The second-order Raman spectrum of AlAs has been measured on samples grown by molecular-beam epitaxy (MBE). The results show significant discrepancies with model calculations of the phonon density of states, but are consistent with recent first-principles calculations. This first-principles approach has been used to compute the phonon energies in GaAs-AlAs superlattices, with controversial implications regarding the sharpness of MBE-grown interfaces. The Raman results presented here support the conclusions of the first-principles analysis.

We describe the mechanism of solid-phase epitaxy (SPE) of Ge on a Si(111)-7×7 surface using reflection high-energy electron diffraction and scanning tunneling microscopy. Amorphouslike Ge layers crystallize in the registry of the 7×7 reconstruction preserved at the Ge/Si interface. The preferred basic unit of epitaxy is a triangular domain corresponding to four half-units of the 7×7 reconstruction. Faultedly stacked (twinned) domains cover almost half of the surface at the initial stage of SPE because Ge grows epitaxially on both the unfaulted and faulted halves of the 7×7 reconstruction. These twinned Ge layers are transformed into normally stacked Ge layers above 400 °C.

A simple model of epitaxial growth proposed by Wolf and Villain is investigated using extensive computer simulations. We find an unexpectedly complex crossover behavior of the original model in both 1+1 and 2+1 dimensions. A crossover from the effective growth exponent ${\mathrm{\beta }}_{\mathrm{eff}}$≊0.37 to ${\mathrm{\beta }}_{\mathrm{eff}}$≊0.33 is observed in 1+1 dimensions, whereas additional crossovers, which we believe are to the scaling behavior of an Edwards-Wilkinson type, are observed in both 1+1 and 2+1 dimensions. Anomalous scaling due to power-law growth of the average step height is found in 1+1 dimensions, and also at short time and length scales in 2+1 dimensions. The roughness exponents ${\mathrm{\zeta }}_{\mathrm{eff}}^{\mathit{c}}$ obtained from the height-height correlation functions in 1+1 dimensions (≊3/4) and 2+1 dimensions (≊2/3) cannot be simultaneously explained by any of the continuum equations proposed so far to describe epitaxial growth.

Several theoretical studies of the stability of (111)-terminated ionic crystals with the NaCl structure have predicted that the crystal Madelung energy can be stabilized by a simple surface reconstruction. However, previous attempts to grow (111)-terminated crystals have resulted in either unreconstructed or faceted surfaces. We demonstrate that deposition of Ni in an oxygen atmosphere onto a Au(111) substrate held at T∼300 °C results in the growth of a stable p(2×2) reconstructed NiO(111) and resolve the atomic details of this surface with our scanning tunneling microscope.

The temperature dependence of the Rayleigh surface-phonon energies and widths have been measured with high-resolution helium-atom scattering for the Γ¯-X¯ direction and the Γ¯-M¯ directions on Al(100) and Al(111), respectively. Over a wide range of wave vectors the measured shifts and widths are in good agreement with a molecular-dynamics simulation based on a simple nearest-neighbor Morse potential. Contrary to previous reports of an enhanced shift in frequency with temperature on Cu(110), these results indicate that this measure of anharmonicity on close-packed aluminum surfaces is about 30% greater than that of the bulk.

