The electrical conductivity of electrochemically oxidized Langmuir-Blodgett films of tridecylmethylammonium-Au(dmit${)}_2$ and icosanoic acid has been studied. The measurements were performed at temperatures down to 3 K and hydrostatic pressures up to 17 kbar, and also at high electric fields up to 3.2 kV/cm. The results support the view that the films consist of relatively large metallic islands, separated by thin barriers, and indicate that the low-temperature conductivity with dσ/dT>0 originates from thermal-fluctuation-induced tunneling. At higher temperatures dσ/dT<0 because of the finite resistance of the metallic islands. The conductivity increased upon applied pressure, and the temperature of the maximum conductivity shifted to lower temperature without any pronounced saturation. The ambient-pressure room-temperature conductivity of the best conducting samples could be increased by a factor of about 2 at 10-kbar hydrostatic pressure and further to 140 S/cm at the temperature of the maximum conductivity at 51 K.

An extended fine structure (EFS) has been observed in the electron-excited secondary-electron-emission (SEE) spectrum of graphite. Measurements were performed in the 0–150-eV electron kinetic range as a function of primary-electron energy above and below the threshold for carbon core-level 1s ionization. For primary electron energies of 200, 500, and 1000 eV the spectra display a well-defined fine structure that does not depend strongly on the primary electron energy. However, a dependence of the spectrum on orientation of the sample with respect to the primary electron beam and the electron analyzer was found. The effect of structural disorder on the EFS-SEE spectrum of graphite was investigated by performing in situ measurements as a function of the 2-keV Ar-ion irradiation dose. It was found that the EFS-SEE is very sensitive to imperfections created by the ion beam for very low dose levels of ion irradiation. The sensitivity of the spectrum to short-range crystalline order was assessed by comparing the spectrum obtained for graphite to that of glassy carbon. It is concluded that the EFS-SEE of graphite originates from interband electronic excitation processes involving excitations of valence electrons by primary electrons and secondary electrons of sufficient energy into unoccupied electronic states of high density in the graphite crystal field. This conclusion is supported by comparison with other electron spectroscopic measurements of graphite.

The effect of alkali-metal chemisorption on the surface electronic structure of Ta(110) is presented. A d-derived surface state on clean Ta(110) is strongly modified, as manifested by a level shift and lateral delocalization. Strong mixing between alkali-metal and substrate valence bands is observed. It is suggested that alkali-metal overlayers on the more complex metals have fundamentally different behavior than on simple metal substrates.

The binary phase diagrams of the alkali-metal fullerides, ${\mathit{A}}_{\mathit{x}}$${\mathrm{C}}_{60}$, are discussed in light of recent experimental results concerning the ${\mathit{A}}_1$${\mathrm{C}}_{60}$ phases and their transformations. The ${\mathrm{Rb}}_1$${\mathrm{C}}_{60}$ and ${\mathrm{Cs}}_1$${\mathrm{C}}_{60}$ phases undergo diffusionless structural transformations upon cooling and their phase diagrams require solutions that differ from the eutectoid transformation of ${\mathrm{K}}_1$${\mathrm{C}}_{60}$. X-ray photoelectron spectroscopy has been used to characterize these transformations. Phase diagrams incorporating two invariant reactions are proposed. The equilibrium vapor pressure of ${\mathrm{C}}_{60}$ has been determined at lower temperatures than prevously examined and the pure ${\mathrm{C}}_{60}$ phase diagram has been assembled for a large range of temperature and pressure. The predicted effects of solid-solution formation on the fcc to simple cubic transition of α-${\mathrm{C}}_{60}$ are discussed in terms of eutectoid and peritectoid phase-diagram topologies. High-temperature decomposition with fractional sublimation has been used to gain insight into the relative thermal stabilities of the various compounds. Using this information and data from the literature, a ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{60}$ phase diagram extending to vaporization temperatures is proposed.

Silver microclusters display giant dipole collective resonances with an unusual size dependence of the resonance frequency. This behavior is due to the fact that the clusters contain polarizable atomic-core d electrons in addition to a delocalized electron cloud. We describe a self-consistent field analysis of the response properties of such a system. It is shown that the efficiency of core screening is controlled by the overlap between d-electron and delocalized-electron wave functions. This provides microscopic justification for the two-region model in which core dielectric screening is missing in a layer near the particle surface. The dipole-excitation energies of such particles are calculated, and it is demonstrated that the model can account for the unusual blueshift of ${\mathrm{Ag}}_{\mathit{N}}^{+}$ resonances. The discussion of the polarization-free layer is also applicable to metallic surfaces.

