Showing papers in "Physical Review B in 1987"

Electronic and atomic structure of amorphous carbon.

Abstract: The electronic structure of amorphous carbon and hydrogenated amorphous carbon (a-C:H) has been investigated through calculations on a number of model structures containing different configurations of ${\mathrm{sp}}^{2}$ and ${\mathrm{sp}}^{3}$ sites. We find that the most stable arrangement of ${\mathrm{sp}}^{2}$ sites is in compact clusters of fused sixfold rings, i.e., graphitic layers. The width of the optical gap is found to vary inversely with the ${\mathrm{sp}}^{2}$ cluster size, and the \ensuremath{\sim}0.5-eV optical gap of evaporated amorphous carbon is found to be consistent with a model of disordered graphitic layers of about 15 A\r{} in diameter, bounded by ${\mathrm{sp}}^{3}$ sites. It is argued that a-C forms such finite clusters in order to relieve strain. It is then shown that the 1.5--2.5-eV optical gap of a-C:H is unusually small and requires that both its valence and conduction band consist of \ensuremath{\pi} states on ${\mathrm{sp}}^{2}$ sites and that these sites must also be significantly clustered, such as in graphitic clusters containing four or more rings. In other words, the optical gap of both a-C and a-C:H depends on their degree of medium-range order, rather than just on their short-range order as is the case in most amorphous semiconductors. We have also studied the nature of states away from the gap in order to interpret the photoemission data and the carbon 1s core-level absorption spectra. The nature of defects and midgap states is discussed, and it is predicted that the defect density decreases with increasing band gap. Finally it is argued that the doping of a-C:H by group-III and -V elements proceeds via a substitution mechanism, as in a-Si:H, in spite of the coordination disorder present in a-C:H. Doping is also expected to be accompanied by an increase in gap states, as in a-Si:H.

Oxygen ordering and the orthorhombic-to-tetragonal phase transition in YBa2Cu

Abstract: In situ neutron powder diffraction measurements show that the orthorhombic-to-tetragonal phase transition in $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$, which occurs near 700\ifmmode^\circ\else\textdegree\fi{}C in a pure oxygen atmosphere, is an order-disorder transition in which the disordering of oxygen atoms into a normally vacant site destroys the one-dimensional Cu-O chains present in the room-temperature orthorhombic structure. For both structures, the oxygen stoichiometry decreases monotonically with increasing temperature. The transition temperature depends on the oxygen partial pressure and occurs when the stoichiometry is near Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.5}$. The tetragonal structure has a partially occupied, nearly octahedral Cu-O arrangement, in contrast to the orthorhombic structure which has one-dimensional Cu-O chains. The observed depression of the superconducting transition temperature in tetragonal $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$, which has been quenched from high temperature, could result either from the disordering of oxygen atoms which destroys the one-dimensional chains or from the absence of ${\mathrm{Cu}}^{3+}$ ions.

Random-field model of exchange anisotropy at rough ferromagnetic-antiferromagnetic interfaces.

Abstract: A field-asymmetric offset of the hysteresis loop in ferromagnetic-antiferromagnetic sandwiches, one of the manifestations of so-called exchange anisotropy, can be predicted from the presence of random interface roughness giving rise to a random field acting on the interface spins. The antiferromagnet breaks up into domains of size determined by the competition of exchange and an additional uniaxial in-plane anisotropy, and this size sets the scale for averaging of the random field.

Theory of the linear and nonlinear optical properties of semiconductor microcrystallites

Abstract: We analyze theoretically the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)]. In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and we consider the phonon broadening of these lines. The lowest interband transition will saturate like a two-level system, without exchange and Coulomb screening. Depending on the broadening, the absorption and the changes in absorption and refractive index resulting from saturation can become very large, and the local-field effects can become so strong as to give optical bistability without external feedback. The small QD limit is more readily achieved with narrow-band-gap semiconductors.

Thermal conductivity of amorphous solids above the plateau.

