Physisorbed ${\mathrm{N}}_2$ on Ni(111) has been studied by uv photoemission using rare-gas spacer layers. For 1${\mathrm{\pi }}_{\mathit{u}}$ ionization, vibrational fine structure has been resolved experimentally and analyzed in detail. The line shape of the electronic levels cannot be described by simple broadening of the corresponding gas-phase spectrum. Nearly no Lorentzian lifetime broadening could be observed; rather the vibrational peaks have a Gaussian line shape (narrowest FWHM, 0.21 eV). The observed peculiarities can be understood within a hopping model taking into account the deexcitation of the ionized molecule by hole transfer to neighbors.

We determine the atomic-scale friction associated with a layer of Pd atoms moving across a graphite substrate from ab initio total-energy calculations. We evaluate the friction energy caused by variations of the chemical bond strength and work against an external force (load) due to variations of the bond length along the trajectory. We find only a very small dependence of the Pd-graphite interaction on the adsorption site which gives rise to a very small friction coefficient μ≊${10}^{\mathrm{-}2}$ for loads near ${10}^{\mathrm{-}8}$ N. We also find μ to increase with load in agreement with recent experiments.

We show that the absorption spectrum of a superlattice in the presence of electric and magnetic fields parallel to the growth axis has striking properties. The spectrum contains resonances only and its structure can be simple or very complex depending on the ratio between the fields. As a function of inverse magnetic field the absorption per unit field is periodic and the integrated absorption forms a Devil’s staircase within each period.

Depletion-width-modulated ESR spectroscopy together with junction-capacitance techniques yield the total spin and charge change associated with the emission of electrons from deep defects (D) in intrinsic a-Si:H samples. We find that, in contrast to n-type doped samples, the predominant transition is from the ${\mathit{D}}^0$ to ${\mathit{D}}^{+}$ charge state near midgap. However, the magnitude of spin to charge change is too small for a defect having a significantly positive correlation energy, ${\mathit{U}}_{\mathit{e}\mathit{f}\mathit{f}}$; rather, it agrees with a nearly zero value of ${\mathit{U}}_{\mathit{e}\mathit{f}\mathit{f}}$ (-50<${\mathit{U}}_{\mathit{e}\mathit{f}\mathit{f}}$<+80 meV) between the singly and doubly occupied charge states.

We have observed a sharp reduction of the photoluminescence linewidth from a two-dimensional exciton system (coupled GaAs/AlGaAs quantum wells), at a certain critical temperature (${\mathit{T}}_{\mathit{c}}$) and under the influence of an electric field. We attribute this narrowing to a phase transition of the exciton system into an ordered state. The experiments suggest that a coherence length long enough to wash out the effect of the fluctuations, which cause the line broadening, is introduced below ${\mathit{T}}_{\mathit{c}}$.

Valence-hole relaxation by acoustic-phonon emission yields spin relaxation through the spin mixing of the hole-band states. In quantum wells, the hole-spin relaxation is incomplete. This leads to a unified theory of the polarization spectra for undoped and doped quantum wells, which explains the diverse features in the spectra.

A phenomenological free energy which includes a novel coupling between vector superconducting and magnetic order parameters is deduced for ${\mathrm{UPt}}_3$. Analysis of this free energy leads to a phase diagram in the H-T plane in agreement with experiment, and identifies the symmetry of the different phases. Some further experiments are suggested.

We report resistivity and superconducting transition temperature measurements on ultrathin MoC films (down to 4.0 Å). We find that the critical sheet resistance separating the superconducting and insulating states at T=0 lies in the range 2.8–3.5 kΩ, which is about one-half of that reported for Bi films.

The superconductive transition of c-axis-oriented ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ thin films, as characterized by measurements of the temperature dependence of the magnetic penetration depth, is found to occur below the Ginzburg-Landau mean-field transition. The divergence of the penetration depth at the transition is in good quantitative agreement with the predictions of a percolation model.

The effect of coupling between layers of the high-${\mathit{T}}_{\mathit{c}}$ superconductors was studied by varying both the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ and ${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ layer thicknesses in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$/${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ superlattices. Superconductivity was found even when the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ layers had nominal thicknesses as small as 12 Å (one unit-cell thickness) but coupling between unit-cell layers is needed to achieve a ${\mathit{T}}_{\mathit{c}}$ of 90 K. The decrease of ${\mathit{T}}_{\mathit{c}}$ in isolated ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ layers with increasing sheet resistance is on the same scale of about ħ/${\mathit{e}}^2$ as observed for ordinary superconductors.

We report measurements of entropy transport due to vortex motion in single-cyrstal ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$. From these magnetothermal measurements we are able to determine thermodynamic properties, such as the transport line energy and superconducting phase boundary ${\mathit{T}}_{\mathit{c}}$(H), as well as transport parameters associated with flux motion. Below ${\mathit{T}}_{\mathit{c}}$, the data are compared with the microscopic theory of Caroli and Maki. Interestingly, we also find entropy transport in the fluctuation regime above ${\mathit{T}}_{\mathit{c}}$, which grows with applied field and for which no theory exists.

