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We show with a recently devised extended first-principles molecular dynamics method that calculated Hugoniots of poly-$\ensuremath{\alpha}$-methylstyrene agree well with precision experimental results of Kritcher et al. [Nature (London) 584, 51 (2020)] and D\"oppner et al. [Phys. Rev. Lett. 121, 025001 (2018)]. The deviation is smaller than 0.8%. This agreement does not sensitively rely on the approximations in the employed first-principles methods as long as underlying physics are well described, as illustrated in the calculation of equation of state for polystyrene covering the warm dense regime. These results may stimulate a broad range of quantitative investigations on warm dense matter that were not thought possible before, and may thus afford a new prospect to the field of inertial confinement fusion and high-energy-density physics.

We present time-dependent density functional molecular dynamics (TDDFT-MD) simulations with an adiabatic approximation to the exchange-correlation potential for hyperthermal protons (${\mathrm{H}}^{+}$) with initial kinetic energies in the range of 2--50 eV impinging on the fcc-hollow site and the on-top site of an Al(111) surface. The surface is modeled by a finite-size cluster and the results are generalized to ${\mathrm{H}}^{+}$-Al surface scattering. From the simulation, neutralization distances are determined and the time development of the kinetic energy and the electronic excitation energy are derived. The results can be rationalized on the basis of the ground-state potential energy surface and the ${\mathrm{H}}^{+}$-Al(111) interaction potential. Furthermore, the difference in initial kinetic energy between ${\mathrm{H}}^{+}$ and ${\mathrm{H}}^{0}$ projectiles required to yield identical exit velocities is derived. Notably, this difference changes sign within the studied range of kinetic energies. This is traced back to the neutralization distances and the difference between both the ground-state potential energy surface and the ${\mathrm{H}}^{+}$-Al(111) interaction potential at those distances.

As a potential high-efficiency XUV light source, high-order harmonic generation (HHG) in solids contains the dynamic information of electrons. The interactions among many particles in solids are ubiquitous and, to some extent, modulate the motion of electrons. However, the role of electron-electron interaction (EEI) effect in solid HHG has not been well understood. In this paper, we study the laser-intensity-dependent EEI effect under the given laser pulse durations. By comparing the finite chain model with the infinite extension model, we find that, if the electric field is strong enough, the induced electric field caused by EEI is nontrivial for the finite chain model. We also find that EEI only can enhance HHG spectra in a limited frequency range under some specific intensities of the laser field in both models, and the EEI effect is related to the configuration of the system. This study can provide a reference for whether EEI can be ignored when simulating the interaction between one-dimensional solids and lasers.

A method is presented for quantifying quantum-mechanical exchange effects within the dielectric theory of electron scattering, the prevailing model for electron energy losses in condensed-matter systems. The approach utilizes a uniquely symmetric view of direct and exchange scattering events along with a generalized interference phase to account physically for the reduced scattering rates required by the Pauli exclusion of final-state fermions with corresponding spin, energy, and momentum. It is found that existing implementations either neglect or substantially overestimate the impact of exchange interference for condensed materials with broad optical loss spectra due to mathematical approximations or physical assumptions which are largely avoidable. The results suggest that the impact of exchange may alter inelastic scattering cross sections by between 10% and 20% for incident energies below 200 eV, a critical energy range for current investigations in electron and photoelectron spectroscopies.

Optical charge state switching was previously observed in photoluminescence experiments for the divacancy defect in $4H$-SiC. The participating dark charge state could not be identified with certainty. We use constrained density-functional theory to investigate the mechanism of charge state conversion from the bright neutral charge state of the divacancy defect to the positive and negative charge states including corresponding recovery of the neutral charge state. While we can confirm that the positive charge state is dark, we do not find evidence that the negative charge state is dark. We compute similar absorption energies required for conversion of the neutral defect to both charge states. However, the formation of the positive charge state requires a series of excitations involving a 2-photon excitation, while the creation of the negative charge state is achieved through a single 2-photon process. Calculated absorption energies for the recovery of the neutral defect from the positive charge state fit the experimental value better than those from the negative charge state. Defect formation energies as a function of the Fermi energy show a very small Fermi energy range in which the negative charge state is most stable, while the positive charge state exhibits a wide stability range. Overall, our computational results give more support to the identification of the dark charge state as the positive over the negative charge state in the mechanism of optical charge state switching.