Abstract: We have measured the thermal conductivity of polymethylmethacrylate (PMMA), amorphous ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$, Ca-K nitrate glass, three ${\mathrm{SiO}}_{2}$-based glasses, and glycerol in the temperature range 30--300 K, using an ac technique that eliminates errors from blackbody radiation. This technique and the problems of blackbody radiation in thermal measurements are discussed in detail. Our data do not support a recent prediction of heat transport by fractons. Instead, we find that the thermal conductivity of these glasses above \ensuremath{\sim}50 K is well described by the minimum thermal conductivity suggested by Slack.

Single-phase 60-K bulk superconductor in annealed Ba2YCu

Abstract: Employing a gettered annealing technique, we have prepared homogeneous polycrystalline samples of oxygen deficient Ba/sub 2/YCu/sub 3/O/sub 7-//sub delta/ for 0less than or equal todeltaless than or equal to0.7. Measurements of resistive T/sub c/, resistivity, magnetization, and lattice parameter indicate that a distinct bulk superconducting phase with a T/sub c/ of 60 K occurs in the range of 0.3

Interatomic interactions in the effective-medium theory

Abstract: An expression is derived for the total energy of a system of interacting atoms based on an ansatz for the total electron density of the system as a superposition of atom densities taken from calculations for the atoms embedded in a homogeneous electron gas. This leads to an expression for the interaction energy in terms of the embedding energy of the atoms in a homogeneous electron gas, and corrections accounting, for instance, for the d-d hybridization in the transition metals. The density of the homogeneous electron gas is chosen as the average of the density from the surrounding atoms. Due to the variational property of the total-energy functional, the errors in the interaction energy are second order in the deviation of the ansatz density from the true ground-state value. The applicability of the approach is illustrated by calculations of the cohesive properties of some simple metals and all the 3d transition metals. The interaction energy can be expressed in a form simple enough to allow calculations for low-symmetry systems and is very well suited for simulations of time-dependent and finite-temperature problems. Preliminary results for the phonon-dispersion relations and the surface energies and relaxations for Al are used to illustrate the versatility of the approach. The division of the total energy into a density-dependent part, an electrostatic ``pair-potential'' part, and a hybridization part provides a very simple way of understanding a number of these phenomena.

Interband critical points of GaAs and their temperature dependence

Abstract: The complex dielectric function \ensuremath{\epsilon}(\ensuremath{\omega}) of GaAs was measured from 20 to 750 K with a scanning rotating-analyzer ellipsometer. The structures observed in the 1.3\char21{}5.5-eV photon-energy region, attributed to transitions near the \ensuremath{\Gamma} point of the Brillouin zone (${E}_{0}$, ${E}_{0}$+${\ensuremath{\Delta}}_{0}$, ${E}_{0}^{\mathcal{'}}$), along the \ensuremath{\Lambda} direction (${E}_{1}$, ${E}_{1}$+${\ensuremath{\Delta}}_{1}$), and near the X point (${E}_{2}$), are analyzed by fitting the second-derivative spectrum ${d}^{2}$\ensuremath{\epsilon}(\ensuremath{\omega})/d${\ensuremath{\omega}}^{2}$ to analytic critical-point line shapes. The ${E}_{0}^{\mathcal{'}}$ and ${E}_{2}$ critical points are best fitted in the whole temperature region by a two-dimensional line shape, whereas the ${E}_{1}$ and ${E}_{1}$+${\ensuremath{\Delta}}_{1}$ transitions are best fitted up to room temperature by a Lorentzian interacting with a continuum of interband transitions (Fano line shape). The excitonic character of the ${E}_{1}$ and ${E}_{1}$+${\ensuremath{\Delta}}_{1}$ transitions is discussed within several theoretical approaches. The experiments indicate that up to room temperature the localized Lorentzian interacting with the continuum is dominant, whereas at higher temperatures the modification of the two-dimensional Van Hove singularity due to the electron-hole attractive perturbation is a better description of the measurements. For all critical points, the energy decreases with increasing temperature while the broadening increases. This dependence on temperature is analyzed in terms of averaged phonon frequencies which cause a renormalization of the energies and a broadening of the band gaps.

