We demonstrate that the conventional picture of crystal structure stabilities of transition metals, namely, as being determined by the degree of filling of the ‘‘canonical’’ d bands, breaks down at very high pressures. We show, by means of first-principles calculations, that for extreme compressions the pseudocore p states become broad and start to hybridize with the valence states. This results in a modification of the electronic structure and consequently unexpected crystal structures become stabilized. Simple model calculations using p- and d-canonical bands, which are allowed to hybridize with each other, predict that the bcc structure is the most stable crystallographic phase at high pressure for most of the transition elements, in agreement with our first-principles calculations.

A general tight-binding total-energy (TBTE) method is presented that accurately predicts energy differences caused by small perturbations such as those occurring in the evaluation of elastic constants and phonon spectra as well as energy differences from large changes in structure such as structural energy differences, equations of state, and defect energies. Unlike other quantitative TBTE models, no pair potential is required. In addition, we only fit to the band structure and total energies obtained from fcc and bcc first-principles calculations, although we could fit to other data as needed. We have applied the method to 11 transition and noble metals and obtained elastic constants, phonon spectra, and vacancy formation energies in very good agreement with both local-density theory and experiment.

Landau quantization of the π energy bands of a carbon nanotube is calculated within the tight-binding approximation. The energy bands do not show explicit Landau levels, but they do have energy dispersion for all values of magnetic field. The energy bandwidth shows oscillations with a period that is scaled by a cross section of the unit cell of the tubule, which is specified by the symmetry of the nanotube.

We show that one-electron band theory fails to describe optical absorption in PPV and the absorption spectrum can be described only within a Coulomb-correlated model. The lowest optical state is an exciton, whose binding energy is estimated theoretically to be 0.90±0.15 eV. Measurements of the photoconductivity quantum efficiency in poly[2-methoxy,5-($2^{\mathcal{’}}$-ethyl-hexyloxy)-1,4 phenylenevinylene] reveal a binding energy in good agreement with the theoretical value.

We report cryogenic high-pressure measurements of a defect-related emission at 1.25 eV in silicon-doped GaAs. The pressure measurements prove that the 1.25-eV photon energy is relative to the conduction band, implying a deep defect level 0.30 eV above the valence band and an electron-capture process from the conduction band into the defect. The defect level moves up in the band gap at a rate of 23±3 meV/GPa. These results are consistent with a vacancy-related defect level, possibly stemming from a gallium-vacancy–silicon-at-gallium (second-nearest-neighbor) defect complex.

We have found evidence for hole-transfer events between oxygen-vacancy-related defects in thermal ${\mathrm{SiO}}_2$ films selectively injected with holes using electron paramagnetic resonance (EPR) and capacitance-voltage measurements. We find that hole injection into ${\mathrm{SiO}}_2$ films reveals two types of EPR centers: oxygen-vacancy-related ${\mathit{E}}_{\mathrm{\delta }}^{\mathcal{’}}$ and ${\mathit{E}}_{\gamma }^{\mathcal{’}}$ centers; both centers are positively charged. Upon annealing the Si/${\mathrm{SiO}}_2$ structures at room temperature, the ${\mathit{E}}_{\mathrm{\delta }}^{\mathcal{’}}$ defect transfers its hole to a neutral oxygen-vacancy (${\mathrm{O}}_3$≡Si—Si≡${\mathrm{O}}_3$) site forming the classic ${\mathit{E}}_{\gamma }^{\mathcal{’}}$ center. Electrically, we observe a concomitant growth in the border-trap density, suggesting that some of the transfer-created ${\mathit{E}}_{\gamma }^{\mathcal{’}}$ centers may be the microscopic entities responsible for the border traps. These results constitute spectroscopic evidence that hole transfer between defect sites can occur over extremely long time scales (hours) in ${\mathrm{SiO}}_2$ films.