The electronic stopping power of nickel for protons and alpha particles at velocities below and around the Fermi velocity has been obtained to high accuracy using time-dependent density functional theory. For the wide range of projectile velocities considered, we observed different regimes of electronic stopping due to the alternative participation of $s$- and $d$-band electrons. Despite the sharp discontinuity in the electronic density of states near the Fermi energy characteristic of the nickel band structure, we do not find an anomalous nonlinear electronic stopping power limit as a function of velocity. However, we find a crossover region above $v=0.15\phantom{\rule{0.28em}{0ex}}\mathrm{a}.\mathrm{u}.$ both for protons and alpha particles, related to the increase in participating host electrons and, in the case of alpha particles, to an increase of the charge state. We compare our calculated results with widely available experimental data and analyze the low velocity limits in the context of Lindhard's linear response theory and previous nonlinear density functional calculations. The comparison shows good accord with the lowest velocity experiments available. This may indicate that the adiabatic local density approximation is already a good theory to calculate electronic stopping power in materials at low velocity.

We develop a theoretical frame to investigate the spectral dependence of the brightness of a single-photon source made of a solid-state nanoemitter embedded in a high-quality factor microcavity. This study encompasses the cases of localized excitons embedded in a one-, two-, or three-dimensional matrix. The population evolution is calculated based on a spin-boson model, using the noninteracting blip approximation. We find that the spectral dependence of the single-photon source brightness (hereafter called spectral efficiency) can be expressed analytically through the free-space emission and absorption spectra of the emitter, the vacuum Rabi splitting, and the loss rates of the system. In other words, the free-space spectrum of the emitter encodes all the relevant information on the interaction between the exciton and the phonon bath to obtain the dynamics of the cavity-coupled system. We compute numerically the spectral efficiency for several types of localized emitters differing by the phonon bath dimensionality. In particular, in low-dimensional systems where this interaction is enhanced, a pronounced asymmetric energy exchange between the emitter and the cavity on the phonon sidebands yields a considerable extension of the tuning range of the source through phonon-assisted cavity feeding, possibly surpassing that of a purely resonant system.

The influence of excitonic and local-field effects on the second-harmonic-generation spectrum of zinc-blende ZnS, ZnSe, and ZnTe is studied from first principles using the Bethe-Salpeter equation. The calculations are based on the theory of R. Leitsmann et al. [Phys. Rev. B 71, 195209 (2005)], which we extended to the low-frequency range. The dielectric function and the second-harmonic-generation spectrum have been obtained within the independent (quasi)particle approximation and within the Bethe-Salpeter approach. The calculations demonstrate that the linear and the nonlinear optical properties are similarly affected by excitonic and local-field effects. The computed spectra are furthermore compared with measurements and calculations within the time-dependent density-polarization functional theory (TD-DPFT) in the real-time framework. The present approach most commonly suggests stronger excitonic effects than observed in the real-time TD-DPFT calculations, while local-field corrections are comparably described. Although agreement is found between the Bethe-Salpeter spectra and measurements for the dielectric function, deviations from the experimental data are observed for second-harmonic generation.

We have designed and fabricated a metamaterial consisting of planar metamolecules which exhibit unusual, almost perfect anapole behavior in the sense that the electric dipole radiation is almost canceled by the toroidal dipole one, producing thus an extremely high $Q$-factor at the resonance frequency. Thus we have demonstrated theoretically and experimentally that metamaterials approaching ideal anapole behavior have very high $Q$-factor. The size of the system, at the millimeter range, and the parasitic magnetic quadrupole radiation are the factors limiting the size of the $Q$-factor. In spite of the very low radiation losses the estimated local fields at the metamolecules are extremely high, of the order of ${10}^{4}$ higher than the external incoming field.

We present results for the lifetime of the orbital transitions of Bi donors in Si, measured using both frequency domain and time-domain techniques, allowing us to distinguish between homogeneous and inhomogeneous processes. The proximity of the energy of the optically allowed transitions to the optical phonon energy means that there is an unusually wide variation in the lifetimes and broadening mechanisms for this impurity, from fully homogeneous lifetime-broadened transitions to fully inhomogeneously broadened lines. The relaxation lifetime (T1) of the states range from the low 10’s to 100’s of ps, and we find that there is little extra dephasing (so that T1 is of the order of T2/2) in each case.