A theoretical model is proposed to explain the magnitude of excess 1/f magnetic flux noise in high-${\mathit{T}}_{\mathit{c}}$ superconducting SQUID’s. Based on a model for 1/f noise generation due to the multiple elastic scattering of quasiparticles with disordered mobile vortices, we show that the 1/f flux noise magnitude has the following features: It increases with T below ${\mathit{T}}_{\mathit{c}}$, sharply falls off with T above ${\mathit{T}}_{\mathit{c}}$, and decreases with an increasing energy gap. Its magnetic-field dependence is also shown to be inversely proportional to the square of the voltage-to-flux transfer function. Our results are shown to be in good agreement with recent experimental measurements.

A renormalization-group recursion formula in a two-dimensional O(N)-invariant model is described by an integral kernel ${\mathit{K}}_{\mathit{N}}$, a probability density exhibiting perturbative asymptotic freedom for all N>1. The ${\mathit{K}}_{\mathit{N}}$’s are qualitatively the same for all N>1 and thus there may be Kosterlitz-Thouless–type transitions in some non-Abelian systems. In fact, we establish that the correlation functions of the O(4) σ model are bounded from below by ones of a random-coupling XY model in which the strong-coupling regions percolate over ${\mathit{Z}}^2$ as the original coupling constant becomes large.

We analyze the static spherically symmetric Einstein-Yang-Mills equations with SU(2) gauge group and show numerically that the equations possess asymptotically flat solutions with regular event horizon and nontrivial Yang-Mills (YM) connection. The solutions have zero global YM charges and asymptotically approximate the Schwarzschild solution with quantized values of the Arnowitt-Deser-Misner mass. Our result questions the validity of the ‘‘no-hair’’ conjecture for YM black holes. This work complements the recent study of Bartnik and McKinnon on static spherically symmetric Einstein-Yang-Mills soliton solutions.

It is argued that, at a first-order phase transition, false-vacuum bubbles may occasionally collapse to become black holes. If the critical temperature ${\mathit{T}}_{\mathit{c}}$ is less than a TeV, these black holes, which have a mass proportional to ${\mathit{M}}_{\mathrm{Pl}}^2$/${\mathit{T}}_{\mathit{c}}$, could survive until today to be the dark matter. Alternatively, evaporation of black holes could give rise to relic particle abundances.

We study numerically the onset of inflation under inhomogeneous initial conditions for various inflationary scenarios and various initial parameters. We solve the full Einstein equations for the gravitational field coupled to scalar-field and radiation-field sources, under the assumptions of spherical symmetry and a closed-universe background. We find that a large initial inhomogeneity suppresses the onset of inflation, and that the scalar field must have values suitable for inflation over a region of several horizon sizes for the inflationary phase to begin.

Neutrino dipole moments ${\mathrm{\mu }}_{\nu }$ would increase the core mass of red giants at the helium flash by δ${\mathit{M}}_{\mathit{C}}$=0.015${\mathit{M}}_{\mathrm{\odot }}$${\mathrm{\mu }}_{\nu }$/${10}^{\mathrm{-}12}$${\mathrm{\mu }}_{\mathit{B}}$ (where ${\mathrm{\mu }}_{\mathit{B}}$ is the Bohr magneton) because of enhanced neutrino losses. Existing measurements of the bolometric magnitudes of the brightest red giants in 26 globular clusters, number counts of horizontal-branch stars and red giants in 15 globular clusters, and statistical parallax determinations of field RR Lyrae luminosities yield δ${\mathit{M}}_{\mathit{C}}$=(0.009±0.012)${\mathit{M}}_{\mathrm{\odot }}$ so that conservatively ${\mathrm{\mu }}_{\nu }$<3×${10}^{\mathrm{-}12}$${\mathrm{\mu }}_{\mathit{B}}$.

Assuming that charged massive particles (CHAMPs) have a Maxwell-Boltzmann velocity distribution, truncated at the galactic escape velocity, and that CHAMPs comprise the galactic dark matter, we can rule out CHAMPs with masses between 350 and 8.6×${10}^4$ TeV/${\mathit{c}}^2$ in the solar neighborhood. In addition, we can rule out masses of neutral CHAMP-proton composites between 100 and 4×${10}^4$ TeV/${\mathit{c}}^2$, provided that they charge exchange with C or O nuclei with a cross section having a value in the interval 30 mb–30 b.

We discuss the Becchi-Rouet-Stora-Tyutin (BRST) cohomology of compact Lie algebras and their infinite extensions in gauge theories. The co-BRST operator is used to construct a BRST-invariant operator W, the zero modes of which are in one-to-one correspondence with the BRST cohomology. Nontrivial solutions to the BRST cohomology conditions differing only in ghost content are shown to exist. A possible connection with supersymmetric topological quantum theories is observed.

Assuming that only the top quark gets a tree-level mass due to electroweak gauge-symmetry breaking, a model is presented where all other known quarks and leptons acquire radiative masses in a hierarchical manner. This is the first such model where the 3×3 charged-current quark mixing matrix is realistically generated without the fine tuning of parameters.