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We investigate effects of a magnetic flux $\ensuremath{\Phi}$ threading mesoscopic Peierls rings. Both isolated rings and ensembles of rings are considered and quantum phonon fluctuations are accounted for. Significant qualitative and quantitative modifications to the mean field results are obtained. Quantum fluctuations yield a continuous structural Peierls change occurring without symmetry breaking and a $\ensuremath{\Phi}$-dependent tunneling splitting. The latter yields a $\ensuremath{\Phi}$-modulated ultrasound or hypersound absorption that could be detected experimentally. Important differences exist between an isolated ring and an ensemble of rings. To perform ensemble averaging, we propose a new and simple method. Unlike isolated rings, ensembles possess structural and magnetic properties that do not distinguish between average sizes $N=4n$ and $N=4n+2$ but rather between smaller and larger N. In ensembles, the flux period is $hc/2e,$ half the period for an isolated ring. An appealing magnetic behavior with an interplay between dia- and paramagnetism is found in ensembles that could be experimentally investigated by means of a SQUID technique.

A phenomenon of partial localization of core holes has been discovered. Fourth-order Green's function and large-scale configuration interaction calculations have been carried out on the N $1s$ x-ray photoelectron spectrum (XPS) of a linear ${\mathrm{NiN}}_{2}$ cluster. The N $1s$ core holes in the ionized states of the cluster are to varying extent delocalized between the two geometrically inequivalent nitrogen atoms. In some of the states the core holes are significantly delocalized. Good agreement with the experimental spectra of the ${\mathrm{N}}_{2}/\mathrm{Ni}(100)c(2\ifmmode\times\else\texttimes\fi{}2)$ chemisorption system is obtained, which makes clear that the effect is to be expected also in this solid-state system. The partial localization of core holes leads to distinct interference effects in the angularly resolved XPS. This can be used for the experimental detection of the effect. The effect is more pronounced for ${\mathrm{NiP}}_{2}$, which is also studied here.

Adsorbate core-hole screening in weak chemisorption systems is studied by the example of the ${N}_{2}/Ni(100)$ system. Fourth-order Green's-function calculations have been carried out on the $\mathrm{N}1s$ x-ray photoelectron spectra (XPS) of a linear ${\mathrm{NiN}}_{2}$ cluster used as a model of the chemisorption system. Good agreement with the experimental spectra is obtained. A powerful method, based on exact effective Hamiltonians (EEH's) constructed by means of a technique for a unique block diagonalization of Hermitian matrices [J. Phys. A 22, 2427 (1989)] is used for a detailed analysis of many-electron core-hole states. Two lowest-energy states in the $\mathrm{N}1s$ XPS of the ${\mathrm{NiN}}_{2}$ cluster, split by 1.47 eV (the experimental splitting in the chemisorption system is 1.3 eV), are classified as $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\pi}}{\ensuremath{\pi}}^{*}$ shakedown satellites, whereas two states dominating the region of the so-called ``giant satellite'' are attributed to the main states. The EEH technique allows us to study in detail the dynamical screening of core holes, which is shown to be responsible for the formation of the strong shakedown satellites. A comparative EEH analysis of the core-hole states of the ${\mathrm{NiN}}_{2}$ and NiCO clusters explains in simple terms the origin of the very different intensities of the giant satellites observed experimentally in adsorbate spectra of weak and strong chemisorption systems.

We study effects of quantum phonons on the optical conductivity \ensuremath{\sigma}(\ensuremath{\omega}) for finite dimerized chains by using two numerical methods: exact numerical diagonalization and by factorizing the electron-phonon wave function (AA, adiabatic ansatz). For small chains, we compare the results of these methods and find a criterion for a valid AA. AA turns out to be: (a) a substantial improvement with respect to the adiabatic approximation usually employed in condensed-matter physics, and (b) much less affected by computer storage limitations than exact diagonalization. We use AA and get \ensuremath{\sigma}(\ensuremath{\omega}) reliably for sufficiently large chains, which could be realized experimentally. The optical gap as well as the \ensuremath{\sigma}(\ensuremath{\omega}) curve near the absorption edge possess a surprisingly strong dependence on the chain size; this could be relevant for experiments on mesoscopic systems.