We present an ab initio study of the electronic stopping power of protons in copper over a wide range of proton velocities $v = 0.02-10~\mathrm{a.u.}$ where we take into account non-linear effects. Time-dependent density functional theory coupled with molecular dynamics is used to study electronic excitations produced by energetic protons. A plane-wave pseudopotential scheme is employed to solve the time-dependent Kohn-Sham equations for a moving ion in a periodic crystal. The electronic excitations and the band structure determine the stopping power of the material and alter the interatomic forces for both channeling and off-channeling trajectories. Our off-channeling results are in quantitative agreement with experiments, and at low velocity they unveil a crossover region of superlinear velocity dependence (with a power of $\sim 1.5$) in the velocity range $v = 0.07-0.3~\mathrm{a.u.}$, which we associate to the copper crystalline electronic band structure. The results are rationalized by simple band models connecting two separate regimes. We find that the limit of electronic stopping $v\to 0$ is not as simple as phenomenological models suggest and it plagued by band-structure effects., 8 pages, 7 figures

We find that if solid xenon is formed from liquid xenon, denoted ``ice,'' there is a 10% increase in $^{129}\mathrm{Xe}$ longitudinal relaxation ${T}_{1}$ time (taken at 77 K and 2 T) over a trickle-freeze formation, denoted ``snow.'' Forming xenon ice also gives an unprecedented reproducibility of $^{129}\mathrm{Xe}$ ${T}_{1}$ measurements across a range of 77--150 K. This temperature dependence roughly follows the theory of spin rotation mediated by Raman scattering of harmonic phonons, though it results in a smaller-than-predicted spin-rotation coupling strength ${c}_{K0}/h$. Enriched ice $^{129}\mathrm{Xe}$ ${T}_{1}$ experiments show no isotopic dependence of bulk relaxation mechanisms at 77 K and at kilogauss fields.

Raman spectroscopy is used to study magnetic excitations in the quasi-one-dimensional $S=1/2$ quantum spin systems $\text{Cu}(\mathrm{Qnx}){({\mathrm{Cl}}_{1\ensuremath{-}x}{\mathrm{Br}}_{x})}_{2}$. The low energy spectrum is found to be dominated by a two-magnon continuum as expected from the numerical calculations for the Heisenberg spin ladder model. The continuum shifts to higher energies as more Br is introduced. The cutoff of the scattering increases faster than the onset indicating that the increase of the exchange constant along the leg is the main effect on the magnetic properties. The upper and lower continuum thresholds are measured as a function of Br content across the entire range and compared to estimates based on previous bulk studies. We observe small systematic deviations that are discussed.

In time-dependent density functional theory and Ehrenfest dynamics are used to calculate the electronic excitations produced by a moving Ni ion in a Ni crystal in the case of energetic MeV range (electronic stopping power regime), as well as thermal energy meV range (electron-phonon interaction regime). Results at high energy compare well to experimental databases of stopping power, and at low energy the electron-phonon interaction strength determined in this way is very similar to the linear response calculation and experimental measurements. Our approach to electron-phonon interaction as an electronic stopping process provides the basis for a unified framework to perform classical molecular dynamics of ion-solid interaction with ab initio type nonadiabatic terms in a wide range of energies.

Angle-resolved photoemission spectroscopy experiments reveal that the valence band splitting at $K$ point in transition metal dichalcogenides is primarily due to the spin-orbit coupling rather than inter-layer interactions. The authors have achieved convincing agreement between experimental results and theoretical calculations over a wide energy range.

We study the biexciton formation process in a single CdTe/ZnTe quantum dot. Consistently with previous studies, we find subquadratic dependence of the biexciton photoluminescence intensity on the excitation power. We quantitatively explain this dependence in terms of the interplay between two alternative biexciton formation mechanisms, including the formation from a long-lived dark exciton residing in the quantum dot. This mechanism is demonstrated to be a dominant one for a wide range of excitation intensities. Our study is complemented by determination of a characteristic biexciton formation time, which is shown to be governed by the spin relaxation dynamics of an excited electron pair.

