We investigate the interaction effects for double-layer two-dimensional electron-gas systems in the absence of a magnetic field using both the random phase approximation (RPA) and Singwi-Tosi-Land-Sjölander (STLS) approximations. We show that the RPA overestimates correlation effects and that one needs, at a minimum, to include local-field corrections in order to get reliable estimates for the correlation energy and pair-distribution functions. The electron momentum distribution in double-layer systems is also studied by evaluating the electron self-energy from the screened exchange interaction. We find that RPA and STLS dielectric functions give quantitatively similar results for this quantity. The momentum distribution function depends very weakly on the layer separation of double-layer systems.

Magnetoconductivity of the two-dimensional electron gas occupying two size-quantization subbands is studied theoretically. When the bottoms of subbands are separated by an integer number of Landau levels, the staircases of Landau levels in both subbands are completely aligned. For such values of magnetic field the intersubband scattering is enhanced. As it was pointed out by Polyanovsky, this results in additional Shubnikov–de Haas oscillations of conductivity with magnetic field, with period depending on subband separation, and amplitude depending weakly on temperature, provided that a large number of Landau levels in each subband are occupied. In the calculation of conductivity we make use of the self-consistent Born approximation generalized to the case of two subbands. An analytical theory for the case of strong disorder and numerical results for the case of weak disorder are presented.

Dimensional crossover between one-dimensional (1D) and two-dimensional (2D) excitons has been studied with a two-band tight-binding model on a square lattice by introducing anisotropy to both the transfer energies and the Coulomb interaction between an electron and a hole. It has been found that the binding energy of the lowest exciton (${\mathit{E}}_{\mathit{B}}$) takes a minimum value in the anisotropic regime rather than in the isotropic 2D case. In other words, a weak anisotropy reduces ${\mathit{E}}_{\mathit{B}}$ while a strong anisotropy increases ${\mathit{E}}_{\mathit{B}}$ as shown by comparing the isotropic 2D case with the 1D limit. On the other hand, the energy levels of higher exciton states have been found to cross each other as the degree of anisotropy increases, and a phase diagram is drawn with respect to the character of the lowest few excitons. These behaviors in the anisotropic region do not appear without the anisotropy of either the transfer energies or the Coulomb interaction which is present in real materials. The effect of dimensional crossover on optical properties is also discussed.

The methylamine and ethylamine intercalates of ${\mathrm{FePS}}_3$ were prepared, characterized by x-ray diffraction (XRD) and investigated by ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ Mössbauer spectroscopy. The XRD results show that the intercalate remains monoclinic but exhibits a fairly large expansion of its c axis. The original Mössbauer line of the host ${\mathrm{FePS}}_3$ is absent in all the compounds. The Mössbauer spectra consist of lines originating from modified host lattice and two types of reduced sites created due to transfer of electrons during intercalation. The origin of the observation of two kinds of reduced sites has been suggested. Mössbauer results highlight that beyond a certain concentration of guest species the transferred electrons in the amine intercalates are primarily of delocalized character whereas it is of localized nature in the case of Li intercalated ${\mathrm{FePS}}_3$.

We study in detail the impurity mechanism suggested by Frank for step bunching instabilities on crystal surfaces during crystal growth and evaporation. A two-dimensional model in which the impurities are treated microscopically is proposed. We perform a numerical simulation of the model and show that it leads to step bunching. In this paper we examine the large line tension limit, where the step train remains effectively one dimensional. Using a mean-field theory, we express the velocity of a step in terms of the widths of adjacent terraces and the parameters of the microscopic model. It is shown that the theory is valid over a wide range of physical parameters, and only outside this range does one have to use a more complicated exposure time formalism. We compare the velocity function predicted by the theory with results from Monte Carlo simulations of the two-dimensional model and find remarkable agreement. Our theory predicts a logarithmic growth of the average terrace width with time for noninteracting impurities, in agreement with Monte Carlo simulations. Lastly, we suggest new physical realizations of the impurity mechanism. We illustrate the robustness of the idea by considering generalized impurities, which are created by the kinetic process itself without involving an external impurity source. In some of these cases a power-law coarsening of the terrace widths may arise.