To clarify the physical picture of adsorbate ionization, the ligand core-hole spectra of a NiCO cluster used as a model of the CO/Ni(100) chemisorption system were obtained by large scale ab initio configuration interaction calculations. The computed C1s and O1s spectra are consistent with the experimental finding for the CO/Ni(100)c(2\ifmmode\times\else\texttimes\fi{}2) system. The results support the picture that the screening mechanism of the core hole on the ligand atoms is dominantly metal-ligand \ensuremath{\pi}\ensuremath{\rightarrow}\ensuremath{\pi}* charge transfer. \textcopyright{} 1996 The American Physical Society.

We reexamine the problem of phase pinning of a three-dimensional charge-density wave (CDW) weakly coupled to impurities. Within an analytical approach, we find that the phase is adjusted both at the short scale, around individual impurity sites, and at the large scale, over extended domains containing many impurities. Both phase adjustments are important for the physical behavior and contribute to the understanding of experimental data as well as numerical simulation results in CDW systems. The former give the main contribution to energy and are responsible for the white-line effect observed in x-ray patterns. The latter determine the functional dependence of the threshold field ${\mathit{E}}_{\mathrm{th}}$ for nonlinear conduction. Unlike what was widely accepted, we find that ${\mathit{E}}_{\mathrm{th}}$ is not related to the total energy gained by phase spatial variations. This picture is consistent with that emerging from the recent experimental investigations of doped ${\mathrm{NbSe}}_{3}$ and could contribute to clarifying the controversy on the pinning type in this material.

Etude de la structure electronique des etats fondamentaux et excites et des etats ioniques de cœur et de valence de cet amas. Cet amas peut servir de modele pour des oxydes Cu(II) a l'etat solide. Les etats a un electron de l'amas sont fortement localises, soit sur le site du cuivre, soit sur celui de l'oxygene

We report numerical results demonstrating the existence of a hidden dynamical quasisymmetry in the optical absorption of finite nanorings consisting of metallic quantum dots described by means of the extended Hubbard model for closed shells, causing, out of a multitude of transitions allowed by spatial symmetry, all but one to be practically forbidden. We show that, by shining only a part of the nanoring, one can get more information than by irradiating the whole nanoring, and suggest that experimentalists apply this method to measure optical absorption spectra.

Exact numerical results are obtained within the extended Hubbard Hamiltonian for nanorings consisting of Ag quantum dots (QD’s ) with C6v symmetry which interact with a circularly polarized light. The results show that the high-harmonic generation (HHG) spectra obtained from such artificial “molecules” are more pronounced than the HHG spectra obtained from a real molecule such as benzene. Our studies show that the HHG spectra obtained from the QD nanorings consist of two plateaus while only one plateau appears for benzene. The role of electron correlations in the generation of the high-order harmonics is studied, and it is shown that it can increase the intensity of the high-order harmonics. Mainly affected are the harmonics which are located in the second plateau. Selection rules for the produced high harmonics and a new “synergetic” selection rule for the symmetry of the states contributing to the HHG spectrum, a combined effect of spatial and charge conjugation symmetries, are discussed.

CDW-to-CDW and CDW-to-SDW (CDW/SDW=charge-spin-density wave) quantum phase transitions have been recently reported for finite rings described by an extended Hubbard model [I. B\^aldea, H. K\"oppel, and L. S. Cederbaum, Eur. Phys. J. B 20, 289 (2001)]. We present exact (Lanczos) diagonalization results which demonstrate that these transitions survive in the presence of a dynamic Su-Schrieffer-Heeger (SSH) electron-phonon coupling. The two transitions are affected in a different way discussed in detail. By treating the SSH electron-phonon coupling dynamically, two levels of different symmetries do cross, allowing one to define precisely a critical point. Because the ground-state symmetry changes at the critical points, we suggest to study the quantum phase transitions by optical methods. Molecular rings such as polyenes (annulenes) turn out to be too far from the critical points, where the most interesting phenomena occur. However, we present an analysis revealing that, if metallic quantum dots of the type already fabricated can be assembled in nanorings, model parameters can be tuned and quantum phase transitions can become observable.