Universal conductance fluctuations in metals: Effects of finite temperature, interactions, and magnetic field.

Abstract: The conductance of any metallic sample has been shown to fluctuate as a function of chemical potential, magnetic field, or impurity configuration by an amount of order ${e}^{2}$/h independent of sample size and degree of disorder at zero temperature. We discuss the relationship of these results to other results in the theory of weak and strong localization, and discuss its physical implications. We discuss the physical assumptions underlying the ergodic hypothesis used to relate theory to experiment. We review the zero-temperature theory and provide a detailed discussion of the conductance correlation functions in magnetic field and Fermi energy. We show that the zero-temperature amplitude of the fluctuations is unaffected by electron-electron interactions to lowest order in (${k}_{f}$l${)}^{\mathrm{\ensuremath{-}}1}$, and at finite temperature interactions only enter insofar as they contribute to the inelastic scattering rate. We calculate the effects of finite temperature on both the amplitude of the fluctuations and their scale. We discuss the conditions for dimensional crossover at finite temperature, and the behavior of different experimental measures of the fluctuation amplitude, in order to facilitate quantitative comparisons of experiment and theory.

Electron mean-free-path calculations using a model dielectric function

Abstract: The inelastic electron mean free path as a function of energy is calculated for Cu, Ag, Au, and Al. The calculations are based on a model dielectric function \ensuremath{\epsilon}(q,\ensuremath{\omega}), which is obtained from a modification of the statistical approximation. In this approach \ensuremath{\epsilon}(0,\ensuremath{\omega}) is determined by the experimentally measured optical dielectric function. Calculated mean free paths are compared to experimental data and to other theories.

Temperature dependence of the dielectric function and interband critical points in silicon

Abstract: The complex dielectric function \ensuremath{\epsilon}(\ensuremath{\omega}) of Si was measured ellipsometrically in the 1.7--5.7-eV photon-energy range at temperatures between 30 and 820 K. The observed structures are analyzed by fitting the second-derivative spectrum ${d}^{2}$\ensuremath{\epsilon}/d${\ensuremath{\omega}}^{2}$ with analytic critical-point line shapes. Results for the temperature dependence of the parameters of these critical points, labeled ${E}_{0}^{\mathcal{'}}$, ${E}_{1}$, ${E}_{2}$, and ${E}_{1}^{\mathcal{'}}$, are presented. The data show good agreement with microscopic calculations for the energy shift and the broadening of interband transitions with temperature based on the electron-phonon interaction. The character of the ${E}_{1}$ transitions in semiconductors is analyzed. We find that for Si and light III-V or II-VI compounds an excitonic line shape represents best the experimental data, whereas for Ge, \ensuremath{\alpha}-Sn, and heavy III-V or II-VI compounds a two-dimensional critical point yields the best representation.

Oxygen and rare-earth doping of the 90-K superconducting perovskite YBa2Cu3O7-x.

Abstract: Structural, magnetic and electronic properties of compounds in the series R${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{x}}$ (R=Nd,Sm,..., and Lu) were studied. Resistivity, Meissner-effect, and shielding measurements have revealed superconductivity among all the rare-earth compounds, except La, Pr, and Tb, with critical temperatures ${T}_{c}$ measured at the midpoint of the resistive transition ranging from 87 to 95 K. No depression of ${T}_{c}$ was observed upon introduction of most of the rare-earth magnetic ions. Susceptibility measurements down to 1.6 K have shown that an antiferromagnetic ordering (most likely due to dipole-dipole interactions) occurs only for the Gd compound. Changes in oxygen content in these materials drastically affect their physical properties. The importance of the cooling rate during the synthesis of the sample has been correlated to oxygen content. ${T}_{c}$'s are optimized by slow cooling. Annealing at 700\ifmmode^\circ\else\textdegree\fi{}C in oxygen pressure of 40 atmospheres slightly increase ${T}_{c}$, while annealing under vacuum at 420\ifmmode^\circ\else\textdegree\fi{}C destroys ${T}_{c}$ and induces a semiconducting behavior. These changes in oxygen content and ${T}_{c}$ are perfectly reversible.