We report direct spectral evidence of photoinduced electron transfer from the excited state of poly(3-octylthiophene), P3OT, onto ${\mathrm{C}}_{60}$ by infrared photoexcitation spectroscopy, 0.01 eV (100 ${\mathrm{cm}}^{\mathrm{-}1}$) to 1.3 eV (11 000 ${\mathrm{cm}}^{\mathrm{-}1}$). The photoinduced absorption spectrum of P3OT, characterized by subgap electronic absorptions and associated infrared-active vibrational modes, is significantly enhanced in magnitude upon adding a few percent of ${\mathrm{C}}_{60}$. Two new peaks are found in the photoexcited spectrum of the P3OT/${\mathrm{C}}_{60}$ composite and assigned to the allowed HOMO (${\mathit{T}}_{1\mathit{u}}$)-LUMO (${\mathit{T}}_{1\mathit{g}}$) transitions of ${\mathrm{C}}_{60}^{\mathrm{-}}$ with energies, 1.15 and 1.25 eV, in good agreement with calculated values. The photoinduced charge transfer from P3OT onto ${\mathrm{C}}_{60}$ enhances the quantum efficiency for photogeneration of charge carriers resulting in charge separation in the excited state. As a result, the lifetime of the excited-state configuration, (P3OT${)}^{+}$ and ${\mathrm{C}}_{60}^{\mathrm{-}}$, is extended; thereby leading to both the enhanced photoinduced response of P3OT and the appearance of the 1.15- and 1.25-eV signatures associated with ${\mathrm{C}}_{60}^{\mathrm{-}}$.

Intensity-dependent optical dephasing is shown to arise from nonresonant excitation of 1% ${\mathrm{Pr}}^{3+}$-doped yttrium aluminum garnet (YAG) with UV radiation between 325–355 nm. UV irradiation of the sample with intensities greater than 30 MW/${\mathrm{cm}}^2$ produces persistent changes in the optical dephasing. Two-pulse photon echo decays on the ${}_{}{}^3{}_{}{}^{}$${\mathit{H}}_4$${\to }^1$${\mathit{D}}_2$ transition of ${\mathrm{Pr}}^{3+}$ were measured at 1.5 K before and after exposure to UV radiation. Changes persisting for at least several hours are observed in the echo intensity, echo decay rate, and temporal form of the echo decay. Changes persist if the sample is kept below 10 K, but warming to 77 K results in the echo decays resuming their initial intensity and duration. No change is detected in the optical absorption spectrum of the ${\mathrm{Pr}}^{3+}$ ions after UV exposure, indicating that most of the ions remain in that charge state. Although the mechanism giving rise to the increased optical dephasing is not completely understood, the additional dynamics may arise from the UV radiation freeing electrons that become trapped in lattice defects. The presence of the dephasing at low temperatures indicates the availability of low-energy excitations of the electronic system.

We measured the 2p and 2s exciton polariton energies in KI versus pressure and temperature using two- and three-photon excitation spectroscopy. The results give reliable values for the 2s and 2p energies of the excitons in the same crystal at the same time under substantially identical conditions to reduce systematic errors in small energy differences. The hydrostatic pressure shift of the 2p excitons in KI was measured. From our results we obtain improved values for the excitonic binding energy ${\mathit{R}}_0$ of KI, the phonon-induced Lamb shift between the 2s and 2p exciton states and the spherical parameter μ and cubic parameter δ characterizing the splitting of the 2p exciton. We also report a value for the excitonic phonon self-energy of the 2s exciton. The data also enable us to extract values for the Kohn-Luttinger parameters ${\gamma }_1$, ${\gamma }_2$, and ${\gamma }_3$ and the light- and heavy-hole masses of the highest valence band.

The dc resistance R, critical current ${\mathit{I}}_{\mathit{c}}$, ac dielectric constant ε(ω), and ac conductivity σ(ω) have been studied in a Pt-film percolation system deposited on α-${\mathrm{Al}}_2$${\mathrm{O}}_3$ fracture surfaces by dc- and rf-magnetron sputtering methods. The fractal dimension of the surfaces is found to be 2.20±0.06. Although the percolation system is entirely different from both the lattice-percolation system and the continuum-percolation system, the power-law behaviors, ${\mathit{I}}_{\mathit{c}}$∝${\mathit{R}}^{\mathrm{-}\mathrm{\alpha }}$, σ(ω)∝${\mathrm{\omega }}^{\mathit{x}}$, and ε(ω)∝${\mathrm{\omega }}^{\mathrm{-}\mathit{y}}$, are observed near the percolation threshold. The exponent α is determined to be 1.12±0.05, which is significantly different from both predictions of two-dimensional (2D) percolation theories and the recent experimental results on Au-film and Ag-film continuum-percolation systems. The exponents x and y are found to be 0.89±0.06 and 0.05±0.02, respectively, in agreement with the general scaling relation x+y=1. These results suggest that the power-law behaviors as well as the general scaling relation in percolation systems could not be affected by the statistical fractal structure of the substrates. The experimental results also support the prediction that the exponents x and y are universal.

