Near-ir absorption spectra are obtained for neutral InP clusters in the range 1.0–1.6 eV using two-color resonant two-photon dissociation spectroscopy. All stoichiometric and many off-stoichiometric InP clusters exhibit a distinct continuum absorption with an onset of 1.3–1.5 eV, close to the band gap of crystalline InP. Additional weaker absorptions are also seen at lower energies. An explanation is given for why these spectra are so similar to those of III-V semiconductor solids.

High-resolution core-level x-ray photoemission and photoabsorption studies of potassium adsorption on the Si(111)(√3 × √3 )R(30°)-B surface show a large core-level shift (∼1.2 eV) for both B(1s) and Si(2p) states, in contrast to results from alkali adsorption on other Si surfaces where little or no shift is observed for Si(2p) states. We interpret these large shifts in terms of a large charge transfer from potassium to the Si-substrate dangling-bond state, possible because of the B-induced removal of filled clean-surface band-gap states. These new results suggest that, in general, charge transfer in the alkali-semiconductor surface bonding is determined by the occupation of band-gap states at the clean surface.

Electronic excitation spectra (0≤ħω<4 eV) of an ultrathin Cs/GaAs(110) interface have been measured at various stages of development with electron-energy-loss spectroscopy. Spectral features which appear as a function of Cs coverage clearly identify the semiconductor-to-metal transition. The semiconducting interface observed up to one Cs layer is characterized as a highly correlated electronic system—a two-dimensional Mott insulator. Interfacial metallization upon Cs multilayer growth is identified by a metallic-excitation continuum and a collective excitation related to the Cs surface plasmon.

The long hot-electron mean free path observed by Sivan, Heiblum, and Umbach in high-mobility 2D electron-gas heterostructures is explained by electron transport in the second subband. Our estimates show that the mean free path in the second subband is longer by 2 orders of magnitude than that in the first subband. New magnetic-focusing measurements reveal hot-electron velocities lower than the Fermi velocity, as expected for a higher-subband transport. The electrostatic potential near a biased constriction used as the hot-electron injector is shown to induce nonadiabatic intersubband transfer.

We define the relative index, Index(P,Q), for a pair of infinite-dimensional projections on a Hilbert space to be the integer that is the natural generalization of dim(P)-dim(Q) in finite-dimensional vector spaces. We show that the Hall conductance for independent electrons in the plane is the relative index where P and Q project on the states below the Fermi energy for Hamiltonians that differ by a quantum flux and the Fermi energy is appropriately placed. This approach is closely related to, and sheds light on, Bellissard’s interpretation of the Hall conductance as an index.

We report observations of an electric-field threshold conduction and of related ac voltage (broad-band noise) generation in low-disorder two-dimensioanl electron systems in the extreme magnetic quantum limit. We interpret these phenomena as definitive evidence for formation of a pinned quantum Wigner crystal and determine its melting phase diagram from the disappearance of threshold and noise behavior at higher temperatures.

We show that the profiles of the Ni K${\mathrm{\beta }}_{1,3}$,β’ x-ray fluorescence spectra of ${\mathrm{NiF}}_2$ and NiO, which were long believed to originate from the multiplet splittings due to an exchange interaction between the x-ray hole and valence unpaired electrons, are clearly interpreted by a charge-transfer effect during the photoionization of one of the 1s electrons. The profiles of the Kβ x-ray fluorescence spectra of ${\mathrm{NiCl}}_2$, ${\mathrm{NiBr}}_2$, and ${\mathrm{NiI}}_2$ are also predicted from the charge-transfer effect. The range of validity of the present theory and its relation to the shakeoff satellites are also discussed.

We have measured the magnetic relaxation of several Au:Fe alloys from ${10}^{\mathrm{-}5}$ to ${10}^4$ sec after removing an applied field. The quality and range of the data are sufficient to demonstrate significant deviations from all functions previously used to characterize the dynamics of similar systems. A simple model, activated relaxation of dispersive excitations on a percolation distribution of finite-sized domains, gives excellent agreement with the measurements and may provide a common link between fundamental excitations and observed behavior in random systems.

We have observed anomalously enhanced self- (tracer) diffusion in systems of polymerlike breakable micelles. We argue that it provides the first experimental realization of a random walk for which the second moment of the jump-size distribution fails to exist (‘‘Levy flight’’). The basic mechanism is the following: Due to reptation, short micelles diffuse much more rapidly than long ones. As times goes on, shorter and shorter micelles are encountered by the tracer, and hence the effective diffusion constant increases with time.

Gravity was measured at eight different heights on a 300-m meterological tower using LaCoste and Romberg gravimeters. The observed values were adjusted for tides, drift, and gravimeter screw errors, and tested for systematic effects due to tower motion. These results are compared with values predicted using Newton’s inverse-square law from surface gravity. The differences exhibit no systematic trends and their rms value is only 10×${10}^{\mathrm{-}8}$ m ${\mathrm{s}}^{\mathrm{-}2}$, well within the estimated errors of the experiment. This result places new constraints on the possible strength and range of any non-Newtonian force.