Critical wave functions and a Cantor-set spectrum of a one-dimensional quasicrystal model

Abstract: The electronic properties of a tight-binding model which possesses two types of hopping matrix element (or on-site energy) arranged in a Fibonacci sequence are studied. The wave functions are either self-similar (fractal) or chaotic and show ``critical'' (or ``exotic'') behavior. Scaling analysis for the self-similar wave functions at the center of the band and also at the edge of the band is performed. The energy spectrum is a Cantor set with zero Lebesque measure. The density of states is singularly concentrated with an index ${\ensuremath{\alpha}}_{E}$ which takes a value in the range [${\ensuremath{\alpha}}_{E}^{\mathrm{min}}$,${\ensuremath{\alpha}}_{E}^{\mathrm{max}}$]. The fractal dimensions f(${\ensuremath{\alpha}}_{E}$) of these singularities in the Cantor set are calculated. This function f(${\ensuremath{\alpha}}_{E}$) represents the global scaling properties of the Cantor-set spectrum.

Phenomenological theory of optical second- and third-harmonic generation from cubic centrosymmetric crystals

Abstract: We present a macroscopic theory for anisotropic second- and third-harmonic generation obtained in reflection from the surface and bulk of cubic centrosymmetric single crystals. The theory is based on possible electric dipole, electric quadrupole, and magnetic dipole sources. Completely general expressions for the harmonic fields are obtained for (100), (111), and (110) faces independent of the details of the surface response but consistent with crystal symmetry. The results obtained agree with all existing experimental data obtained by various groups during the past few years. The possibility of separating out surface and bulk responses is considered using symmetry, polarization, or geometry arguments and it is concluded that for second-harmonic generation this cannot be done in general without additional information. Third-harmonic generation, barring any strong resonantly enhanced surface electric dipole effects, is essentially a bulk probe.

Theoretical study of band offsets at semiconductor interfaces

Abstract: We present a first-principles approach to deriving the relative energies of valence and conduction bands at semiconductor interfaces, along with a model which permits a simple interpretation of these band offsets. Self-consistent density-functional calculations, using ab initio nonlocal pseudo-potentials, allow us to derive the minimum-energy structure and band offsets for specific interfaces. Here we report results for a large number of lattice-matched interfaces, which are in reasonable agreement with reported experimental values. In addition, our systematic analysis leads to the important conclusions that, for the cases considered, the offsets are independent of interface orientation and obey the transitivity rule, to within the accuracy of our calculations. These are necessary conditions for the offsets to be expressible as differences between quantities which are intrinsic to each of the materials. Based on the information obtained from the full interface calculations, we have developed a new and simple approach to derive such intrinsic band offsets. We define a reference energy for each material as the average (pseudo)potential in a “model solid,” in which the charge density is constructed as a superposition of neutral (pseudo)atomic densities. This reference depends on the density of each type of atom and the detailed form of the atomic charge density, which must be chosen consistently for the different materials. The bulk band structures of the two semiconductors are then aligned according to these average potential positions. For many cases, these model lineups yield results close to those obtained from full self-consistent interface calculations. We discuss the comparison with experiments and with other model theories.

Topology of the resonating valence-bond state: Solitons and high-T c superconductivity

Abstract: We study the topological order in the resonating valence-bond state. The elementary excitations have reversed charge-statistics relations: There are neutral spin-1/2 fermions and charge \ifmmode\pm\else\textpm\fi{}e spinless bosons, analogous to the solitons in polyacetylene. The charged excitations are very light, and form a degenerate Bose gas even at high temperatures. We discuss this model in the context of the recently discovered oxide superconductors.

Electronic excitations in finite and infinite polyenes.

