Inclusive ${\Omega }^{-}$ production in $e^{+}{e}^{-}$ annihilation at 29 GeV has been measured with the Mark II detector. From an integrated luminosity of 207 ${\mathrm{pb}}^{-1\mathrm{}}$, we determine a production rate of 0.014±0.006±0.004 ${\Omega }^{-}$, ${\overline{\Omega }}^{+}$ per hadronic event. This is roughly 35 times the Lund-model prediction of 0.0004 ${\Omega }^{-}$, ${\overline{\Omega }}^{+}$ per hadronic event, but comparable to the Webber-model prediction of 0.006 ${\Omega }^{-}$, ${\overline{\Omega }}^{+}$ per hadronic event. The large rate of ${\Omega }^{-}$ production, compared with production rates for other baryons, and with theoretical predictions based on diquark models, indicates that spin suppression does not hold for ${\Omega }^{-}$ production.

The observed pattern of the population and decay of the superdeformed band of ${}_{}{}^{152}{}_{}{}^{}\mathrm{Dy}$ is related to the large splitting of the giant dipole resonance based on the superdeformed minimum as well as to the low level density associated with it, and to the sudden onset of static pairing correlations taking place at the rotational frequency $\hslash \omega \approx 0.3$ MeV. A new method for spectroscopic studies of superdeformed nuclei is suggested.

Relativistic mean-field theory is used to describe ground-state deformations of nuclei. The relativistic Hartree equations are solved by expansion of the nucleon wave functions and the meson field in a harmonic-oscillator basis. Using parameters adjusted in spherical nuclei we find in axially symmetric deformed nuclei good agreement with conventional nonrelativistic calculations based on density-dependent Skyrme forces.

We demonstrate the feasibility of using $\delta$ electrons to study the influence of time-delay effects associated with isolated nuclear resonances on atomic excitation. With present techniques, variations in atomic ionization probabilities on the order of 20% can be detected. We present results for the case of $\delta$-electron emission across the 12-keV-wide $p$+${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ elastic-scattering resonance at 4.808 MeV. Such studies can extend measurements of nuclear-atomic time-delay effects to a range of time and energy previously inaccessible to experimental investigation.

High-resolution extreme-ultraviolet spectra from solid targets irradiated by a picosecond KrF laser focused to ${10}^{16}$ W/${\mathrm{cm}}^2$ have been recorded. The line profiles of transitions in Li-like fluorine and oxygen are asymmetric and up to 2 Å in width. Calculations indicate the presence of transitions of the type $2p-3p$ and other forbidden Stark components.

We report thermal-convection experiments using helium I where the vertical temperature difference has an externally imposed time dependence. Both positive and negative threshold shifts are observed depending on the strength and rate of modulation, as well as a depression of the initial slope of the heat transfer above threshold, in qualitative agreement with predictions of Ahlers, Hohenberg, and Lücke and of Rosenblat and Herbert.

Surface extended x-ray-absorption fine-structure measurements have been performed on ${\mathrm{C}}_2$${\mathrm{H}}_2$ and ${\mathrm{C}}_2$${\mathrm{H}}_4$ on Cu(100) at 60 K. Clear evidence is found for the elongation of C-C bonds of 0.13(7) Å for ${\mathrm{C}}_2$${\mathrm{H}}_4$ and 0.22(5) Å for ${\mathrm{C}}_2$${\mathrm{H}}_2$. This is the first direct evidence from an appropriate structural probe for the stretching of molecules upon chemisorption. It solves a long-standing debate and helps to calibrate other indirect information on intramolecular bond lengths as taken from vibrational spectroscopy and shifts of the $\sigma$-shape-resonance position.

The novel $\beta$-incommensurate transition in quartz and berlinite is studied in the $\epsilon =4-d$ expansion. It has no stable fixed point, indicating that the transition is first order.

A combination of magnetic susceptibility and Mössbauer measurements on quasicrystalline $i-{\mathrm{Al}}_{74}{\mathrm{Mn}}_{20-x}{\mathrm{Fe}}_x{\mathrm{Si}}_6$ ($0.02\le x\le 7.5$) establishes that Mn atoms in $i$-${\mathrm{Al}}_{74}$${\mathrm{Mn}}_{20}$${\mathrm{Si}}_6$ occupy two distinct classes of sites, and that Fe substitutes for only one of them. Thw two classes are distinguished by the possession or otherwise of a localized magnetic moment. The data are consistent with a structure of interconnecting Mackay icosahedra (MI) in which localized moments are possessed only by Mn atoms adjacent to "broken" MI connections. The implied connectivity of the resulting MI network is close to that anticipated for a packing of MI on a three-dimensional Penrose-tile lattice.