Shock-compression experiments were performed on ${\mathrm{NbH}}_{0.75}$ to 150 GPa and ${\mathrm{TaH}}_{0.50}$ to 210 GPa using a 25-mm propellant gun and a 20-mm two-stage light-gas gun. Linear relations were found between the shock velocity ${\mathit{U}}_{\mathit{s}}$ and the particle velocity ${\mathit{u}}_{\mathit{p}}$: ${\mathit{U}}_{\mathit{s}}$=4.79+1.14${\mathit{u}}_{\mathit{p}}$ for ${\mathrm{NbH}}_{0.75}$ and ${\mathit{U}}_{\mathit{s}}$=3.44+1.23${\mathit{u}}_{\mathit{p}}$ for ${\mathrm{TaH}}_{0.50}$. Compression curves of interstitial hydrogen, deduced from the Hugoniots of these hydrides and original metals, come close to the one calculated for metallic hydrogen. These data provide additional examples of a universal compression behavior of hydrogen in a metallic environment.

We have modeled single-atom radiation damage events in diamond by a molecular-dynamics simulation, using an empirical interatomic potential to describe the interaction between the atoms in diamond. We find that the damage threshold energy needed to displace a single atom is well above simple estimates based on the diamond cohesive energy. The threshold derived from our simulations is approximately 50 eV, and is relatively insensitive to the direction of initial motion of the displaced atom. The high threshold energy is due to a rapid dissipation of kinetic energy from the bombarded atom into incoherent vibrational energy of its neighboring atoms before the displaced atom can overcome the structural energy barrier to defect formation. This rapid dissipation can be understood qualitatively by noting that when the kinetic energy of a carbon atom is comparable to the damage threshold energy, its velocity is comparable to the speed of sound in diamond.

Investigation of a 30-Å iron oxyhydroxide catalyst by Mössbauer spectroscopy revealed that the recoilless fraction decreased dramatically with increasing temperature, becoming effectively zero at 50 K. This effect is explained on the basis of a combined effect of the recoil of the particle and the particle motion due to thermal agitation. The mean-square amplitude of particle motion is derived as a function of temperature. Application of uniaxial pressure to the sample or freezing the sample in carnuba wax increased the recoilless fraction markedly by enhancing the interparticle contact forces.

An improved phase diagram for the polymorphic fcc-bcc phase transition of inverse-power-potential crystals is obtained through an exact analytic calculation of the crystalline free energy including the first-order [O(${\mathit{T}}^2$), where T is the temperature] anharmonic term in the internal energy. The anharmonic correction reduces the region of bcc stability relative to the harmonic result, in qualitative agreement with recent Monte Carlo results for the 1/${\mathit{r}}^6$ potential.

We derive a universal equation of state for compressed solids, based on thermodynamic arguments applied to virial expansions of E(ρ,T) and p(ρ,T), of the form p(v/${\mathit{v}}_0$${)}^2$=${\mathit{A}}_0$+${\mathit{A}}_1$(ρ/${\mathrm{\rho }}_0$)+${\mathit{A}}_2$(ρ/${\mathrm{\rho }}_0$${)}^2$. The thermal pressure is included from the beginning, and the only essential approximation is the truncation of expansions, which is justified by molecular arguments. Agreement with experiment is very good for a wide range of materials, including quantum solids, noble-gas and polar-gas solids, metals, ionic compounds, and hydrocarbons. A separate assumption gives the temperature dependence of the parameters as ${\mathit{A}}_{\mathit{i}}$(T)=${\mathit{A}}_{\mathit{i}}$+${\mathit{b}}_{\mathit{i}}$T-${\mathit{c}}_{\mathit{i}}$T lnT, for T>${\mathrm{\theta }}_{\mathit{D}}$. The usual behavior of the Grüneisen number as a function of temperature and density is accounted for in a simple way by these results.