Abstract: We study electronic excitations in long polyenes, i.e., in one-dimensional strongly correlated electron systems which are neither infinite nor small. The excitations are described within Hubbard and Pariser-Parr-Pople (PPP) models by means of a multiple-reference double-excitation expansion [P. Tavan and K. Schulten, J. Chem. Phys. 85, 6602 (1986)]. We find that quantized ``transition'' momenta can be assigned to electronic excitations in finite chains. These momenta link excitation energies of finite chains to dispersion relations of infinite chains, i.e., they bridge the gap between finite and infinite systems. A key result is the following: Excitation energies E in polyenes with N carbon atoms are described very accurately by the formula ${E}^{\ensuremath{\beta}}$=\ensuremath{\Delta}${E}_{0}^{\ensuremath{\beta}}$+${\ensuremath{\alpha}}^{\ensuremath{\beta}}$k(N)q, q=1,2,..., where \ensuremath{\beta} denotes the excitation class, \ensuremath{\Delta}${E}_{0}^{\ensuremath{\beta}}$ the energy gap in the infinite system [${\ensuremath{\alpha}}^{\ensuremath{\beta}}$k(N)g0], and k(N) the elementary transition momentum. The parameters \ensuremath{\Delta}${E}_{0}^{\ensuremath{\beta}}$ and ${\ensuremath{\alpha}}^{\ensuremath{\beta}}$ are determined for covalent and ionic excitations in alternating and nonalternating polyenes. The covalent excitations are combinations of triplet excitations T, i.e., T, TT, TTT, . . . . The lowest singlet excitations in the infinite polyene, e.g., in polyacetylene or polydiacetylene, are TT states. Available evidence proves that these states can dissociate into separate triplets. The bond structure of TT states is that of a neutral soliton-antisoliton pair. The level density of TT states in long polyenes is high enough to allow dissociation into separate solitons.

Temperature effects on the universal equation of state of solids

Abstract: Recently it has been argued based on theoretical calculations and experimental data that there is a universal form for the equation of state of solids. This observation was restricted to the range of temperatures and pressures such that there are no phase transitions. The use of this universal relation to estimate pressure-volume relations (i.e., isotherms) required three input parameters at each fixed temperature. It is shown that for many solids the input data needed to predict high temperature thermodynamical properties can be dramatically reduced. In particular, only four numbers are needed: (1) the zero pressure (P=0) isothermal bulk modulus; (2)it P=0 pressure derivative; (3) the P=0 volume; and (4) the P=0 thermal expansion; all evaluated at a single (reference) temperature. Explicit predictions are made for the high temperature isotherms, the thermal expansion as a function of temperature, and the temperature variation of the isothermal bulk modulus and its pressure derivative. These predictions are tested using experimental data for three representative solids: gold, sodium chloride, and xenon. Good agreement between theory and experiment is found.

Electronic structure of MoSe2, MoS2, and WSe2. I. Band-structure calculations and photoelectron spectroscopy

Abstract: The band structures of the semiconducting layered compounds ${\mathrm{MoSe}}_{2}$, ${\mathrm{MoS}}_{2}$, and ${\mathrm{WSe}}_{2}$ have been calculated self-consistently with the augmented-spherical-wave method. Angle-resolved photoelec- tron spectroscopy of ${\mathrm{MoSe}}_{2}$ using He i, He ii, and Ne i radiation, and photon-energy-dependent normal-emission photoelectron spectroscopy using synchrotron radiation, show that the calculational results give a good description of the valence-band structure. At about 1 eV below the top of the valence band a dispersionless state was measured, almost completely of Mo 4d character. Such a state, which is not predicted by band-structure calculations, has also been observed in metallic layered compounds. Suggestions are given for the explanation of this phenomenon.

Experimental tests for the quantum behavior of a macroscopic degree of freedom: The phase difference across a Josephson junction