To map the interfacial structure of ${\mathrm{CoSi}}_2$/Si(111), the atomic thickness variations of epitaxial ${\mathrm{CoSi}}_2$ films were spatially resolved and determined using ballistic-electron-emission microscopy. Modulation spectroscopy was also used, and it showed not only thickness variations, but also strong lateral variations near the interfacial steps and dislocations. Also, a strong energy dependence of the quantum-size effects due to the transition from bound quantized subbands to resonances in ${\mathrm{CoSi}}_2$ was seen.

The photodynamics of a photodarkened ${\mathrm{CdS}}_{\mathit{x}}$${\mathrm{Se}}_{1\mathrm{-}\mathit{x}}$-doped glass has been studied by pump-probe time-resolved Raman spectroscopy. A marked decrease in the phonon Raman intensities was found when the photodarkened sample was preexcited by the pump pulse. Such changes were not found for a fresh (not photodarkened) sample. The decrease in the Raman intensities, which is related to the decrease in the number of electrons in the upper edge of the valence band, recovered with a time constant of 45 ms. This means that the electrons photoexcited by the pump pulse are trapped in a certain photodarkening-induced state and that the lifetime of this state is 45 ms. It is pointed out that we now have a means to control the population of valence electrons in semiconductor microcrystallites by adjusting the laser power and/or the delay time in pump-probe experiments.

We present a study of ground-state energies and densities of quantum dots in a magnetic field, which takes into account correlation effects through the current-density-functional theory. The method is first tested against exact results for the energy and density of two- and three-electron quantum dots, and it is found to yield an accuracy better than 3%. Then we extend the study to larger dots and compare the results with available experimental data. The orbital and spin angular momenta of the ground state, and the evolution of the density profile as a function of the magnetic field are calculated. Quantitative evidence of edge reconstruction at high magnetic field is presented.

We report magnetoresistance measurements on a two-dimensional electron gas made from a high-mobility GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As heterostructure, where the externally applied magnetic field was expelled from regions of the semiconductor by means of superconducting lead grains randomly distributed on the surface of the sample. A theoretical explanation in excellent agreement with the experiment is given within the framework of the semiclassical Boltzmann equation.

A three-pulse degenerate four-wave-mixing experiment and its theoretical analysis reveal contributions of local field and biexciton effects to the nonlinear optical response of the two-dimensional exciton. Quantum beats with a frequency equivalent to the biexciton binding energy appear only for specific polarizations. Polarization-dependent spectrally resolved four-wave mixing shows an additional peak that is separated from the 1s heavy-hole exciton line by the biexciton binding energy. These experimental results are in good agreement with the optical selection rules of the theoretical model, providing evidence for biexcitonic contributions to the four-wave mixing.

The cyclotron resonance absorption spectrum in a Wigner crystal is calculated. Effects of spin splitting are modeled by substitutional disorder, and calculated in the coherent-potential approximation. Due to the increasing strength of the dipole-dipole interaction, the results show a crossover from a double-peak spectrum at small filling factors to a single-peak spectrum at filling factors ≳1/6. Radiation damping and magnetophonon scattering can also influence the cyclotron resonance. The results are in very good agreement with experiments.

The modification of the surface-plasmon dispersion relation in a thin foil by the near-zone effects of a scanned probe offers optical data that yields interesting forms of microscopy and spectroscopy. An exact solution for this effect is obtained here by taking advantage of the applicability of nonretarded electrodynamics in prolate spheroidal coordinates. Using the known optical properties of the materials, the modifying parameters and dispersion relations are discussed. Both planar and curved foils are examined.

The motion of small Pt clusters at a step edge on the Pt(111) surface was studied by molecular-dynamics simulations using the corrected effective-medium theory. It was found that interlayer diffusion of the clusters proceeds via dissociation and consecutive single-atom exchanges with the edge atoms, rather than hopping over the edge as a unit. The dissociation and the exchanges occur in a coherent way. We therefore propose that surface defects, like steps or large vacancy pits, may facilitate cluster dissociation via incorporation of the cluster atoms into the edges.