Using the extended Thomas-Fermi version of density-functional theory (DFT), calculations are presented for the barrier for the reaction ${\mathrm{Na}}_{20}^{+}$+${\mathrm{Na}}_{20}^{+}$→${\mathrm{Na}}_{40}^{2+}$. The deviation from the simple Coulomb barrier is shown to be proportional to the electron density at the bond midpoint of the supermolecule (${\mathrm{Na}}_{20}^{+}$${)}_2$. An extension of conventional quantum-chemical studies of homonuclear diatomic molecular ions is then effected to apply to the supermolecular ions of the alkali metals. This then allows the Na results to be utilized to make semiquantitative predictions of position and height of the maximum of the fusion barrier for other alkali clusters. These predictions are confirmed by means of similar DFT calculations for the K clusters.

The resonant nuclear reaction ${}_{}{}^1{}_{}{}^{}\mathrm{H}$${(}^{15}$N,αγ${)}^{12}$C was used to measure hydrogen concentrations in thin Nb films as a function of the external ${\mathrm{H}}_2$ pressure (H solubility). Experiments were performed in the pressure range up to 1 mbar and temperatures between 166 and 200 °C, i.e., in the region around the critical temperature (177 °C) of the α-α’ phase transition. We find that the hydrogen solubility in thin epitaxial films is considerably reduced compared to bulk data. The analysis of the data within mean-field theory for a lattice gas shows that the H-Nb interaction remains, within the experimental error, unchanged with respect to the bulk value but that the attractive H-H interaction energy is strongly reduced (0.1 eV in the film compared to 0.2 eV in the bulk). The difference is attributed to strain effects due to the clamping of the films to the substrate.

Field-emission and photofield-emission measurements of the surface density of electronic states have been carried out for copper chemisorbed on the (100) and (111) facets of a thermally annealed tungsten field emitter, in the range of temperature, and coverage where the adsorbed atoms form spatially homogeneous overlayers. Such overlayers are found to modify the intrinsic surface states of the substrate, and to introduce, on the (100) facet, additional features in the surface density of states. The observation of a final-state effect in surface photofield emission is reported, demonstrating that this technique can provide useful information on the surface density of states of an adsorbed overlayer in the energy range between the Fermi level and the vacuum level. The present work offers insight into earlier conflicting results concerning the effect of adsorbed copper on the intrinsic surface states of tungsten surfaces.

The (2×2)p2mg and c(2×2n) structures induced by oxygen adsorption on Rh(110) have been examined by means of scanning tunneling microscopy. The real-space images show the glide and centered symmetry required by the diffraction patterns, as well as substrate reconstruction. In addition, a metastable (6×2) structure and a number of defects have been identified and characterized, including an antiphase domain wall in the (2×2) structure. Where a step defect separates two c(2×8) domains, they tend to adopt a specific morphological relationship.

${\mathrm{K}}_2$${\mathrm{NiF}}_4$-type solid solution system ${\mathrm{Sr}}_{2\mathrm{-}\mathit{x}}$${\mathrm{La}}_{\mathit{x}}$${\mathrm{RhO}}_4$ was prepared and the structure and electrical and magnetic properties were investigated. The result of powder x-ray-diffraction analysis shows stretching of the a and b axes and shortening of the c axis with an increase of x. The positive temperature dependence of the resistivity in the range x≤0.15 disappears with an increase of x greater than 0.15. The temperature dependence of the activation energy of carriers suggests two-dimensional variable range hopping-type conduction in the semiconducting range. The localized magnetic moment of the Rh ion is greater than the calculated value based on the low spin state, and the antiferromagnetic interaction among Rh ions decreases with an increase of x. A magnetic anomaly around 60 K observed for ${\mathrm{Sr}}_2$${\mathrm{RhO}}_4$ diminishes with an increase of x and vanishes at x=0.20. A positive Seebeck coefficient suggests hole conduction in this system and the Fermi energy obtained from the linear temperature dependence of the Seebeck coefficient on T decreases with x, which can be explained from the decrease of holes with the substitution of low spin ${\mathrm{Rh}}^{3+}$(4${\mathit{d}}^6$) for ${\mathrm{Rh}}^{4+}$(4${\mathit{d}}^5$).

Temperature stability and diffusion properties of ultrathin iron films grown at room temperature on a Cu(001) single-crystal surface have been investigated using low-energy ion scattering. None of the as-grown films is found to be thermally stable. Instead an enrichment of the surface with substrate atoms is observed. Below two monolayers, where the Cu substrate is not completely covered by Fe, the onset of Cu diffusion occurs at 300 K. Iron films wetting the substrate entirely appear to be more resistant against thermal treatment. It is shown explicitly that initially a single monolayer of copper migrates onto the films, leaving the underlying Fe layers mainly intact. This overlayer adopts the same crystal lattice as the as-grown films; in particular a monolayer of Cu atoms is found in bcc lattice sites on top of iron films exhibiting bcc structures.

