experiment also suggests that thermal equilibrium among the excited electrons is not fully established in the first ∼500 fs after excitation.

The field-ion–scanning-tunneling microscope has been employed to study the ${\mathrm{Sc}}_2$${\mathrm{C}}_{84}$ metallofullerenes as individual-molecule adsorbates and as a solid-phase crystalline film on the Si(100)2×1 surface. The results suggest that the Sc atoms are captured inside the ${\mathrm{C}}_{84}$ cage and that a significant amount of charge transfer to the cage takes place. The properties of ${\mathrm{Sc}}_2$${\mathrm{C}}_{84}$ in the solid phase are similar to those of the pristine ${\mathrm{C}}_{84}$ fullerene, i.e., the intermolecular bonding is from the van der Waals interaction. However, the intermolecular distance becomes smaller compared to the pristine ${\mathrm{C}}_{84}$.

Image states on Pd(111) with various coverages of Ag were investigated by two-photon photoemission spectroscopy for temperatures between 90 and 1300 K. The formation of three-dimensional islands, smooth layers, and surface alloys is identified. The transition from one-dimensional localization in front of the surface to three-dimensional localization on top of Ag islands with diameters between 10 and 100 Å is observed. The high-resolution spectroscopy of image states provides a new tool to study the growth mode, morphology, and island size distribution of ultrathin metal films.

The global phase diagram of the spin-1/2 Ising model with nearest- and next-nearest-neighbor interactions on the square lattice is considered, including the fully anisotropic cases. A closed-form expression is given that accurately represents the phase boundaries when the crossed next-nearest-neighbor interactions are not of opposite signs. The phase diagram includes disordered, ferromagnetic, antiferromagnetic, axially single-striped, and diagonally double-striped phases.

The angular distributions of Ag atoms ejected from a Ag{111} surface due to 2-keV Ar bombardment have been calculated from molecular-dynamics simulations of the particle impact event. For atoms ejected with kinetic energies of above 15 eV, the distributions are found to be sensitive to variations in the ejection energy. The direction along which the maximum sputter intensity is detected alternates between two different 〈211〉 azimuths as the kinetic energy of the ejected atom is increased. A time-exposure representation of the surface-collision cascade is introduced for examining the collision mechanisms that cause atoms to eject along specific crystallographic directions. It shows that, in addition to the presence of the open spacings between the first-layer surface atoms, there are subtle collision effects that determine the preferred direction of ejection. Atomic ejection along those 〈211〉 azimuths which have the non-close-packed row of atoms extending to the bulk is mainly induced by the up-down-, the head-on-, and the sideswipe-collision processes. The up-down-collision process may result in ejection of atoms at a distance of more than 20 Å away from the point of the primary impact. Further, the low-energy atoms are usually emitted from the surface due to sideswipe collisions by the moving surface atoms. The dependence of the preferred direction of ejection on the collision time of the ejected atom is also discussed.

The atomic and electronic structures of lattice defects in oxygen-deficient rutile ${\mathrm{TiO}}_{2\mathrm{-}\mathit{x}}$ (1×${10}^{\mathrm{-}4}$≲x≲3×${10}^{\mathrm{-}3}$) are discussed on the basis of electron-paramagnetic-resonance measurements. Titanium interstitials with a single paramagnetic electron (S=1/2), which are the main lattice defects for small x (≲3×${10}^{\mathrm{-}4}$), strongly interact with each other for large x (≳4×${10}^{\mathrm{-}4}$) resulting in Ti interstitial pairs with two paramagnetic electrons (S=1) whose Ti-Ti distance and orientation are 3.25 Å and 〈110〉, respectively. At the same time, a considerable fraction of the Ti interstitials are ordered in cooperation with an oxygen deficit, resulting in {121} plane defects. The (121) plane defect contains Ti-ion pairs whose Ti-Ti orientation is 〈100〉, and the Ti-ion pair has two covalent-bond electrons and an additional paramagnetic electron (S=1/2).

We present Monte Carlo calculations of the pressure-composition isotherms of palladium hydride using recently constructed embedded-atom-method potentials and a Monte Carlo method that allows for volume equilibration as the composition varies.