The atomic and electronic structures of the 4Hb polytype (alternating layers of T and H types) of ${\mathrm{TaS}}_2$ have been studied with a scanning tunneling microscope, operating in air at room temperature. On areas with presumably the T-type layer being the surface, a commensurate √13×√13 charge-density wave (CDW) superlattice is observed, independent of sample bias. On the presumably H-type surface, however, images are bias dependent. Increase of the positive sample bias from about 50 to 170 mV results in a continuous change of images from a dominant atomic pattern with a very weak superimposed CDW modulation to a fully developed commensurate √13×√13 CDW superlattice. With a negative bias (down to -300 mV), however, there is hardly any indication of a CDW modulation. The bias-dependent behavior can be explained by the energy dependence of the probability of electron tunneling from the tip through top (H type) to the second (T type) layer.

The formation of polarons and bipolarons in conjugated polymers with nondegenerate ground states is studied theoretically. We use the Pariser-Parr-Pople model combined with the Su-Schrieffer-Heeger model of electron-lattice coupling and a Brazovskii-Kirova-type symmetry-breaking interaction. The relative stability of polarons and bipolarons is systematically studied within the unrestricted Hartree-Fock approximation as a function of the model parameters and the doping concentration. Coulomb interactions significantly suppress the stability of bipolarons, and for realistic parameters the doped carriers are most likely in the form of polarons at low doping levels. On the other hand, the high concentration of carriers tends to favor a bipolaron lattice, and our model predicts a possible doping-induced phase transition from a polaron lattice to a bipolaron lattice. The significance of these results is discussed in connection with experiments in polypyrrole and other polymers.

Resonant photoemission in the rare-earth 4d→4f prethreshold absorption region has been investigated in ${\mathrm{LaCl}}_3$ and ${\mathrm{CeCl}}_3$. It is found that the intensity ratio of the rare-earth 5${\mathit{p}}_{1/2}$ photoemission to the total 5p emission deviates from the statistically expected value in the prethreshold absorption region and depends on the final multiplet states of the 4d→4f photoexcitation. The ratio reaches 0.7 at the 4${\mathit{d}}^9$4${\mathit{f}}^1$ ${(}^3$${\mathit{D}}_1$) photoexcitation in ${\mathrm{LaCl}}_3$ and nearly 0.5 at the 106-eV photoexcitation in ${\mathrm{CeCl}}_3$. In ${\mathrm{CeCl}}_3$, it is also found that the magnitudes of the resonant enhancements of the Ce 4f emission are not proportional to the strengths of the 4d→4f photoabsorption lines. These resonant aspects are explained by the multiplet dependence of the Auger transition probability involved in the resonant photoemission.

Linear equidistant and bond-alternating infinite chains of hydrogen atoms have been investigated by the ab initio crystal-orbital method at the Hartree-Fock (HF) level, by including electron correlation up to the complete fourth order of the Mo/ller-Plesset perturbation theory (MP4-PT), and by using different versions of density-functional theory (DFT). The Bloch functions have been expanded in all cases in a series of high-quality atomic-orbital basis sets and complemented by extended sets of polarization functions up to 6s3p2d1f per H atom. In order to compare the performance of the PT and DFT methods, several physical properties have been computed at all theoretical levels including lattice geometry, cohesive energy, mechanisms of bond alternation (Peierls instability), and energetic features of nonequilibrium configurations (dissociation). For these latter quantities, both spin-restricted (RHF) and unrestricted (UHF) wave functions have been employed in all orders of PT. The methods described have been used parallel to infinite chains and to the ${\mathit{H}}_2$ molecule, to be able to check their accuracy on experiments. In the case of the DFT, six different functionals (combining Slater and Becke exchange with local and gradient-corrected correlation potentials) have been utilized to test their accuracy in comparison with the MP4 results.

A detailed spectroscopic study with crystal-field analysis of the doped laser crystal Nd:${\mathrm{GdVO}}_4$ is presented. The crystal field is expanded in terms of operators transforming as the irreducible representation of the ${\mathit{T}}_{\mathit{d}}$ group. This allows us to interpret the crystal-field parameters in terms of a simple point-ion model. A similar analysis of Nd:${\mathrm{YVO}}_4$ is also made for comparison.