We performed positron annihilation spectroscopy measurements on uranium dioxide irradiated with 45 MeV $\ensuremath{\alpha}$ particles. The positron lifetime was measured as a function of the temperature in the 15--300 K range. The experimental results were combined with electronic structure calculations of positron lifetimes of vacancies and vacancy clusters in ${\mathrm{UO}}_{2}$. Neutral and charged defects consisting of from one to six vacancies were studied computationally using the $\mathrm{DFT}\phantom{\rule{0.16em}{0ex}}+\phantom{\rule{0.16em}{0ex}}U$ method to take into account strong correlations between the $5f$ electrons of uranium. The two-component density functional theory with two different fully self-consistent schemes was used to calculate the positron lifetimes. All defects were relaxed taking into account the forces due to the creation of defects and the positron localized in the vacancy. The interpretation of the experimental observations in the light of the DFT + $U$ results and the positron trapping model indicates that neutral ${\mathrm{V}}_{\text{U}}\phantom{\rule{0.16em}{0ex}}+\phantom{\rule{0.16em}{0ex}}2{\mathrm{V}}_{\text{O}}$ trivacancies (bound Schottky defects) are the predominant defects detected in the 45 MeV $\ensuremath{\alpha}$ irradiated ${\mathrm{UO}}_{2}$ samples. Our results show that the coupling of a precise experimental work and calculations using carefully chosen assumptions is an effective method to bring further insight into the subject of irradiation induced defects in ${\mathrm{UO}}_{2}$.

We consider theoretically the dynamics of electric currents optically injected in carbon nanotubes. Although the plasma oscillations are not seen in these systems, the main effect on the carrier's motion is due to strongly nonuniform space-charge Coulomb forces produced by time-dependent separation of injected electron and hole densities. We calculate evolution of the dipole moment characterizing the time- and coordinate-dependent charge density distributions and analyze different regimes of the dynamics. The developed time-dependent dipole moment leads to a dipole radiation in the THz frequency range for typical parameters of injected currents., 7 pages, 6 figures

Ge-based spin-photodiodes have been employed to investigate the spectral dependence of optical spin orientation in germanium, in the range 400--1550 nm. We found the expected maximum in the spin polarization of photocarriers for excitation at the direct gap in $\ensuremath{\Gamma}$ (1550 nm) and a second sizable peak due to photogeneration in the $L$ valleys (530 nm). Data suggest distinct spin depolarization mechanisms for excitation at $\ensuremath{\Gamma}$ and $L$, with shorter spin relaxation times whether the $X$ point is involved. These devices can be used as integrated photon-helicity detectors over a wide spectral range.

Transitions between the two states of an AC Stark-split doublet are forbidden in centro-symmetric systems, and thus almost impossible to observe in experiments performed with atomic clouds. However, electrons trapped in nanoscopic heterostructures can behave as artificial atoms, with the advantage that the wavefunction symmetry can be broken by using asymmetric confining potentials. Here we develop the many-body theory describing the intra-doublet emission of a resonantly pumped intersubband transition in a doped asymmetric quantum well, showing that in such a system the intra-doublet emission can be orders of magnitude higher than in previously studied systems. This emission channel, which lies in the terahertz range, and whose frequency depends upon the pump power, opens the way to the realization of a new class of monolithic and tunable terahertz emitters., Comment: 10 pages, 7 figures

due to the presence of the Ag 0 impurity. Moreover, from the present results, it is predicted that on passing to NaCl:Ag 0 the CT onset would be out of the optical range while the 5s-5p transition would undergo a redshift of 0.3 eV. These conclusions, which underline the different character of involved orbitals, are consistent with experimental findings. The existence of a CT transition in the optical range for an atom inside an ionic host is explained by a simple model, which also accounts for the differences with the more common 3d systems. The present study sheds also some light on the R dependence of the s 2 - sp transitions due to s 2 ions like Tl 1 .

Likely candidates for the ionic structure and lowest-energy minima of cationic sodium clusters ${\mathrm{Na}}_{n}^{+}$ are presented in the size range $n=4\char21{}40.$ We perform a systematic and unconstrained global optimization by the basin-hopping method with three different energy functions: a many-body empirical classical potential modified to account for the extra ionic charge, a simple quantal H\"uckel-type model, and a density-functional-theory-based orbital-free model. The similarities and differences between the models are discussed, and the global shapes are analyzed with the Hill-Wheeler parameters.

The optical properties of bimetallic silver-gold clusters are investigated in the size range 1.9--2.8 nm. Clusters, produced by laser vaporization of an alloy target Au-Ag (50%-50% atomic composition) are embedded with a low concentration in an alumina matrix. The mixed particles have the same stoichiometry as the rod. Absorption spectra exhibit a plasmon resonance band between those of pure Au and Ag embedded clusters, shifting towards higher energies with decreasing cluster size. Theoretical calculations in the frame of the time-dependent-local-density-approximation, including an inner skin of ineffective screening and the porosity of the matrix, are in good agreement with experimental results.

