X-ray studies of thick, free-standing films of the liquid-crystal compound $n$-butyl 4′-$n$-hexyloxybiphenyl-4-carboxylate (46OBC) have determined the critical behavior at the three-dimensional hexatic-$B$-to-smectic-$A$ phase transition. The position and width of the scattering peaks exhibited $1-\alpha$ singularities in agreement with theory and heat-capacity results. Measurements on two-layer films of 46OBC provided evidence for a continuous, two-dimensional, hexatic-to-liquid transition.

We study the evolution of liquid structure factors with changing bond-orientational (hexatic) order. The form of the radial structure factor is found to depend strongly on the mean square amplitude of fluctuations in the hexatic order parameter. The effects of substrates on the hexatic ordering process in two dimensions are also examined. Our results apply not only to hexatic fluids, but also to other liquids whose local densities are coupled to $x-y$-like primary order parameters.

We report high-precision measurements of the shear modulus, viscosity, and specific heat of cholesteryl nonanoate through its blue phases (BPI-BPIII). For BPI and BPII we find a shear modulus which remains finite as the frequency approaches zero, indicating that they are viscoelastic solids. We find that the cholesteric and BPIII have a viscoelastic behavior similar to each other and that as expected the isotropic phase is a Newtonian fluid. Both our mechanical and calorimetric data show for the first time that BPIII is a stable and distinct thermodynamic phase.

The first observation of the nematic-smectic-$A$-smectic-$C$ multicritical point in a single-component liquid-crystal system is reported. High-resolution pressure-temperature data are presented, which support the view that the phase diagram is universal near the nematic-smectic-$A$-smectic-$C$ point.

Tracer diffusion coefficients $D^{*}$ of deuterated polystyrene of molecular weight $M$ diffusing into polystyrene of molecular weight $P$ are measured by means of an ion-beam analysis method. While for $P>\mathrm{}200\mathrm{}000$, these $D^{*}$ values are independent of $P$, consistent with a reptation mechanism ($D^{*}=D_R$) for the diffusion of $M$ chains, $D^{*}$ increases rapidly as $P$ is decreased, in quantitative agreement with $D^{*}=D_R+D_{\mathrm{CR}}$, where $D_{\mathrm{CR}}$ is the diffusion coefficient of the $M$ chain by constraint release.

A renormalization-group theoretical result for the concentration dependence of the cooperative diffusion constant of semidilute polymer solutions is given. The result satisfactorily explains the recent experimental result by Wiltzius et al. This is the first application of the renormalization-group theory to semidilute-solution transport properties.

An analysis of Kossel lines is reported for colloidal crystals of highly charged polystyrene particles in semidilute aqueous solutions. The phase transformation between fcc and bcc structures and their coexistence is observed. Lattice constants are found to be systematically smaller than those calculated from the uniform particle distribution throughout solution. This observation shows that a long-range weak attraction also exists between colloidal particles. The fine structure of the Kossel line is explained by use of dynamical theory.

Transmission and electron-yield measurements of the Si $L_{2,3}$ edge as a function of alloy composition $x$ in ${\mathrm{Si}}_x{\mathrm{Ge}}_{1-x}$ alloys reveal anomalous spectra for $x\approx 0.15$, the composition of the crossover of the conduction-band minima from $L$ in the Brillouin zone (for $x<\mathrm{}0.15\mathrm{}$) to $\Delta$ (for $x>\mathrm{}0.15\mathrm{}$). We interpret this as evidence for a sudden change of the ground state of the exciton from an extended effective-mass Wannier-type state for $x<\mathrm{}0.1\mathrm{}$, to a deep localized state near $x=0.15\mathrm{}$, and back to an extended state for $x>\mathrm{}0.2\mathrm{}$.

The temperature dependence of near-edge x-ray-absorption fine-structure spectra above the C $K$ edge and S $L_{2,3}$ edge for thiophene (${\mathrm{C}}_4$${\mathrm{H}}_4$S) on Pt(111) reveals changes in the chemisorption geometry and the cleavage of the carbon-sulfur bond. Our results suggest the formation of a metallocycle intermediate after desulfurization with the S atom replaced by a Pt surface atom. The near-edge x-ray-absorption fine-structure results are corroborated by thermal-desorption, x-ray photoemission, and electron-energy-loss data.

Energetic positrons implanted into various ionic crystal surfaces are reemitted with a maximum kinetic energy (14.7 ± 0.7 eV for LiF) near the band-gap energy. The evidence suggests that, at least for LiF and NaF, the emission originates from bulk-formed positronium (Ps) breaking up after diffusing to the surface. In our model, the positrons are Auger emitted when the Ps electrons recombine with holes at the surface, and the spectrum of positron energies thus reflects the density of empty surface states.