Angular anisotropies in the source wave and scattering factor are shown to cause artifacts in the images reconstructed from multiple-energy photoelectron holograms. A variable-axis small-angular-cone method is introduced to eliminate such anisotropies. We show that within the small angular cone, the anisotropy of the source wave towards an atom is canceled by that towards the detector. Also, the cone only samples the flat part of the slope of the scattering factor’s phase. The remaining shift in the image position can be quantitatively corrected for the near-neighbor atoms.

The linear transport behavior of metallic multilayers is studied by using the real-space Kubo formula, for a model Hamiltonian consisting of zero-range spin-dependent impurity-scattering potentials. The resulting theory allows a direct comparison with the Boltzmann equation approach in the quasiclassical limit, which is expected to be a good approximation for most magnetic multilayers. Furthermore, the regimes for which quantum corrections might be needed are explicitly indicated. It is explicitly shown that: (i) periodicity is not required as a mechanism for giant magnetoresistance, and (ii) the two main geometries, current in the plane of the layers and perpendicular to the plane of the layers, exhibit very different size effects, the latter yielding the so-called series resistor model.

The (e,2e) technique is well known to be able to measure the momentum profiles of the electron orbitals in molecules. In crystalline solids energy levels are replaced by bands, and the momentum profiles simplify to energy-dependent δ functions. In this paper the development from a molecular to a crystalline picture of the electronic structure is illustrated using a simple model of a linear chain of atoms of increasing length. From this model we try to get some insight into the (e,2e) momentum profiles expected for disordered solids. These results are compared to the experimental data for carbon films with 6 different degrees of order, i.e., amorphous carbon films, annealed amorphous carbon films, and highly oriented pyrolitic graphite (HOPG) films. The intensity of the π-electron contribution is suppressed in HOPG, due to the orientation chosen. In the annealed evaporated samples, the planes of graphite atoms have random orientation and the π electrons are clearly seen. With increasing order the momentum profiles show increasingly well defined peaks.

The effect of an octupole term in a quadrupole deformed single-particle potential is studied from the classical and quantum-mechanical viewpoint. Whereas the problem is nonintegrable, the quantum-mechanical spectrum nevertheless shows some shell structure in the superdeformed prolate case for particular, yet fairly large octupole strengths; for spherical or oblate deformation the shell structure disappears. This result is associated with classical periodic orbits that are found by employing the removal of resonances method; this approximation method allows determination of the shape of the orbit and of the approximate octupole coupling strength for which it occurs. The validity of the method is confirmed by solving numerically the classical equations of motion. The quantum-mechanical shell structure is analyzed using the particle-number dependence of the fluctuating part of the total energy. In accordance with the classical result, this dependence turns out to be very similar for a superdeformed prolate potential plus octupole term and a hyperdeformed prolate potential without octupole term. In this way the shell structure is explained at least for some few hundred levels. The Fourier transform of the level density further corroborates these findings.

We have computed the diffusion constant for hydrogen and deuterium on Ni(001) using the quantum version of transition-state theory formulated in terms of the transition rate for the path centroid. The embedded-atom method was used for the potential. We find a transition beween thermally activated diffusion at high temperatures and almost temperature-independent diffusion at lower temperatures. We interpret the transition as being from semiclassical motion over the barrier to quantum tunneling through the barrier. One-dimensional model calculations suggest that the distinct transition is due to the shape of the potential barrier along the surface. The calculated values for the diffusion constant at high temperatures agree with the experimental results; however, the transition is located at a too low temperature (40 K) compared with experiments (100 K). For deuterium the transition occurs at approximately 25 K, which is in disagreement with the weak isotope effect found in the experiments.

A deformed-jellium model is used to calculate the fission barrier height of positive doubly charged sodium clusters within an extended Thomas-Fermi approximation. The fissioning cluster is continuously deformed from the parent configuration until it splits into two fragments. Although the shape of the fission barrier obviously depends on the parametrization of the fission path, we have found that remarkably, the maximum of the barrier corresponds to a configuration in which the emerging fragments are already formed and rather well apart. The implication of this finding in the calculation of critical numbers for fission is illustrated in the case of multiply charged Na clusters.