Using profilometry, large out-of-plane swelling was found when irradiating lithium fluoride with MeV to GeV heavy ions. The effect scales with the range of ions and is linked to the electronic energy loss. Above a critical energy loss of 4.2 keV/nm, ion-induced swelling increases by two to three orders of magnitude. The threshold is much lower than 10 keV/nm required for track etching, suggesting swelling measurements as a suitable technique for testing in particular nonamorphisable material sensitivity versus swift heavy-ion irradiation. A quantitative comparison with data available from x-ray and neutron irradiations indicates that swelling in LiF results from an intermediate defect zone in between the track halo (10--20 nm) consisting mainly of single color centers and the track core (1--2 nm) identified by small-angle x-ray scattering.

We have, in total-energy calculations and electron-energy-loss measurements, identified a molecular adsorption state of hydrogen residing above a step atom on a stepped copper surface. Rotational and vibrational properties of the molecular motion in the bound adsorption state have been determined from an effective three-dimensional potential-energy surface, derived from density-functional calculations. Our calculations reveal that the axially symmetric orientational barrier confines the molecule to rotate in a two-dimensional (2D) manner, and corroborate the spectral assignment of our electron-energy-loss measurements in terms of 2D quantum rotor states. Further support for the existence of this exotic hydrogen adsorption state comes from the good agreement between calculated and measured values of the adsorption energy, the internal vibrational energy, and the dipole activity of the internal vibration. The calculated adsorption energy is small, and in the range for physical adsorption, whereas the strongly perturbed molecular bond and the short hydrogen-copper distance suggest the formation of a surface chemical bond.

We present a comparative experimental and theoretical study of the frequency shift in ultrahigh vacuum dynamic force microscopy at 80 K on graphite(0001) measured as a function of the tip-sample distance for different resonance amplitudes A in the repulsive and attractive regime of the tip-sample forces. The resulting frequency shift versus distance curves scale with ${1/A}^{3/2}$ over the full range. We determined the tip-sample force from the frequency shift versus distance curves and found good agreement with specific force laws for long-range (van der Waals), short-range (Lennard-Jones), and contact (Hertz) forces.

We have analyzed the problem of the secondary electrons emitted from a solid irradiated by a positron or electron particle beam. The relevant concepts that form the basis for a theoretical understanding of the secondary electron emission problem are reanalyzed by looking at Wolff's theory. However, to attain analytical results, such a theory must include some simplification, which may be valid in only a limited number of situations. To overcome such a limitation we have elaborated a Monte Carlo procedure for the calculation of the secondary electrons emitted from a solid, irradiated with a positron beam. The choice of a primary positron beam is justified because the experiments involving electron emission are not contaminated by the reemitted primary electrons. The calculations were performed in the positron primary energy range between 50 eV and 2 keV, and for different incidence angles with respect to the surface of a copper sample. Many numerical results are reported, namely: i) the elastic mean-free paths of positrons in copper, ii) the mean number of electrons emitted per positron, iii) the penetration depth of the positrons, and iv) the depths from which the secondary electrons are emitted. Finally, the numerical results concerning the secondary electron energy distribution are compared with the experimental data recently presented by Overton and Coleman showing a general good agreement.

We calculate the singlet-triplet splitting for a couple of two-dimensional electrons in the potential of two positively charged impurities which are located out of plane. We consider different relations between vertical distances of impurities $h_1$ and $h_2$ and their lateral distance $R$. Such a system has never been studied in atomic physics but the methods, worked out for regular two-atomic molecules and helium atom, have been found to be useful. Analytical expressions for several different limiting configurations of impurities are obtained an interpolated formula for intermediate range of parameters is proposed. The $R$-dependence of the splitting is shown to become weaker with increasing $h_1,h_2$., Comment: 14 pages, RevTeX, 5 figures. Submitted to Phys Rev. B

