A structural model for intermediate-size silicon clusters is proposed that is able to generate unique structures without any dangling bonds. This structural model consists of a bulklike core of five atoms surrounded by a fullerenelike surface. Reconstruction of the ideal fullerene geometry results in the formation of crown atoms surrounded by π-bonded dimers. This model yields unique structures for ${\mathrm{Si}}_{33}$, ${\mathrm{Si}}_{39}$, and ${\mathrm{Si}}_{45}$ clusters without any dangling bonds and hence explains why these clusters are the least reactive towards chemisorption of ammonia. Finally, reagent-specific chemisorption reactivities observed experimentally are explained based on the electronic structures of the reagents.

Methods for characterization of self-affine surfaces and measurements of their roughness exponents H are developed. It is shown that for smoothed surfaces, which underwent particular coarse graining or averaging of the small-scale fluctuations, the excess surface area ${\mathit{S}}_{\mathrm{ex}}$ and the mean square root radius of curvature ${\mathit{a}}_{\mathit{c}}$ are related by two distinct asymptotic power laws if ${\mathit{a}}_{\mathit{c}}$ is well below or well above a certain crossover scale ${\mathit{a}}_{\mathrm{cr}}$. In the local regime of self-affinity, when ${\mathit{a}}_{\mathit{c}}$≪${\mathit{a}}_{\mathrm{cr}}$, ${\mathit{S}}_{\mathrm{ex}}$∝(${\mathit{a}}_{\mathit{c}}$/${\mathit{a}}_{\mathrm{cr}}$${)}^{\mathrm{-}\left(1\mathrm{}\mathrm{-}\mathit{H}\right)}$. In the global regime of self-affinity, when ${\mathit{a}}_{\mathit{c}}$≫${\mathit{a}}_{\mathrm{cr}}$, ${\mathit{S}}_{\mathrm{ex}}$∝(${\mathit{a}}_{\mathit{c}}$/${\mathit{a}}_{\mathrm{cr}}$${)}^{\mathrm{-}2\left(1\mathrm{-}\mathit{H}\right)/\left(2\mathrm{}\mathrm{-}\mathit{H}\right)}$. The former scaling relationship is consistent with the well known definition of local fractal dimensions ${\mathit{d}}_{\mathrm{loc}}$=${\mathit{d}}_{\mathrm{top}}$+1-H. The latter scaling relationship offers alternatives for characterization of self-affinity over large scales by means of excess dimensions defined as ${\mathit{d}}_{\mathrm{ex}}$=${\mathit{d}}_{\mathrm{top}}$+2(1-H)/(2-H) and can be used for determination of roughness exponents from the measurements provided in the global regime. The thermodynamic method of fractal analysis, proposed earlier for self-similar surfaces (A.V. Neimark, Pis’ma Zh. Eksp. Teor. Fiz. 51, 535 (1990) [JETP Lett. 51, 607 (1990)]; Physica A 191, 258 (1992)), is extended for self-affine surfaces for determination of fractal dimensions and roughness exponents from adsorption and capillary experimental data.

We present a study of the charge-induced vibrational frequency shifts of the Raman active modes in the ${\mathrm{C}}_{60}^{\mathit{n}\mathrm{-}}$ molecules performed in the framework of a four-parameter transferable bond-charge model. The calculated relative shifts compare quite well with the existing experimental data and the residual discrepancies are in reasonable agreement with the vibronic shifts as given by ab initio calculations. For the half-filled conduction electronic shell (${\mathrm{C}}_{60}^{3\mathrm{-}}$ configuration), the dominant contribution to the electron-phonon coupling constant is seen to arise from low-frequency ${\mathit{H}}_{\mathit{g}}$ modes.

We analyze the experimental results of Hoen, Burk, Zettl, and M. Inui [Phys. Rev. B 46, 1874 (1991)], and show that the observed length change of ${\mathrm{TaS}}_3$ due to the applied electric field is consistent with the modern understanding of the elastic properties of the charge-density waves.

The initial nonthermal electron distribution at densities ≤${10}^{14}$ ${\mathrm{cm}}^{\mathrm{-}3}$ is studied in GaAs:Be by investigating the band-to-acceptor luminescence after excitation by picosecond laser pulses. Two peaks are observed originating from electrons generated out of the heavy- and light-hole valence bands, respectively. This assignment is confirmed by polarization dependence. Variation of excitation energy yields the valence-band dispersions for wave vectors <0.04(π/a), where purely parabolic terms dominate. The light- and heavy-hole masses relevant for optical interband excitation of a 100 GaAs surface close to the band edge are determined to be ${\mathit{m}}_{\mathrm{lh}}$=0.093${\mathit{m}}_0$±0.007${\mathit{m}}_0$ and ${\mathit{m}}_{\mathrm{hh}}$=0.54${\mathit{m}}_0$±0.05${\mathit{m}}_0$.