We develop and apply a semiempirical shell-correction method to calculate the binding energies of open-shell, neutral and charged, simple-metal clusters, which can be modeled as triaxially deformed jellium droplets. Systematics of ground-state properties of clusters with sizes up to 100 atoms, such as ionization potentials and electron affinities, are studied and compared to available experimental measurements on sodium, potassium, and copper clusters. We also report on systematics of the energetics of fission channels for doubly charged cationic and anionic species, as well as the energetics of monomer and dimer separation channels, and compare them to experimental data. Pertaining to characteristic patterns as a function of cluster size in the above quantities, triaxial shell effects exhibit a rich structure, yielding overall substantial improvement in the agreement between theory and experiment. In particular, we show that the lifting of the degeneracies in the electron spectra via cluster triaxial-shape deformations underlies the appearance of odd-even alternations in such patterns. Furthermore, our analysis of ground-state properties can lead to unambiguous assignments of equilibrium cluster shapes, as well as shape isomers.

A fast linearized method for electronic-structure calculations is introduced and shown to be applicable to disordered alloys and a variety of other systems. The method allows a full-potential solution to the local-density-approximation equations for both ordered and substitutional alloys. The method was used for the challenging problems of evaluating the elastic constants and the relaxation around impurities. As a demonstration of the applicability of the method for more open structures, the vibrational frequencies of doped fullerenes were successfully calculated.

We consider the large-U limit of the one-band Hubbard model at half-filling on a nonbipartite two-dimensional lattice. An external magnetic field can induce a three-spin chiral interaction at order 1/${\mathit{U}}^2$. We discuss situations in which, at low temperatures, the chiral term may have a larger effect than the Pauli coupling of electron spins to a magnetic field. We present a model that explicitly demonstrates this. The ground state is a singlet with a gap; hence the spin susceptibility is zero while the chiral susceptibility is finite and paramagnetic.

The dielectric functions (ε̃) of ${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{Rh}}_{1\mathrm{-}\mathit{x}}$ alloys with x=0.48, 0.5, and 0.54 were measured by spectroscopic ellipsometry in the energy range 1.5–5.4 eV. The optical properties do not change drastically as the antiferromagnetic (AF)-ferromagnetic (F) phase transition occurs.

Positron-lifetime and Doppler-broadening spectroscopies were used to investigate vacancies formed by 2.2- and 10-MeV electrons. Carbon vacancies yield a positron lifetime of 160 ps, only 15 ps longer than the bulk lifetime, and the Doppler-broadening S parameter is very close to that for the bulk. For silicon vacancies the positron lifetime is 260 ps and the S parameter is 1.12 times the bulk value. In n-type materials the defects are neutral, whereas in p-type materials no vacancies are detected. Introduction rates of carbon and silicon vacancies were also determined.

We present a resonance formalism that allows us to express the transmission probability for symmetrical resonant-tunneling structures exclusively in terms of their transmission poles and reflection zeros (transmission 1s). The predictions of our approach are compared to the numerically calculated transmission probability for a number of structures, and excellent agreement is obtained. Using several examples, we demonstrate that our formalism works equally well for isolated resonant peaks, minibands, and strongly overlapping resonances.

We suggest a theoretical method for the determination of valence-band offsets at alloy-type heterojunctions that is based on average-bond-energy theory in conjunction with a cluster expansion method. The application of this method to ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs produces results in very good agreement with relevant experimental data.

We report on a strong modulation of the resonantly excited (e1:hh1)1S exciton photoluminescence and LO-phonon Raman scattering by an additional, weak, above-gap radiation at low temperatures. These effects are attributed to the interaction between excitons and cold, free carriers (in the e-h pair density range of ${10}^5$<n<${10}^7$ ${\mathrm{cm}}^{\mathrm{-}2}$) that causes an exciton population redistribution and an increased dephasing rate. The analysis indicates that localized and delocalized exciton states span the same spectral range. It also yields the spectral dependence of the additional exciton dephasing that is due to the interaction with free carriers.

We have studied the photoluminescence spectra from GaAs/AlAs quantum wells doped with silicon donors in the AlAs barriers. The GaAs well width in these structures was chosen so that the lowest conduction subband ${\mathit{e}}_1$ lies above the donor states. An impurity-related feature is identified as the donor-to-valence-band transition. A comparison of the energy of this feature with that of the AlAs X-valley-to-valence-band transition yields the donor binding energy.