Photomodulation of Si-H stretching vibration can be observed in hydrogenated amorphous silicon $(a\ensuremath{-}\mathrm{S}:\mathrm{H})$ as a photoinduced bleaching (PB) band around 2000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ which is embedded in a photoinduced absorption spectrum coming from excess carriers trapped at tail states. Since this vibronic modulation is a real-time change following a modulated excitation, it is different from a well known metastability in $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ as induced by strong light soaking. It is suggested from the theoretical work reported by Yonezawa et al. [J. Non-Cryst. Solids 137a138, 135 (1991)] that the modulation of a Si-H vibration reflects a change in length of a neighboring weak Si-Si bond following a modulated excitation. Based on this picture, we have tried to see how the spectrum of the PB band changes when the weak Si-Si bonds are broken or reformed by light soaking or thermal annealing, respectively. Change in the spectrum after the treatments could be observed over almost a whole range of the PB band and this vibronic modulation seems to be related selectively with the monohydride configuration of Si-H bond. By evaluating quantitatively the spectral change in the PB band combined with simultaneous change in dangling-bond density, density of the weak Si-Si bonds or neighboring Si-H bonds responsible for the vibronic PB band can be estimated to be about $5\ifmmode\times\else\texttimes\fi{}{10}^{17}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}.$ This value is much smaller than the amount of the monohydride Si-H bonds contained in specimens used here by about four orders of magnitude, indicating that only a very small part of the existing Si-H bonds participates in the structural flexibility in $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}.$

It is shown that the La $5p$ photoemission (PE) spectra consist of a superposition of two final states reflecting predominantly ${5p}^{5}{4f}^{0}$ and ${5p}^{5}{4f}^{1}$ character. In contrast to La $3d$ spectra, the ${4f}^{1}$-derived component appears at the high-binding-energy side of the ${4f}^{0}$-derived emission due to a weaker interaction between the photohole and the $4f$ electron. This splitting into two components is crucial for the quantitative understanding of the resonant behavior of the $5p$ states at the $4\stackrel{\ensuremath{\rightarrow}}{d}4f$ excitation threshold. In addition to the participator decay channel usually considered for the description of the resonant PE, a spectator decay channel must be taken into account that leads particularly to an enhancement of the ${4f}^{1}$ originating emission. The contributions of the spectator transitions are particularly important in the range of the so-called preresonances, e.g., at the ${}^{3}{P}_{1}$ and ${}^{3}{D}_{1}$ excitation thresholds.

The dephasing of excitons and the formation of biexcitons in self-organized CdSe/ZnSe islands grown by molecular-beam epitaxy is investigated using spectrally resolved four-wave mixing. A distribution of exciton-exciton scattering efficiencies and dephasing times in the range of 0.5-10 ps are observed. This indicates the presence of differently localized exciton states at comparable transition energies. Polarization-dependent measurements identify the formation of biexcitons with a biexciton binding energy of more than four times the bulk value. With decreasing exciton energy, the binding energy slightly increases from 21.5 to 23 meV, while its broadening decreases from 5.5 to 3 meV. This is attributed to a strong three-dimensional confinement with improving shape uniformity for decreasing exciton energy. [S0163-1829(99)04739-6].

The charge of divacancies in undoped float-zone silicon has been investigated as a function of photon energies between 0.2 and 1.2 eV by means of positron lifetime spectroscopy. For photon energies between 0.25 and 0.65 eV and above 0.75 eV, negatively charged vacancies were formed, whereas in the 0.65 to 0.75 eV range there was no effect from illumination. The results are explainable on the basis of Watkins and Corbett's model of divacancy [Phys. Rev. 138, A543 (1965)]. Evidence was found that the radioactive positron source significantly reduces the stability of the photon-induced population of negatively charged divacancies, an effect which is invoked to explain the apparently anomalous results reported by Kawasuso and Okada [Jpn. J. Appl. Phys., Part 1 36, 605 (1997)].

We have determined the dielectric function of ${\mathrm{Be}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}\mathrm{Se}$ above the fundamental absorption edge for the full composition range $(0l~xl~1)$ by spectroscopic ellipsometry in the UV and VUV range. The spectra show pronounced features in the photon energy range between 2.7 and 25 eV. By comparing the ZnSe spectra $(x=0)$ to literature data, we assign the observed structures to interband transitions at specific points or regions of the Brillouin zone. The corresponding assignment for BeSe is made by tracking the systematic dependence of these structures on the Be content x from $x=0$ to $x=1. {E}_{0}$ has the steepest slope ${\mathrm{dE}}_{0}/dx$ and exhibits a bowing vs x. On samples with a large Zn content $(xl0.2)$ two structures show up at high energies (15.7 and 16.9 eV) which have not been observed before. We assign these structures to transitions from the $\mathrm{Zn} 3d$ bands to a higher conduction band at the L point and at the $\ensuremath{\Gamma}$ point.