The ${\mathit{P}}_{\mathit{b}}$ center at the (111) Si-${\mathrm{SiO}}_2$ interface has been studied with electron-nuclear-double-resonance (ENDOR) spectroscopy taking advantage of the high specific interface area of oxidized porous Si samples. The ENDOR spectrum consists only of two structureless lines at the Larmor frequencies of ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ and ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$. No hyperfine interaction with ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ could be detected. The ${\mathit{P}}_{\mathit{b}}$-center${–}^1$H${,}^{19}$F distances have been estimated in the dipole-dipole approximation to ≊13 Å, which corresponds to the average ${\mathit{P}}_{\mathit{b}}$-center separation. The intensity ratio of the ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ and ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ ENDOR lines is stable under high-temperature vacuum annealings and hydrogen treatments. Quantitative electron-paramagnetic-resonance measurements of the central hyperfine-structure intensity confirm the assignment of the ${\mathit{P}}_{\mathit{b}}$ center to the Si dangling-bond defect, a model recently questioned by the suggestion of a hydrogen-molecule defect.

We calculate the current fluctuation of a mesoscopic ring in the ballistic-quantum-transport regime and in the presence of a magnetic field as a function of the Fermi energy, the applied external magnetic field, and the frequency. The system is represented by a tight-binding Hamiltonian, and nonequilibrium Green functions are obtained by the Keldysh formalism. The behavior of the shot noise as a function of the magnetic flux is very sensitive to the value of the Fermi energy and frequency. We find that for zero frequency and Fermi level such that the transmission is maximum the shot noise has oscillations with period hc/2e and vanishes when the current is maximum. This behavior holds for frequencies up to ${10}^9$–${10}^{10}$ Hz, for a ring of length of about 1500 Å and Fermi energy of 5 meV. On the other hand, for a Fermi level corresponding to a minimum in the transmission the period of the oscillations is hc/e, as for the current.

Population dynamics of exciton polaritons in a mesoscopic one-dimensional system is theoretically studied, in which the quantization of the polariton plays important roles. Then the polaritons are shown to occupy dominantly the specified quantum state above a critical pump rate, reflecting their bosonic nature. These can give rise to coherent light emission.

We obtain hot-carrier cooling rates in CdSe using upconversion luminescence with 2.5-psec time resolution. The carrier energy-loss rates are significantly reduced in comparison with those expected in a simple theory of carrier energy transfer to the lattice. The slow cooling can be explained to be caused by hot-phonon effects provided that the lattice dynamical lifetime of optical phonons in CdSe is about 6 ps at 8 K.

We have measured the tunneling between two two-dimensional electron gases at high magnetic fields B, when the carrier densities of the two electron layers are matched. For filling factors ν<1, there is a gap in the current-voltage characteristics centered about V=0, followed by a tunneling peak at ∼6 mV. Both features have been observed before and have been attributed to electron-electron interactions within a layer. We have measured high field tunneling peak positions and fitted gap parameters that are proportional to B, and independent of the carrier densities of the two layers. This suggests a different origin for the gap to that proposed by current theories, which predict a √B dependence.

The surface structure of a 4°-vicinal Si(001) sample tilted toward the [110] direction has been studied with low-energy electron diffraction. The correlation of dimer rows across double-atomic-height steps has been investigated by analyzing angular profiles of the diffracted beams. An irreversible phase transition has been discovered by observing the change of the splitting distances in individual (2×1) beams after heating the sample to above 1320°C. This change is considered as a result of converting a surface consisting of regions of terraces with dimer-row shifts all in the same direction from one terrace to the next within each region to a surface containing only single-direction dimer-row shifts for the entire surface. The phase transition is explained by the reduction of surface-stress-induced strain energy of the new phase.

Raman spectra of purified carbon deposits containing multishelled carbon nanotubes of average diameter 10–12 nm show narrow vibrational bands and no phonon softening in contrast to unpurified material. This is in agreement with lattice-dynamical calculations using a semiempirical pseudopotential, which predicts no significant phonon softening in this size range. Only a very weak disorder-induced vibrational band at 1356 ${\mathrm{cm}}^{\mathrm{-}1}$ (${\mathit{I}}_{\mathit{G}}$/${\mathit{I}}_{\mathit{D}}$=0.04) is found for the unpurified as well as purified carbon deposits, which suggests that the scattering is originated by structural defects rather than by size effects.

We have measured the structure of ${\mathrm{D}}_2$ adsorbed in zeolite 13X at temperatures in the range 5.5 to 80 K at a variety of fillings. We find that the ${\mathrm{D}}_2$ molecules adsorb into well-defined adsorbtion sites in the zeolite structure. Two distinct adsorbtion sites, with binding energies on the order of 40 and 80 K, have been identified. No evidence for bulklike liquid or solid behavior of the ${\mathrm{D}}_2$ adsorbed in the pores has been observed. Once the adsorbtion sites are saturated ${\mathrm{D}}_2$ is no longer adsorbed into the pores and forms a bulk phase on the surface of the zeolite.