The study of the roughening transition of a stepped surface of Cu(113) has been performed with the use of synchrotron grazing-incidence x-ray scattering. Surface roughening was observed directly from x-ray scans of the step superlattice reflection between 300 and 750 K. The results are interpreted with a theory of Villain, Grempel, and Lapujoulade, and the values of the transition temperature, the step-step repulsive energy, and the kink energy are determined to be 620±10 K, 7.4±0.9 meV, and 181±6.5 meV, respectively.

A simple method is proposed to calculate total energies of solids with lattice defects. Lattice-defect energies are determined via perturbation theory on a crystal whose lattice constant is chosen optimally. The method is applied to the computation of surface energies of a series of transition metals as a function of surface orientation. An inverse correlation between surface-atom density and surface energy per surface atom is obeyed in every case, although electronic rearrangement effects are significant. Good agreement is obtained with available experimental and theoretical results.

We have measured the temperature-dependent onset of strain relief in metastable ${\mathrm{Si}}_x{\mathrm{Ge}}_{1-x}$ strained layers grown on Ge substrates. On the basis of these measurements, and physical arguments, we propose that strained-layer breakdown is most directly determined not by thickness and lattice mismatch, but rather by (1) an "excess" stress (the difference between that due to misfit strain and that due to dislocation line tension) and (2) temperature. With use of these parameters, observed regimes of stability and metastability are shown to be described within a simple, unified framework.

We have observed unambiguously the coherent tunneling of photoexcited electrons through an unequal ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ double-barrier superlattice structure. This observation confirms the Fabry-Perot quantum-mechanical interference mechanism in this system. On the basis of this result, a high-responsivity infrared photodetector can be designed with low dark current.

Conversion-electron Mössbauer measurements have been used to study hyperfine fields in Fe/Ag(100) single-crystal multilayer films at temperatures down to 15 K. The Fe layer thicknesses corresponded to 1.0, 2.4, and 5.5 monolayers (ML). From the relative intensity of the Mössbauer lines we show conclusively that the orientation of the magnetic moment of the 1.0- and 2.4-ML films in zero applied field is perpendicular to the film plane while the orientation of the 5.5-ML sample is in plane at room temperature and partially out of plane at low temperatures. The low-temperature hyperfine fields are enhanced compared to bulk Fe.

Through the calculations of the diagonal and off-diagonal elements of the full $N$-particle density matrix we show that all essential features of the Laughlin variational wave functions can be derived in a generalized cooperative-ring-exchange theory of the fractional quantum Hall fluid.

The electronic structure of five antimony chalcogenides, Sb${\mathrm{I}}_3$, ${\mathrm{Sb}}_2$${\mathrm{Te}}_3$, SbTeI, TlSb${\mathrm{S}}_2$, and ${\mathrm{Tl}}_3$Sb${\mathrm{S}}_3$, is investigated. Information from x-ray diffraction, Mössbauer spectroscopy, electrical measurements, and photoemission spectroscopy is combined with band-structure calculations to get a coherent picture of these materials. The chemical notion of the asymmetry and delocalization of the $5s^2$ antimony lone pair is given a quantitative interpretation both from the full band-structure calculation and from a simple molecular description.

An Ansatz is proposed for the joint probability distribution of the eigenvalues of the transfer matrix in the quantum transport problem, based on symmetry arguments and a "maximum entropy" hypothesis. The local statistical behavior of the distribution is predicted to be that of the well-known random-matrix ensembles of Wigner and Dyson; and this result is confirmed by independent numerical calculations. For metals this behavior leads to size- and disorder-independent conductance fluctuations, and this approach suggests an alternative framework for the scaling theory of localization.

Two-dimensional-angular-correlation experiments using positron-annihilation spectroscopy were performed on a series of disordered Cu-rich CuPd-alloy single crystals. The results are compared with theoretical calculations based on the Korringa-Kohn-Rostoker coherent-potential approximation. Our experiments confirm the theoretically predicted flattening of the alloy Fermi surface near [110] with increasing Pd concentration. The momentum densities and the two-dimensional-angular-correlation spectra around zero momentum exhibit a characteristic signature of the electronic states near the valence-band edge in the alloy.

Epitaxial bcc Fe films grown on Ag(001) are ferromagnetic. At $T=30\mathrm{}$ K, the 3- to 4-monolayer films are perpendicularly magnetized at remanence, whereas thinner and thicker films have their magnetization in plane. Above $T=100\mathrm{}$ K no perpendicular remanence has been observed for any film thickness. Films consisting of more than 5 monolayers have a Curie temperature equal to that of bulk bcc Fe.

The infinitesimal stability of a spherical vesicle (closed membrane) is studied as a function of the pressure difference between the outer and inner media. It is found that above some threshold pressure the spherical vesicle can be deformed into a shape associated with $l\mathrm{th}$-order spherical harmonics. The comparison with numerical examples calculated previously by Deuling and Helfrich shows good agreement. Some applications to red blood cells are discussed.