The electronic response of a two-dimensional (2D) electron system represents a key quantity in discussing one-electron properties of electrons in semiconductor heterojunctions, on the surface of liquid helium and in copper-oxide planes of high-temperature superconductors. We here report an evaluation of the wave-vector and frequency dependent dynamical quadratic density-response function of a 2D electron gas (2DEG), within a self-consistent field approximation. We use this result to find the $Z_1^3$ correction to the stopping power of a 2DEG for charged particles moving at a fixed distance from the plane of the 2D sheet, $Z_1$ being the projectile charge. We reproduce, in the high-density limit, previous full nonlinear calculations of the stopping power of a 2DEG for slow antiprotons, and we go further to calculate the $Z_1^3$ correction to the stopping power of a 2DEG for a wide range of projectile velocities. Our results indicate that linear response calculations are, for all projectile velocities, less reliable in two dimensions than in three dimensions., 17 pages, 5 figures, to appear in Phys. Rev. B

The probability of the ${L}_{2}{L}_{3}V$ Coster-Kronig (CK) decay process in Ni following the creation of a ${2p}_{1/2}$ core hole has been measured by photoemission spectroscopy using synchrotron radiation. The branching ratio between CK and ${L}_{2}\mathrm{VV}$ Auger decay has been determined as a function of the photon energy in the proximity of the ${L}_{2}$ edge (870\char21{}890 eV). It is found to become independent upon the photon energy range already a few eV above threshold. The direct low-energy (about 7 eV) CK emission spectrum has also been observed.

Recent direct mechanical measurements of atomic force microscopy showed that the force between the silicon tip and the silicon sample is long range in the attractive region and its magnitude at maximum is relatively smaller. These features disagree with previous theoretical predictions based on the {ital ab initio} calculations. We investigated the nature of forces between a silicon tip and the silicon (111)-(2{times}1) surface by performing first-principles pseudopotential and classical molecular dynamics calculations and by calculating the van der Waals interaction. The first two methods give forces that are short range in nature. Fair agreement between the experiment and theory is obtained when the van der Waals interaction is included. The effect of the tip induced deformation is analyzed. {copyright} {ital 1999} {ital The American Physical Society}

Molecular-dynamics simulations of sodium silicate glasses in a range of alkali concentration going from 1.8% to 33.33% molar of ${\mathrm{Na}}_{2}\mathrm{O}$ are reported. Our simulations show that there is a tight relationship between Na atoms and nonbridging oxygens which are mainly located in the first Na coordination shell. In the whole range of composition, Na and nonbridging oxygen atoms appear to segregate giving rise to a heterogeneous distribution. For the higher alkali concentrations, formation of microchannels is observed. The activation energies for alkali diffusion have been computed and found in agreement with the experiment. The mechanism for the diffusion has also been investigated and found to occur through the microchannels, if present, or across the network for low compositions, but the Na motions always appear to be assisted by nonbridging oxygen atoms.

Fully microscopic random phase approximation/local density approximation calculations of the dipole plasmon for very large neutral and charged sodium clusters ${\mathrm{Na}}_{N}^{Z+}$ in the size range $950\ensuremath{\lesssim}N\ensuremath{\lesssim}12050$ are presented. Sixty different sizes are considered altogether, which allows for an in-depth investigation of the asymptotic behavior of both the width and the position of the plasmon.

A large number of elemental and compound semiconductors in bulk form become metallic upon melting due to changes in the local atomic structure occurring in the transition from the solid to the liquid state. We report studies of the vibrational properties of liquid Bi clusters that suggest that for the smaller sizes this atomic structure transition does not occur. Using interference enhanced Raman spectroscopy, in situ studies of bismuth ultrathin films, supported on weakly interacting carbon substrates, have been performed. Changes in the vibrational spectra with liquid cluster size have been correlated with ex situ transmission electron microscopy to estimate a critical size below which densification of the structure does not occur. For various models this yields clusters of diameter in the range of \ensuremath{\sim}45\char21{}75 \AA{}.