The optical absorption, luminescence, luminescence excitation, and luminescence decay kinetics of a dopant with a 3${\mathit{d}}^3$ shell (most probably ${\mathrm{Cr}}^{3+}$) and ${\mathrm{V}}^{3+}$ ions in natural ${\mathrm{Ca}}_2$${\mathrm{Al}}_3$${\mathrm{Si}}_3$${\mathrm{O}}_{12}$(OH) zoisite crystal have been measured. The splitting of the R lines for a dopant with a 3${\mathit{d}}^3$ shell, equal to 340 ${\mathrm{cm}}^{\mathrm{-}1}$, is one of the largest known. The very broad infrared luminescence spectrum is associated with various dopants present in the crystal. We show that possible application of zoisite as a laser material is hindered by efficient nonradiative transitions at room temperature related to the presence of trace impurities.

By performing large-scale molecular-dynamics simulations of clusters of MgO we investigate the fundamental physics of melting, and the effects of pressure on melt and crystal structure along the melting curve from zero pressure to 300 GPa. We find that melting occurs at constant root-mean-squared (rms) displacements relative to the average near-neighbor distance over the entire pressure range, in agreement with Lindemann’s predictions of 1910, and contrary to previous studies that indicate failure of Lindemann’s law for nonmonatomic substances. The high-pressure validity of Lindemann’s law supports one-phase models for melting. The liquid structure varies along the melting curve, becoming more crystal-like with increasing pressure with average coordinate number changing from 4.5 to 6, and ΔV/${\mathit{V}}_{\mathit{c}}$ tends to zero with increasing pressure. Trends in thermodynamic functions and structure indicate that in the extreme pressure limit, melting is characterized only by dynamical changes such as onset of rapid diffusion, and not by local structural changes, since high pressure favors efficient packing of the liquid as well as the solid.

We have performed density functional theory–linear combination of atomic orbitals molecular-dynamics statistical trajectory ensemble simulations of the 500 eV cross-beam collision of ${\mathrm{C}}_{60}^{+}$+${\mathrm{C}}_{60}$, where the charged collision partner is thermally excited with an average vibrational energy of 30 eV. The time-resolved fragmentation cross section shows the immediate sputtering of carbon dimers, and the formation of small fragments (N≤9) during the early stage of collision where there are still large cluster compounds (110≤N≤120). Medium-size cluster fragments (N≊10–40) are formed mainly at the very end of the collision, i.e., as products of a multifragmentation process. The mean coordination number cross section is introduced as a means of characterizing structures of fragmentation products. Though our simulation statistics are calculated from a limited number of trajectories only (n=96–1024), odd-even alterations in the fragmentation cross section are clearly resolved.

The self-consistent scalar-relativistic band structures for BaS, BaSe, and BaTe in NaCl-type and CsCl-type structures were obtained using the tight-binding linear muffin-tin orbital method. From atomic-sphere approximation (ASA) total-energy calculations the structural phase stability in these compounds has been studied. The equilibrium lattice constant for all three compounds agrees well with the experimental results. The pressures at which these compounds undergo a structural phase transition from NaCl-type to CsCl-type were calculated. The calculated pressures and volumes at transition agree well with the experimental results. The energy band gaps at ambient conditions in the NaCl-type structure and the volume dependence of band gaps in the CsCl-type structure were calculated. In addition the pressures and the volumes at which band overlap metallization occurs were also calculated and compared with experimental and other theoretical results.

The EPR fine- and hyperfine-structure tensors of ${\mathrm{Co}}^{2+}$, embedded in MgO, are calculated using a molecular-orbital approach within the density-functional-theory framework. Two methods, namely the multiple-scattering local-spin-density approximation and the linear combinations of atomic orbitals within local-spin-density approximations have been used for this purpose, leading to quite correct assignment of the optical spectrum and the EPR parameters. The pros and cons of both methods are underlined.

X-ray-photoemission spectra of 4d(${\mathit{N}}_{4,5}$) core level in heavy rare-earth elements are studied theoretically with the consideration of the 4d-4f4f(${\mathit{N}}_{4,5\mathrm{-}}$${\mathit{N}}_{6,7}$${\mathit{N}}_{6,7}$) super-Coster-Kronig decay process in the excited states. It is shown that the lifetime broadening of the states in the multiplets strongly varies with their binding energies. The calculation considering the decay effect reproduces the experimental spectrum fairly well. The generally adopted assumption of the constant core-hole lifetime broadening breaks down completely if the coupling between a core-hole and unfilled valence shell is strong and yet the decay process is dominated by the coupling between them. The tendency in the lifetime broadening, that the larger binding energy states have the wider width, can be understood from a point of view of electron correlation.

Minima, transition states, and rearrangement pathways have been calculated by eigenvector following for model face-centered-cubic solids. Results are reported for systems bound by three empirical potentials representing Au, Ag, and ${\mathrm{C}}_{60}$. Various vacancy and interstitial diffusion processes are investigated, and diffusion constants, formation energies, and migration energies are compared between the different systems. In addition to these generally localized mechanisms, several highly cooperative rearrangements were found, and the possible significance of these results, and of the effects of lattice relaxation, is discussed.