Abstract: Experiments are described that demonstrate the quantum behavior of a macroscopic degree of freedom, namely the phase difference \ensuremath{\delta} across a current-biased Josephson tunnel junction. The behavior of \ensuremath{\delta} was deduced from measurements of the escape rate \ensuremath{\Gamma} of the junction from its zero-voltage state. The relevant parameters of the junction, that is, its critical current and shunting admittance, were determined in situ in the thermal regime from the dependence of \ensuremath{\Gamma} on bias current and from resonant activation in the presence of microwaves. It was found that the shunting capacitance was dominated by the self-capacitance of the junction while the shunting conductance was dominated by the bias circuitry. For an underdamped junction in the quantum regime, \ensuremath{\Gamma} became independent of temperature at low temperatures with a value that, with no adjustable parameters, was in excellent agreement with predictions for macroscopic quantum tunneling at T=0. When the critical current was reduced with a magnetic field so that the junction remained in the thermal regime at low temperatures, \ensuremath{\Gamma} followed the predictions of the thermal model, thereby showing the influence of extraneous noise to be negligible. In a further series of experiments, the existence of quantized energy levels in the potential well of the junction was demonstrated spectroscopically. The positions of the energy levels agreed quantitatively with quantum-mechanical predictions involving junction parameters measured in the thermal regime. The relative heights and widths of the resonances are in reasonable agreement with the predictions of a simple model.

Path-integral computation of superfluid densities

Abstract: The normal and superfluid densities are defined by the response of a liquid to sample boundary motion. The free-energy change due to uniform boundary motion can be calculated by path-integral methods from the distribution of the winding number of the paths around a periodic cell. This provides a conceptually and computationally simple way of calculating the superfluid density for any Bose system. The linear-response formulation relates the superfluid density to the momentum-density correlation function, which has a short-ranged part related to the normal density and, in the case of a superfluid, a long-ranged part whose strength is proportional to the superfluid density. These facts are discussed in the context of path-integral computations and demonstrated for liquid $^{4}\mathrm{He}$ along the saturated vapor-pressure curve. Below the experimental superfluid transition temperature the computed superfluid fractions agree with the experimental values to within the statistical uncertainties of a few percent in the computations. The computed transition is broadened by finite-sample-size effects.

Surface vibrational spectroscopy by infrared-visible sum frequency generation

Abstract: We report the first observation of a vibrational spectrum of a monolayer of molecular adsorbates by infrared-visible sum frequency generation. The results pave the way for future dynamic studies of surface phonons and molecular vibrations.

p -spin-interaction spin-glass models: Connections with the structural glass problem

Abstract: The static and the dynamical theories for the mean-field p-spin (pg2) interaction spin-glass model are studied. A broken-replica-symmetric equilibrium solution leads to a glass transition at a temperature ${T}_{g}^{\mathcal{'}}$ where the Edwards-Anderson order parameter is discontinuous but where there is no latent heat and there is a discontinuous specific heat. The dynamical theory leads to a continuous slowing down and predicts a glass transition at ${T}_{g}$g${T}_{g}^{\mathcal{'}}$. A reinterpretation of the equilibrium solution allows us to relate the dynamical transition to the equilibrium theory. The mathematical structure of the mean-field dynamical theory is closely related to certain recent dynamical theories of the structural glass transition.

Critical theory of quantum spin chains.

Abstract: The relation between quantum spin chains and conformal field theories is reexamined. Using a generalized Hubbard model representation it is argued that the critical theory for generic half-odd-integer spin antiferromagnets is the Wess-Zumino-Witten model (WZW model) with topological coupling, k=1, whereas generic integer spin antiferromagnets have an energy gap. The higher-k WZW models (which describe integrable higher spin models) are multicritical points in the space of all spin Hamiltonians. The k=1 WZW model represents a stable fixed point for many theories including WZW models of arbitrary odd k with relevant operators added, generalized Hubbard or Thirring models with an odd number of colors and the O(3) \ensuremath{\sigma} model at topological angle \ensuremath{\theta}=\ensuremath{\pi}.

Determination of molecular orientations on surfaces from the angular dependence of near-edge x-ray-absorption fine-structure spectra.