We have measured the diffusion of CO on flat and stepped Pt(111) surfaces using the linear optical diffraction method from an adsorbate grating. Measurements over a wide range of temperatures (133--313 K) and CO coverages (0.1--0.67 ML) indicate Arrhenius behavior with diffusion activation energies of 3.0--4.7 kcal/mol and 7.3--7.9 kcal/mol for flat and stepped surfaces, respectively. Comparisons of our measurements on flat and stepped (defected) surfaces with previous experiments by various groups using various techniques resolves several discrepancies and illustrates the importance of defect-induced effects on variations in measured diffusion parameters.

The photon-energy dependences of photoemission intensities of ${\mathrm{C}}_{60}$ were quantitatively calculated by the single-scattering approximation for the final state and the ab initio molecular orbital calculation for the initial state. The calculated results agreed well with the measured intensity oscillation in the photon-energy range of $h\ensuremath{\nu}=18--110\mathrm{eV}.$ The calculation by the plane-wave approximation for the final state also gave similar oscillations, which suggests that the oscillations are independent of the accuracy of the final state. These results indicated that the oscillations originate from the interference of photoelectron waves emanating from the 60 carbon atoms, i.e., the multicentered photoemission with the phase difference of each wave. Further, the analytical calculation with a simplified spherical-shell-like initial state revealed that the spherical structure of ${\mathrm{C}}_{60}$ molecule and its large radius dominate the oscillations.

Conductivity measurements from 300 to 1200 K of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ thin films, with doping from $x=0$ to $x=1,$ show that the entire doping range fits the adiabatic small polaron model $\ensuremath{\sigma}=(A/T)\mathrm{exp}(\ensuremath{-}{E}_{a}/kT).$ Furthermore the $x$ dependence of $A$ explicitly shows the effects of on-site Coulomb repulsion, i.e., a polaron cannot hop into an occupied site. Instead of increasing monotonically as more carriers are introduced, $A$ starts to decrease at $x=0.2$ and, as expected for Hubbard band splitting, is reduced to almost zero when the lattice is full.

A dynamical optimization of the minimum energy cluster structures of Na, K, Rb, and Cs clusters was performed using a many-body potential based on local density calculations. The energetics and vibrational analysis of the neutral clusters in the size range $8lNl310$ were calculated, including the free energy as a function of the cluster size and the melting temperature. The fission process due to Coulomb forces of 2+, 3+, and 4+ charged alkali-metal clusters was studied extensively using molecular dynamics. We show that the cluster size at which multiply charged clusters undergo fission depends strongly on the cluster temperature. Three phases in the temperature-size-phase plane are identified corresponding to unstable, metastable, and stable clusters. These regions are bound by the spontaneous size at zero temperature and the critical size at the critical temperature. The cluster critical size exhibits a power law dependence on the total charge, which is in excellent agreement with experiments. The energy barriers that the clusters need to undergo fission are reported as a function of cluster size. The limitations of the liquid drop model are indicated in light of the dynamical findings.

The (110) surface of many transition metals undergoes reconstructions either on the clean substrate or induced by adsorbates. The reconstructions can be induced by a variety of strongly bound adsorbates, e.g., H, O, S, and alkali-metal atoms. In this work, H/Pd(110) is chosen as a prototype of such reconstructions. The pairing-row and missing-row reconstructions are studied in a wide range of coverages, $\ensuremath{\theta}=0.5--1.5\mathrm{ML},$ by using density-functional theory with the local-density approximation and the generalized gradient approximation. The driving force for the reconstructions is also analyzed in detail. The pairing-row reconstruction is driven essentially by the repulsion between the hydrogen atoms adsorbed in the same trough while the driving force for the missing-row reconstruction is the better adsorbate-substrate interaction on the reconstructed surface.

Photoemission spectra of the high-temperature superconductor Bi2Sr2CaCu2O8 near (pi,0) show a dramatic change when cooling below T-c: the broad peak. in the normal state turns into a sharp low-energy peak followed by a higher binding-energy hump. Recent experiments fmd that this row-energy peak persists over a significant range in momentum space. We show in this paper that these data are well described by a simple model of electrons interacting with a collective mode that appears only below T-c.

The electron-positron correlation energy in an electron gas has been calculated many times, and quite different results have been obtained. Therefore we found it useful to compute correlation energies using the more recent approach of Stachowiak labeled PHNC (perturbed hypernetted chain) which is self-consistent and describes quite well the experiment as concerns partial annihilation rates, at least in simple metals. This approach passed the additional test successfully, leading to reasonable results for the correlation energy in the whole range of investigated electron densities $(0.1l{r}_{s}l25)$.