The Hellmann-Feynman theorem is extended to account for degenerate states. Given a point $\ensuremath{\lambda}={\ensuremath{\lambda}}_{0}$ in parameter space where the energy $E({\ensuremath{\lambda}}_{0})$ is n-fold degenerate, it is shown that the corresponding n forces (slopes) are obtained by diagonalizing the derivative of the Hamiltonian, $\ensuremath{-}\ensuremath{\partial}H(\ensuremath{\lambda})/\ensuremath{\partial}\ensuremath{\lambda}{|}_{\ensuremath{\lambda}={\ensuremath{\lambda}}_{0}},$ in the subspace of degenerate eigenstates. Such a rotation within the subspace of degenerate eigenfunctions is easy and simple to apply in practical calculations and should be performed separately for each independent direction in parameter space for which the forces are to be calculated.

The two available Mg 2p - 1 core-level spectra of the recently discovered high-temperature superconducting MgB 2 crystal exhibit interesting structures, but contradict each other. This motivated us to perform ah initio calculations on the core-level spectra of cluster models. The computed spectra reveal unusual rich and intense structures triggered by a B 2p z →Mg 3s,3p charge transfer.

Following innervalence ionization of a cluster, the system can relax by electron emission, a phenomenon called intermolecular Coulombic decay. This process is characterized by an efficient energy transfer mechanism between neighboring monomers in the cluster. A theoretical description within the framework of Wigner-Weisskopf theory is developed, thus enabling a detailed analysis of the decay process. The main result of the formal treatment, a simple, approximate expression for the electronic decay width of an innervalence hole state, is applied to investigate the effect of cluster size. On the basis of extensive ab initio calculations, pronounced size effects are found in the concrete example of neon clusters. The decay lifetime decreases in a monotonic fashion from hundred femtoseconds in ${\mathrm{Ne}}_{2}$ down to less than ten femtoseconds in ${\mathrm{Ne}}_{13}.$ Suggestions are made how to facilitate the experimental observation of intermolecular Coulombic decay in clusters and condensed matter.

In mesoscopic systems, the symmetry of the wave function cannot be broken and must be considered. We construct symmetry-adapted trial wave functions of the Bardeen-Cooper-Schrieffer-type for mesoscopic rings described by an extended-Hubbard model at half-filling. The comparison with exact numerical (Lanczos) diagonalization in small rings indicates that this variational approach is reasonably accurate. Within this approach, we demonstrate a crossover from a weak-correlation regime to a strong correlation one. Particularly interesting is the behavior of the lowest-excitation energy, which switches from a highest-occupied-molecular-orbital--lowest-unoccupied-molecular-orbital gap to a splitting energy related to a collective tunneling of electrons.

The coupling of a surface plasmon to an adsorbate core level in x-ray photoelectron spectroscopy has been observed for the $1s$ level of oxygen adsorbed on an aluminum (111) surface. The coupling parameter was found to be strongly coverage dependent. To distinguish between intrinsic and extrinsic effects the dependence of the coupling parameter on the photoelectron escape angle was also measured. In this way both extrinsic and intrinsic terms could be identified, as well as the interference term. Within the framework of the model used for the interpretation of results, the intrinsic effect was found to be low in magnitude with a coupling parameter of \ensuremath{\sim}0.04, which implies a surface-plasmon screening energy of about 0.4 eV. This unexpectedly weak core-hole-plasmon coupling is explained by considering the repulsion of the screening charge by the valence electrons of the adsorbed atom.