Abstract: A model is presented which allows the determination of molecular orientations on surfaces from analysis of the angle-dependent resonance intensities in K-shell near-edge x-ray-absorption fine-structure (NEXAFS) spectra. In particular, we discuss the origin and angular dependence of so-called \ensuremath{\sigma} and \ensuremath{\pi} resonances which, in a molecular-orbital picture, correspond to transitions from 1s initial to empty ${\ensuremath{\pi}}^{\mathrm{*}}$ and ${\ensuremath{\sigma}}^{\mathrm{*}}$ molecular-orbital final states. All molecules are classified into two general groups, depending on whether the final-state molecular orbital points into a specific direction (``vector'' case) or whether energetically degenerate orbitals span a plane (``plane'' case). General equations are derived for these cases which describe the dependence of the resonance intensities on the \ensuremath{\pi} and \ensuremath{\sigma} symmetry of the final state, the substrate symmetry, the molecular orientation on the surface, and the incidence angle of the elliptically polarized synchrotron radiation with respect to the surface. Model calculations are presented to elucidate the sensitivity of the resonance intensities to the molecular orientation on the surface and the degree of linear x-ray polarization. The capability of NEXAFS to accurately determine molecular orientations on surfaces is illustrated by two examples. Molecular ${\mathrm{O}}_{2}$ on Ag(110) is shown to lie down on the surface with the O-O axis along the [11\ifmmode\bar\else\textasciimacron\fi{}0] azimuth, and the plane of the aromatic ring in benzenethiol (${\mathrm{C}}_{6}$${\mathrm{H}}_{5}$SH) on Mo(110) is tilted by 23\ifmmode^\circ\else\textdegree\fi{} from the surface normal.

Transition from the tunneling regime to point contact studied using scanning tunneling microscopy

Abstract: Using scanning tunneling microscopy, we have studied local surface modifications induced by point contact of a tunnel tip with metallic surfaces. Two distinct types of topographical modification are found which correlate directly with the chemical condition of the tip as determined by tunneling spectroscopy. Observation of the dynamics of the transition from the tunneling regime to contact permits an evaluation of the gap distance prior to point contact. At gap spacings of s\ensuremath{\lesssim}3 A\r{} a significant decrease in the apparent tunnel barrier height is observed just before touching. Contact was found to initiate with the intimate contact of several atoms only. The implications of these results for lithography on a nanometer scale, and on the investigation of the mechanical properties of surfaces over ranges of a few nanometers are discussed.

Effect of grain boundaries on the Raman spectra, optical absorption, and elastic light scattering in nanometer-sized crystalline silicon

Abstract: The intensity of the Raman-active ${\ensuremath{\Gamma}}_{25\mathcal{'}}$ mode of nanometer-sized crystalline silicon, nc-Si, normalized to that of calcium fluoride, ${\mathrm{CaF}}_{2}$, at 322 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ was measured for samples deposited under controllably varied conditions. Changes of the intensity by a factor of up to approximately 6.7 were found. These are correlated with the lattice expansion and with the compressive stress in thin films of the material. It is suggested that the enhancement of the scattering cross section, which scales with the observed optical-absorption coefficient and diffuse elastic light scattering, is due to enhanced coupling of the electromagnetic field of the incident light to the charge-density fluctuations at the grain boundaries of the quasi-isolated crystallites.

Self-consistent large-N expansion for normal-state properties of dilute magnetic alloys.

Abstract: Finite-temperature properties of the infinite-$U$ Anderson model for rare-earth alloys are calculated within a unified approach. The impurity-electron density of states and magnetic moment spectrum provide a natural framework for describing both static and dynamic properties. The density of states and moment spectrum exhibit low-energy "Kondo resonances" with approximate single-parameter scaling, which persists for impurity valences in the range 1.0-0.7. The position of the resonance in the zero-temperature density of states, ${T}_{0}$, sets the scale for all low-temperature properties. Results are reported for the impurity valence, resistivity, thermopower, thermal conductivity, magnetic susceptibility, specific heat, photoemission and inverse-photoemission spectra, and neutron scattering linewidth. The effect of spin-orbit interactions is incorporated in the theory. The calculation is a diagrammatic approximation motivated by the simplifying concept of large angular momentum degeneracy ($N$). The approximate solution is thermodynamically consistent and satisfies all pertinent sum rules. Static properties (magnetic susceptibility and specific heat) are in good agreement with exact results of the Bethe ansatz. Experimental results on both dilute and concentrated Ce alloys are described quantitatively with use of a one-parameter (${T}_{0}$) theory.