Classically, the unimodular theory of gravity with a constrained determinant ${\mathit{g}}_{\mathrm{\mu }\nu }$ is equivalent to general relativity augmented by an arbitrary cosmological constant Λ, which arises as an integration constant. At the quantum level we argue that an integration over Λ should be included in the Euclidean path integral for the vacuum functional. The fully renormalized Λ=0 overwhelmingly dominates all other contributions yielding a zero observed cosmological constant. While the technical part of our argument is similar to that of the ‘‘wormhole’’ argument, the two approaches are logically different.

A possible particle-physics solution is proposed to the recently reported discrepancy between the Homestake and Kamiokande II experiments concerning the ‘‘time variation’’ of the solar-neutrino flux. It is pointed out that some of the models giving rise to a large magnetic moment of electron neutrinos include the possibility that the right-handed neutrino interacts with electrons strongly enough so that Kamiokande does not see the time variation, while the Homestake experiment shows the time variation in anticorrelation with solar activity due to the neutrino helicity flip in the sun.

We present a series of new one-loop nonsupersymmetric four-dimensional heterotic-string partition functions that we conjecture to lead to vanishing one-loop cosmological constants. We demonstrate that our partition functions satisfy various requirements imposed by the physical consistency of possible underlying string models, and we discuss the results of an automated search for such models. Although we find no such models directly, we surprisingly discover many sets of models whose partition functions algebraically differ precisely by our expressions. These models thus represent new and unexpectedly degenerate points in the space of string-field-theory vacua.

We have found a one-complex-parameter family of Dirac actions which interpolates between the Minkowski and a Euclidean Dirac action. The interpolating action is invariant under the ‘‘interpolating Lorentz transformations.’’ The resultant Euclidean action is Hermitian and SO(4) invariant. There is no doubling of degrees of freedom of the Dirac fermion and no contradiction between the SO(4) invariance and the Hermiticity property of the Euclidean propagator. The Euclidean theory so obtained also satisfies the Osterwalder-Schrader positivity condition.

Measurements of the vector analyzing power ${\mathit{iT}}_{11}$ in ${\mathrm{\pi }}^{+}$d elastic scattering at 50 MeV have been made at pion laboratory scattering angles between 60° and 130° using a dynamically polarized target and a magnetic spectrometer. In contrast to measurements of spin observables at higher energies, we find agreement with models that include the πN ${\mathit{P}}_{11}$ amplitude and disagreement with models that exclude or suppress it.

Quasielastic (e,e’p) cross sections have been calculated with a full partial-wave expansion of both Coulomb-distorted electron waves and distorted proton waves in the framework of the relativistic σ-ω model. Results are compared with those from currently used nonrelativistic approximations and with data. Spectroscopic factors are derived for the valence-proton knockout of ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ and ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ and are compared with predictions from recent theoretical approaches.

We have studied indirect excitation processes contributing to the x-ray line emission of neonlike barium (${\mathrm{Ba}}^{46+}$) interacting with an electron beam. Techniques are presented which resolve the contributions due to electron capture by fluorinelike ions and inner-shell ionization of sodiumlike ions from contributions due to direct electron-impact excitation and radiative cascades. The strength of resonance excitation of the 2p–3s magnetic quadrupole line through intermediate levels 4lnl’ (n=5,6) is measured to equal S=(2.8±0.8)×${10}^{\mathrm{-}19}$ ${\mathrm{cm}}^2$ eV.

We report the first experimental observation of the route to chaos for semiconductor lasers with weak optical feedback. For increasing feedback level, the laser undergoes a quasiperiodic route interrupted by frequency locking, which is also predicted by a comprehensive numerical analysis. Experiments further demonstrate the occurrence of quantum-noise-induced transitions between two attractors with different relaxation oscillation frequencies. The attractors are created by two subsequent Hopf bifurcations of the same external cavity mode.

The scattering length for electron-neon scattering is calculated very accurately for the first time by an ab initio method resulting in a rigorous upper bound of 0.2218${\mathit{a}}_0$. The calculation uses the multiconfiguration Hartree-Fock method which includes the effects of dynamical target polarization and electron correlation through the configuration interaction procedure. Phase shifts are also calculated for very low energies, an extremely difficult region for accurate theoretical investigation. The results are found to be in excellent agreement with the most accurate experimental results of O’Malley and Crompton.

The first experimental study is reported of a plasma produced by a heavy-ion beam. Relevant parameters for heating with heavy ions are described, temperature and density of the plasma are determined, and the hydrodynamic motion in the target induced by the beam is studied. The measured temperature and the free-electron density are compared with a two-dimensional hydrodynamic-model calculation. In accordance with the model, a radial rarefaction wave reaching the center of the target was observed and the penetration velocity of the ion beam into the xenon-gas target was measured.