Nanometer size ${\mathrm{ZrO}}_2$ clusters are generated using the hydrothermal method. The physical properties of nanoclusters of ${\mathrm{ZrO}}_2$ of different sizes have been studied by using the photoacoustic spectrum (PAS). The results show the blueshift of the cutoff wavelength of the PAS absorption edge and the absorption peak of the first-order differential spectrum. When the average grain size of the ${\mathrm{ZrO}}_2$ nanoclusters is less than 5 nm, the blueshift of the absorption peak is 9.8 nm with respect to that corresponding to an average grain size of 90 nm. At the same time, we have observed the fine structures of the absorption edge when decreasing the average size of the nanoclusters of ${\mathrm{ZrO}}_2$. Moreover, we have found that the thermal constant of the nanoclusters increases significantly when their average grain size is decreased. The above phenomena are related to the quantum size effect, the super surface effect, and the interaction of nanoclusters.

Mass-selected ${\mathrm{Pt}}_{\mathit{n}}$ and ${\mathrm{Pd}}_{\mathit{n}}$ (n=1–15) clusters, generated by ion bombardment, are deposited at room temperature in submonolayer quantities on an Ag(110) single-crystal surface and characterized by electron spectroscopy (x-ray-photoemission spectroscopy and ultraviolet photoemission spectroscopy). The monodispersed clusters indicate individual discrete electronic structure features of the Pt 5d and Pd 4d emission. For the monomers virtual bound-state formation as in dilute (3%) PtAg and PdAg alloys is observed. With increasing cluster size, first, the increasing splitting between the bondinglike and antibondinglike d states reflects the increase in cohesive energy due to molecular interaction and second, the shift of the center of gravity of the d emission towards the Fermi energy indicates the trend to metal formation. The Pt 4f and Pd 3d core levels show practically no shift with increasing cluster size indicating a two-dimensional cluster structure. Total-energy calculations, performed within this study using the embedded-atom method, suggest a chain structure for small ${\mathrm{Pt}}_{\mathit{n}}$ (n=1–7) and ${\mathrm{Pd}}_{\mathit{n}}$(n=1–17) clusters on the Ag(110) surface, lying along the [1¯10] direction.

Selective Rydberg-level population of multiply charged ions (e.g., Z=6, 7, and 8) at solid surfaces is treated in normal emergence geometry. For the intermediate ionic velocity region (between v≊1 and 3 a.u.) a molecular-dynamics-type model of the electron pickup process from the solid valence band into low-angular-momentum ionic states (l=0, 1, and 2) is proposed. Specific features of the Rydberg states and ions (large size, high degeneracy with respect to l, high value of Z) are included in the model. The electron transition amplitude is calculated as a mixed electron-density flux through a moving Firsov plane, whose kinematics is determined by a variational requirement. A multichannel character of the process is taken into account in the framework of a statistical treatment of decoupled channels, based on the approximation of small transition probabilities. The population probability ${\mathit{P}}_{\mathit{n}\mathit{l}}$=${\mathit{P}}_{\mathit{n}\mathit{l}}$(v,Z) of the (n,l) state is in sufficiently good agreement with available beam-foil experimental data (S VI, Cl VII, Ar VIII) not only as a function of the principal quantum number n, but also as a function of l and v. An ‘‘anomalous’’ peak at n=11 in the population probability of Ar VIII is briefly discussed from the standpoint of the developed formalism. The predicted maxima in the v dependence of ${\mathit{P}}_{\mathit{n}\mathit{l}}$(v,Z) in the intermediate velocity region calls for further more refined experimental studies.

We discuss the concept of the surface energy of a solid. Our point of view is that the surface enrgy, as classically defined, is but one attribute of a more general distance-dependent interaction between bodies. We argue that the mechanics of adhesion and fracture of solids can be correctly modeled by the incorporation of classical surface energy into a conventional elastic model. As evidence of this, we reconsider a classical model of Obreimoff for the cleaving of mica. Our analysis shows that the introduction of a short-range distance-dependent interaction leads to the system being governed by an effective surface energy which is 2.25 times larger than the surface energy as classically defined.