A spectroscopic study of surface waves was made on gels of gelatin and agarose over the frequency range ${10}^1$–${10}^6$ Hz. Two different optical techniques, the ripplon light scattering and the induced-surface-wave measurement were used for the appropriate frequency range. The dispersion curve determined for the gelatin gel showed that surface-tension waves appear in a frequency range higher than 20 kHz in spite of its solidity, while surface elastic waves propagate at frequencies lower than 300 Hz. As for the waves in the intermediate region, a theoretical prediction suggested coexistence of both waves though it was not confirmed experimentally. A similar result was obtained also in an agarose gel.

A first principles, general study of the thermodynamic and kinetic aspects of the B1-B2 phase transition in alkali halides is presented. Particular attention is paid to (a) how to construct models of increasing complexity to be used with generic quantum-chemistry techniques and (b) the topological and symmetry-dependent features of the energetic and the Gibbs potential surfaces analyzed. Our results indicate that the transition may be thought of as involving the simultaneous opening of the rhombohedral angle in the primitive B1 crystallographic cell while a contraction of the lattice parameter takes place. Transition paths depend strongly on pressure and show large and asymmetric Gibbs barriers that qualitatively account for many of the empirical facts around the phenomenon of hysteresis.

Pressure-induced amorphization has been observed in a number of oxide crystals, including α-quartz. We have examined the properties of quartz as a function of pressure using classical interatomic potentials, and ab initio pseudopotentials. With both approaches we find that the quartz structure becomes mechanically unstable in the 20–35 GPa range. The instability is reflected in a soft zone-edge phonon mode which occurs in the same pressure range. We have analyzed the microscopic origin of the softenings, and relate them to the presence of an oxygen close-packed cubic arrangement in the quartz high-pressure structure. Within this close-packed arrangement, the silicon cations exhibit an instability between fourfold and sixfold coordination sites.

The angular and temperature dependence of the linear light scattering and the temperature dependence of the second-harmonic nonlinear light scattering were studied in two different samples of ${\mathrm{KH}}_2$${\mathrm{PO}}_4$ (KDP) crystal: one from a zone close to the seed and another from a zone far from the seed. The angular dependence of the intensity of the linearly scattered light from the sample from the zone close to the seed shows a strong peak for the near Z(YX)Y scattering. This scattering originates from growth bands parallel to the (01¯1) lattice planes in the pyramidal growth sector. The temperature dependence of the magnitude of this peak shows a considerable increase in the vicinity of the phase transition from the paraelectric to ferroelectric phase. Such an increase in the linear light scattering was not observed in the sample from the zone far from the seed. The anomalies related to the growth bands parallel to the (010) lattice planes in the prismatic growth sector were investigated also by second-harmonic light scattering. Here again a considerable increase in the intensity of the frequency-doubled scattered light was observed in the vicinity of the phase-transition temperature. The results of the light-scattering experiments are compared with an x-ray topographic study of the lattice defects.

An empirical potential-energy function, consisting of two- and three-body terms, has been derived for the study of calcium solids and clusters. With a single set of parameters, this potential reproduces the phonon frequencies and elastic constants of both the low-temperature fcc and the high-temperature bcc crystalline phases to a high degree of accuracy. It also gives a sensible energy profile for the tetragonal (Bain path) interconversion of the two structures, which are both minima along the path. The potential predicts that the fcc and hcp solids have almost equal cohesive energies, with that of the bcc phase being 0.02 eV lower. Other cubic phases and various two-dimensional and one-dimensional structures have lower cohesive energies. The potential leads to the conclusion that the most stable clusters tend to be polytetrahedral, leading to icosahedral packing, thereby maximizing coordination number.