The Pbnm→P$2_1$/n ferroelastic phase transition in phenothiazine has been studied using experimental intermolecular energy functions of the form -A/${\mathit{r}}^6$+${\mathit{Be}}^{\mathrm{-}\mathit{C}\mathit{r}}$ and a quadratic term for intramolecular distortion energy. A change of the molecular dihedral angle during the phase transition is found and a low-frequency mode that could be responsible for it has been identified. A split-molecule disorder model has been applied and predicts by free-energy arguments a first-order order-disorder phase transition at a temperature that is in good agreement with experiment. The phase transition under pressure is also studied and the calculated p-T phase diagram also agrees with experiment.

We consider the energy dependence of the Mn 3s and 3p multiplets from gas-phase atomic Mn and crystalline ${\mathrm{MnF}}_2$ and ${\mathrm{KMnF}}_3$ over the range from x-ray photoelectron spectroscopy (XPS) energies down to energies near threshold. First comparing atomic and solid-state spectra for these multiplets permits concluding that the splittings in the compounds ${\mathrm{MnF}}_2$, MnO, and ${\mathrm{Cd}}_{0.3}$${\mathrm{Mn}}_{0.7}$Te are highly atomic in character, with no significant effects due to extra-atomic screening. Measuring the energy dependence for atomic Mn, ${\mathrm{MnF}}_2$, and ${\mathrm{KMnF}}_3$ then shows for both the 3s and 3p multiplets that there is a decrease in the intensities of the higher-binding-energy quintet states relative to those of the corresponding septet states as the excitation energy is lowered. This effect on the quintet:septet branching ratios is also found to extend to rather high energies, with the ratios at the XPS limit of ≊1400 eV above threshold being approximately 25–30 % greater than those at ≊200 eV above threshold. We show that this energy-dependent final-state branching ratio is not due simply to spin-dependent dipole matrix elements as derived from single-configuration Hartree-Fock calculations. We suggest that this effect is caused by the sudden-to-adiabatic transition, which at lower energies favors the exchange-stabilized septet states that are the ground states of the ions formed. However, two prior theoretical models for such sudden-to-adiabatic intensity changes [Stohr, Jaeger, and Rehr, Phys. Rev. Lett. 51, 821 (1983) and Thomas, Phys. Rev. Lett. 54, 182 (1985)] were not found to describe our results well, particularly in the extension of the effect to higher energies. We consider qualitatively a configuration-interaction model with quintet-septet interchannel coupling that may better describe these effects and form the basis for more quantitative calculations.

It has been proposed that single crystals of phenothiazine undergo a proper ferroelastic phase transition: Pnma(${\mathit{D}}_{2\mathit{h}}$)→P$2_1$/c(${\mathit{C}}_{2\mathit{h}}$). This premise is in conflict with Brillouin-scattering studies from both quasi- and high-symmetry longitudinal and transverse acoustic modes reported here. The ${\mathit{C}}_{55}$ elastic constant of phenothiazine as an orthorhombic crystal at ambient conditions shows no temperature dependence. The inelastic, light-scattering spectra support an order-disorder phase transition; no significantly unstable acoustic phonons were observed through the phase transition. Laser diffraction from structural domains reveals that the crystal is polysynthetically twinned above and below ${\mathit{T}}_{\mathit{c}}$. At room temperature, domain walls are made indistinct by disorder but become visible as the sample is cooled. Stress-strain hysteresis below ${\mathit{T}}_{\mathit{c}}$ is attributed to a ferrobielastic effect.

X-ray-absorption fine-structure (XAFS) data of RbBr, RbCl, and KBr at 30 K and 125 K were measured and analyzed. An ionized-atom multiple-scattering calculation and a correlated Debye model were used for fitting the theory used in the computer code feff5 to data. The modifications of feff5 necessary to obtain good fits to the data are discussed. The results demonstrate the domination of single-scattering and focusing paths in XAFS and the determination of vibrational information through at least 10 Å around the center atom. Numerical calculations were performed to analyze the cause of the difference found between the forward-scattering amplitudes of ${\mathrm{Rb}}^{+}$ and ${\mathrm{Br}}^{\mathrm{-}}$ focusing atoms. The second and third cumulants were determined of the first and second neighbors and were used to calculate the temperature dependence of the thermal-expansion coefficient including quantum effects at low temperature. Agreement with macroscopic thermal-expansion measurements was found.

A renormalization method is proposed to analyze the electronic band structure of disordered Fibonacci chains. The perfect (quasiperiodic) chain is contemplated as a particular case. Both bond and on-site problems are considered. The method is computationally efficient and suitable to deal with large chains in real space. It can be useful for the study of electronic properties of random tilings.