Two methods for relating different carbon tubules are described. A single graphene ribbon of infinite length but finite width can be coiled to form an infinite set of tubules, each of which has a unique pitch or helicity. This maps in a one-to-one fashion the translation/rotation operations of the group of each tubule in the set. Within the set all irreducible representations are collected into the same number of bands. Alternatively, tubules can be imagined to be pressed flat so that centers of six-member rings lie along the crease. The direction of their creases on a graphene sheet relate tubules having the same helicity but different numbers of identical rotationally symmetric subunits around their circumference. These sets help to reconcile the different expressions for band structure of tubules. These sets also help sort the various ways to join semi-infinite tubules. A perfect infinite tubule is composed entirely of hexagons. Adding one heptagon and one pentagon transforms half of the tubule into a different tubule. The two ways to group tubules suggest that the least distortion of neighboring hexagons occurs if the heptagon and pentagon are joined together or are separated to opposite sides of the tubule. In the latter case, the tubule could be imagined to be flattened so that the heptagon and pentagon are folded in half, one along each crease. This heptagon-pentagon defect best connects sets of tubules in a pairwise fashion. The paired sets of tubules have axis vectors that meet at a 30° angle on a graphene sheet. This analysis and experimental considerations suggest that the ideal bend in a tubule caused by a heptagon-pentagon pair is likely to be 30°. Because entire sets of tubules are joined in similar fashion, tailoring of tubule electronic properties can be imagined.

The surfaces of in situ cleaved NaF crystals have been studied by scanning force microscopy (SFM) in ultrahigh vacuum (UHV). Operating the UHV force microscope in the contact mode, the surface structure with extended terraces interrupted by steps of typically 0.25 nm and 0.5 nm in height is revealed. Atomic-scale resolution is achieved on the terraces as well as at monatomic steps of the ionic crystal. The square surface lattice shows a periodicity of 0.45±0.04 nm that corresponds well to the spacing between equally charged ions along the 〈100〉 directions in the bulk phase. From the SFM data at cleavage steps the imaging area of the sensing tip is estimated to be less than 1 ${\mathrm{nm}}^2$. Some aspects of the contrast mechanism in the lateral and normal force images are discussed. The SFM data are complemented by low-energy-electron-diffraction measurements on NaF(001) carried out in the same compact UHV system.

The use of free-atom-to-metal Auger-parameter shifts to determine atomic-potential parameters for Na, Mg, and Zn is shown to lead to an apparent inconsistency between the effects of perfect screening and the influence of valence-wave-function compression in the metallic state. This conflict is traced to the neglect in standard potential models of the quadratic dependence of the atomic-core potential on valence charge. We propose a generalized model which gives a good description of the atomic-core potential, and show that this can be used to derive empirical solid-state potential parameters from metal-atom Auger-parameter shifts.

Classical trajectory calculations were employed to study diamond (100) surface reconstructions. Atomic forces were computed from two types of potential-energy functions. A quantum mechanical potential based on the semiempirical PM3 (parametric method number three) Hamiltonian was used to describe the central core of the surface model, while an empirically parametrized potential was developed to extend the size of the model surface. The results indicate that the most energetically favorable surface consists of alternating monohydride dimers and dihydride sites. The global topology of reconstructed (100) surfaces can include dimer rows and zigzag dimer sites. The two configurations are close in energy, for both dehydrogenated and monohydride surfaces, with the row configuration being slightly more favorable. The minimum-energy dehydrogenated (100) diamond surface was found to consist of dimers with biradical electronic structures. The presence of atomic-level steps was found to prevent the formation of nearby dimer bonds, even when each of the available carbon atoms has a free valence.

We propose a very simple model of lattice dynamics of graphite and its intercalation compounds (GIC’s): our model only uses five force constants for graphite, and seven for the GIC. It is used to calculate the dynamical matrix of the structure, and to obtain by matrix diagonalization the phonon vibration frequencies for each element of the first Brillouin zone. Making use of available experimental results concerning graphite as well as ${\mathrm{LiC}}_6$, we may approximate the parameters introduced in the model. Finally, with the results so obtained, we calculate a great number of physical properties of these compounds: specific heat, Debye temperature ${\mathrm{\theta }}_{\mathit{D}}$, bidimensional or tridimensional character, as well as elastic constants.