A frequency regime in which electromagnetic waves in a strongly disordered medium undergo Anderson localization in $d=3\mathrm{}$ dimensions is suggested. In the presence of weak dissipation in $d=2+\epsilon$ it is shown that the renormalized energy absorption coefficient increases as the photon frequency $\omega$ approaches a mobility edge ${\omega }^{*}$ from the conducting side as $\alpha \sim {({\omega }^{*}-\omega )}^{-\frac{\mathrm{}(d-2\mathrm{})\mathrm{}\nu }{2}}$, $\nu =\frac{1}{\epsilon }$. This mobility edge occurs at a frequency compatible with the Ioffe-Regel condition.

We present theory and extended experimental results for the large shift in optical absorption in GaAs-AlGaAs quantum well structures with electric field perpendicular to the layers. In contrast to the Stark effect on atoms or on excitons in bulk semiconductors, the exciton resonances remain resolved even for shifts much larger than the zero-field binding energy and fields > 50 times the classical ionization field. The model explains these results as a consequence of the quantum confinement of carriers.

A quantum Monte Carlo method is developed to treat path integrals at finite temperature and with topological constraints to study a periodic model of a granular superconductor. The results obtained provide strong evidence for a low-temperature first-order reentrant transition induced by zero-point quantum fluctuations.

The spatial expansion of electron-hole plasma created by nanosecond pulsed-laser excitation of Si is examined by time-resolved luminescence imaging. On a several-nanosecond time scale, the plasma created by 0.5-μJ excitation expands from the excitation point at subsonic velocities, contrary to a fast-diffusing plasma model recently proposed to explain spectroscopic anomalies. For 50-μJ excitation, time-resolved images show a shell of plasma expanding at near-sonic velocities, indicating phonon-wind-driven transport with highly non-linear damping.

The $N$-color Ashkin-Teller model is solved exactly in the large-$N$ limit in two dimensions. The phase diagram is found. It is shown that for positive four-spin coupling the transition is first order while for negative four-spin coupling the transition is continuous and Ising type. The specific heat near the second-order transition is calculated and it is found to be finite at the transition because of large corrections to scaling. The latent heat and correlation length at the first-order transition are also calculated.

We have observed inclusive production of $D^0$ and $D^{+}$ mesons, and their charge conjugates, in $e^{+}{e}^{-}$ annihilation at 29 GeV on the basis of a data sample of 106 p${\mathrm{b}}^{-1\mathrm{}}$. These signals correspond to $R$ values of $R(D^0+{\bar{D}}^0)=1.8\mathrm{}\pm 0.5$ and $R(D^{+}+D^{-})=1.2\mathrm{}\pm 0.4$. Taking the $D^{+}$ and $D^0$ data together, we measure a charge asymmetry of $A=-0.08\mathrm{}\pm 0.12$ for charmed quarks. A comparison of $R(D+\bar{D})$ with $R(D^{*}+{\bar{D}}^{*})$ obtained via the process $D^{*+}\to D^0{\pi }^{+}$ gives a $\frac{D}{D^{*}}$ ratio of ${1.0}_{-0.2\mathrm{}}^{+0.3\mathrm{}}$, indicating that direct $D^{*}$ production dominates over direct $D$ production.

Recent experimental results give an upper bound of 164 MeV for the tau neutrino mass. By combination of experimental limits on neutrino mixing angles with constraints on the destruction of deuterium after nucleosynthesis it is possible to raise our previous lower bound of 23 MeV to 50 MeV, and modulo recent less certain results, to 120 MeV. If data on mixing angles improve by several orders of magnitude it should be possible to rule out completely a heavy [$>O\left(1\mathrm{} \mathrm{MeV}\right)$] tau neutrino mass.

It is claimed that extended monopoles of the 't Hooft-Polyakov type admit parastatistics in the sense of Finkelstein and Rubenstein.

Measurements of the spin observables $A_{\mathrm{NN}}\left({90}^{\circ }\right)$ and $A_{N0\mathrm{}}\left({90}^{\circ }\right)$ for the reaction $\mathrm{pp}\to d{\pi }^{+}$ between 500 and 800 MeV are presented and compared with previous measurements and with predictions from theories and a partial-wave analysis. These are the first available measurements of $A_{\mathrm{NN}}$ above 590 MeV.

The invariant cross section for production of jet pairs in 400-GeV/c $\mathrm{pp}$ interactions has been measured as a function of $p_T$ in the $p_T$ range 4 to 9 GeV/c. The results are in good agreement with predictions of perturbative QCD models.