We study the electronic and Fourier-spectral properties of one-dimensional generalized Fibonacci lattices generated by the stacking rule ${\mathit{S}}_{\mathit{l}+1\mathrm{}}$=${\mathit{S}}_{\mathit{l}}^{\mathit{n}}$${\mathit{S}}_{\mathit{l}\mathrm{-}1}^{\mathit{m}}$ with positive integers n and m, where ${\mathit{S}}_{\mathit{l}}$ is the lth generational binary sequence. After showing that, in the limit of the large potential strength, the energy spectrum of a lattice with certain specific n and m can be determined by the associated characteristic value τ(n,m), we investigate the relation between the electronic band structure and the Fourier spectrum. When the lattice possesses the Pisot-Vijayaraghavan (PV) property (i.e., when n+1>m), the Fourier spectrum is closely related to the electronic band structure; the location and the relative strength of the Fourier spectral peak is in agreement with the location and the relative width of the energy spectral gap. On the other hand, when the lattice possesses no PV property (i.e., when n+1≤m), the Fourier spectrum is not directly related to the electronic band structure; the strength of the Fourier spectral peak is irrelevant to the width of the energy spectral gap, while the location of the peak corresponds to that of the gap. We also study the dependence of the electronic and Fourier-spectral properties on the initial conditions of the stacking rule through detailed study of the copper mean lattice (n=1,m=2) with initial conditions ${\mathit{S}}_1$={A} and ${\mathit{S}}_2$={${\mathit{AB}}^{\mathit{p}}$}. It is found that the fractal structure of the energy spectrum is independent of the integer p, while some local electronic properties depend on p. It is also found that the global structure of the Fourier spectrum depends on p; it looks more blurred, and thus the aperiodic nature of the lattice becomes clearer with the increase of p.

A closed expression is obtained for the pair-correlation function of a polymer system consisting of a charged core surrounded by solute ions in solution. Two propagating modes are predicted from microscopic theory. The first can be detected by microwave-frequency waves. The second has a velocity c of the order of meters per second, which can be measured through the solute isothermal compressibility in the system. Measurement of the solute isothermal compressibility would provide justification of an appropriate thermodynamic identity in the mean spherical approximation (MSA). The theory also gives the result that, in the Ornstein-Zernike equation the unscaled pair-correlation functions satisfy the k→0 sum rule for the velocity of the second mode mentioned above, not the scaled pair-correlation functions (as commonly written in the MSA).

Measurements of the dielectric permittivities in the mixed proton-glass crystal ${\mathrm{K}}_{0.61}$(${\mathrm{NH}}_4$${)}_{0.39}$${\mathrm{H}}_2$${\mathrm{PO}}_4$ are reported from 20 Hz to 200.8 MHz and for fields along both the a and c axes. Between 14 and 50 K, the c-axis permittivity outside the piezoelectric-resonance range shows a temperature-dependent spread in relaxation times but cannot be fit accurately with the Cole-Cole formula. Strong piezoelectric resonance for a field along the c axis was observed in the region between 100 kHz and 10 MHz.

X-ray-absorption near-edge structure (XANES) data have since long been expected to provide information on third- (or higher-) order correlations in materials. In the present study, three-body correlations in amorphous alloys are studied by taking advantage of XANES data. The detailed analysis of XANES results computed for amorphous alloys of the ${\mathrm{Ni}}_{80}$${\mathrm{B}}_{20}$ and the ${\mathrm{Fe}}_{90}$${\mathrm{Zr}}_{10}$ type provide evidence that a fraction of the atoms in the structure are likely to contribute more to the final shape of the XANES result than the other atoms. In this connection, significant contributions of electron multiple-scattering processes are responsible for the shapes. In pursuit of a structural property to link short-range geometry and multiple scattering, together with significant contrast related to the different XANES shapes, atomic triangles of defined size are introduced as a criterion to interpret XANES results. An approach introduced to display graphically the abundance of such triangles serves as a basis for further discussions. In the case of ${\mathrm{Ni}}_{80}$${\mathrm{B}}_{20}$, the XANES results threw light on metal-metal-boron triangles with certain measures, which are likely to be of particular interest in the structure. These triangles are characterized by one metal-boron distance between 0.19 and 0.21 nm and to a smaller fraction between 0.26 and 0.29 nm, whereas the triangles’ other two sides are of average value within the ranges of the peaks of the partial pair-correlation functions. For the Zr neighborhood in ${\mathrm{Fe}}_{90}$${\mathrm{Zr}}_{10}$ it turned out that triangles with measures deviating from average values should occur less numerously than in the discussed model.

The infrared-reflectance spectra of silica glasses were measured and analyzed with a Kramers-Kronig transformation for a better understanding of the response of silica, especially in the high-frequency region (1000–1300 ${\mathrm{cm}}^{\mathrm{-}1}$). The silica samples were prepared by the sol-gel technique and subsequently subjected to heat treatments at selected temperatures to induce the formation of structures with a variable connectivity of the silicate network. The infrared reflectivity was found to increase with heat-treatment temperature. This was attributed to glass densification, resulting from condensation of Si-OH groups into Si-O-Si network bridging units. This densification mechanism was also confirmed by Raman measurements. The profiles of the high-frequency parts of the transverse-optic (TO) and longitudinal-optic (LO) spectra were found consistent with the presence of two coupled modes (${\mathrm{AS}}_1$ and ${\mathrm{AS}}_2$) for the asymmetric stretching of Si-O-Si bridges, as proposed in previous works. Along these lines, the shoulder at ∼1200 ${\mathrm{cm}}^{\mathrm{-}1}$ in the infrared-absorption spectrum of silica, which is particularly enhanced in the spectra of silica gels, can be understood as arising from the TO response of the ${\mathrm{AS}}_2$ mode. The LO-TO splitting of the various infrared-active modes was found to depend on heat-treatment temperature. This effect was attributed to variations of silicate network connectivity with temperature, which is presumably affecting the range of the Coulombic forces inducing the LO-TO splitting.

Extensive numerical simulations of wave packets and pulses in two-dimensional (2D) random systems exhibit a subdiffusion at intermediate times, shown to be linked to the fractal structure of 2D eigenstates. The mean-square pulse width 〈‖r${\mathrm{\Vert }}^2$〉 scales as ${\mathit{t}}^{2\nu }$, with 0≤ν≤1/2 being a continuous function of the disorder strength. Good agreement is found between numerical values of ν and weak-localization predictions. At very long times, the subdiffusive regime crosses over to localization with long power-law asymptotics.

This paper discusses a macroscopic model for ac conduction in electronically or ionically conducting disordered solids. The model considers ac conduction in an inhomogeneous solid that is characterized by a spatially randomly varying thermally activated (frequency-independent) conductivity. Discretizing Maxwell’s equations leads to an equivalent electrical circuit that is a simple-cubic lattice where each pair of nodes are linked by a resistor and a capacitor in parallel. The values of the resistors are determined by the local resistivity while the capacitors are all equal, given by the infinite-frequency dielectric constant. It is shown that the capacitor currents are Maxwell’s displacement currents. Assuming uncorrelated resistances, the model is solved analytically at low temperatures in the effective-medium approximation (EMA) and in a naive percolation-path approximation. Both approximations predict similar universal ac responses as T→0, where the macroscopic frequency-dependent conductivity becomes independent of the activation-energy probability distribution. The universality represents an unusual type of regularity appearing in the extreme disorder limit. The universality prediction is tested by computer simulations of 200×200 lattices in two dimensions and of 50×50×50 lattices in three dimensions. The computer simulations show that the EMA works very well in two dimensions in the whole temperature range studied; in particular, the low-temperature universality prediction is confirmed. In three dimensions the universality prediction is confirmed as well.

Kapton polyimide and high-modulus polyimide (PPT) films were carbonized and graphitized at various temperatures from 600 to 3000 °C. Their surface was studied by atomic-force microscopy and/or by scanning tunneling microscopy in order to follow the modification of the large-scale morphology and the atomic structure as a function of the heat-treatment temperature (HTT). On the pristine Kapton films, the local order of the molecules is brought to the fore. With increasing HTT (600 to 1000 °C) the structure becomes more disordered while at larger scale a bumpy morphology appears. During graphitization, the bumpy morphology gradually disappears and is replaced by graphitized terraces whose size increases with HTT. At atomic scale, it is shown that the graphene layers progressively grow for HTT higher than 1800 °C. On the films treated between 1800 and 2400 °C, graphene layers containing point defects are imaged and (√3 × √3 )R 30° superstructures are observed near large defects. On the samples treated at 2400 and 2600 °C, moiré patterns are observed and are attributed to stacking faults (turbostratic structure).